摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of Materials Chemistry Scientific Editor Staff Editor Professor Anthony R. West Mrs. Janet M. Leader Department of Chemistry The Royal Society of Chemistry University of Aberdeen Thomas Graham House M eston Wa Ik Science Park Aberdeen AB9 2UE, UK Cambridge CB4 4WF, UK Editorial Secretary: Miss J. E. Chapman Materials Chemistry Editorial Board Anthony R. West (Aberdeen) (Chairman) C. Richard A. Catlow (London) David A. Rice (Reading) David A. Dunmur (Sheffield) Rodney P. Townsend (Bebington) H. Monty Frey (Reading) Allan E. Underhill (Bangor) John W. Goodby (Hull) Graham Williams (Swansea) John D. Wright (Canterbury) International Advisory Editorial Board M. A. Alario-Franco (Madrid) D.Kohl (Aachen) K. Bechgaard (Copenhagen) M. Lahav (Rehovot) J. D. Birchall (Runcorn) A. J. Leadbetter (Daresbury) D. Bloor (Durham) P. M. Maitlis (Sheffield) A. K. Cheetham (Oxford) J. S. Miller (Wilmington) E. Chiellini (Pisa) P. S. Nicholson (Hamilton) M. G. Clark (Wembley) M. Nygren (Stockholm) P. Day (Grenoble) V. Percec (Cleveland) D. Demus (Halle) C. N. R. Rao (Bangalore) B. Dunn (Los Angeles) M. Ratner (Evanston) W. J. Feast (Durham) J. Rouxel (Nantes) A. Fukuda (Tokyo) R. Roy (University Park, PA) D. Gatteschi (Florence) J. L. Serrano (Zaragoza) A. M. Glass (Murray Hill) J. N. Sherwood (Glasgow) J. B. Goodenough (Austin) J. Simon (Paris) G. W. Gray (Poole) J. F. Stoddart (Sheffield) A. C. Griffin (Cambridge) S. Takahashi (Osaka) S-i.Hirano (Nagoya) G. J. T. Tiddy (Bebington and Salford) P. Hodge (Manchester) B. J. Tighe (Birmingham) H. lnokuchi (Okazaki) Yu. D. Tretyakov (Moscow) W. Jeitschko (Munster) R. J. P. Williams (Oxford) 0. Kahn (Orsay) R. Xu (Changchun) Journal of Materials Chemistry (ISSN 0959-9428) is published six times a year by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Transactions Ltd., Blackhorse Road, Letchworth, Herts SG6 1 HN, UK. NB Turpin Transactions Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1991 Annual subscription rate EC (inc. UK) f175.00,USA $395.00, Rest of World f 195.00.Customers should make payments by cheque in stirling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11 003. USA Postmaster: send address changes to Journal of Materials Chemistry, Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11 003. Second Class postage paid at Jamaica, NY 11431. All other dispatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. PRINTED IN THE UK. @ The Royal Society of Chemistry, 1991. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the publishers.Professor A. R. West, Scientific Editor Mrs. J. M. Leader, Staff Editor Tel.: Aberdeen (0224) 272918 Tel.: Cambridge (0223) 420066 Fax: (0224) 272938 E-Mail (JANET): Telex: 73458 UNIABN G RSCI @UK.AC.RL.GB Fax: (0223) 420247 or 423623 Telex: 818293 ROYAL G INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication in two forms, Articles and Materials Chemistry Communications. These should describe original work of high quality dealing with the synthesis, structures, properties and applications of materials, particularly those associated with advanced technology.Art icles Full papers contain original scientific work that has not been published previously. How- ever, work that has appeared in print in a short form such as a Materials Chemistry Com- munication is normally acceptable. Four copies of Articles including a top copy with figures etc. should be sent to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB44WF, UK. Materials Chemistry Communications Ma ter iaIs C h em ist ry Com m u n icat ions contain novel scientific work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restric- ted to two pages of the double-column A4 format.The manuscript will be returned for reduction if this length is exceeded. For a Communication consisting entirely of text and ten references, with no figures, equations or tables, this corresponds to approximately 1600 words plus an abstract of up tc; 40 words. Submission of a Materials Chemistry Com- munication can bg made either to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK, or viaa member of the Interna- tional Advisory Editorial Board. In the latter case, the top copy of the manuscript includ- ing any figures etc., together with the name of the person to whom the Communication is being submitted, should be sent simultan- eously to the Editor at the Cambridge address. Authors may wish to contact the Board mem- ber to ensure that he is available to arrange review of the manuscript within reasonable time. In order to avoid delay in publication, proofs of Communications are not sent to authors unless this is specifically requested. Full details of the form of manuscripts for Articles and Materials Chemistry Communi- cations, conditions for acceptance etc. are given in issue number one of Journal of Materials Chemistry published in January of each year, or may be obtained from the Staff Editor. There is no page charge for papers published in Journal of Materials Chemistry. Fifty reprints are supplied free of charge. Any author who is publishing in Journal of Materials Chemistry for the first time is entitled to a free copy of the issue in which the paper appears.

ISSN:0959-9428    

DOI:10.1039/JM99101FX005

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99101BX007

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ISSN 0959-9428 JMACEP(2) 157-306 (1991) Journal of Materials Chemistry Synthesis, structures, properties and applications of materials, particularly those associated with advanced technology CONTENTS 157 FEATURE ARTICLE. Solid electrolytes and mixed ionic-electronic conductors: an applications overview A. R. West 163 Compound and solid-solution formation, phase equilibria and electrical properties in the ceramic system Zr0,-La,O,- Ta,O, C. Zheng and A. R. West 169 Thermal and X-ray diffraction studies of liquid crystals incorporating a perfluoroalkyl group T. Doi, Y. Sakurai, A. Tamatani, S. Takenaka, S. Kusabayashi, Y. Nishihata and H. Terauchi 175 Structure evolution during thermal processing of high- T, ceramic superconductors produced using sol-gel techniques G.Kordas, G. A. Moore, J. D. Jorgensen, F. Rotella, R. L. Hitterman, K. J. Volin and J. Faber 181 Role of fractal structure on thin-film processing of YBa2Cu30,-, using alkoxide sols G. Kordas, G. A. Moore, M. B. Salamon and J. B. Hayter 191 Depth profiles and electrochemical properties of IrO, electrocatalysts stabilized with TiO, A. De Battisti, A. Barbieri, A. Giatti, G. Battaglin, S. Daolio and A. Boscolo Boscoletto 197 Paramagnetic rod-like liquid crystals, bis[5-(4-alkoxybenzoyloxy)salicylaldehyde]copper(11) E. Campillos, M. Marcos, J. L. Serrano and P. J. Alonso 20 1 Database analysis of crystal-structure-determining interactions involving sulphur: implications for the design of organic metals G. R. Desiraju and V. Nalini 205 Intercalation of 2-aminoethylferrocene into the layered host lattices MOO,, 2H-TaS, and a-Zr(HPO,), HzO K.Chatakondu, C. Formstone, M. L. H. Green, D. O’Hare, J. M. Twyman and P. J. Wiseman 213 X-Ray photoelectron spectroscopy of new soluble polyaniline perchlorates: evidence for the coexistence of polarons and bipolarons M. B. Inoue, K. W. Nebesny, Q. Fernando and M. Inoue 217 Alkyloxy-substituted CTTV derivatives that exhibit columnar mesophases V. Percec, C. G. Cho and C. Pugh 223 Oxonol dyes: X-ray crystallographic and solid-state ’ nuclear magnetic resonance studies of some organic semiconductors M. C. Grossel, D. J. Edwards, A. K. Cheetham, M. M. Eddy, 0.Johnson and S. R. Postle 233 Elastic and Coulombic contributions to real-space hole pairing in doped La,CuO, X.Zhang and C.R. A. Catlow 239 Preparation and crystal structure of U-phase Ln3(Si,-,A13 +x)012+xNz-x(~ x0.5,Ln =La, Nd) P-0. Kall, J. Grins, P-0. Olsson, K. Liddell, P. Korgul and D. P. Thompson 245 Non-linear optical properties of Group 10 metal alkynyls and their polymers W. J. Blau, H. J. Byrne, D. J. Cardin and A. P. Davey 25 1 Synthesis and mesomorphism of stilbazole complexes of rhodium(1) and iridium@) D. W. Bruce, D. A. Dunmur, M. A. Esteruelas, S. E. Hunt, R.Le Lagadec, P. M. Maitlis, J. R. Marsden, E. Sola and J. M. Stacey 255 High-birefringence materials using metal-containing liquid crystals D. W. Bruce, D. A. Dunmur, P. M. Maitlis, M. M. Manterfield and R. Orr 259 Analytical characterization by X-ray photoelectron spectroscopy of quaternary chalcogenides for cathodes in lithium cells C.Malitesta, D. Centonze, L. Sabbatini, P. G. Zambonin, L. P. Bicelli and S. Maff 265 Electron paramagnetic resonance spectroscopy of Cu+/Cu2+ p”-alumina D. Gourier, D. Vivien, B. Dunn and L. Salmon 27 1 Aromatic ether-ketone-‘)(’ polymers. Part I.-Synthesis and properties P. J. Horner and R. H. Whiteley 28 1 Ammonium-ion motions in the hexagonal tungsten trioxide framework. A neutron scattering study of the bronze (NH,),,,,WO, and of [(NH4),0]o,08,W0, R. C.T. Slade, P. R. Hirst and B. C. West 289 Solid-solution formation, electrical properties and zero-resistance behaviour in the spinel system MgzTi04-MgTi204 T. J. Cogle, C. A. S. Mateus, J. H. Binks and J. T. S. Irvine 293 Synthesis of a crowned azobenzene liquid crystal and its application to thermoresponsive ion-conducting films K.Kimura, M. Hirao and M. Yokoyama MATERIALS CHEMISTRY COMMUNICATIONS 295 Structure of La,,,Sr,,,CuO,,,,. Powder neutron diffraction on very small sample volumes D. B. Currie and M. T. Weller 297 Isomorphous substitution in non-linear optical KTiOPO,. Time-of-flight neutron powder diffraction study of K,,,Rb,,,TiOPO, S. J. Crennell, A. K. Cheetharn, J. A. Kaduk and R. H. Jarrnan 299 Promotion of the metal-oxide support interaction in the Ni/TiO, catalyst. Crucial role of the method of preparation, the structure of TiO, and the NiTiO, intermediate G. Sankar, C. N. R. Rao and T. Rayrnent 301 Novel chemical preparative route for semiconducting MoSe, thin films K. C. Mandal and 0.Savadogo 303 Photoinduced phase transitions in novel liquid-crystalline copolymers C. H. Legge, M. J. Whitcornbe, A. Gilbert and G. R. Mitchell 305 Book Reviews: R. M. Barrer; F. P. Glasser; W. C. Thompson Note: Where an asterisk appears against the name of one or more authors, it is included with the authors’ approval to indicate that correspondence may be addressed to this person.

ISSN:0959-9428    

DOI:10.1039/JM99101FP021

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

Cumulative Author Index 1991 Adams J. M., 43 Adams P. N., 141 Allen Sir Geoffrey, 1 Allen G. C., 69, 73 Alonso P. J., 197 Annen M. J.. 79 Apblett A. W., 143 Arhancet J. P., 79 Ball R. G. J., 105 Barbieri A., 191 Barrer R. M.. 305 Barron A. R., 143 Battaglin G., 191 Bicelli L. P., 259 Binks J. H.. 289 Blau W. J., 145 Boscolo Boscoletto A., 191 Brock T., 151 Brown I.T., 69 Bruce D. W., 251, 255 Byrne H. J., 245 Campillos E., 197 Cardin D. J., 245 Catlow C. R. A,, 233 Centonze D., 259 Chambers R. D., 59 Chatakondu K., 205 Cheatham L. K., 143 Cheetham A. K.. 113, 223, Chen C-Y., 79 Cho C.G., 217 Chvatal Z., 59 Cogle T. J.. 289 Cook M. J., 121 Cox P. A., 51 Crennell S. J.. 113, 297 Currie D. B.. 295 297 Davis M. E., 79 De Battisti A., 191 Dent A. J., 103 Desiraju G. R., 201 Dickens P.G., 105, 137 Dodd S.M., I1 Doi T., 169 Dunmur D. A., 251, 255 Dunn B., 265 Dyer A., 43 Eddy M. M., 223 Edge S., 103 Edwards D. J., 223 Egdell R. G., 63 Esteruelas M. A., 251 Faber J., 175 Fernando Q., 213 FitzGerald E. T., 51 Flavell W. R., 63 Formstone C. A., 51, 205 Giatti A., 191 Gibb T.C., 23 Gier T. E., 153 Gilbert A., 303 Glasser F. P., 305 Golden M. S., 63 Golden S. J., 63 Goodby J. W., 5 Gourier D., 265 Greaves C., 17 Green M. L. H., 205 Grey C. P., I13 Grins J., 239 Grossel M. C., 223 Harrison K. J., 121 Harrison W. T. A., 153 Hayter J. B., 181 Hirao M., 293 Horner P. J., 271 Howe S.D., 29 Howlin B., 29 Hunt S. E., 251 Hursthouse M. B., 139 Imaeda K., 37 Inokuchi H., 37 Inoue M., 213 Inoue M. B., 213 Irvine J. T. S., 147, 289 Iwasawa N., 37 Jarman R.H., 113, 297 Jiang M. R. M., 11 Johnson O., 223 Jones A. C., 139 Jorgensen J. D., 175 Jutson J. A., 73 Kaduk J. A., 113,297 Kaharu T., 145 Kathirgamanathan P., 103, Kimura K., 293 Kordas G., 97, 175, 181 Korgul P., 239 Kurmoo M., 51 Kusabayashi S., 169 Lee C. K., 149 Legge C.H., 303 Le Lagadec R., 251 Li H-X., 79 Liddell K., 239 Lo Jacono M., 129 Lukes P., 29 Maffi S., 259 Maitlis P. M., 251, 255 Male S. E., 69 Malitesta C., 259 Kall P-O., 239 141 Marsden J. R., 251 Maruyama Y., 37 Mateus C.A. S., 289 Matijevib E., 87 Matsubara H., 145 McKeown N. B., 121 Milburn G. H. W., 155 Mitchell G. K., 303 Moon B. M., 97 Moore G. A., 175, 181 Moretti G., 129 Mori T., 37 Morris M., 43 Motevalli M., 139 Musicanti M., 129 Nakano C., 37 Nalini V., 201 Nardella A., 129 Nebesny K.W., 213 Nishihata Y., 169 O’Brien P., 139 Ocaiia M., 87 O’Connor P., 103 O’Hare D., 51, 205 Olsson P-O., 239 Orr R., 255 Owen J. J., 113 Percec V., 217 Porta P., 129 Postle S. R., 223 Powell A. V., 137 Pugh C., 217 Quill K., 141 Rao C.N. R., 299 Rayment T., 299 Richardson R. M., 121 Roser S. J., 121 Rotella F., 175 Salamon M. B., 181 Salmon L., 265 Sankar G., 299 Savadogo O., 301 Serrano J. L., 197 Sherrington D. C., 151 Silver J., 29 Simmons J. M., 121 Sinclair D. C., 147 Slade R. C.T., 281 Slaney A. J., 5 Slater P. R., 17 Smith R. I., 91 Sola E., 251 Stacey J. M., 251 Stucky G. D., 153 Takahashi S., 145 Takenaka S., 169 Tamatani A., 169 Templeton-Knight R., 59 Terauchi H., 169 Thompson D. P., 239 Thompson W. C.. 305 Thomson A. J., 121 Tilley R.J. D., 155 Twyman J. M., 205 Underhill A. E., 103, 141 Vivien D., 265 Volin K. J., 175 Walsh J. R., 139 Weller M. T., 11, 295 West A. R., 91, 147, 149, West B. C., 281 Whitcombe M. J., 303 Whiteley R. H., 271 Wiseman P. J., 205 Yokoyama M., 293 157, 163 Daniel M. F.. 121 Hirst P. R., 281 Mandal K. C., 301 Sabbatini L., 259 Zambonin P. G., 259 Daolio S., I9 I Hitterman R. L., 175 Manterfield M. M., 255 Saito G., 37 Zhang X., 233 Davey A. P.. 245 Hoffman D., 87 Marcos M., 197 Sakurai Y., 169 Zheng C., 163 1 -~-~~ ~~ Conference Diary 1991 March 9-16 Electronic Properties of Polymers: Orientation and Dimensionality of Conjugated Systems Kirchberg, Austria Professor H. Kumany, Inst. fur Festkoqxrphysik der Universitiit Wien, StNdlhofgasse 4, A-1090 Vienna, Austria March 10-17 European Conference on Liquid Crystals Coumayeur. Italy Professor M.P. Fontana, ECLC 1991, c/o TECDIS S.P.A., 41, rue dela Gare, 11024 Chatillon, Vde d'Aosta, Italy March 11-15 International Congress on Optical Science and Engineering The Hague, The Netherlands Sylvie Trehais, Program Coordinator,Europtica-ServicesIC, 16Avenue Bugeaud, 751 16Pans, France March 12-14 First Arab Conference on "Polymers and their Applications" Mansoura University, Mansoura, Egypt Professor Dr Abbas A. Yehia, Department of Polymers and Pignents,Natid Research Center, Dokki,Cairo, Egyp; Professor Dr E. M. Abdel-Bary, Chemistry Department,Faculty of Science. Mansoura UniversityJMansoura, Egypt March 12-15 4th International Conference on Ceramic Powder Processing Sdence Nagoya, Japan Shin-ichi Hirano, Nagoya University, Department of Applied Chemisq, School of Engineering, Furo-cho, Qlikusa-ku, Nagoya 46441, Japan.Tel.: (052) 781-5111 Ext. 3343. FAX: (052)782-5170 March 18-22 Conducting Polymers Materials Physics Topical Grwp, APS Meeting, Cincinnati, Ohio,USA DrR. H. Baughman, Allied-Signal Inc., Research and Technology, POBox l(nlR, Monistown, NJ 07962, USA Tel.: (201) 455-2375. FAX: (201) 455-3934 March 19 Piezoelectric and Pyroelectric Materials and their Applications London, UK The Meetings Office, The Institute of Physics, 47 Belgrave Square, London SW 1X SQX, UK March 25-27 British Liquid Crystal Society Annual Conference 1991 University of Reading, UK Dr G.Mitchell, Department of Physics, University of Reading, Reading RG6 2AH, UK March 25-27 1st International Conference on Deformation and Fracture of Composites Manchester, UK Conference Depament (300325, Plastics and Rubber Institute, 11 Hobart Place, London SWlW OHL March 25-27 Workshop on Ions in Polymeric Materials Oxford, UK Professor Richard Catlow. The Royal Institution, 21 Albemade Street, London WlX 4BS March 25-28 British Crystallographic Assodation Spring Meeting Sheffield University, UK DrA. J. Smith, Department of Chemisuy, University of Sheffield, Sheffield S3 7HF. UK April 2-6 MacroLux '91 Luxembourg Mme. Pauline Pechner, Macmlux '91. IFCEB-Luxembourg, B.P. 667, L-2016 Luxembourg April 3 New Applications of Electrochromism -Display, Light Modulation and Printing London, UK Dr D.R.Rosseinsky, Department of Chemistry, The University, Exeter EX4 4QD, UK April 3-5 Polymer Physics: Conference to Mark the Retirement of Professor A. Keller Bristol, UK Peter Barham and Jeff Well, Polymer Physics Conference, H H Was Physics Laboratory, Tyndall Avenue, Bristd BS8 1% UK April 8-11 Deformation, Yield and Fracture of Polymers VIII Cambridge, UK The Conference mice, "he Plastics and Rubber Institute, 11 Hobart Place, London SW 1 W OHL, UK April 8-1 1 150th Anniversary Annual Chemical Congress of The Royal Society &Chemistry London. UK DrJ. EGibson, "he Royal Societyof Chemistry, Burlington House, London W1V OBN,UK April 9-12 Mineral and Organic Functional Fillers in Polymers LeMans, France The Secretariat, L.C.O.M.CongressMOFFIS 91, i l'attention du Pr.J.-C. Bmsse, Facult6 de Sciences, Universite du Maine, Route de Laval, F-72017 -Le Mans Cedex, France April 10-12 Polar Solids Discussion Group Meeting: Defect and EJectronic Properties of Complex Ceramic Oxides Oxford, UK Professor Richard Catlow, The Royal Institution, 21 Albemarle Street, London WlX 4BS, UK. Tel.: 071-409 2992 April 14-19 ACS Meeting: Electrically Conductive Polymers Sessions Atlanta, Georgia, USA DrRonald L. Elsenbaumer, Allied -Signal Inc., CRL 251, Columbia Road, Morristown, NJ 07%2, USA. Tel.: (201) 455-5295. FAX: (201) 455-5159 11 April 15-17 April 15-19 April 18-19 April 21 -24 April 29 -May 4 May 5-9 May 5-10 May 5-11 May 14-16 May 19-29 May 20-24 May 21-25 May 22-23 May 22-24 May 26-30 May 26-June 2 May 27-3 1 May 27-3 1 May 27-3 1 Advances in Particulate Technology Guildford, UK Mrs Jean Iibaett, Department 0fChnical and PlDCessJiqpeering, Univedy of Surrey, Guildford, Surrey GU2 5XH, UK International Workshop on HTCSThin Films: Properties and Applications Rome, Italy Mrs Liu' Catena, HTCS Thin Films Workshop, Dipartimento di Fisica, Universiti di Tor Vergata, Via E.Camevale, 1-00 173 Roma, Italy Thermoplastic Eiastomers Brussels, Belgium Kay Royle, Rapra Technology Limited, Shawbury, Shrewsbury, Shropshire SY4 4NR, UK Euro MBE '91: Sixth European Conference on Mdecular Beam Epitaxy and Related Growth Methods Tampere, Finland Ms Raili Siekkinen, TampereUniversity of Technology, PO Box 527, SF-33101 Tampere, Finland Materials Research Society Spring Meeting Anaheim, California, USA Materials Research Society, 9800 McKnight Road, Pittsburgh, PA 15237, USA.Tel.: (412) 367-3003. FAX: (412) 367-4373 Rolduc Polymer Meeting 6 Rolduc, The Netherlands P. J. Lemstra, Eindhoven University of Technology, PO Box 513,5600 MB Eindhoven, The Netherlands Sensors: 179th Meeting of the E(ectrochemica1 Society. Sensors Based on Organic Electroactive Materials Washington DC,USA Dr P. Kathir, Cookson Technology Centre, Sandy Lane, Yamton, Oxford OX5 lPF, UK ECCG-3: 3rd European Conference on Crystal Growth Budapest, Hungary A. L~rinczy, Conference Secretary, Res.Inst. for Technical Physics, Budapest, Ujptst 1. Pf. 76, Hungary-1325 Sensor 91 Nuremberg, Germany ACS Organisations GmbH, Von-Miinchhausen-Strasse29, D-3050 Wunstorf 2, Germany International Workshop on Modern Magnetic Materials and their Technological Impact La Habana, Cuba Professor C. Rdriguez Castellanos, Dean Physics Faculty, La Habana University, Vedado-Collina Universitaria, La Habana, Cuba New and Alternative Materials for the Automotive Industries Florence, Italy ISATA Secretariat, 42 Lloyd Park Avenue, Croydon CRO SSB, UK Tenth International Conference on Solid Compounds of "bansition Elements Munster, Germany SCTE-10, Anorganisch-Chemisches hstitut der Universitiit, Wilhelm-Klemm-Str. 8, D4400Miinster, Germany Tel.: (+ 49 251) 83 3121; FAX: (+ 49 251) 83 3169 Second International Symposium on Metal-Containing Liquid Crystals St.Pierre de Chartreuse, France Dr A-M. Gimud-Godquin, DRF/LCH. Centre d'Etudes Nucleaires de Grenoble, 85X, 38041 Grenoble Cedex, France Thirteenth International Conference on Advances in Stabilization and Controlled Degradation of Polymers Lucerne, Switzerland In Europe: Dr N. C. Billingham, MOLS, University of Sussex, Brighton BN19QJ. UK In the USA: Professor A. V. Patsis, Materials Laboratory, CSB 209, State University of New York, New Paltz, New York 12561, USA MatTech '91 -The Second European hst-West Symposium on Materials and Processes Helsinki, Finland Professor Kaj Lilius, PL 121, SF-02101 Espoo, Finland.Tel.: + 358-0451 2769. FAX: + 358-0-4512799; + 358-0-4552250; + 358-0-4512 660 National Conference -Work Shop: "Progress and Problems in Liquid Crystals'' Leningrad, USSR S. I. Vavilov State Optical Institute, 199034 Universitetskaja nab., 5, Lmingrad, USSR FAX: 2184172 International Conference on Advanced Materials ICAM -91 Strasbrg, France E.MRS/P. SifYert, C. R. N., B. P. 20, F-67037 Strasbourg-Cedex, France 6th International Symposium on Intercalation Compounds Orleans, France Secretariat ISIC 6, CRSOCICNRS,45071 Orleans Cedex 02,France The Second International Conference on Rare Earth Development and Applications (ICRE'91) Beijing, China Senior Engineer Liu Aisheng, &Nor Engineer Jin Jinghong, The Chinese Sodety of Rare Earths,76 Xueyuan Nan Lu,Beijing 100081, China.Tel.: 8312541 or 891666. FAX: 8312 144 ... 111 June 10-14 8th Bratislava International Conference on Modified Polymers High Tatras, Czechoslovakia DrD. Lath, Polymer Institute, Slovak Academy of Sciences, 842-36 Bratislava, Dubravska cesta, Czechoslovakia June 17-21 Gordon Conference on Liquid Crystals Brewster Academy, Wolfebom, New Hampshire, USA Professor N. A. Clark, Department of Physics, Condensed Matter Laboratory, Campus Box 390, Boulder, CO903094390,USA June 17-21 3rd International Symposium on Polymer Electrolytes hey, France A. Gandini, EFPG BP 65,38402 St Martin d'Heres, France J~ne19-21 1991 E(ectronic Materials Conference Boulder,Colorado, USA BarbaraJ. Kamperman, Meetings Manager, The Mineral Metals and Materials Society, 420 Commonwealth Drive, Warrendale, PA 15086,USA. Tel.: (412) 776-9050.FAX: (412) 776-3770 June 19-21 Conferenceon Fracture Fhcesses in Brittle Disordered Materials Noordwijk, 7he Netherlands Congress OBlice ASD, PO Box 54,2640 AB Pijnacker, The Netherlands. Tel.: 31-1736 95356. FAX: 31-1736 92242 June 24-26 Fifth International Symposium on Catalyst Deactivation Northwestern University, Illinois, USA DrK. K. Robinson, Am- Research Center, Research and Development Department, PO Box 3011, Napewile, IL 60566, USA. Tel.: (708)-420-4964. FAX: (708)-420-5303 June 24-28 Third International Conference on Ferroelectric Liquid Crystals Boulder, Colorado, USA Professor N.A. Clarlt, FLC 91, Office of Conference SeMces, Campus Box 454, University of Colorado, Boulder, Colorado, CO 80309,USA June 24-28 'Ikansducers '91 (6th International Conference on Sensors and Actuators) San Francisco, California, USA Mrs Linda Reid, University Extension, University of California, Berkeley, CA 94720, USA.Tel: (415) 642-4151 July 1-4 ICIM 91 (Second International Conference on Inorganic Membranes) MontpeUier, France Professor LCot, c/o ICIMz-91, ENSCM, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 1, France Tel.: 33 67540085. FAX: 33 67635970 July 2-5 International Symposium: Supported Reagent Chemistry York, UK DrJohn F. Gibson, The Royal Society of Chemistry, Burlingtan House, London WlV OBN, UK July 3-5 Understanding Self-Assembly and Organisation in Uquid Crystals (Joint British Uquid Crystal Society and Statistical Mechanics and Thermodynamics Group ofthe RSC) Leeds, UK DrJ.R Henderson, School of Chemistry, University of Leeds, Leeds LS2 9JT July 7-12 10th International Conference on the Chemistry ofthe Organic Wid State University of British Columbia, Vancouver, Canada Conference Secretariat, ICCOSS X, c/o Venue West Ltd, W5-375 Water Street, Vancouver, B.C. Canada V6B 5c6. Tel.: (604) 681-5226. FAX: (604) 681-2503 July 7-12 7th International Conference on Surface and Cdldd Science Compikgne, France Secretariat of the 7th ICSCS, c/o Wagons-Lits Tourisme, BP 244, F-92307 Levallois-Perret Cedex, France July 9-10 Polymers in Extreme Environments Nottingham, UK conference Department (3129, The Plastics and Rubber Institute, 11 Hobart Place, London SW 1 W OHL,UK July 9-13 The VIth International Conference on the Chemistry ofSelenium and Tellurium Osaka, Japan Professor Noboru Sonoda,Osaka University, Dept of Applied Chemistry, Faculty of Engineering,Suita, Osaka 565, Japan.Tel.: (81) 6-877-5 11 1 Ext. 4276. FAX: (81) 6-876-4754 July 15-18 Rhedogy ofPolymer Melts Prague, Czechoslovakia PMM Secretariat, c/o Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206Prague, Czechoslovakia July 17-19 DCEM 11, Deposition and Characterisation of Electronic Materials (ASSCGlRSC Dalton) Manchester, UK DrM. E. Pemble, Department of Chemistry, UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, UK or MrsE.S. Wellingham, Field End House, Bude Close, Nailsea, Bristol BS19 2FQ, UK July 21 -26 Polymer Surfaces and Interfaces I1 Durham, UK Professor W.J. Feast, Department of Chemistry, University of Durham, SouthRoad, Durham DH13LE, UK July 22-24 EUROMAT 91: The 2nd European Conference on Advanced Materials and Processes Cambridge, UK Euromat 91, Conference Department, The Institute of Metals, 1 Carlton HouseTerrace, bndon SWlY 5DB, UK iv August 17-22 3rd IUPAC Congress Budapest, Hungary E. Pungor, c/o Hungarian Academy of Sciences, H-11 1 1Budapest, Gellert ter 4, Hungary August 19-22 InternationalTopical Conference on Optical Probes ofConjugated Polymers Snowbird, Utah, USA Department of conferences and Institutes. fivision of continuing Education, 2174 Annex.University of Utah, salt Lake City, UT 84112, USA. Tel.: (801) 581-5809. FAX: (801) 581-3165 August 20-24 International Conference on Polytypes, Modulated Structures and Quasicrystals (MOSPQQ '91) Balatonszieplak. Hungary Secretary of MOSPOQ '91, Roland Eotvos Physical Society,PO Box 433, H-1371 Budapest, Hungary August 25-30 ACS Autumn Meeting New Yorlc, USA ACS, International Activities Office, 155 16th Street NW,Washington DC 20036, USA August 26-30 Flfth International Conference on Langmuir-BlodgettFilms Paris, France Annie Ruaudel-Teixier, Chhan, Servicede ChimieMolkulaire, Bit. 125, C.E.N. Saclay, 91 191, Gif-sur-Yvette Cedex, France. Tel: 69-08-54-55. FAX:69-08-79-63 August 26-30 Summer European Liquid Crystal Conference Viius University, Lithuania Dr P.Adomenas, 1991 SELCC, Department of Chemistry, Vilnius University, Naugarduko 24,232006 Vilnius, Lithuania Tel.: Vilnius 661553. Telex: 261212 VUSU. FAX: (007-012-2) 613473 September 1-6 XVI International Symposium on Macrocyclic Chemistry (ISMC 1991) Sheffield, UK Dr Norma A. Stoddart, Department of Chemistry, The University, Sheffield S3 7HF. UK. Tel.: (0742) 768555 Ex. 4522. FAX: (0742) 739826 September 2-4 Crystalline Structures and Defects in Ceramics Ljubljana, Yugoslavia M. Drofenik, Institute JozefStefan, Yu-61001 Ljubljana, PO Box 100,Yugoslavia September 4-6 IWASES-I1 (International Workshop on Auger Spectroscopy and Electronic Structure) Lund, Sweden Dr C-0. Almbladh, Department of Theoretical Physics, Sijlvegatan 14 A, S-22362 Lund, Sweden E-mail:COA@SELDC52.BITNET.FAX: 46-(0)46-104710 September 4-6 Polymer Stabilization: Mechanisms and Applications Birmingham, UK Professor N. S. Allen, Department of Chemistry, Manchester Polytechnic, John Dalton Building, Chester Street, Manchester M15GD September 9-11 6th Biennial Polymer Conference Leeds, UK Meetings office, The Institute of Physics, 47 Belgrave Square, London SWlX SQX, UK September 9- 14 Physics of Polymer Networks Ballenstedt. Germany Organizing Committee, Physics of Polymer Networks, Technical University of Merseburg, Dept of Physics, Merseburg, DDR-4200, Germany September 18-20 Recyding of Polymers Marbella, Spain W. Heitz, Philipps-Universitiit Marburg FB 14 Physikalische Chmie, Polymere, Hans-Meerweinstrasse, D-3550 Marburg, Germany September 22-25 5th Sensors and their Applications Conference Edinburgh,UK Meetings office, The Institute of Physics, 47 Belgrave Square, London SWlX SQX, UK September 23-27 7th International Symposium on EIedrets Potsdam, Germany Professor Detlev Geiss, Academy of Sciences, Institute of Polymer Chemistry 'En& Corns', Kantstrasse 55, 1530TeltowSeehof, Germany September 30- Eurosensors V October 2 Rome, Italy Professor A.D'Amico, Universita' di Roma "TOR VERGATA, Dipartimento di Ingegneria Elettronica, Via Orazio Raimondo, 1-00173 Roma, Italy September 30- Speciality Polymers '91 October 2 Mainz, Germany conference Organizer, SP'91, Butterworth Scientific Ltd.PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 SBH, UK September 30-Macromdecule-Metal Complexes IV October 5 Siena, Italy Professor Roland0 Barbucci, Dipartimento di Chimica, Universita di Siena, Pianodei Mantellini 44.53 lo0 Siena, Italy V Odober6-11 October 13-17 Oaober 14-18 October 14-18 Odober 20-26 October 23-25 October 28-November 2 November 30-December 1 December 14 December 24 December 2-7 December 16-17 January 6-8 February 3-5 March 30-31 Apnl5-10 July 13-18 July 14-16 Sixth International Workshopon Glasses and Ceramics from Gels SeviUe, SpainDr Luis Esquivias Fedriani (Workshop Chairman), Departamento de Estrudura y Propiedades de 10s Materiales, Apartado, 40-1 15 10 Puem Real (Wiz),Spain Unconventional Photoactive Solids (UPS) Symposium on Molecular Systems Okazaki, Japan Tadaoki Mitani, Institute for Molecular Science, Myodaiji, Okazaki 444,Japan ECASIA 91 (4th European Conference on Applications of Surface and Interface Analysis) Budapest, Hungary ECASIA 91, MTA ATOMMI, Pf.51, Ha1 Debmen, Hungary. Tel.: (36)-52-16181. Telex: 72210 (atom h). FAX: (36)-52- 161 81 Olefin and Vinyl Polymerization and Functionalization Hangzhou, China Professor Kun-Yuan Qiu, Secretary-General of Symposium91 Hangzhou, Qlanistry Department, Peking University, Beijing 100871,China Solid State Ionics 8th International Conference Lake Lwise, Canada Dr P. S. Nicholson, McMaster University, JHE-249,Hdton, Ontario, Canada LSS 4L7.Tel.: (416) 529-7575. FAX: (416) 529-5994 Polymers in a Marine Environment III London, UK Rhian Bufton, conference Organiser, 'Ihe Institute of Marine Engineers, The Memorial Building, 76 Mark he, London EC3R 7JN, UK EurophysicsConference on Macromolecular Physics: Polymer Networks Wemigerode, Germany S. Wartewig, 'Carl Schorlemmer' Technical University Leuna-Merseburg, Department of Physics, DDR-4200 Merseburg, Germany International Symposium on New Polymers Kyoto, Japan Professor Toshinobu Higashiura, Department of Polymer Chemistry, Kyoto University, Yoshida, Sakyoku, Kyoto 606,Japan Electrical, Optical, and Magnetic Properties of Organic Solid StateMaterials(MRSSymposium) Boston,USA D. J. Sandman, GTE Laboratories Incorporated, 40 Sylvan Road, Waltham, MA 02254, USA.Tel.: (617) 466-4216 Cellulose '91 New Orleans, USA Dr Noelie R. Bertoniere, USDA AR!3 Southern Regional Research Center, 1100 Robert E. Lee Boulevard, PO Box 19687, New Orleans, LA 70179-0687, USA Materials Research Society Fall Meeting Prague, Czechoslovakia J. Heymvsky, Institute of Physical Chemistry and Electrochemistry, Dolejskova 3,182 23 Prague 8, Czechoslovakia Mein Engineering: Structure Prediction and Ugand Interactions London, UK Dr Neera Brokakoti, c/o Roche Products Lul,Broadwater Road, Welwyn Garden City, Heas AL7 3AY, UK Conference Diary 1992 Fluoropdymers '92 Manchester, UK Dr E. Smith or DrS. Dunn,Department of Chemistry, UMIST, Manchester M60 lQD, UK 19thAustralian Polymer Symposium Fmantle, Australia Dr Graeme George, Secretary RACI Polymer Division, chemimy Department, University of Queensland, St.Lucia QLD 4067, Australia Kautschukelastische Pdymersysteme Bad Nauheim, Germany GDCh-Geschaftsstelle, Abteilung Tagungen, Postfach 90 0440,D-6000Frankfurt am Main, Germany ACS Spring Meeting San Francisco, USA ACS, International Activities Office, 155 16th Street NW,Washington DC 20036, USA 34th IUPAC International Symposium on Macromdecules Prague, Czechoslovakia IUPAC Macro '92 Secretariat, Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, Heyrovkeho nam. 1888/2,16206 Prague 6, Czechoslovakia Speciality Polymers '92 London, UK conference Organizer, Speciality Polymers '92, Butterworth Scientific Limited, PO Box 63, Westbury House, Bury Streer, Guildford, Surrey GU2 5BH, UK vi August 23-28 ACS Autumn Meeting Washington,USA ACS, Intematimal Activities Office, 155 16th Street NW,Washington DC 20036, USA August 24-28 European Conference on Molecular Electnnrics Padua, Italy professor R Bozio, ECME 92,Department of Physical chemistry, 2 Via bredan, 1-35131 Padua, Italy August31-Macromdecules '92 sptember 4 Canterbury, UK Dr A Amass, Department of ChemicalEngineering and Applied Chemistry, University of Asm, Aston Triangle,BirminghamB4 7ET.UK vii

ISSN:0959-9428    

DOI:10.1039/JM99101BP023

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(2), 157-162 FEATURE ARTICLE Solid Electrolytes and Mixed Ionic-Electronic Conductors: An Applications Overview Anthony R. West Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB9 2UE, UK A review of the various applications of solid electrolytes and mixed conductors is presented. Discussion focuses on their uses in: sensors; high-density batteries; solid-oxide fuel cells; ion-exchange materials; optical waveguides; solid-state batteries; thin-film electrochromic devices; and high-T, ceramic superconductors. The potential for future development is also discussed. Keywords: Feature article; Solid electrolyte; Ionic-electronic conductor Solid electrolytes are an unusual group of materials which have high ionic conductivity with negligible electronic conduc- tivity.Examples are now known involving high conductivity of most monovalent and some divalent ions, e.g. Ag+ in RbAg415, Na' in /?-alumina, Li' in H-doped Li,N, 02-in 9Zr0, * 1Y 203(yttria-stabilised zirconia), F-in PbSnF4 and H+ in HU02P04*4H20 (hydrogen uranyl phosphate, HUP) (Fig. 1). There is another group of materials, mixed ionic- electronic conductors, that have high conductivities of both ions and electrons. Solid electrolytes and mixed conductors have been surveyed recently in ref. 1 and more comprehen- sively in ref. 2. The purpose of this review is to summarise the current status of the applications of these materials (Table 1) and to Table I Applications of solid electrolytes and mixed ionic/electronic conductors batteries, primary or secondary sensors, gas pumps fuel cells, especially solid-oxide fuel cells electrochemical reactors supercapacitors synthesis of new materials by ion exchange waveguide fabrication by ion exchange optimisation of superconductivity by oxygen (de)intercalation lithium (de)intercalation, new materials, solid solution electrodes electrochromics, smart windows and displays indicate some likely future developments.Specifically excluded from consideration here are polymeric electrolytes and poly- meric mixed conductors; these have been reviewed recently in ref. 3 and 4. Batteries The great upsurge of research into solid electrolytes (also called superionic conductors or fast-ion conductors) in the 1960s, which commenced with the discovery of the ion- conducting properties of fl-alumina' and various Ag salts,6 was motivated to a great extent by the possibility of building new high-density secondary battery systems.The concept of the Na/S cell, using molten electrodes separated by a solid p-alumina electrolyte, originated from the Ford Motor Com- pany and has since been developed vigorously in several major laboratories worldwide. The main Na/S cell designs that have been tested contain a p-alumina tube, closed at one end, with one molten electrode inside and one outside (Fig. 2). The cell operating temperature is 300-350 "C, in order to maintain both electrodes and the sodium polysulphide dis- charge products in the liquid state (Fig.3). The cell voltage Ak03 insulator Na Al can j3-aluminaelectrolyte t \ sulphur + C felt 1 103 KIT Fig. 2 Schematic drawing of a sodium-core, Na/S cell: Fig. 1 Some conductivity Arrhenius plots 2Na +XS eNa,S, I I I I I I I I i cell400 -operatingregion -i= I Ill I 1 1+Na Na2S2 Na2S4Na2S5 S composition Fig. 3 Phase diagram for the Na-S system and cell open-circuit voltage at 350 "C is 2.08 V during initial discharge, while the discharge products are in the region of liquid immiscibility (Fig. 3), and then falls gradually to 1.78 V, at which point Na2S2 starts to precipitate. For practical reasons, the onset of such precipitation is taken as the fully discharged state.Na/S cells have a theoretical power density of 760 W h kg-'; in practice, power densities of 150-200 W h kg-' are routinely achieved in individual cells. The principal research and development groups that have been assembling and testing multicell batteries, including Brown Boveri (West Germany), Chloride Silent Power and Beta R & D (UK), Ford (USA) and Yuasa/NGK (Japan), report performance levels that approach the 1000 cycles of reliable operation that are required for electric vehicle applications. For instance, the most recent data available on the 360 cell battery from Brown Boveri reveal densities of 85 W h kg-' and 120 W kg-'.7 This compares favourably with the power density of lead acid batteries, typically 20-40 W h kg-'.There are many problems that can lead to a decrease in battery performance, e.g. short circuiting through the electro- lyte walls, resistance rise associated with the precipitation of impurities (especially Ca leached from the ceramic electrolyte), corrosion of the container and consequent loss of sulphur. Individually these problems appear not to be insurmountable; however, there is still the necessity to improve cell reliability. The early fears that the p-alumina electrolyte may be thermo- dynamically unstable when in contact with molten Na (owing to the leaching of oxygen from the conduction planes, followed by partial collapse of the crystal structure and loss of ionic conductivity) appear to be groundless. Given the twin factors of diminishing fossil-fuel supplies and increasing environmental awareness surrounding atmos- pheric pollution, it seems reasonable to expect some degree of commercialisation of Na/S batteries within the next decade, either for electric-vehicle or power-station load-levelling appli- cations.An interesting alternative to the Na/S cell that has recently been announced*-" is the Zebra cell, Na/MCl,: M =Fe, Ni. It uses much the same design and technology as the Na/S cell but the cathode compartment has, instead, a mixture of liquid NaAlC1, and solid Fe (or Ni) Cl,. The cell discharge J. MATER. CHEM., 1991, VOL. 1 reaction is 2Na +(Ni, Fe)Cl, +2NaC1+ (Ni, Fe) The purpose of the NaAlCl, is to act as an ionically con- ducting liquid contact between the solid electrolyte and the solid electrode, i.e.the NaC1-FeC1,-Fe, mixture. The cell may be readily assembled in the discharge state, from a mixture of NaCl and Fe/Ni, impregnated with NaAlCl,. The cell has a higher voltage, 2.35 V for Fe and 2.57 V for Ni, and lower operating temperature, 250 "C,than the Na/S cell; prototype batteries giving several hundred cycles of operation, have been tested. It remains to be seen whether the perform- ance (densities of 88 W h kg-' and 65 W kg-' have been quoted for a 66 cell Na/NiCl, battery) can be optimised to exceed that of Na/S batteries. A variety of alternative battery systems using solid electro- lytes have been proposed over the years but none has received the same amount of attention as the 'a batteries' described above. The only example that has achieved commercialisation is the Li/12 miniature heart pacemaker primary battery.It contains as the solid electrolyte a thin film of LiI electrolyte which forms in situ on assembly when the two electrodes are placed in contact. This and other potential batteries, many based on polymeric materials, have been reviewed.' ' Gas Sensors and Pumps Oxygen sensors have been important in various applications for determining oxygen contents of gases and liquids. Most are fabricated from a tube of an oxide ion conductor such as yttria-stabilised zirconia (YSZ), bismuth oxide (in oxidising environments) or thoria (in reducing environments), Fig. 4. The tube is coated with inner and outer electrodes of porous Pt and the potential difference that develops between the electrodes may be related to the difference in oxygen partial pressure in the two compartments.One of the compartments contains a reference gas, e.g. air, 0, or a metal-metal oxide mixture such as Ni-NiO. Oxygen sensors are used commercially for monitoring gas compositions in combustion-plant and metallurgical processes and for determining the amount of oxygen dissolved in molten metals.12 They are also used in car exhaust systems (2 probe) to help optimise the fuel :air ratio. The same cell principle that is illustrated in Fig. 4 is also used in oxygen pumps.13 The two electrodes are short-circuited and oxygen gas may then be pumped from one electrode compartment to the other.Commercial devices are available and are used to purify oxygen or to provide con- trolled oxygen atmospheres in studies of, for example, the corrosion of metals and the cultivation of micro-organisms. Yl environment /c to be measured-1 Fig. 4 Design of an oxygen sensor based on yttria-stabilised zirconia solid electrolyte. I/= (RT/nF)In [p"(O,)/p'(O,)] J. MATER. CHEM., 1991, VOL. 1 Solid Oxide Fuel Cells (SOFC) Dramatic advances have been made in the development of solid oxide fuel cells in the past few years, as signalled by the first international SOFC conference in 1989.14 Much of this impetus has come from Westinghouse, USA but recently, major EEC and Japanese programmes have also commenced.The basic principle of the SOFC is shown in Fig. 5." It uses the oxide ion conducting ceramic, yttria-stabilised zir- conia, to act as separator and solid electrolyte between the fuel (CO, H,, CH, etc.) and air/oxygen. The air electrode is based on the mixed conductor perovskite, LaMnO,, and the fuel electrode is an electronically conducting Ni/ZrO, cermet. Cell operating temperature is 1000 "C, at which the electrolyte, the most resistive component in the cell, has a conductivity of 0.1 $2-'cm -Typical cell voltage is 0.7 V. The choice of cell components is such that they should be compatible with each other over long periods of time at high temperatures and it may be that the compositions of the components are not yet optimised. The original Westinghouse design, which has operated at the 3 kW level for 2500 h, is a tubular design, but there is now increasing interest in a flat- plate configuration (Fig.6). This is made feasible by advances in ceramic fabrication, using tape-casting methods to fabricate the planar components, although there are still doubts about the long-term mechanical stability of such structures. The individual three-layer cell 'sandwiches' are separated by a bipolar plate of electronically conducting LaCrO, which has a corrugated structure on either side to permit the easy flow of air and fuel over the respective electrode surfaces. There are several intrinsic features of SOFCs that make them attractive as power sources. (1) At the temperature of cell operation, 1000°C, methane (natural gas) may be used directly as the fuel.In the competing molten carbonate fuel cell (MCFC) fuel reforming is necessary thereby leading to a reduction in its efficiency. (2) Fuel conversion efficiencies of 50-60% should be possible, which is much higher than is obtainable with, e.g. MCFCs. (3)There should be few problems 10 Ni.Zr02 cermet anode 1 I I -40 LaMn03 cathode EXCESS e-02' 4e -20~-* /AIR AIR Fig. 5 Schematic of a solid oxide fuel cell. Adapted from ref. 15 CURRENT FLOW y YSZ PLATECELLf REPEAT 1 ,, FUEL Fig. 6 Schematic of a parallel plate SOFC design. Adapted from ref. 15 with electrolyte management, corrosion and maintenance. (4) Air pollution should be small.(5) High-grade waste heat is produced, leading to possible combined heat and power (CHP) applications for SOFCs. Recent targets for both Japan and the USA include 25 kW cells for 1990; it therefore seems likely that, within a few years, fuel cells based on the SOFC principle, may finally make a major contribution to energy management programmes. Electrochemical Reactors Yet another application of YSZ and similar oxide ion conduc- tors, in a configuration similar to that of Fig.4, is for the electrochemical partial oxidation of hydrocarbons, e.g. natural gas, to give industrially useful products such as CH30H and C2H4. Development work is still at an early stage and yields are low, but this is, nevertheless, seen as an important growth a~ea.'~,'~Choice of electrode is critical so as to catalyse the oxidation.Vayenas has shown, in the NEMCA (non-Faradaic electrochemically modified catalytic activity) technique, that application of a voltage to mixed conducting electrodes such as Bi2O3-Pr60,, leads to greatly enhanced rates for the oxidation of methane.I7 The mechanism of oxidation is unclear but may involve the active 0-species as an inter- mediate. Supercapacitors The amount of charge that can be stored in a capacitor is limited by the area of the electrodes; double-layer capacitances formed between an ionically conducting electrolyte and a metal electrode are typically optimised at a value between 1 and 10 pF cmV2, since capacitance, C, is proportional to A/d, where A is the electrode area and dis the double-layer thickness.Greatly enhanced apparent capacitances have been achieved by using as the electrode a fine mixture of electrode and solid electrolyte. Thus, in cells of the type: graphite, RbAg41 ,/R bAg,I ,/grap hi te, R bAg,I , a finely ground mixture of electronically conducting graphite and ionically conducting RbAg,I, is used as the electrode material. With this, interfacial contact areas of many square metres per gram are obtained and capacitances as high as 1-10 F are achieved in small, gram-size devices.18 The time constant, z,of a capacitor is given by the magni- tude of the RC product, which for example may have a value of 100 for a supercapacitor containing an electrolyte of resistance 1OR.Such a capacitance can be effective only at low frequencies, since from the relation or=1, co corresponds to the frequency at which the charge stored on a capacitor reaches lje of its limiting value. In the above case, the full magnitude of the capacitance would be observed only at angular frequencies, o,considerably less than 10-'Hz. Synthesis of New Materials by Ion Exchange Solid electrolytes are ideal materials for carrying out ion- exchange reactions since they have mobile ions of one type within a rigid host framework. Using ion-exchange methods, new materials can be synthesised that, thermodynamically, are metastable and could not be synthesised by other means, such as direct reaction of the components. Sometimes, the new materials have propertiesjstructures that are of techno- logical importance.Following on from the discovery of the high Na+ ion conductivity in /?-alumina, there was considerable interest in studying the structures and properties of ion-exchanged /?-aluminas.l9 This remained a topic of essentially academic interest until the discovery, by Farrington and Dunn, that Na+ ions in Nap"-alumina could be ion exchanged for a range of divalent and trivalent cation^.^'-^^ These materials provided the first examples of mobile divalent cations in solid electrolytes; the most remarkable is Pb2 +p"-alumina whose conductivity is comparable to that of Nab"-alumina over a very wide temperature range (Fig. 7). The synthesis of trivalent p"-aluminas has led to a new family of solid-state laser materials.In particular, Nd3 +pff-alumina has luminescence properties that compare very favourably with those of Nd-YAG. For instance, its absorption spectrum contains an anomalously large absorption coefficient at 573 nm, with an oscillator strength nearly 10 times that of Nd-YAG at the same wavelength. Other transitions in the two materials have comparable oscillator strengths.24 The high oscillator strength, coupled with long fluorescence life- times at high Nd concentrations, leads to potential appli- cations as lasers. One of the current problems that prevents full commercial exploitation is associated with the difficulty in growing large single crystals of p"-alumina: crystals tend to be small plates of cross-sectional area several mm2 but of thickness <1 mm.The wide range of optical properties exhib- ited by the lanthanide, transition metal and Cu' /I"-aluminas have been reviewed in ref. 23. Fabrication of Waveguide Materials by Ion Exchange The ion-exchange process discussed above may be used, under closely controlled conditions, to give inhomogeneous mater- ials. The associated compositional variations may lead to variations in refractive index and therefore, potential appli- cations in waveguides. LiNb03 single crystals may be converted into high-index optical waveguides by proton exchange of the surface lay- er~.~~.~~The surface layers form a solid solution (Li, -xHx)Nb03 whose structure depends on both the crystal- lographic orientation of the surface and the composition x.~' LiNb03 is not usually regarded as a solid electrolyte, but the Li+ ions have sufficient mobility under the conditions used, 200 "C in benzoic acid for several days, for partial ion exchange to occur.Waveguides have been fabricated from p-alumina single I I I I 1 2 3 4 5 6 lo3KIT Fig. 7Conductivity Arrhenius plots of p"-aluminas, from ref. 23 J. MATER. CHEM., 1991, VOL. 1 crystals by partial Na Ag exchange,28 but the interiors only of the crystals are allowed to ion exchange. This is done by first coating the crystals with a diffusion mask of Ni-Cr, with a polyamide overcoat, and then etching the crystal end faces uia laser lithography to expose a selected set of conduc- tion planes.The masked crystal is immersed in a bath of molten AgN03 and these inner conduction planes undergo ion exchange. The resulting material has a high refractive index inner region, associated with the Ag p-alumina and this buried waveguide structure is found to be very efficient for coupling and guiding light. Optimisation of Superconductivity by Oxygen (De)intercalation The high T, ceramic superconductors such as YBa2Cu30, and Bi2Sr2CaCu20d are mixed oxide ionic/electronic conduc- tors. Their composition 6 is variable and the different values are achieved by processing the materials at different tem- peratures and oxygen partial pressures. The critical tempera- tures, T,, generally vary greatly with oxygen content.In Bi2Sr2CaCu,0B,T, is optimised at 87 K for 6 =8.185 (Fig. 8). Such a value of 6 can be obtained, for example, in air at 820 "C or in N2 at 400 0C.29 At temperatures above 400-500 "C, especially in fine-grained powders, oxide-ion diffusion rates are sufficiently rapid that samples can respond fairly rapidly to changing T and p(02), but in dense ceramics full equilibration and hom- ogenisation may be difficult. In the well studied YBa2CuJOd materials, there is still confusion as to how T, varies with 6 (Fig. 9). In samples that have been quenched after high- temperature equilibration, T, varies approximately linearly with 8.30But in samples that have been prepared by oxygen deintercalation at relatively low temperatures (400 "C) there is evidence for plateaux at 90 and 60 K in the plot of T, us.8.31Very recent results suggest that the plateaux are caused by ordering of oxygen ions at low temperatures, together with associated changes in the electronic structure. Thus, it is possible to take a quenched material, anneal it for several days at 100-200 "C and generate increased T, values, with plateaux in plots of T, us. x similar to those in Fig. 9. It is clear that oxide ion conduction is fundamental to the processing and optimisation of the properties of the ceramic superconductors, and is important not only in controlling the overall oxygen content, 8, and therefore the hole concen- 85 80 s L" 75 70 I I I 1 I 8.15 8.16 8.17 8.18 8.19 8.20 6 in Bi,Sr,CaCu,O, Fig.8 Critical temperature us. composition for Bi,Sr,CaCu,O, (ref. 29) J. MATER. CHEM., 1991, VOL. 1 7.0 6.8 6.6 6.4 6 in YBa,Cu,O, Fig. 9 Critical temperature us. composition for Y Ba,Cu,O, for samples prepared by (0)quenching from high temperatures3' and by (0)low-temperature deintercalation of oxygen3' tration, or average oxidation state of copper, but also in achieving structures with ordered defect arrangements and modified T, values. Lithium (De)intercalation, New Materials, Solid Solution Electrodes The ability to introduce lithium ions into or remove lithium ions from certain transition metal compounds gives rise to a variety of new phases and solid solutions, some of which find application as reversible electrodes in prototype high-density battery systems.32 The principle is illustrated in Fig.10 in which TiS, behaves as an intercalation host, accepting lithium ions from the electrolyte/anode and electrons from the external circuit to form solid solutions, LixTiS2. The requirements for the host structure (TiS,) are to (a) accept electrons and be electronically conducting and (b)accept Li' ions and for them to exhibit high mobility. Many host (oxide, sulphide, etc.) structures have been intercalated successfully with lithium. Currently there is much interest in V,OI3 as a possible battery cathode since its Li+ diffusion rate is higher than that of TiS2. A convenient means of carrying out 'lithiation' is to immerse samples in n-butyl lithium dissolved in hexane.The n-butyl lithium acts as a source of lithium and the residual n-butyl useful power electrolyte TIS~ LI LI+ +Li+ + Fig. 10 Intercalation of Li into TiS2, adapted from ref. 32 groups dimerise to forin octane. This is essentially an internal redox/intercalation reaction. The reverse process of deintercalation or delithiation may be carried out using a variety of methods, e.g. electrochemical deintercalation or treatment of a sample with a solution of I2 in acetone (LiI is insoluble in acetone and gradually precipitates as Li is removed). As well as acting to reverse intercalation reactions, this method may be used to synthesise entirely new materials. For example, a new form of COO, has been synthesised by deintercalation of Li from LiCoO,.Electrochromics, Smart Windows and Displays Commercial electrochromic devices are now available based on the reversible intercalation of protons into thin films of W03, yielding a coloured tungsten bronze.33 The device structure is shown schematically in Fig. 11 and is made by evaporation onto a glass substrate of successive layers of indium tin oxide electrodes and W03. The proton source is hydrated Ta205. In the OFF state, the device is colourless and transparent. In the ON state, H+ ions intercalate the W03 and the accompanying electrons enter the 5d band of W, giving a bronze of nominal stoichiometry HXW~!-,W~O3.Absorption of light by the 5d electrons is responsible for darkening, which occurs in a matter of seconds.Such structures have been proposed as large-scale coatings on 'smart windows' for better heating/lighting management of buildings, but none are yet at the commercial stage owing to problems in fabricating large-area thin-film structures of sufficient quality and uniformity. Two small-area devices are commercially available, antidazzle car mirrors from Schotts and electrochromic spectacles from Nikon. Future Prospects All of the possible applications listed in Table 1 have been demonstrated convincingly in laboratory-scale experiments. Several are undergoing development and testing on a larger scale, and others, such as sensors and electrochromic thin- film devices, are actually on the market place.Since these various feasibility studies have already been successfully car- ried out, future developments are likely to depend on economic and environmental factors and on whether sufficient resources are made available to turn laboratory-scale devices into commercial products. It is therefore difficult to make predic- tions as to which applications are likely to achieve full commercialisation, other than for those which are already undergoing large-scale development. Of more interest is to enquire whether any further develop- ments in new materials or improved properties are possible or desirable. Many research groups are looking for new solid electrolytes with the particular objectives of finding (a) high oxide ion conductivity at intermediate temperatures (200-500 "C), (b) high protonic conductivity at similar tem- peratures, since most materials with high proton conductivity at room temperature have a high water content and are not I TO I 1 H+SOURCE Fig.11 Schematic construction of a thin-film electrochromic device 162 J. MATER. CHEM., 1991, VOL. 1 stable at high temperatures, (c) high lithium ion conductivity in atmosphere stable materials at ambient temperature. Such materials could find applications in improved fuel cells (a),(b) or batteries (c). There is also much interest in finding new mixed conductors especially those with (a) high oxide ion 10 11 12 13 R. J. Bones, J. Coetzer, R. C. Galloway and D. A. Teagle, J. Electrochem. SOC., 1987, 134, 2379. R.G. Linford, p. 564 in ref. 2. P. Jagannathan et al., in Solid Electrolytes and Their Applications, ed. E. C. Subbarao, Plenum Press, New York, 1980, p. 201. H. Iwahara, in Solid State Ionic Devices, ed. B. V. R. Chaudari conductivity for catalyst and reversible electrode applications and (b)high Li', Na' ion conductivity for reversible elec- trodes in solid-state batteries. New sensor materials and devices are required that are selective to, for instance, oxides of sulphur, oxides of nitrogen or C02. There is also much activity in developing multilayer, thin- film solid-state devices particularly for miniature power 14 15 16 17 18 and S. Radhakrishna, World Scientific, Singapore, 1989, p. 309. Proc. Int. Symp. Solid Oxide Fuel Cells, Nagoya, Japan, 1989, Science House, Tokyo.J. T. Brown, p. 630 in ref. 2. B. C. H. Steele, I. Kelly, H. Middleton and R. Rudkin, Solid State Zonics, 1988, 28-30, 1547. C. G. Vayenas, Solid State Zonics, 1988, 28-30, 1521. R. A. Huggins, p. 664 in ref. 2. sources. An advantage of miniaturisation is that the electrolyte resistance decreases linearly as thickness decreases and there- fore, the normally stringent requirements of having low specific resistivity for ionic conduction can be relaxed somewhat. In summary, there are probably a considerable number of 19 20 21 22 J. T. Kummer, Prog. Solid State Chem., 1972, 7, 141. B. Dunn and G. C. Farrington, Mater. Res. Bull., 1980, 15, 1773. G. C. Farrington and B. Dunn, Solid State Zonics, 1982, 7, 267. G. C. Farrington, B. Dunn and J. 0.Thomas, Appl. Phys. A, 1983, 32, 159. potential ionic conductors/mixed conductors waiting to be discovered and a sufficient number of perceived applications to ensure continued vigorous activity in this area. 23 24 25 G. C. Farrington, B. Dunn and J. 0.Thomas, p. 327 in ref. 2. M. Jansen, A. J. Alfrey, 0.M. Stafsudd, D. L. Yang, B. Dunn and G. C. Farrington, Opt. Lett., 1984, 9, 119. J. L. Jackel, A. M. Glass, G. E. Peterson, C. E. Rice, D. H. Olson and J. J. Veeselka, J. Appl. Phys., 1984, 55, 269. References 26 J. L. Jackel, C. E. Rice and J. J. Veeselka, Electron. Lett., 1983, 19, 387. A. R. West, Ber. Bunsenges. Phys. Chem., 1989,93, 1235. 27 M. Ito and H. Takei, Jpn. J. Appl. Phys., 1989, 28, 144. High Conductivity Solid Ionic Conductors, Recent Trends and Applications, ed. T. Takahashi, World Scientific, Singapore, 1989. 28 B. Dunn, G. C. Farrington and J. 0. Thomas, ZSSZ Lett., 1990, 1, 1. J. R. MacCallum and C. A. Vincent, Polymer Electrolyte Reviews, 29 C. Namgung, J.T. S. Irvine, J.H. Binks, E. E. Lachowski and Elsevier, Barking, 1987, 1989, vol. 1 and 2. S. Etemad, A. J. Heeger and A.G. MacDiarmid, Rev. Phys. 30 A. R. West, Supercond. Sci. Tech., 1989, 2, 181. C. Namgung, J.T. S. Irvine and A. R. West, Physica C, 1990, Chem., 1982, 33, 443. 168, 346. Y. F. T. Yao and J. T. Kummer, J. Znorg. Nucl. Chem., 1967, 29 31 R. J. Cava, B. Batlogg, C. H. Chen, E. A. Rietman, S. M. Zahurak 2453. T. Takahashi, p. 1 in ref. 2. W. Fischer, p. 595 in ref. 2. J. Coetzer, J. Power Sources, 1986, 18, 377. 32 33 and D. Werder, Phys. Rev. B, 1987, 36, 5719. K. West, p. 447 in ref. 2. F. G. K. Baucke and J. A. Duffy, Chem. Br., 1985, 643. R. C. Galloway, J. Electrochem. SOC., 1987, 134, 256. Paper 0/02205E;Received 17th May, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100157

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(2), 163-167 Compound and Solid-solution Formation, Phase Equilibria and Electrical Properties in the Ceramic System Zr0,-La,O,-Ta,O,t Chaogui ZhengS and Anthony R. West University of Aberdeen, Department of Chemistry, Meston Walk, Aberdeen AB9 2UE, UK One new compound, ZrLaTa,O,, has been synthesized in the system Zr0,-La,O,-Ta,O,, with an orthorhombic unit cell, a= 10.890(3) A, b= 12.450(3)A and c=6.282(2) A. Extensive ternary solid solutions are formed by five of the binary phases: La,Zr,O,, La,TaO,, LaTaO,, LaTa,O, and LaTa,O,,. Two main solid-solution mechanisms are in evidence: La +Ta e2Zr and La +3Zr 3Ta. The solid-solution phase previously described as being based on Ta,Zr,O,, has, instead, the ideal stoichiometry, TaZr,.,,O,. A significant correction is made to the unit cell dimensions of La,TaO,. The subsolidus phase diagram of the Zr0,-La,O,-Ta,O, system has been determined at 1500°C.The electrical conductivity of a selection of the phases has been determined. Most are very poor semiconductors, but one, LaTa,O,, has a high conductivity, lop4S2-' cm-' at 400 "C, which appears to be due to oxide ions. ZrLaTa,O,, by contrast is an excellent insulator with a resistivity 31O6R cm at 800 "C. Keywords: Zr02-La20,-Ta20, system; Ceramic oxide; Oxide ion conduction We are interested in studying multicomponent ceramic oxide systems with the particular objective of synthesizing new ceramic phases or solid solutions and in measuring their electrical properties. Results on the system Zr02-La203- Nb205 have been reported.' New ternary solid solutions based on LazZr207, La,NbO, and LaNbO, were found and their electrical conductivities determined; all appear to be modest, p-type semiconductors.Here, the corresponding Ta20s system is reported. Major differences between the Taz05- and Nb20,-containing systems are found, including the existence of a new phase, LaZrTa,Oll. The system La20,-ZrO, was reviewed in ref. (1) and is not described again here. Briefly, it contains the cubic pyroch- lore phase La,Zr,O,, which forms a limited range of solid solutions over the range 30-35 mol% La203 at 1500 "C. Solid solution of the end-members is very limited (i.e. <1% La203 in ZrO, and <2% ZrO, in La203) at 1500 "C, the temperature of present interest.A phase diagram for the system La203-Ta205 has been reported,* showing the existence of four congruently melting phases La,TaO, (2020& 20 "C), LaTaO, (1930& 20 "C), LaTa,09 (1 850 20 "C) and La,Ta, ,033 (1890f20 "C). LaTa30, has a distorted perovskite structure with 2/3 of the large A cation sites vacant.,., La,TaO, has a weberite, distorted fluorite LaTaSO1, may have a distorted ReO, structure;8 and the structure of LaTaO, is not kno~n.~.~.'~ Partial phase diagrams for the binary system Zr02-Ta205 have been reported.' At Ta,O,-rich compositions a range of H-Ta20, solid solution forms at high temperatures; solid- solution limits were not determined accurately and are esti- mated as ca.20 mol% Zr02 l1 and ca. 11 YOZrO,' at 1500 "C. It appears that, at lower temperatures especially, these solid solutions may be better represented as a homologous series of line phases.12 The phase Ta,ZrO17 was reported,', but appears to have an upper limit of stability at 1500 "C.The phase Ta2Zr6OI7 is given in ref. (12) and this appears as a solid-solution phase in ref. (1 1) covering the range 67-86 mol% Zr02. Solidus temperatures in the system t Supplementary data available (SUP 56823, 10 pages); details from Editorial Office. $ Permanent address: Department of Chemistry, Peking University, Beijing 100871, People's Republic of China. Zr02-Ta205 appear to be at least 1750 "C; no solid solution of Ta205 in ZrO, has been indicated. Experimental The methods used to react mixtures (in Pt crucibles at 1500 "C) and to analyse the products (by X-ray powder diffraction, with internal standard KCI, as necessary) are essentially the same as those used in the study of the Zr02-La203-Nb205 system.' In this case, however, the entire study was carried out at 1500 "C because there was no evidence of partial melting at this temperature in any of the compositions.Results and Discussion Results of heating experiments on 119 compositions in the binary edges Zr0,-Ta205, La2O3-Ta2O5 and in the ternary system ZrO ,-La2 0,-Ta2 0 are available as Supplementary Data. Only those results in which the samples were deemed to have reached equilbrium are included. System La203-Ta20, Results on this binary edge (Fig.1) are in general consistent with those of most previous reports but with the difference that all four of the phases on this join have variable stoichi- ometry. The phases, with their solid-solution limits at 1500 "C in parentheses, are La,TaO, (24 & 1-28 & 1YOTa20s), LaTaO, (47& 1-50.5 & 0.5Y0), LaTa30g (74.5& 0.5-76.5 +0.5%), LaTaSO1, (81 & 1-84.5 k0.5Y0).The formation of such solid solutions is perhaps not surprising since at least three of these phases have distorted, cation-deficient structures, based on fluorite (La,TaO,), perovskite (LaTa,O,) and ReO, (LaTaSO1,). It seems likely that, at higher temperatures, the individual solid-solution ranges may be more extensive, especially for the latter two, whose structures (perovskite and ReO,) are closely related.There are two possible simple mechanisms that could be responsible for these solid solutions: 5La e3Ta (1) La eTa +0 (2) Very accurate density measurements would be required to show which mechanism was applicable to each of these solid J. MATER. CHEM., 1991 VOL. 1 Fig. 1 Subsolidus phase diagram Zr0,-La,O,-Ta,O, (mole%) at 1500 "C. 0,single phase; 0,two phases; 0,three phases solutions, since they are all of only limited extent. Such measurements have not been attempted. Indexed X-ray powder patterns for LaTa309 and La3Ta07 are available as Supplementary Data; such data are not available in the literature. The refined unit cell parameters for La,TaO,, a= 7.628(2) A, b= 7.749(2) 8, and c= 11.163(5)A, are considerably different from those reported in ref.(5), a= 7.84 A, b= 10.56 A, c =7.70 A. Using those cell parameters in ref.(5), it is possible to index the powder pattern but agreement between observed and calculated d spacings is often poor and cannot be improved by refinement. We believe that the parameters in ref.(5) are incorrect and arose owing to an incorrect indexing of the powder pattern. System Zr02-Ta205 Results on this join (Fig. 1) confirm the existence of the Ta,O, solid solutions containing <13& 1YOZrO,, similar to that suggested in ref. (12). A limited amount of solid solution of Ta205 in ZrO, appears to form, containing up to 2f0.5% Ta205. The Ta,Zr,O, ,phase was found over the solid-solution range 83 ?1-89 IfllYOZr02 which is much less extensive than that given in ref.( 11) and also extends to higher ZrO, contents.We believe that the ideal stoichiometry of this phase is TaZr,.,,08 (i.e. with 84.62 moly0 Zr.0,) instead of Ta,Zr6017 (with 85.72% ZrO,). The reported X-ray data for this phase are unindexed.', We collected fresh data and indexed them by trial and error Visser methods, to give a primitive ortho- rhombic unit cell with high figure of merit, a =5.1 17( 1) A, b = 5.279(1) A, c=4.976(1) A. Again these are available as Sup- plementary Data. Density measurements on two compositions gave values of 7.20 and 7.46 g ~m-~. Consideration of these density values and the unit cell volume indicated that the unit cell contents are much less than one formula unit of Ta2Zr6OI7.Instead, it seems highly likely that the unit cell contains a single unit of formula TaZ2,,,08. The variation in composition, over the range 83-89% ZrO,, could occur by one of two possible simple mechanisms: 4Ta e5Zr (3) 0+2Ta e2Zr (4) The expected variation in density with composition is I E 7.00 B I \ I I I I I I 0 0.20 0.40 Y Fig. 2 Density data for TaZr,,,,O, solid solutions, showing the most likely substitution mechanism to be 5Zr e4Ta shown in Fig. 2 for each of these mechanisms; the experimental data are added for comparison. Given that experimental densities are often a few percent less than theoretical values, the data indicate mechanism (3) as being the most likely.The solid-solution formula may therefore be written as + 5x08:Ta, -4xZr2,75 -O.O2092<x <0.06857. The crystal structure of TaZr2,7508 is not known but it could be a defective version of one of the ABO, structure types. Ternary System Zr02-La20,-Ta205 The results for the 1500°C isothermal phase diagram are shown in Fig. 1. Most of the binary phases form extensive ternary solid-solution series and a new compound, ZrLaTa,O ,,,was found. ZrLaTa,O, , has been indexed using Visser methods (Table 1). The data fit an orthorhombic unit cell, a= 10.890(3)A, b= 12.450(3)A, c= 6.282(2)A. Systematic absences indicate a face-centred cell belonging to one of three possible space groups, Frnrnrn, Fmm2 or F222.The density, determined by dispacement of toluene in a specific gravity bottle, was 7.64 g cm- which compares reasonably well with a calculated value of 7.40gcm-,, assuming unit cell contents of four formula units. From the compositional location of the ternary solid solu- tions in Fig. 1, it appears that several solid-solution mechan- isms are important. On the join, La,TaO,-La,Zr,O,, both end-members form partial solid solutions with each other and the mechanism is clearly La +Ta 2Zr (5) This gives rise to the solid solutions La,-,Ta, -zZr2x07: 0 <x <0.165 and La, +,Ta,Zr2 -2y07:0 <y <0.44. This mech- anism also appears to operate in the LaTaO, solid solutions on the join LaTa0,-ZrO,, giving La, -xTal -xZr2x04: 0<x <0.06.Similar mechanisms were apparent in the solid solutions in the corresponding system La2O3-ZrO2-Nb2O5.' J. MATER. CHEM., 1991 VOL. 1 Table 1 Powder X-ray data for ZrLaTa,O,, dobs./A dcalc.lA h k 1 I 6.224 6.225 0 2 0 17 5.438 5.442 1 0 1 39 4.984 4.986 1 1 1 3 4.097 4.097 1 2 1 18 4.098 2 2 0 3.113 3.113 0 4 0 52 3.047 3.047 3 1 1 77 2.805 2.804 0 2 2 100 2.806 3 2 1 2.702 2.702 2 4 0 12 2.702 1 4 1 2.505 2.506 3 3 1 28 2.505 0 3 2 2.493 2.493 2 2 2 10 2.074 2.075 0 6 0 4 2.056 2.057 4 0 2 7 2.056 1 0 3 2.030 2.030 5 1 1 8 2.030 4 1 2 1.952 1.952 3 5 1 23 1.814 1.815 6 0 0 33 1.814 3 0 3 1.732 1.731 0 6 2 30 1.732 3 6 1 1.716 1.716 4 4 2 7 1.716 1 4 3 1.651 1.650 4 6 0 6 1.650 2 6 2 1.568 1.568 6 4 0 35 1.557 1.556 0 8 0 14 1.548 1.548 3 7 1 12 1.548 0 7 2 1.523 1.524 6 2 2 13 1.523 0 2 4 1.509 1.509 2 0 4 6 1.509 7 2 0 1.494 1.496 1 8 1 5 1.468 1.468 7 2 1 8 1.460 1.461 1 6 3 4 1.357 1.358 2 4 4 6 1.342 1.341 1 9 1 3 1.341 2 9 0 1.328 1.328 0 5 4 6 1.265 1.266 3 9 1 4 1.266 0 9 2 1.253 1.253 6 6 2 9 1.224 1.224 2 0 5 4 1.182 1.182 3 1 5 14 1.182 6 1 4 1.167 1.167 6 2 4 10 1.167 9 2 1 1.158 1.158 1 4 5 7 1.143 1.142 9 3 1 6 Unit cell: a=10.890(3) A; b=12.450(3) A; c=6.282(2) A; 1/=851.73A3; D,b,=7.637gcm-3, ~,,,,,=7.40gcm-~, for ~=4.Occurrence of mechanism (5) may be understood from size considerations. Zr is slightly larger than Ta [octahedral bond lengths to oxygen are: Ta-0, 2.04 A, Zr-0, 2.12 8, ref. (14)] but is rather smaller than La (for CN=8: Zr-0, 2.24 A, La-0, 2.56 A). Since size differences are not too great, coupled substitution of Zr onto Ta/La sites, and vice versa, is possible. In the so!id solutions based on LaTa309, the mechanism appears to be: 3Ta S La + 3Zr (6) The resulting solid solution formula is La, +.Ta3 -3xZr3x09: O< x <0.104. The end-member of this series, with x = 1 would be the hypothetical phase 'La2Zr309': hence this solid solution runs parallel to the Zr02-Ta205 edge at a constant 25% La203 content.Mechanism (6)appears unlikely at first sight since it involves the creation of interstitial La3 + ions. However, the defective perovskite structure of LaTa309, with only 1/3 of the La sites occupied, contains plenty of interstitial sites for additional La3+ ions. It is possible that mechanism (6)is also responsible for the LaTa501, solid solutions, but since these extend to lower Zr contents and have a wider range of La:Ta ratios, this is less clear-cut. Since LaTa5014 appears to have an Re0,-related structure, there are plenty of interstitial sites available for La3+ ions. The complete isothermal section of the phase diagram for the system Zr0,-La203-Ta205 at 1500 "C has been deter- mined (Fig.1). In addition to the five ternary solid-solution fields and the new phase described above, it contains a large number of two- and three-phase regions. Electrical Properties The electrical conductivities of a selection of the new materials have been measured using the a.c. impedance technique, as previously described.' Most have low conductivity at high temperatures, but one, LaTa309, appears to have a high conductivity of oxide ions, comparable to that of yttria-stabilised zirconia at 400 "C. Details are as follows. The data for LaTa309 and its solid solutions displayed features that indicated it could be an oxide ion conductor. A typical impedance data set is shown in Fig.3 for stoichiometric LaTa309 at one temperature, 508 "C and spanning the fre- quency range 10-'-106 Hz (data recorded with a Solartron 125011286 set-up and a Hewlett Packard RF bridge). The data show three clear features: (i) a high-frequency arc, with associated parallel capacitance 6 pF, which is attributable to the bulk response of the ceramic and the associated resistance of which is given from the low-frequency intercept of the arc on the Z' axis, ca. 5.5 kR; (ii) an intermediate-frequency arc, with capacitance ca. 1 nF, which is attributable to a resistive grain boundary of resistance ca. 200 kR at 508 "C; (iii) a low- frequency inclined spike, of capacitance ca. 0.5 pF, which is attributable to electrode polarisation and the rate-limiting diffusion of oxygen molecules through the electrodes to the elec trode/ceramic interface.Such behaviour is typical of oxide ion conductors. In addition, the bulk resistance value, Rb, was found to be uninfluenced by changing the ambient atmosphere from air to argon during the conductivity measurements, again typical of oxide (or other) ion conductors. Bulk conductivity data obtained from plots such as those shown in Fig. 3 for LaTa309 and its solid solutions are presented in Arrhenius format in Fig. 4 and Table 2. Highest conductivity was found for LaTa309 and a solid solution Lal.033Ta2,9Zro. but decreased at higher Zr contents. On the join La203-Ta205, the conductivity decreased for solid solutions to either side of LaTa309 (Lao,955Ta3,02709 and La, ,045Ta2~97309). For one composition, data measured in both air and argon are shown and are essentially coincident.Further experiments are in progress to determine the transport number of oxide ions in LaTa309. A combined conductivity Arrhenius plot for the other compositions that were measured is given in Fig. 5, with activation energies and conductivity values at 1000 K listed in Table 2. Of these, La3Ta07 and its solid solutions have the highest conductivity, but since their conductivity decreases on changing the atmosphere from air to argon (Fig. 6) it is concluded that their conduction is electronic and p-type. The other phases, LaTaSOl4, La2Zr207, LaTaO,, TaZr2.,,04 and Table 2 Conductivity data composition (mol%) ZrO, La20, Ta205 a/lt-'cm-' at 1OOOK E,/eV" La,Zr ,O 54.0 40.0 6.0 4.oX10-7 1.274 La TaO 75.0 25.0 2.7 x10-5 0.969 6.0 71.0 23.0 1.8 x lo-' 0.8 19 13.0 67.0 20.0 5.4 x lo-' 0.790 20.0 62.0 18.0 2.0 x (in air) 6.0 x SO-' (in Ar) LaTaO, 0.679 0.543 4.0 48.0 48.0 1.1 x 1.387 LaTa,O, 24.0 76.0 2.2 x 10-4 0.420 25.0 75.0 4.9 x 10-4 0.385 26.0 74.0 3.8 x 10-5b 0.543 10.0 25.0 65.0 6.3 x 10-4 0.385 22.0 25.0 53.0 1.8 x lop4 (in air-Ar) LaTa,O 0.470 3.0 16.0 81.0 1.8 x so-5 0.983 TaZr2.7 5O8 88.0 12.0 2.1 x 1.110 33.3 16.7 50.0 7.0 x lop8 ZrLaTa,O, , 1.337 a 1 eVz1.602 x J. Sample contained LaTa,O, (solid solution) plus trace LaTaO,.6.3xl 02Hz O. 07.2 0 0 E 0 C 0 -$.4.8 0 h 0 0 0 2.4-0 0 0 5x105Hz I I I I 1 f 2.4 4.8 7.2 9.6 12.0 Rb 2'110~R cm 3.6 5 2.4 0c: 0 0 0-0 0. 0 0 k I 1.2 6.3xl 02Hz 0 0 -.----_.-- - I7 00 O 0 0 00~ 50Hz ooooo ffo ; Ow046.3~104p 1 I 1 1.2 12.4 3.6 4.8 6.0 Fig.3 Impedance data for LaTa,O, in air at 508 "C: (a) high-frequency data, >630 Hz; (b)low-frequency data, (65 kHz. Data sets overlap at intermediate frequencies J. MATER. CHEM., 1991 VOL. 1 TI"C 1000 700 500 300 i i i 1 1 1 1 .o 1.5 0 103~1T Fig. 4 Conductivity data for LaTa,09 and its solid solutions: (i) ZrO, (0.9): Y,O, (0.1); (ii) La, ,033Ta2,9Zro, (iii) LaTa,O,; (iv) La0.955Ta3.02709; (v) La1.07Ta2.78Zr0.2209; (vl) La1.045Ta2.97309. Data for one composition, (v), were measured both in air (+) and in argon (0).Data for yttria-stabilised zirconia are shown for com-par i son TI'C 1500 1000 700 500 300 1.o 1.5 2.0 lo3KIT Fig.5 Conductivity data for various materials in the system Zr0,- La,O,-Ta,O,. Data for yttria-stabilised zirconia are shown for comparison. 0, LaTa309; 0, La,Ta07; 0, LaTa,O,,; +,TaZr,,,,08; A, LaTaO,; A,La2Zr,07; (>, ZrLaTa,O,, ZrLaTa,O, all have much lower conductivity. Indeed, the conductivity of the new phase ZrLaTa,O,, is so low (3 x lO-'R-l cm-' at 800 "C)that it may find applications as an electrical insulator. Permittivity data for all the new materials were determined at the same time as their conductivities.In all cases, the high- frequency permittivity was small with no indication of ferro- electric behaviour. J. MATER. CHEM., 1991 VOL. 1 TI“C 1000 700 500 300 I I I I 1 -3 -4 I E -5 -. b cn --6 -7 1.o 1.5 2.0 lo3 K/ T Fig. 6 Conductivity data for La,TaO, solid solution of composition La2.747Zro.443Tao,,9707showing atmosphere dependence and p-type conduction. 0,In argon; 0,in air A.R.W. thanks SERC for a research grant. We thank J. T. S. Irvine for advice on conductivity measurements. References 1 C. Zheng and A. R. West, Br. Ceram. Trans. J., 1990, 89, 138. 2 N. S. Afonskii and M. Neiman, Znorg. Muter., 1967, 3, 1132. 3 P. N.Iyer and A. J. Smith, Acta Crystallogr., 23, 1967, 740. 4 H.P. Rooksby, E. A. D. White and S. A. Langston, J. Am. Ceram. SOC., 1965, 48, 447. 5 H. P. Rooksby and E. A. D. White, J. Am. Ceram. SOC., 1964, 47, 95. 6 A. J. Dyer and E. A. D. White, Trans. Br. Ceram. Soc., 1964, 63, 301. 7 M. M. Pinaeva, V. V. Kuznetsova, A. B. Ustimovich and L. D. Shul’man, Znorg. Muter., 1973, 9, 563. 8 L. N. Lykova and L. M. Kovba, Russ. J. Znorg. Chem. (Engl. Trans.), 1971, 16, 459. 9 C. Keller, 2. Anorg. Allg. Chem., 1962, 318, 89. 10 V. S. Stubican, J. Am. Ceram. Soc., 1964, 47, 55. 11 B. W. King, J. Schultz, E. A. Durbin and W. H. Duckworth, in Phase Diagrams for Ceramists, American Ceramic Society, Col- umbus, 1964, diagram 374. 12 R. S. Roth and J. L. Waring, in Phase Diagramsfor Ceramists, American Ceramic Society, Columbus, 1975, diagram 4458. 13 JCPDS Powder Diffraction File, Card 8-246. 14 R. D. Shannon, Acta Crystallogr., Sect. A, 32,751. Paper 0/02053B; Received 9th May, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100163

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(2), 169-173 I69 Thermal and X-Ray Diffraction Studies of Liquid Crystals incorporating a Perfluoroalkyl Group Takashi Doi," Yoshiaki Sakurai,a Akira Tamatani," Shunsuke Takenaka,*a Shigekazu Kusabayashi," Yasuo Nishihatab and Hikaru Terauchib a Chemical Process Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan Department of Physics, Faculty of Science, Kwansei-Gakuin University, Nishinomiya, Hyogo 661, Japan Thermal properties of perfluoroalkyl 4-(4-alkoxybenzoyloxy)benzoates (l), 4-(4-alkoxyphenoxycarbonyl)ben-zoates (2), and 4-trans-(4-alkoxyphenoxycarbonyl)cyclohexanecarboxylates (3) have been examined, and the layer spacings of the smectic A and C phases were obtained by X-ray diffraction experiments. The perfluoroalkyl group facilitates the formation of smectic A and C phases (S, and S,, respectively).For compound 1, the molecular arrangements in the S, and S, phases are concerned with the chain length of the alkoxy group, that is the layer spacing of the S, phase agrees with the calculated molecular length when the alkoxy group is the octyloxy group, and exceeds it when it is the butoxy one. The derivatives of compound 2 show only the S, phase, having a somewhat interdigited arrangement. Compound 3 preferentially forms the S, phase. The results are discussed in terms of the molecular structures and local interactions. Keywords: Liquid crystal; X-Ray diffraction; Smectic phase; Perfluoralkyl compound Liquid crystals incorporating fluorine atoms frequently show very interesting characteristics for display devices, and their development is currently of great interest.'7' Generally, liquid crystals incorporating a perfluoroalkyl chain are known to show remarkable smectic proper tie^.^-^ Interestingly, these liquid crystals show an SA phase having a monolayer or partial bilayer arrangement of the molecule^.^*^ A fluoropho-bic interaction has been supposed to play an important role in this. We prepared three compounds having a perfluoroalkyl group: where R'=C,H2,+ ,(n= 1-8), RZ=CH2(CF2),F or C2H4(CF2),F (m=1-10). In this paper, we describe the ther- mal properties of some derivatives of compounds 1-3, and the molecular arrangements in the SA and Sc phases studied by X-ray diffraction. The results have been discussed in terms of the molecular structures and the short-range interactions around the perfluoroalkyl chain.Experimental Materials 1,l-Dihydroperfluoroheptyl 4-(4-Octyloxybenzoyloxy)benzoate (Ij1 A mixture of 4-octyloxybenzoyl chloride (5.4 g, 0.02 mol) and benzyl 4-hydroxybenzoate (5.55 g, 0.024 mol) in dry pyridine (30 cm3) was stirred at 70 "C for 6 h, and was evaporated to dryness under vacuum. The residue was dissolved in chloro- form and washed with water. After drying over anhydrous sodium sulphate, the solution was evaporated to dryness, and the residue was poured onto the top of the chromaLography column on silica gel, and was developed with chloroform. The first eluant was recrystallized from ethanol, giving benzyl 4-(4-octyloxybenzoyloxy)benzoateas colourless needles (6.0 g), mp.63 "C (Found: C, 75.5; H, 7.0%. Calc. for C29H3205: C, 75.6; H, 7.0%). Benzyl 4-(4-octyloxybenzoyloxy)benzoate(5.9 g, 0.0 13 mol) was hydrogenated under the presence of palladium/carbon (100 mg, 10%) in a solvent mixture of toluene and ethanol (l:l, 100 cm3). After removal of the catalyst by filtration, the reaction mixture was concentrated. The precipitates were collected by filtration, and recrystallized from ethanol, giving 4-(4-octyloxybenzoyloxy)benzoicacid as colourless needles (4.3g) (Found: C, 71.2; H, 7.1%. Calc. for C22H2605: C, 71.3; H, 7.1 %). This compound underwent the phase transition of C*146*Sc*183.N.238.1 (T/"C).A mixture of 4-(4-octyloxybenzoyloxy)benzoicacid (0.8 g, 0.002 mol) and phosphorous pentachloride (0.5 g, 0.002 mol) in thionyl chloride (0.5 cm3) was stirred at 50 "C for 2 h, and then the mixture was dried under vacuum. The residue and 1,l-dihydroperfluoroheptanol(1.0 g, 0.003 mol) in 15 cm3 of dry pyridine was stirred at 70 "C for 5 h. The resulting solution was evaporated to dryness under vacuum, and the residue was treated with column chromatography on silica gel, where chlor- oform was used as a developing solvent. The first eluant was recrystallized from a solvent mixture of hexane and ethanol (5:1 w/w), giving 1,1-dihydroperfluoroheptyl 4-(4-octyloxy- benzoy1oxy)benzoate as colourless needles (1.38 8); 6, (solvent CDCl,; standard SiMe,) 0.89 (3 H, t, 6.7), 1.30-1.39 (8 H, m), 1.45-1.50 (2 H, m), 1.83 (2 H, q, 6.70), 4.05 (2 H, t, 13.2), 4.83 (2 H, t, 6.54), 6.98 (2 H, d, 8.50), 7.34 (2 H, d.8.50), 8.14 (4 H, d, 8.50); 6, (solvent CDCl,; standard trifluoromethyl- benzene) -17.98 (3 F, t, 9.88), 56.46 (2 F, m), 59.31 (2 F, m), 59.98 (2 F, m), 60.41 (2 F, m), 63.30 (2 F, m). Other fluor- inated materials were identified with 'H and I9F NMR. 1,1,2,2- Tetrahydroperjuorohexyl trans-4-(4-Octyloxy- benzoy1oxy)cyclohexanecarboxylate (3c) A mixture of trans-cyclohexanedicarboxylic acid (20 g, 0.12 mol) and phosphorous pentachloride (56.3 g, 0.27 mol) was heated at 50 "C for 1 h, and then evaporated to dryness under vacuum. The residue and benzyl alcohol (32 g, 0.29 mol) in dry pyridine (100 cm3) were stirred at 70 "C for 4 h.The resulting solution was evaporated to dryness under vacuum, and the residue was extracted with ether. The ether layer was washed with water and dried over anhydrous sodium sulphate. The extract was purified with column chromatography on silica gel, where chloroform was used as a developing solvent. The first eluant was recrystallized from a solvent mixture of ethanol and hexane (1: I), giving dibenzyl trans-cyclohexane- 1,4-dicarboxylate as colourless needles (43.0 g), m.p. 84 "C. Dibenzyl trans-cyclohexane-l,4-dicarboxylate(10 g) in a sol- vent mixture of ethanol and toluene (150 cm3, 1:1) was hydro- genated under the presence of palladium/carbon (100 mg, After uptaking hydrogen (37 mol% of theoretical amount), the catalyst was removed by filtration, and the resulting solution was evaporated to dryness under vacuum.The residue was purified with column chromatography on silica gel, where chloroform was used as a developing solvent. The first eluant gave the recovered material (5.2 g). The second eluant was recrystallized from a solvent mixture of ethanol and hexane (1:l), giving trans-4-benzyloxycarbonylcyclohex-anecarboxylic acid as colourless needles: 3.6 g, m.p. 82 "C. A mixture of trans-4-benzyloxycarbonylcyclohexanecarboxylic acid (2.85 g, 0.01 mol) and phosphorous pentachloride (2.5 g, 0.012 mol) was heated at 50 "C for 1 h, and the resulting solution was evaporated to dryness under vacuum. The residue and 4-octyloxyphenol (2.9 g, 0.013 mol) in dry pyridine (20 cm3) were stirred at 70 "C for 4 h.The resulting solution was evaporated to dryness under vacuum, and the residue was extracted with ether. The ether layer was washed with water and dried over anhydrous sodium sulphate. The extract was purified with column chromatography on silica gel, where chloroform was used as a developing solvent. The first eluant was recrystallized from hexane, giving benzyl trans-4-(4- octyloxyphenoxycarbonyl) cyclohexanecarboxylate (5.1 g), m.p. 47 "C (Found: C, 74.6; H, 8.2%. Calc. for C29H3805: C, 74.7; H, 8.2%). Benzyl trans-4-(4-octyloxyphenoxycarbonyl)cyclohexane-carboxylate (1.3 g) in a solvent mixture of ethanol and toluene (l:l, 100 cm3) was hydrogenated under the presence of pal- ladium/carbon (100 mg, 10%).After removal of the catalyst, the resulting solution was evaporated to dryness under vac- uum, and the residue was purified with column chromatogra- phy on silica gel, where chloroform was used as a developing solvent. The eluant was recrystallized from a solvent mixture of ethanol and hexane (1: l), giving trans-4-(4-octyloxy- phenoxycarbony1)cyclohexanecarboxylic acid (0.85 g). This material underwent the phase transition of C.ll4.plastic.183- SA*199-I(T/"C).(Found: C, 70.0; H, 8.5%. Calc. for C22H3205: C, 70.2; H, 8.6%. trans-4-(4-Octyloxyphenoxycarbonyl) cyclohexanecarboxylic acid and 1,1,2,2-Tetrahydroper-fluorohexanol were reacted by a similar method mentioned above, giving 1,1,2,2-tetrahydroperfluorohexyltrans-4-(4-octyl-oxyphenoxycarbony1)cyclohexanecarboxylate as colourless needles.Method Phase transitions were observed by using a Nikon Model POH polarizing microscope fitted with a Mettler FP 52 heating stage. Transition temperatures and the latent heats were measured with a Daini-Seikosha SSC-5200 differential scanning calorimeter, where indium (99.9%) was used as a calibration standard with a heating rate of 5 "C min-' (m.p. 156.6 "C, AH =6.80 cal mg- I)?. 'r 1 calz4.2 J J. MATER. CHEM., 1991 VOL. 1 X-Ray Diffraction The sample sealed in a square cell (5 x 5 mm') was gradually cooled from the isotropic liquid under a ca. 1 kG magnetic field in order to obtain a single domain in the cell.The cell temperature was controlled automatically within & 0.05 "C by a microcomputer. Cu-Ka radiation monochromatized by a pyrolytic graphite crystal was employed as an X-ray beam, scattered from the sample and directed to the scintillation counter through two receiving slits. Results Transition temperatures and latent heats for the compounds 1-3 are summarized in Tables 1-3. The transition tempera- tures for unfluorinated derivatives are also indicated for the comparative study. A nematic phase is not formed through these fluorinated derivatives. The transition temperatures for compounds 1 and 2 are plotted against the fluorocarbon number (m),in Fig. 1. Some phase diagrams for the mixtures are shown in Fig. 2. In Fig.2(a) the fluorinated derivative is well miscible with the unfluorinated liquid crystal. In the figure, both Sc phases have no affinity, and the Sc-SA transition temperatures steeply decrease with an increase in the concentration of each com- ponent. A similar trend can be seen in Fig. 2(b),where the Sc-SA transitions also show a steep depression. In Fig. 2(c), on the other hand, both N and SA phases have wide two-phase regions, and the Sc-SA transition is broad, while both components are similar to those in Fig. 2(a) and 2(b).A similar phase separation in mesophases is also observed in Fig. 2(d), where the reference is a so-called 'polar liquid crystal', and shows a partial bilayer arrangement of the molecules in the SA phase. In the mixture of the cyano compound and In, all the mesophases showed a phase separ- ation, and the phase diagram was difficult to draw.X-Ray Diffraction The molecular arrangements in SA and Sc phases were exam- ined by X-ray diffraction. The Laue pictures showed a ring because of insufficient orientation of the molecules within the cell. The X-ray profiles for derivative lm are shown in Fig. 3. For lm the diffraction angles in the SA and Sc phases are 2.36 and 2.34, and the layer spacings are calculated to be 37 and 38 A, respectively. Similarly, the layer spacings for some derivatives of the present compounds were obtained, and the results are summa- rized in Table 4. a 0 0 0 a R n 0 A a 100 n a n I 46810 46810 m m Fig. 1 Plots of transition temperatures against fluorocarbon numbers (m)for: (a) the octyloxy and (b)the butoxy derivatives of compound 1.0,SA-I; 0,S,-S,; A, melting point J. MATER. CHEM., 1991 VOL. 1 Table 1 Transition temperatures (T/"C)and latent heats for compound 1 compounds R' la CHZc2 F5 '87 -* 106 6.3 CH2C6F1 3 . 81 -* 147 7.4 C2H4C4F9 .94 -. 133 4.8 C2H4C6FI 3 . 104 -. 151 7.7 C2H4C8F17 '117 -. 168 9.1 C2H4C10F21 ' 130 -. 175 -10.7 lg C8H17 CH2C2F5 . 48 . 68 . 84 0 7.7 lh CH2C2F4H . 76 (. 37 * 58) 0 5.9 li CH2C4F8H . 61 (. 59) . 68 0 6.5 1j CH2C6F13 . 71 . 107 . 120 0.2 8.5 C2H4C4F9 . 71 -109 .111 0.25 6.4 C2H4C6F13 . 88 . 122 ' 129 0.2 8.2 C2H4C8F1 7 . 101 . 131 . 145 0.2 8.5 C2H4C10F21 .109 . 136 * 158 0.2 7.9 -lo C2H5CH(CH3)" CZH4C4F9 -85 -IP * 105 (. 104) 136 0.2 6.8 1q * 71 l'a . 51 -. 69 6.3 l'b *54 -. 67 8.6 Parentheses indicate a monotropic transition. S-configuration. Table 2 Transition temperatures (T/"C)and latent heats for compound 2 compounds R' ~ ~~ 2a CH3 C2H5 2c C4H9 C8H17 2e 2f 2g2h C2H5CH(CH3)b2i C2H5CH (CH,) C3H6b 2j (CH&C4H7CH (CH,) CH2b 2'a C8H17 R2 C SA I AHsAP,/kJmol-' C2H4C4F9 * 75 . 106 6.5 C2H4C4F9 . 84 . 117 6.4 C2H4C4F9 C2H4C4F9 . . 78 87 . 107 . 96 5.3 6.9 C2H4C6F1 3 C2H4C8F17 C2H4C10F21 . 98 ' 116 . 128 . 118 136 . 149 6.8 8.0 8.0 CZH4C6F13 C2H4C8Fl 7 ' 97 * 112 . 97 . 119 a 6.6 C2H4C6F1 3 . 88 C6H 13 . 57 "The latent heat was impossible to determine because of recrystallization.bS-configuration. Table 3 Transition temperatures (T/"C)and latent heats for compound 3 AHs,-s,lk J AHs,-,lkJ compounds R' R2 C sc SA I mol-' mol-' 3.23a C4H9O CH2C2F5 4.7C8H 17O CH2C2F5 3.83c 2H4C4 9 C2H4C1 OF, 1 -C8H 17 CH2C2F5 3f c2 4c4 9 10.4 -3g C2H4C10F21 4.9C6H13 Parentheses indicate a monotropic transition. "The transition was impossible to detect by DSC because of recrystallization. Discussion In order to discuss the thermal properties, we have to know the structural characteristics of the present compounds as well as the physical properties of the perfluoroalkyl chain. It is known that a perfluoroalkyl group is more rigid than an alkyl group owing to steric hindrance between fluorine atoms, since the van der Waals radius of fluorine (1.35 A) is larger than that of hydrogen (1.10 A).In fact, in n-decafluoro- butane, the energy difference between gauche and trans confor-mations is calculated to be 2.2 kcal mol- ',while the difference for butane is 0.9 kcal mol- ', indicating that the internal rotation of the perfluoroalkyl group is fairly restricted, and the trans conformation is energetically favourable.' The molecular structures of the present compounds were calculated roughly by using geometrical parameters,' where both alkyl and perfluoroalkyl groups were supposed to have a zigzag conformation in order to keep the best linearity; the results are shown in Fig.4.It would be reasonable to assume that in the mesophases, the molecules maintain molecular rotational and interlayer motion freedoms, and alkyl and perfluoroalkyl chains also keep a rotational freedom within the molecule. We suppose here that the molecule rotates around A, passing through the centres of the aromatic rings, and the alkyl group rotates around AX1,as shown in Fig. 4. As we can see from Fig. 4(a), the molecular structure of derivative lm is strongly dependent on the rotational angle of the alkoxy group. The longitudinal molecular lengths are 38 and 35 A on the trans and cis conformations, respectively, indicating that the cis conformer is fairly bent. Considering J. MATER. CHEM., 1991 VOL. 1 (4 5000 - 5000 0 i= 0 i= cn z % 3000 L - u) 9 L.d3000 cn c u)CI C3 C3 0 0 0 I 50 mol% J 100 0 50 mol% 100 1000 - 1000 1 I t I I 1.0 2.0 3.0 4.0 281" Fig.3 X-Ray profiles of derivative lm at: (a) 137 "C (SA phase); (b) 120 "C (Sc phase)150 0 i= 100 0 50 100 0 50 100 mol% mol% +-32.6A-Fig. 2 Phase diagrams for mixtures of: (a) 4-octyloxyphenyl 4-(4- octyloxybenzoy1oxy)benzoate(on left) and lm (on right); (b) 4-oc-tyloxyphenyl 4-(4-octyloxybenzoyloxy)benzoate(on left) and le (on right);(c)4-octyloxyphenyl4-(4-octyloxybenzoyloxy)benzoate(on left) and lp (on right); (d) N-[4-(4-octyloxybenzoyloxy)benzylidene]-4-cyanoaniline (on left) and 2d (on right). Dashed lines indicate mono- tropic transition Fig.4 Molecular structures of derivatives: (a)lm; (b)lf; (c) 2f; (a') lr. Circles indicate fluorine atoms in the saturated chain the fact that the alkoxy group rotates almost freely in the mesophases, both conformers are very important in determin- ing the mesomorphic properties of the molecule. The confor- the ratios of the layer spacing to the molecular length are mational difference mainly arises from the ester groups, since summarized in Table 4. the bond angle of -CO-0-(110') is less than that of For compound 1, the derivatives with R' =butoxy group -C=C-(in aromatic ring, 120').8 For the butoxy deriva- show only the S, phase, and the Sc phase is not formed even tives [le, Fig. 4(b)], the longitudinal lengths for the trans and in the higher members such as le and If.The derivatives with cis conformers are 34 and 33 A, respectively, and the difference R2 =octyloxy group, on the other hand, show the Sc phase is less than that for the octyloxy derivatives. Although the as well as the SA one even in the lower members such as lg strict molecular length of the molecule should be a statistical and lk. These facts indicate that a long alkoxy group in the average of all the conformers, for the sake of convenience we R1position is indispensable for the formation of the Sc phase. suppose that the longitudinal length of the trans conformer As we can see from Fig. 4(a)and 4(b):the alkoxy chain length is the molecular length. The calculated molecular lengths and at R1 determines the molecular structure of the entire Table 4 Layer spacing and molecular length of compounds molecular' compounds R' R2 length, MIA layer spacing, LIA Ljm T/"C phase lm C8H17 C2H4C8F1 7 38 38 1.o 80 38 1.o 90 38 1.o 100 38 1.o 105 38 1.o 120 38 1.o 125 38 1.o 137 36 1.o 80 35 1.o 109 42 1.2 140 42 1.2 150 42 1.1 110 42 1.1 125 43.5 1.25 95 48 1.4 120 "Refer to text. J.MATER. CHEM., 1991 VOL. 1 molecule; that is, the long alkoxy group is the cause of the bent molecular structure. Ratios of the observed layer spacing to the calculated molecular length show an interesting trend. Although the layer spacings in the SA and Sc phases for compound 1 are a function of the terminal alkyl and perfluoroalkyl chain lengths, the ratio is dependent on the shape of the alkyl group at position R' rather than the length of the perfluoroalkyl group.When R' is an octyloxy group (lm and lj), the ratios are close to unity, indicating that the molecules form a monolayer arrangement in the SA and Sc phases. When R' is the butoxy group (If ), the ratio is ca. 1.2, indicating that the molecules form the interdigited arrangement. When R' is the 4-methylhexyl group, derivative lp also forms the interdi- gited arrangement, where the ratios in the SA and Sc phases are 1.4 and 1.25, respectively. These facts indicate clearly that the structure of the substituent at the R' position determines the molecular arrangement in the smectic phases. An interesting fact for derivatives lm and lj is that the layer spacings for the SA and Sc phases are almost the same.The average long axis of the phenyl benzoate moiety probably arranges perpendicular to the layer in the SA phase, and the tilt in the Sc phase. Therefore, the accordance of the layer spacings is an accidental case. Compound 2 involves a terephthalate core. As we can see from Fig. 4(c), the molecular structure is different from that of compound 1, since the ester groups orient themselves opposite each other. In the cis conformer for compound 2 [Fig. 4(c)], the molecular length is 37 A, while that for trans is 38 A. The difference (1 A) is far smaller than that for derivative lm, indicating that compound 2 keeps the linear shape even for the higher members. Both butoxy and octyloxy derivatives show only the SA phase, the SA-I transition temperatures are a little lower than those for compound 1, and any tilted smectic phases are not formed even in the higher members.The SA phase has the interdigited arrange- ment, where the ratio of the layer spacing to the molecular length is 1.1, which is smaller than those for If and lp. It has been reported that some perfluoroalkyl compounds form the interdigited arrangement owing to a 'fluorophobic' interaction, but others form the monolayer one.'" The present results indicate that an alternation of the molecular arrange- ments in the smectic phases is concerned with the molecular structure rather than the fluorophobic interaction. As we can see from the comparison of derivatives lk and l'a, and lm and l'b, the transition entropies of the SA-I transition for the fluorinated derivatives are smaller than those for the hydrogen ones. From these results, we can assume that the perfluoro- alkyl group is more rigid than the usual alkyl one, and plays the role of a hard core as for an aromatic ring.In connection with the role of the perfluoroalkyl group, an interesting fact is that the SA-I transition temperatures for derivatives lh and li are lower by ca. 30 "C than those for lg and lk. Considering the fact that the dipole moments for 1,1,I -trifluoroethane and 1,l-difluoroethane are 2.32 and 2.24 D," respectively, the change in the transition temperature is impossible to correlate with the change in the dipole moment.Furthermore, trifluoromethyl and difluoromethyl groups would have a similar volume. Therefore, we assume that a fluorophobic interaction at the surface of the smectic layer plays some important roles in enhancing the SA-I transition temperature. A chiral derivative (lp) shows SA and chiral Sc phases, where the SA-I transition temperature is relatively low, prob- ably owing to the branched methyl group. On the other hand, lo and lq having a branched methyl group at positions 1 and 2 are non-mesogenic. Derivatives 2h and 2i show the SA phase, while the transition temperatures are low. Interestingly, 2j is non-mesogenic, probably owing to two branched methyl groups. These results indicate that the chemical modification of the alkyl group at the R' position strongly affects the mesomorphic properties of the molecules.Compound 3 has a cyclohexane core instead of the benzene ring in compound 2. Apparently, the cyclohexyl group decreases the SA-I transition temperature, while it increases the melting point. The derivatives preferentially form the SA phase. Interestingly, derivative 3c shows the Sc as well as the SA phase. The perfluoroalkyl compounds show interesting phase behaviour in the binary mixtures. In Fig. 2(a) and 2(b), the fluorinated derivatives and the reference compound having the SAl phase are miscible in the SA phase, where the SA-I (nematic) transitions have narrow two-phase regions. In Fig. 2(c) and 2(d), on the other hand, where one of the components has the interdigited arrangement in the SA phase, the SA-I transitions show wide two-phase regions, and the phase separation becomes remarkable with an increase in the chain length of the perfluoroalkyl group.Therefore, we assume that the emergence of a wide two-phase region and a non- uniformity of the phases are a result of the difference in the molecular arrangements in the smectic phases. As we can see from Fig. 2, the Sc phase of the fluorinated derivatives has no affinity with that of the reference com- pounds. The difference in the molecular structures may be one of the causes of the phenomenon. Conclusion The fluoroalkyl group increases the smectic properties and enhances the SA-I transition temperature. The perfluoroalkyl compounds show the monolayer or the interdigited arrange- ment in the smectic phases.The rigidity of the perfluoroalkyl chain and the fluorophobic interaction, are important in these phenomena. References 1 Hp. Schadt and S. M. Kelly, J. Phys., 1985, 46, 1395. 2 M. Koden, K. Nakagawa, Y. Ishii, F. Funada, M. Matsuura and K. Awane, Mol. Cryst. Liq. Cryst. Lett., 1989, 6, 185. 3 V. V. Titov, T. I. Zverkova, E. I. Kovshev, Yu. N. Fialkov, S. V. Shelazhenko and L. M. Yagupolski, Mol. Cryst. Liq. Cryst., 1978, 47, 1. 4 S. Misaki, S. Takamatsu, M. Suefuji, T. Mitote and M. Matsu- mura, Mol. Cryst. Liq. Cryst., 1981, 66, 123. 5 A. V. Ivashchenko, E. I. Kovshev, V. T. Lazareva, E. K. Prudni- kova, V. V. Titov, T. I. Zverkova, M. I. Barnik and L. M. Yagupolski, Mol. Cryst. Liq. Cryst., 1981, 67, 235. 6 E. P. Janulis Jr., J. C. Novack, G. A. Papapolymerou, M. Tristani-Kendra and W. A. Huffman, Ferroelectrics, 1988, 85, 375. 7 F. Tournnilhac and J. Simon. Proc. 2nd lnt. Conf Ferroelectric Liquid Crystals, 1989, p. 82. 8 Y. Sakurai, S. Takenaka, H. Miyake, H. Morita and T. Ikemoto, J. Chem. SOC., Perkin Trans. 2, 1989, 1199. 9 M. Koden. T. Kuratate, T. Shinomiya, K. Nakagawa and F. Funada, personal communication. 10 J. A. Riddick, W. B. Bunger and T. K. Sakano, Techniques of Chemistry, Vol. I1 Organic Solvents, ed. A. Weissberger, Wiley, New York. 1986. Paper 0/02705G; Received 18th June, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100169

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(2), 175-180 Structure Evolution during Thermal Processing of High-Tc Ceramic Superconductors produced using Sol-Gel Techniques George Kordas,ta Glenn A. Moore/ J. D. Jorgensen,b F. Rotella,b R. L. Hitterman,b K. J. Volinb and J. Faberc a Department of Material Science and Engineering, Ceramics Division, Science and Technology Center for Superconductivity and Materials Research Laboratory, University of Illinois at Urbana- Champaign, 105 S. Goodwin, Urbana, lL 61801, USA Argonne National Laboratory, Argonne, lL 60439, USA Amoco Corporation, Naperville, lL 60566, USA Characterization of the decomposition and phase development reactions for Cu" methoxyethoxide, 2Ba-3Cu, Y-~CU, and Y-2Ba-3Cu gels has been performed using in situ neutron diffraction and differential thermal analysis (DTA).The structural evolution process of a Y-2Ba-3Cu gel to the superconducting YBa,Cu,O,~, phase was elucidated using the acquired spectra. Precursor gel powder was produced from an alkoxide sol-gel system having a methoxyethanol, methyl ethyl ketone and toluene solvent mixture. Neutron diffraction results indicate initial YBa,Cu,O,-, formation below 700 "C, and that a firing temperature of 850 "C is sufficient to produce essentially single-phase YBa,Cu,O,-,. The conversion of BaCO, to BaCO, polymorph was observed to occur between 790 and 800 "C, after which rapid formation of the YBa,Cu,O,-, tetragonal phase occurred. The results also indicate an incomplete conversion of the non-superconducting tetragonal phase to the supercon- ducting orthorhombic phase during the cool-down and annealing segments of the heat treatment.Keywords: Sol-gel processing; Y-Ba-Cu-0 system; Neutron diffraction; Superconductivity 1. Introduction The processing of single-phase YBa2Cu307-supercon-ducting powder and thin films is paramount to basic materials research studies as well as commercial product developments. Initially, single-phase superconductors were produced by the solid-state reactions of yttrium oxide, barium carbonate, and copper oxide powders.' A severe drawback of this process is the necessary 950 "C processing temperature for several hours and the subsequent annealing process in oxygen near 450 "C to yield the orthorhombic superconducting phase. The high calcination temperature usually results in extensive micro- structural grain growth' and interaction of the supercon- ducting film with the substrate.,q4 Sol-gel processing techniques offer an alternative to the traditional ceramic processing methods.Research on sol-gel processing over the last 20 years has demonstrated that the purity and microstructure of resulting materials can be con- trolled through variation of the starting chemistry and pro- cessing parameters. For example, microstructure control can be achieved by variation of precursor type, water and solvent concentration, pH, and mixing and firing temperatures.' Most frequently, the sol-gel method uses metal alkoxides, which hydrolyse and condense to form polymeric networks. Follow- ing gelation, firing at significantly reduced temperatures, com- pared to those used for traditional oxide sintering, is used to form the appropriate ceramic material.For example, Hirano .~et ~1 have produced partial formation of YBa2Cu,07 -x superconducting powder at 650 "C using yttrium isopropox- ide, barium isopropoxide, and copper(I1) 2-ethoxyethoxide. The gel powder was fired in a flowing 02-0,mixture in order to alleviate formation of BaCO, during the organic group decomposition stages. Horowitz et aL7 synthesized YBa2Cu,07-sol-gel superconductor at 650-700 "C t Present address: Demokritos, National Research Center for Physi- cal Sciences, Institute of Materials Science, 153 10 Ag. Paraskevi Attikis, Athens.Greece. through complete hydrolysis of yttrium diisopropoxide, bar- ium diisopropoxide, and copper(I1) n-butoxide. The ability to produce essentially complete YBa,Cu,07 -x orthorhombic conversion was due to the lack of BaC0,. Kramer et a1.* have produced YBa2Cu307 -sol-gel superconductor pow- ders using Y methoxyethoxide, Ba methoxyethoxide, and copper(I1) ethoxide. Firing of the gels at 700 "C resulted in a YBa2Cu307--x orthorhombic conversion of 8-10% owing to the presence of BaCO,. Kramer et al.' also demonstrated that epitaxial films can be produced on SrTiO, via sol-gel routes. Therefore, the sol-gel technique can lead to high- quality superconductors that can be used for physical property measurements as well as technological applications.This publication reports the structural development and characterization of a sol-gel process that we developed for the production of the YBa2Cu307 -x superconductors exten- sively described in a number of previous publication^.^-' ' 2. Experimental Cu" methoxyethoxide, Y methoxyethoxide, and Ba methoxy- ethoxide precursors were synthesized as previously reported.'-' Stable sols having concentrations between 0.01 and 0.05 mol kg-I were obtained by mixing the desired precursors in a methoxyethanol-methyl ethyl ketone-toluene solvent mixture exposed to air. Gel powder was produced by stripping the solvent mixture from the acquired stable sol using a rotary evaporator. The gel powder was subsequently dried in vacuum at 120-200 "C and then pressed into cylindri- cal pellets, 1.O cm diameter x 1.5 cm length at 20-30 psi$ for the neutron diffraction measurements.Prior to the neutron diffraction experiments, characterization of the gel decompo- sition temperatures were performed using differential thermal analysis (DTA). A Perkin-Elmer Model 1600 was utilized and gel powder samples were heated at a rate of 10 "C min-' to $ 1 psi z 6.895 x lo3 Pa. 176 850 "C in flowing oxygen, 0.5 ft3 h-' (static cubic feet per hour).§ All anneals were carried out after cooling in oxygen. The neutron diffraction experiments were performed on the Special Environment and General Purpose Powder Diffractometer, at the Intense Pulsed Neutron Source (IPNS)." The gel pellets were heat-treated in a controlled atmosphere in the powder diffractometer.For each analysed composition, two sample pellets were stacked vertically on an alumina sample platform and a thermocouple placed directly above the pellets in an alumina cap which rested on the sample. Oxygen or argon gas at a flow rate of 2.0 SCFH was circulated in the furnace. Diffraction data were accumulated at a fixed scattering angle of 28=90" by the time-of-flight technique for 2 h at each temperature in order to achieve adequate counting statistics. The data were processed using a VAX 780 computer. An equilibrium of the sample at each temperature was not achieved prior to the data acquisition owing to instrument time constraints. However, the furnace control parameters allowed a ca.20 min stabilization period within 10 "C of each temperature setting. The experiments were performed using 5 "C min-' heating and cooling rates unless otherwise noted. 3. Results and Discussion Fig. 1 shows DTA results for the copper(I1) methoxyethoxide precursor powder. This measurement reveals a large exother- mic reaction in the range 200-270 "C and two smaller exother- mic peaks in the range 350-450 "C. Based on this result, room temperature, 275,475 and 675 "C were chosen as the tempera- tures for acquisition of neutron diffraction data for the cop- per(I1) methoxyethoxide sample. The diffraction spectra for the copper(I1) precursor pellets are shown in Fig. 2. The room- temperature data indicate the major constituent to be amorph- ous, with minor constituents being CuO, Cu20, and Cu metal.The presence of crystalline phases is attributed to the 120-200 "C temperature used during sample drying. The copper gel sample appears completely crystalline by 275 "C, with CuO being the sole copper constituent. Thus, the large exothermic reaction observed just below 275 "C is attributed to the formation of CuO and the elimination of its correspond- ing organic byproducts. Neither Cu metal nor CuzO were observed in the 275 "C powder pattern. No apparent crystallo- graphic structure variation was observed in the 475 to 675 "C spectra, indicating that the smaller exothermic reactions occurring in the range 350-450 "C were most likely due to a second stage of organic byproduct oxidation.Our single phase 4 1 ft3 h-' ~7.866 m3 s-'x O" I I J. MATER. CHEM., 1991, VOL. 1 04000 I 0 7 9 1 .o 1.5 2.0 2.5 3.0 dspacing /A Fig. 2 Neutron diffraction patterns for copper(I1) alkoxide consoli-dation. (a) Room temp.; (b) 275 "C; (c) 475 "C; (d)675 "C. O=CuO; 0=Cu,O; A =CU; 0 =A1,03 formation of CuO by 275 "C is 200°C lower than that reported by Hirano et a1.,13 in which similar alkoxide precur- sors were used. They observed the presence of both Cu,O and CuO up to 475 "C as determined using X-ray diffraction techniques. In addition to the copper-containing phases, the neutron diffraction patterns recorded above room temperature revealed the presence of Al,03. As discussed previously, the sample pellets were positioned between two alumina dies in the neutron beam.As a result of sample pellet shrinkage during the copper(I1) precursor run, the upper alumina pos- itioning cap entered the neutron beam. Thus, the spectra above room temperature showed the presence of alumina. However, this interference did not limit the identification of the copper oxide phases. Fig. 3 shows the DTA curve for the Y-3Cu gel sample. The data indicate a sharp exothermic reaction below 275 "C followed by a gradual exothermic rise between 300 and 550 "C. Neutron diffraction data were therefore taken at room tem- perature, 275 and 550°C. The diffraction data are shown in Fig. 4. The data indicate that CuO is the dominating phase in the 275 and 550°C spectra with the balance of material being in the form of Y2Cu205. The presence of yttrium oxide was not observed in the patterns, indicating the direct forma- tion of the yttrium copper oxide.Fig. 5 shows the DTA curve for 2Ba-3Cu gel powder. The curve exhibits three exothermic peaks between 150 and 525 "C, and a small endothermic dip at 800 "C corresponding to the BaC0,-BaC03 polymorph phase conversion. Fig. 6 shows neutron diffraction patterns of samples heat-treated at 325, 0160 I 1 220 I 380 I I 540 I I 700 I I temperature/"C temperature/% Fig. 1 Differential thermal analysis of copper(I1) alkoxide Fig. 3 Differential thermal analysis of Y-3Cu gel J. MATER. CHEM., 1991, VOL. 1 10 1.5 2.0 25 3.0 3.5 dspaci ng /A Fig.4 Neutron diffraction patterns for Y-3Cu gel consolidation. (a) Dried gel; (h)275 "C; (c) 550 "C. O=CuO; A =Y,Cu,O, a,ni I st I0 .-C n 1.o 15 2.0 2.5 30 3.5 dspaci ng /A Fig. 6 Neutron diffraction patterns for 2Ba-3Cu gel consolidation. =BaCuO,; 0 =CuO; A =BaCO, 800, and 850 "C.The first spectrum recorded at 325 "C shows CuO and BaCO, as the predominant phases. The 800 and 850 "C sample spectra contained a large alumina contribution from the positioning cap. For these two samples additional neutron diffraction data were obtained after partially cooling the furnace and masking the portion of the neutron beam impinging on the positioning cap. The spectrum representative of the 800 "C sample, recorded at 450 "C, indicates the pres- ence BaCuO, and CuO.These phases were also detected in the 850 "C sample, recorded at 200 "C, Fig. 6. In the 850 "C sample the BaCuO, phase is much more developed than in the 800 "C sample. Fig. 7 shows the DTA analysis of the Y-2Ba-3Cu gel powder produced using Cu" ethoxide obtained from Johnson Matthey (Alfa products) corporation and processed using the methoxyethanol-methyl ethyl ketone-toluene system. The DTA curve shows two exothermic peaks at 200 and 500 "C. The BaC03-BaC03 polymorph conversion endotherm occurred at 795 "C. This DTA measurement closely resembles that of the 2Ba-3Cu gel. For example, the large exothermic peak between 400 and 500 "C appears to have similar intensit- ies and widths in both plots, indicating that the barium and possibly copper constituent reactions are taking place.Fig. 8 shows the neutron diffraction data of the Y-2Ba-3Cu gel heat-treated from 200 to 850 "C. From Fig. 8 it is observed Fig. 7 Differential thermal analysis of Y-2Ba-3Cu gel 5400 . , <25E 0 Ill I I I I I 10 15 20 25 30 35 dspacing/ A Fig. 8 Neutron diffraction patterns for Y-2Ba-3Cu gel consoli- dation. 0 =BaCuO,; 0=CuO; H=BaCO,; A =BaCO,; A = Y2Cu20,; 0 =tetragonal YBa,Cu,O, -x 178 that the BaCO, peaks do not change appreciably in intensity from 325 to 750"C, but at 800°C the phase has been completely replaced by the BaC0, polymorph. Thus, the BaCO, phase appears to be very inert up to the conversion temperature, ca.790-800 "C. The 2Ba-3Cu results indicate that 2 h at 800 "C was sufficient for the reaction of the newly formed BaCO, polymorph with the copper oxide phases to form Ba2Cu0, to go to completion. This presence of Ba2Cu03 has been observed by Wang et at 950°C in 2Ba:3Cu specimen prepared using the solid-state method. Ba2Cu03 was not observed at any temperature in our 2Ba-3Cu samples. From the room-temperature acquired diffraction data shown in Fig. 8, it is apparent that the Y-2Ba-3Cu gel sample was completely amorphous prior to heat treatment. The hump-like nature of the scattering intensity is representative of the under-moderated Maxwellian neutron distribution, characteristic of a pulsed spallation neutron source. The spectra appear quite complex owing to the three-component nature of the gel.However, with the aid of the previously discussed diffraction patterns, the data can be adequately interpreted. Two main phases, CuO and BaC03, were observed at 325 "C with a small amount of Y2Cu205 also present. Free Cu metal and Cu20 were not observed at any temperature. The Y,Cu205 peak intensities increased through the 750°C acquisition, at which time the phase appeared to react readily with the other constituents to form tetragonal YBa2Cu307-,. The BaC03 phase was very stable through the 750°C acquisition, but at 800°C had been converted to BaCO, polymorph. The BaCO, polymorph was observed to react readily with CuO and Y2Cu205 forming tetragonal YBa2Cu307-, and a small amount of BaCu02.After 2 h at 850 "C only small traces of the BaCO, polymorph phase remained. Residual impurities after 4 h at 850 "C were Y2Cu205, CuO and BaCuO,. Both argon and oxygen atmosphere were used to cool the YBa2C~307-xspecimens from the 850 "C sintering tempera- ture to the annealing temperature of 450 "C at 5 "C min-'. Fig. 9 shows the diffraction data taken while annealing. The first sample was cooled from 850 to 450 "C at a rate of 5 "C min-in flowing argon. This treatment retained the tetragonal phase as indicated in Fig. 9. The single-phase nature of this sample is apparent when compared to the 'standard' tetrag- onal YBa,Cu,O,-, spectra of Fig. 10." This is, of course, exactly what would be expected since the tetragonal-ortho- rhombic transition cannot occur unless oxygen is available.' After the initial diffraction data were acquired at 450 "C, oxygen was introduced into the furnace.After 2 h of oxygen 4000 I I I 3200 fn c.-C3 2400 Y >c.-v) 1600 Q)c .-C 800 0 10 1.5 2.0 2.5 3.0 35 d-spacingI A Fig. 9 Neutron diffraction patterns for YBa,Cu,O, -x annealing treatments. (a)Ar-cooled, no anneal; (b)Ar-cooled, 2-4 h anneal; (c) 0,-cooled 0-2 h anneal; (d)0,-cooled, 2-4 h anneal; (e)0,-cooled 4-5 h anneal. 0 =Tetragonal YBa,Cu,O,-x J. MATER. CHEM., 1991, VOL. 1 3600t 0.8 1.2 I .6 2.0 2.4 2.8 3.2 dspaci ng/A 7WVII (b) I I I I I I 1 0.8 I .2 1.6 2.0 2.4 28 3.2 dspaci ng/A Fig. 10 Neutron diffraction patterns for (a) tetragonal and (b)ortho-rhombic YBa,Cu,O, -x produced using the solid-state method" annealing the data acquisition was started.The data show partial conversion of the tetragonal sample to the orthorhom- bic superconductive phase. Fig. 9 also shows data from the second annealing experi- ment. In this experiment a tetragonal YBa2Cu307 -,sample was cooled in oxygen from 850 "C at a rate of 5 "C min-' and then annealed at 450 "C in oxygen. It was observed that partial orthorhombic conversion occurred during cooling. Annealing at 450°C in oxygen for 4 h produced further orthorhombic conversion. However, after 5 h of annealing, the rate of tetragonal-to-orthorhombic conversion was extremely small. From the peak intensity ratios it appears that the acquired tetragonal-orthorhombic mixture is com- posed of ca.30% tetragonal. This is not the behaviour expected of a sintered YBa2Cu307 -,material, prepared near 900 "C. Typically at elevated temperatures, i.e. 850 "C, a single- phase tetragonal material having 06,05-6,20is present.' As the sample is cooled in oxygen or air, oxygen is intercalated into the structure. At ca. 650°C an oxygen concentration of 06,5is achieved and preferential oxygen ordering with respect to the b axis induces a tetragonal-to-orthorhombic phase conversion. It is reported by Manthiram and Gooden-ough16.17 that for their YB~,CU,O,-~ prepared below 800 "C, the presence of 0 2p holes ('paired' oxygen) in the tetragonal structure hinders preferential oxygen ordering, and thus hin- ders the tetragonal-to-orthorhombic conversion.For their material, processed using oxalate precursor at a minimum temperature of 780 "C for 5 days and annealed in oxygen at 450 and 350 "C for 12 h, it was found that a semiconducting tetragonal phase having 06.7was obtained. This material was completely converted to the superconducting orthorhombic phase by first eliminating 'paired' oxygen in the structure. Elimination of 0 2p holes was achieved by heat treating the 06.7tetragonal material in nitrogen at 750-780 "C for 12 h J. MATER. CHEM., 1991, VOL. 1 or in air at 810 "C. This treatment produced a low oxygen content tetragonal phase which upon reannealing at 400"C converted completely to the superconducting orthorhombic phase.This is reportedly due to the reinsertion of monomeric oxygen at a concentration that prevents formation of 0 2p holes. Our neutron diffraction sample cooled in argon, fol- lowed by oxygen annealing was analogous to the oxygen removal/reinsertion cycle used by Manthiram and Gooden- ough.' However, our material did not completely convert to single-phase orthorhombic YBa,Cu30, -x. Our neutron diffraction annealing experiments were designed to see if an increased oxygen intercalation rate was present in the argon-cooled sample, compared to the sample cooled in oxygen, and whether enhanced phase conversion was achieved by retaining the low oxygen content tetragonal phase during cooling. It can be seen from the data in Fig.9 that the low-oxygen-content tetragonal phase was partially converted to a tetragonal-orthorhombic mixture after 2 h of oxygen annealing. The tetragonal-to-orthorhombic peak intensity ratios of this spectra were very similar to those of the oxygen-cooled sample spectra, which were taken during the first 2 h of annealing at 450 "C. Thus, a greater oxygen intercalation rate was observed during the initial stages of the annealing treatment. However, no appreciable increase in the degree of tetragonal-to-orthorhombic phase conversion was obtained. At the present time an exact explanation for the observed incomplete phase conversion behaviour is not available. Poss-ible contributors to the problem may include variations of the initial stoichiometry or residual carbon impurity phases. It is clear from the neutron diffraction data that our phase evolution process is quite different from that observed for oxide sintered processes as well as for other chemical precursor systems and that impurity phases such as Y2Cu205, not reported in those systems, may affect the conversion process. The literature seems to indicate that when consolidation temperatures of 900-950 "C are used, partial conversion is not a problem.' 5*1 However, when lower conversion tempera- tures are used, broad susceptibility transitions characteristic of incomplete phase conversion are generally observed.l9 Fig. 11 shows the susceptibility curve for the YBa2Cu30,-, sample cooled and annealed in oxygen.The curve shows a broad transition due to the two-phase nature of the fired pellet sample. An onset temperature of ca. 65 K was observed. 6 a Oo0' Isi.c c 0 000 -0,i -0Q -0 -0.001 -.-0 c Q,C 0,E -0.002! * I . I . 1 -I ' I . I 0 20 40 60 80 100 120 temperature/K Fig. 11 Susceptibility curve for YBa,Cu,O, --x sample annealed in flowing oxygen for 5 h at 450 "C 179 Summary and Conclusions The amorphous gel to YBazCu307-, oxide conversion was characterized using in situ time-of-flight neutron diffraction techniques. The results indicate that by 325 "C, CuO, BaCO, and Y2Cu205 have formed, (Fig. 12). Tetragonal YBa,Cu,O,-, was observed to be present as a minor phase at 700 "C.After the BaCO3-BaCO3 polymorph conversion near 800 "C the formation of tetragonal YBa2Cu307 -,as the dominant phase rapidly occurred. After 4 h at 850 "C essen- tially single-phase tetragonal YBa2Cu307-,was present. Small amounts of Y2Cu205, CuO, and BaCu02 were observed as residual impurities. Incomplete conversion of the oxide material to the superconducting orthorhombic phase during cool-down and annealing is believed to be due to the creation of a stable tetragonal-orthorhombic phase mixture. This stable mixture is not reported by researchers using consolidation temperatures above 900 "C. In summary, the formation of YBa2Cu307 -x from alkoxide sol-gel derived powder consisted of several consolidation reactions involving five main phases, CuO, BaC03, BaC03 polymorph, Y2Cu205, and BaCu02.The superconducting YBa2Cu307 -,pellet sample showed an onset temperature of 65-70 K, as determined using mag- netic susceptibility measurements. Free gel powder processed in a similar way typically yields 80-85 K superconducting transition onsets. The superconducting transitions were broad owing to incomplete phase conversion and the presence of minor impurity phases. The ability to form high-quality fine-grain superconduct- ing powder appears to be hindered by a partial tetragonal YBa2Cu307 -,-to-orthorhombic phase conversion. For our system we do not believ that the lack of orthorhombic phase formation can be solely attributed to the hindered oxygen intercalation and ordering effects, which are a consequence of paired oxygen in the tetragonal phase.Future work is being performed to follow quantitative changes in the YBa,Cu3 phase ratios, variation in lattice parameters, and oxygen content. The authors would like to thank the following people for their constructive advice and suggestions as well as assistance in performing the reported work: Frank Rotella, Mark Teepe, David Kenzer, Julie Twaddle, and Dr. Fulin Zou. Funding for this project was provided by NSF-DMR 86-12860. The work in Argonne was supported by U.S. Dept. of Energy, Div. of Basic Energy Sciences-Materials Sciences, under con- tract W-31-109ENG-38 and by the NSF-funded Science and Technology Center for Superconductivity under Grant No.DMR-88-09854. temperature/ C phase 120 275 550 650 750 800 850 \ Ctr -1 CUO I cup 4 UaCO, I UaCO, PM ___I UaCuO, 4 YCU 20, I Y1Ua2C~307-x ____I Fig. 12 Summary of phase development and consumption during thermal processing of YBa,Cu, gel 180 J. MATER. CHEM., 1991, VOL. 1 References 11 G. Moore, S. Kramer and G. Kordas, Mater. Lett., 1989, 7, 415. 1 M. K. Wu, J. R. Ashburn, A. J. Torng, P. H. Hor, R. L. Meng, 12 J. D. Jorgensen, J. Faber, J. M. Carpernter, R. K. Crawford, J. R. Haumann, R. L. Hitterman, R. Kleb, G. E. Ostrowki, F. J. L. Gao, Z. J. Huand, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett., 1987, 58, 908. 13 Rotaella and T. G. Worltan, J. Appl. Crystallogr., 1989, 22, 321. S. Hirano, T.Hayashi, R. H. Baney, M. Miura, H. Tomonaga, 2 K. E. Easterling, C. C. Sorrell, A. J. Bourdillon, S. X. Dou, G. J. Siogett and J. C. Macfarlane, Mater. Forum, 1988, 11, 30. 14 Chem. Lett., 1988, 665. A. Manthiram, J. S. Winnea, A. T. Sui, H. Steinfink and J. B. 3 H. Koinuma, K. Fukuda, T. Hashimoto and K. Fueki, Jpn. Goodenough, J. Am. Chem. SOC., 1987, 109, 6667. J. Appl. Phys., 1988, 27, L1216. 4 C. T. Cheung and E. Ruckenstein, Mater. Lett., 1988, 7, 172. 5 R. Roy, Science, 1987, 238, 1664. 6 S. Hirano, T. Hayashi, M. Miura and Hiroyuki Tomonaga, Bull. Chem. SOC.Jpn., 1989, 62, 888. 15 16 J. D. Jorgensen, M. A. Beno, D. G. Hinks, L. Soderholm, K. J. Volin, R. L. Hitterman, J. D. Grace, I. K. Schuller, C. U. Segre, K. Zhang and M. S. Kleefisch, Phys. Rev. B, 1987,36, 3608. A. Manthiram and J. B. Goodenough, Nature (London), 1987, 329, 701. 7 H. S. Horowitz, S. J. McLain, A. W. Sleight, J. D. Druliner, P. L. Gai, M. J. VanKavelaar, J. L. Wagner, D. D. Biggs and S. J. Poon, Science, 1989, 243, 66. 8 S. Kramer, K. Wu and G. Kordas, Mater. Res. SOC. Symp. Proc., 17 18 J. B. Goodenough and A. Manthiram, in Chemistry of Oxide Superconductors, ed. C. N. Rao, Blackwell, 1988, pp. 101-1 12. G. Wang, S. J. Hwu, S. N. Song, J. B. Ketterson, L. D. Marks, K. R. Poeppelmeier and T. 0. Mason, Adv. Ceram. Mater, 1987, 1988, 99, 323. 2, 313. 9 S. A. Kramer, G. Kordas, J. McMillan, G. C. Hilton and D. J. VanHarligen, Appl. Phys. Lett., 1989, 53, 156. 10 S. Kramer, G. Moore, G. Kordas, P. A. Keifer and C. T. G. 19 Witanachi, H. S. Kwok, X. W. Wang and D. T. Shaw, Appl. Phys. Lett., 1988, 53, 234. Knight, Mater. Res. SOC. Symp. Proc., 1988, 121, 643. Paper 0/03010D; Received 4th July, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100175

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(2), 181-189 Role of Fractal Structure on Thin-film Processing of YBa,Cu,O,-, using Alkoxide Sols George Kordas,a Glenn A. Moore,a Myron B. Salamonb and John B. Hayter" a Department of Material Science and Engineering, Ceramics Division, Materials Research Laboratory, and Science and Technology Center for Superconductivity, University of Illinois at Urbana-Champaign, 105 S. Goodwin, Urbana, lL 61801, USA Physics Department and Materials Research Laboratorx University of Illinois at Urbana- Champaign, 104 S. Goodwin, Urbana, lL 61801, USA Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, USA An alkoxide sol-gel route has been developed for producing YBa,Cu,O,_ superconducting powder and oriented thin films.Cu" methoxyethoxide, Y methoxyethoxide and Ba methoxyethoxide precursors were synthesized and processed into stable sols with methoxyethanol (MOE), methyl ethyl ketone (MEK), toluene and diisopropyl ketone (DIK) solvents. Pyridine (C,H,N), lutidine (C,,H,,N) and toluene-2,4-disulphonic acid were used as organic additives to modify the polymeric network structure and corresponding sol behaviour. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) were used to study the structural modifications in sols and gels with changes in solution chemistry parameters, viz. acidity/basicity, moisture exposure and solvent system. Mass-fractal dimensions in the range 2.0-2.5 were obtained for 0.01-0.1 mol kg-' YBa,Cu, sols in methoxyethanol-methyl ethyl ketone-toluene solvent.Significant effects of two organic base additives, pyridine and lutidine, on the sol structure were observed as a function of sol concentration. The ability of the different solutions to cover SrTiO,, ZrO,, sapphire, SiO, and silicon substrates was measured by contact-angle measurements. Thin films produced via spin casting, dip coating and spray coating were characterized using optical, scanning electron microscopy (SEM) and X-ray diffraction techniques, and the results are discussed in terms of the sol structures. Keywords: Oxide superconductor; Fractal; Sol-gel processing; Thin film 1. Introduction Processing of single-phase YBa,Cu,O, -x superconducting powder and oriented thin films has been a major research and development area within the field of high-T, superconduc- tors.Single-phase materials are paramount to investigations on the nature of superconductivity and its technological applications. Owing to the incongruent melting behaviour of -yrYBa2Cu30-processing techniques have been sought that can overcome the relatively high calcination times and tem- peratures involved with conventional oxide-sintering tech- niques.'.2 The use of chemically based or solution preparation techniques has been investigated extensively in the hope of finding systems that allow molecular-level mixing of compo- nents and the formation of compounds that can be trans- formed at relatively low temperatures to superconducting powders, thin films, or fibres.Potential advantages of such systems include the ability to purify easily starting materials, the elimination of milling steps used in the solid-state process, the production of an extremely fine-grain single-phase prod- uct, and the ability to introduce trace amounts of dopants homogeneously into the material., The sol-gel method offers a high degree of microstructural control through the variation of process parameter^.^ Typically, but not exclusively, it uses metal alkoxides, which are hydrolysed and condensed to form polymeric networks. The resulting gel is calcined to remove residual organic by-products yielding the appropriate ceramic material. The use df alkoxides for producing YBa,Cu,O,-, creates several interesting problems.First, the formation of homo- geneous three-component sols having similar reaction rates is a challenging task.5 Secondly, most copper(I1) alkoxides are reported to be insoluble in alcohols and hydrocarbons.6 This is because a very stable three-dimensional olimeric structure is promoted by the desired four-fold, square-planar co-ordi- nation of the copper(r1) ions. One approach to weakening this olimeric structure is to use long-chain alkoxy groups, which separate the copper(I1) ions and which offer greater steric stabilization due to the extended alkyl structure.,q8 Copper(1) species are not typically used because of their extreme oxidat- ive tendency.' With regard to the barium and yttrium alkox- ides, solubility is promoted by using highly polar solvents such as 2-methoxyethoxide and long-chain alkyl groups.For example, barium ethoxide is also reported to be insoluble; however, longer-chain alkoxides such as 2-methoxyethoxide are completely soluble in the parent solvent." Horowitz et a/.' reported an alkoxide-processing route in which Cu' dibutylamine, barium diisopropoxide, and Y triiso-propoxide precursors formed a solution in tetrahydrofuran. The sol was hydrolysed at 313 K using 18 times the required water for complete hydrolysis. The solid product contained 0.2-0.3 wt.% carbon. This low-carbon precursor mixture was converted to YBa2Cu307-, at 923-973 K, owing to the absence of BaC0, formation and the use of an inert calci- nation atmosphere. The use of argon at higher temperatures promotes the formation of a low-oxygen tetragonal phase, 06.05-6.20,which is favourable to the intercalation of oxygen, 06.8-6.9, during oxygen annealing at 946 K.Susceptibility measurement of the processed powder exhibited only ca. 1.3% of the ideal Meissner signal. This small susceptibility signal was attributed to the particle size:penetration depth ratio, size range 40-200 nm. Another disadvantage of this process is that the viscous properties of the sol were eliminated during the complete-hydrolysis step. Hirano er aI." reported on use of partially hydrolysed ethoxyethoxide and isopropoxide alkoxide precursors to form both gel powder and viscous sols favourable to dip coating. In this case 0.25 and 1.0 times the required water for complete hydrolysis were reacted at 313 K for 12 h.It was reported that the use of ozone-containing oxygen allowed direct YBa,Cu,O,-, formation at 923 K. For samples calcined and annealed in a 3000ppm 03/02atmosphere 0.94% carbon remained, as compared with the 2.24% obtained when pure 0, was used. The resulting powder could be used to form high-density superconducting compacts at 1193 K. This pro- cess was reported to form workable viscous sols; however, the residual carbon content after heat treatment was still much greater than desired. Kordas et a!.' have synthesized YBa,Cu,O, -,supercon-ducting powder and films using Cu" ethoxide, Y methoxy-ethoxide and Ba methoxyethoxide. Sols were obtained using toluene and methoxyethanol-methyl ethyl ketone solvent systems.Small-angle X-ray scatterring, differential thermal analysis and scanning electron microscopy have revealed that the fractal dimensions of sols and gels, the crystallization reactions of gels and the microstructure of films are drastically affected by the processing chemistry. The present investigation expands upon our previous re- port13 and aims to develop a foundation for engineering routes to powders and films with low carbon content as well as the ability to produce the network structure of the sols and gels while maintaining the useful viscous properties of the sols. This structural knowledge is important for controlling the rheological properties of the sol, its drying characteristics, thermal decomposition and consolidation behavi0~r.l~ The type of gel network developed aiso affects the morphology of films produced when rapid solvent evaporation occurs, as is the case during spin casting or dip coating." Most sol-gel systems are usually composed of mass fractals.A mass-fractal system consists of self-similar elements, each exhibiting a characteristic power-law relationship between the mass and the radial dimension.' For sol-gel systems, fractal behaviour can be present from the molecular level to the scale of the final gel network components. Within the time period that these elements are evolved, hydrolysis and condensation reac- tions mediate the growth processes. The two most common non-destructive methods of characterizing these structures are small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS).' 2. Experimental 2.1 Precursor Synthesis and Formation Barium methoxyethoxide was synthesized by the reaction of the metal with the solvent.The yttrium methoxyethoxide was prepared by reacting Y metal with an HgC1, catalyst and refluxing the solution for 24 h in methoxyethanol. The cop- per@) ethoxide synthesis involved the reaction of lithium ethoxide with anhydrous CuC1, followed by filtration of an insoluble phase. The resulting material was then washed in the parent alcohol and vacuum dried to form a blue powder. Toluene, methoxyethanol (MOE), methyl ethyl ketone (MEK) and diisopropyl ketone (DIK) were used to solubilize the copper(I1) ethoxide partially.The limited solubility allows the formation of a polydisperse colloidal system which can still undergo hydrolysis and condensation reactions when com- bined with the Y and Ba alkoxides. More detailed precursor synthesis has been reported previously.' *-*' 2.2 YBCO Sol Preparation Fig. 1 shows the sol-gel formation flow diagram. Stable sols were formed using synthesized precursors in several solvents such as MOE, MEK, toluene and DIK. Mixing was performed at room temperature in the presence of oxygen or air until a stable solution was obtained. With further exposure to moist- J. MATER. CHEM., 1991, VOL. 1 barium methoxyethoxide synthesis Bametal+ 2HOC2H40CH3-Ba(OC2H40CH3)2 I purification using L pressure filtration i precursors complexed in highly polar solvent system: toluene J met h ox yet h an 01 methyl ethyl ketone diisopropyl ketone stable threecomponent sol -thin films fiIm application : solvent stripping/gel formation spin coating vacuum drying of gel powder dip coating I *spray coating calcination of gel powder Fig. 1 Alkoxide processing flow chart for YBa,Cu,O, supercon--x ductor ure from the air, viscous sols and gels were obtained.It was determined experimentally that aqueous acids and bases such as HCI, acetic acid and hydrogen peroxide caused decompo- sition or destabilization of the YBa2Cu3 alkoxide solutions, resulting in decomposition or precipitation. Therefore, we modified the solution environment using organic bases and acids, specifically pyridine (C,H,N), lutidine (C, 3H2 1N) and toluene-2,4-disulphonic acid.It was determined experimen- tally that toluene-2,4-disulphonic acid induced precipitation. Precipitation was not obtained using organic bases. 2.3 SAXSlSANS Sample Preparation and Data Collection For SAXS experiments a standard 123 MOE-MEK-toluene sol was concentrated using rotary evaporation. Solutions with concentrations of 0.01, 0.02, 0.03, and 0.10 mol kg-' were prepared for SAXS measurements. These four sols were div- ided into three portions and 10 wt.% additions of pyridine and lutidine were added subsequently. SAXS specimens were prepared by first filtering the sols through 0.5 pm PTFE membranes to remove dust and any unreacted particles.Debye-Scherrer capillaries of 0.7 and 1.0 mm diameter and 0.01 mm wall thickness were filled with sol and flame sealed. J. MATER. CHEM., 1991, VOL. 1 Care was taken to avoid direct contact of the sol with the flame. Gel samples were prepared by exposing sol-containing capillaries to air for several days followed by flame sealing. A Rigaku 18 kW Cu-Ka rotating anode X-ray source, oper- ated at 45 kV and 220mA with pinhole geometry, was used for the SAXS experiments. A horizontal position sensitive detector was 910 mm from the vertically mounted sample capillary. The X-ray path was evacuated using a mechanical vacuum pump. Detector signals were summed with the aid of an IBM AT personal computer. The observable structure range for the SAXS instrument was 16.8-700 A. A parasitic background was established using an MOE-containing capil- lary. Specimen acquisition times of 60min were used.These acquisition times were longer than those reported for investi- gations of silica sols owing to the higher mass absorption coefficients of the yttrium, barium and copper con-stituents.21.22 Data processing consisted of transforming the scattering intensity uersus detector channel data into a scattering inten- sity versus scattering vector, Q =(47r/I.) sin 8. The scattering associated solely with the network structure was obtained after subtracting the parasitic scattering of a solvent-filled capillary, and the data were plotted in log-log fashion. The resulting slope was determined using least-squares fitting procedures. This slope is representative of the mass-fractal dimension, which is characteristic of the network structure over the observed Q range.The range of observable structural features in a given Q range is expressed by 27r/Q, or in this case 16.8 700 A.16After the sol measurements were obtained the top of the sample capillary was broken off to allow the gelation of the sol in air. Gel sample acquisitions were made 2 weeks after the sol sample measurements. The SANS measurements were performed using the NIST research reactor. Neutrons from the reactor core passed through a liquid-nitrogen-cooled filter, consisting of 8 in of beryllium (to scatter fast neutrons from the beam) and 8 in+ of single-crystal bismuth (to stop core prays).A cold source consisting of frozen deuterium oxide was used to slow the incident neutrons further without excessive neutron-beam attenuation. The beam was then monochromatized using a helical-channel velocity selector and was collimated using a series of 12 cadmium apertures placed along the 4.5 m evacu- ated flight lube. A circular 14 mm diameter mask was placed in the collimated beam just in front of the scattering samples to ensure a consistent beam size for all acquisitions. Ca. 3 x lo5 neutrons per s impinged on the sample. The sample holder consisted of a five-position computer-controlled stage and was open to air. The detector was 1.2 m behind the sample and consisted of a 64 cm x 64 cm position-sensitive pro-portional counting array of Borkowski-Kopp-type elements.The scattering information was accumulated using a DEC PDP 11,23 computer. Sample ho!ders for SANS were made from ordinary micro- scope slides, nominally free of boron. PTFE spacing material (1.1 mm thick) was laminated between the edges of two laboratory slides and sealed using Superglue and 15 min epoxy. Sol samples were made by filling the sample holder with ca. 1.5 mm of sol so that the level of sol was above the 14mm mask in front of the specimen. For gel samples, ca. 2.0cm3 of sol were deposited in a sample holder and allowed to gel inside a fume hood. The evaporative loss in most cascs did not create an unfilled area in the beam path.Acquisition times of 2 h were used in order to accumulate sufficient scattering data from the sol and gel samples. In addition to the sol and gel samples, a blank cell, cadmium t 1 in =2.54 x m. (complete scatterer) sheet, and solvent-containing cell were run for use in data processing. Data processing consisted of masking the perimeter array elements and then performing radial averaging of the remain- ing detector elements. The resulting data file was composed of the scattering intensities over the observed Q range and the per cent error for each radial detector grouping. The data were then corrected for the sample cell absorption and solvent scattering. A VAX 780 was used for the above analysis.It was determined that the solvent contribution to the scattering was a constant over the observed Q range. 2.4 Contact-angle Measurements and Film Preparation For contact-angle measurements a projection lens and a rotating goniometer apparatus was used to magnify the interfacial angle, 4, of a sol droplet placed on a test substrate. A 5 x lop6dm3 droplet was placed on a cleaned or coated substrate using a Hamilton microsyringe. The measurement goniometer was then rotated until the measurement cross-hair coincided with the contact angle. Three measurements were taken for each sol and the average value was calculated. Spin, dip and spray coating techniques were used to produce thin films. A1203, SrTiO,, MgO, Si, SO2 and ZrO, were used as substrates.Substrates were cleaned ultrasonically in concentrated HC1, followed by a deionized water rinse, an isopropyl alcohol rinse, and drying using a chlorofluorocarbon jet. For spin coating, several drops of sol were placed on the substrate; the edge of a glass pipette was then used to distribute the sol over the entire substrate surface. The sub- strate was spun at 2400-5000 rpm for 30s. This procedure was repeated up to 10 times for multiple-coat films. Thick- nesses of 0.1-0.5 pm per application were typically obtained, as determined by optical microscopy and scanning electron microscopy (SEM). Film cracking was osberved for coatings greater than 1-2 pm thick. Dip coating was performed by slowly inserting a substrate into a sol at a constant rate of 25 cm min-' using a servo motor apparatus and then withdrawing the substrate at the same rate.The edges of the substrate were made to contact the sides of the dipping vessel, allowing contact forces to draw the excess sol from the surface evenly. This wall-contact technique appeared to produce less of a thickness gradient on the substrate edges. After five coats had been applied a 673-873 K min-' heat treatment was performed so that cracking was avoided. The spray-coating process utilized a Badger Model No. 250-3 air brush and fluorocarbon carrier gas. A microfine tip was found to produce the most controlled application. Substrates were placed in direct contact with an aluminium plate heated to 473-523 K. A sweeping motion of the fine sol spray, ca.5 cm away from the vertically inclined substrate, resulted in a translucent coating of the order of 5 pm thick. For multiple applications, heat treatments (temperature between 673 and 873 K for 1 min) were used to consolidate the films between applications. Film consolidation was performed using rapid thermal heat treatment. Coated substrates were placed vertically in a quartz boat and inserted slowly into the quartz tube furnace using a thermocouple push rod. A typical firing sequence was as follows: a 30-45 s push-in to 673 "C with a 2 min hold, continued slow push-in to the 1023-1173 K hot zone and holding 1-5 min in air, the substrate was then removed over a period of 1 min to 723 K and held for several more minutes to avoid thermal shock cracking.The substrate was then placed in an annealing tube furnace at 723-823 K for several hours under flowing oxygen, 2.5 dm3 min-'. Heat treatments between 400 and 600 "Cwere performed in air. The annealing treatments were done in 02. 3. Results 3.1 Interpretation of the Fractal-dimension Data The structural elements of a sol-gel system can be thought of as a polydisperse ramified polymer network having radial density gradients. These units are termed fractals, which can be classified into surface (S) and mass (M)fractals. Surface fractals can be geometrically defined by a power law: S-rDs; 2<D,<3 where r is the radius or a linear dimension that increases with growth and D, is the fractal dimension.For mass fractals, the mass of an object is related to r by the mass-fractal dimension, DM: M-rDM; 1 <DM<3 The parameter DM is derived from the power-law dependence of the scattering intensity: Z(Q)=Q -DM on the scattering vector Q: Q =(471sin 0)ji where 20 is the scattering angle and A is the X-ray wavelength. The intensity of scattering in the region (QR,) >1, is given by: 4Q) =Q -a where R, is the electronic radius of gyration and ct is defined as the log-log slope. For mass and surface fractals ct is D, and 6 -D,, respectively. For thin-film formation, the fractal dimension is usually less than 1.5 (indicative of linear polymer growth). 3.2 SAXS Data Fig. 2 and 3 show the SAXS log-log plots for the standard sols and gels at various concentrations.The scattering data indicate that total scattering increases with increasing sol or gel concentration. This is expected since the density of the scattering elements is directly proportional to the concen- tration. These plots also show a decrease in slope for Q values less than 0.02k'. This non-linear behaviour has been re- ported in the literature as being due to a lower concentration of very large fractal structures in the sols and gels.2' The high degree of linearity observed in the log-log plots for Q >0.02 A -' indicates consistent self-similar scaled geometry over the observed length scale. Table 1 lists the mass-fractal dimensions and log-log slopes for the measured sols and gels.The accuracy of the data in Table 1 is kO.05, based on curve fit; the reproducibility of the data is & 0.1. Table 1 Mass fractal dimension data for alkoxide sols and gels as determined from SAXS log-log plots" D (soljgel conc.)/mol kg- 0.01 0.02 0.03 0.10 MMT sol 2.39 2.24 2.18 2.19 MMT lutidine sol MMT gel 2.40 2.33 2.28 2.33 2.16 2.09 2.16 2.20 MMT lutidine gel 2.88 3.37 2.82 2.58 MMT pyridine sol MMT pyridine gel 2.4 I 2.50 2.15 2.35 2.04 2.44 2.03 2.46 "MMT =methoxyethanol-methyl ethyl ketone-toluene. J. MATER. CHEM., 1991, VOL. 1 Fig. 2 SAXS plots of four standard 123 sols having various concen- trations (jmol kg-'): +, 0.01; A, 0.02; W, 0.03; +, 0.10 3.3 SANS Data SANS data were obtained for the 123 sols and the yttrium, barium and copper(I1) precursors.Fig. 4 shows the log-log plot of sols formed using diisopropyl ketone as the solvent. Table 2 contains the fractal dimensions obtained from the linear portion of the SANS log-log plots. The Q range of the SANS instrument with the deuterium cold source in operation was 0.0027-0.0772 A-', corresponding to a structural infor- mation range of 81-2300 A. Without the cold source the SANS Q range is 0.0122-0.1 160 kl,which is less than that of the SAXS instrument. 3.4 Thin Films Table 3 shows the contact-angle measurements on a variety of substrates with sols used for SAXS analysis. The contact Table2 Mass fractal dimension data for alkoxide sols and gels as determined using SANS log-log plots solvent methoxyethanol MM MMT DIK 123 sol 0.8 1.9 1.513.6 123 gel 2.4 2.2 123 aged sol 123 aged gel 10% lutidine sol 2.9 2.9 2.6 2.6 2.3 10% lutidine gel 2.4 10% pyridine sol 1.3 Ba,-Cu, sol LOS LOS 3.9 Y sol LOS Y gel 2.0 Ba sol LOS Ba gel 2.0 cu sol 3.6 2.2 LOS =lack of scattering; MM =methoxyethanol-methyl ethyl ketone; MMT =methoxyethanol-methyl ethyl ketone-toluene; DIK =diiso-propyl ketone.J. MATER. CHEM., 1991, VOL. 1 Table 3 Contact-angle mesurements made using standard and organic base-containing sols substrate conc./mol kg- = 0.01 sapphire standard 0.0 pyridine 0.0 lutidine 0.0 -SrTiO, -SrTiO, (gel-coated) ZrO, 2.0 Si (oxide layer) 0.0 SiOz 0.0 Si (etched) 8.0 Oo0 F + + 1 I I I 0.01 0.1 1 Qtk' contact angle/" 0.02 0.03 0.10 8.5 19.5 22.5 4.0 5.0 6.5 2.0 - 8.0 16.5 gelled 33.5 - 16.0 28.0 4.0 14.0 15.0 1.o 18.0 25.5 13.5 22.0 23.5 10.0 17.0 24.0 angles varied from 0 to 22.5" over a solution concentration range of 0.01-0.1 mol kg-'.These data indicate that sols on SrTiO, have a relatively high contact angle when compared to ZrO,, silicon, silica and sapphire substrates. The contact angle was significantly modified with 10 wt.% additions of pyridine and lutidine. Fig. 5 shows a spin-cast film on silicon after being ultra- sonicated in acetone for 20min. Both the toluene and the MOE-MEK processed sols, 0.1 mol kg-', formed uniform thin films ca.1500A thick using spin coating, as determined by optical microscopy.Only slight cavitation effects are visible in this figure. These films had excellent adhesion to the substrate and remained crack-free and continuous during ultrasonication. Inspection of the spin-cast films using an optical microscope equipped with Nomarski optics (which consist of lenses allowing contrast enhancement through differential interference) revealed good thickness uniformity. Accumulation of excess sol on edges produced a variable-thickness region of 0.5 mm into the substrate. Both MOE- MEK and MOE-MEK-toluene solvent systems produced sols having good wettability and film-forming characteristics. Cracking of the films was not observed during further pro- cessing when fewer than four coats were applied.Fig. 6 shows a micrograph of a spin-cast film deposited on a single-crystal (001) SrTi03 substrate. Crystallographic orientation of the film crystallites is clearly visible. The film was heat treated as follows. The substrate/film was inserted into the furnace, in air, from room temperature to 573 K over a period of 2 min and held for 5 min; the substrate was then pushed slowly into the hot zone, 773 to 1123 K in 1 min. The film was held at 1123 K for 5 min in air and then over 1 min pulled back to 773 K and annealed in flowing oxygen for Fig. 3 SAXS plots of four standard 123 gels produced from sols having various concentrations (key as for Fig.2) -2.2 -2.0 -1.8 -1.6 log (Qtk') Fig.4 SANS plot of sols formed using diisopropyl ketone as the solvent. U,Cu (D=2.2); 0,2Ba-3Cu (D=3.9); A,Y-2Ba-3Cu (D= 1.5/ 3.6) Fig. 5 Optical photomicrograph of spin-cast film on silicon, ultra- sonicated for 20 min in acetone Fig. 6 Optical photomicrograph of fired YBa,Cu,O,-, thin film on single-crystal (001) SrTiO, substrate A 2.5 3.0 3.5 4.0 4.5 5.0 Fig. 7 Rocking curve of the 001 diffraction line, for an oriented YBa,Cu,O, -,thin film on single-crystal (001) SrTiO, substrate 3.5 h. Fig. 7 shows the rocking curve of the 001 diffraction line for a YBCO film on SrTiO,. The diffraction data indicate the c axis orientation of the film with respect to the c axis of the substrate. A rocking curve, 8 scan with fixed 28, of the 001 film peak was formed. The width of the peak is termed the mosaic spread and is characteristic of the degree of orientation.The mosaic spread (full width at half maximum) for this layer was 0.28'. The best quality dip-coated films were produced using a dip rate of 25 cm min-'. Film uniformity was similar to that of films produced using spin casting. The film quality was greatly degraded by substrate adhesion of particulate contam- mination from either the sol or the surrounding air. The dip- coating method allowed easy multiple-coat application, thicknesses of 0.1-0.3 pm per dip were obtained. Fig. 8 shows SEM micrographs of unfired DIK spray-coated films. The film was ca. 10pm thick and showed the evidence of cracking.In order to obtain a continuous film structure, multiple coats were applied. 4. Discussion 4.1 SAXS Data The interpretation of small-angle scattering log-log plot slopes in terms of the dimensionality of a given network structure has been a continuously evolving area over the past 10 years. A compact mass fractal having a rough surface would have a slope of 3.0, a disc having a rough surface would have a fractal dimension of 2.0,15 and a smooth compact object would yield a limiting Porod slope of 4.0. For our standard J. MATER. CHEM., 1991, VOL. 1 Fig. 8 SEM photomicrograph of unfired 123,DIK, spray-coated film on a sapphire substrate 0.01 mol kg- ' sol, without organic base addition, a mass-fractal dimension, DM, of 2.39 was determined.Thus, the composing elements of the sol can be thought of as being hybrid structures having features of both spherical alkoxide clusters and disc-like objects. However, the actual geometry of the elements cannot be defined solely on the basis of the scattering behaviour. Information on the chemical reactivity of the precursor elements and information concerning the particle-particle interaction is required. Therefore, geometri- cal models of fractal structures are often composed based on the careful study of reaction kinetics. Such studies take years of concentrated research but do, in some cases, predict the observed structural de~elopment.~~ This approach is valid for relatively simple systems, e.g.silica; however, it is not appli- cable to a complex three-component alkoxide sol-gel system such as ours. Therefore, no mathematical models of the growth processes were constructed. Nevertheless, the acquired fractal dimension data are useful when solution properties such as substrate wettability are also measured. The standard sol's fractal dimension decreased from 2.39 to 2.19 with increasing concentration (Table 1). This indicates that a primarily two-dimensional growth process was present. The corresponding standard gels have very consistent fractal dimensions, exhibiting differences in DM of less than 0.1 throughout the observed concentration range. The lutidine-containing sols exhibited essentially the same D trend as the corresponding standard 123 sols.The measured lutidine-containing gels had somewhat inconsistent DMvalues, ranging from 3.37 to 2.58. The increase in fractal dimensions ~ of the lutidine gels when compared to the corresponding lutidine sols, indicates that a more three-dimensional growth process was present after the organic base addition was made. The observed gelation time for the lutidine-containing sols J. MATER. CHEM., 1991, VOL. 1 was much greater than that for the standard sols; e.g. in the 0.1 mol kg- ' lutidine-containing sol, gelation occurred in 24 h versus ca. 1 week for the 0.1 mol kg-' standard sol. For the pyridine-containing sols, fractal dimensions were smaller than for the corresponding standard sols. For example, a D value of 2.03 was obtained for the 0.1 mol kg-' sol, uersus 2.19 for that of the standard sol.The measured pyridine- containing gels had D, values between 2.50 and 2.35. It was observed that the pyridine additions tended to enhance the solubility and stability of the sols. A colour change from blue- green to dark green was also observed. This colour change seems to indicate that a chemical modification of the alkoxide elements probably occurred. The increased stability of the pyridine-containing sols resulted in gelation times approxi- mately twice as long as that of the same standard sol. The pyridine-containing gels were determined to have a more three-dimensional polymeric network, i.e. higher fractal di- mension, than the corresponding standard gels.4.2 SANS Data Unhydrolysed barium and yttrium stock solutions showed essentially no neutron scattering, indicating that the alkoxide species were less than 54A in size. The yttrium and barium methoxyethoxide gels on the other hand showed mass-fractal dimensions of 2.4 and 2.0, respectively. Thus, the barium gel structure consisted of planar-like structures distributed ran- domly throughout the sample. The SANS data for Cu" methoxyethoxide dispersed in MOE-MEK indicate a fractal dimension of 3.6 (Table 2). This fractal dimension lies between that of compact objects having smooth surfaces, 'colloids' D = 4, and compact objects having rough surfaces, D=3. For another SANS sample, 0.3 mol of synthesized CU" ethoxide was solubilized in log of DIK to produce a completely transparent dark-green solution.The fractal dimension of the solubilized alkoxide was 2.2 (Fig. 4). Thus, the solvent system has a tremendous effect on the solution structure of copper(I1) ethoxide. Ba,Cu, sols were prepared using MOE-MEK, MOE-MEK-toluene, and the DIK solvent systems. For the MOE- MEK and MOE-MEK-toluene-containing sols, a lack of scattering prevented D values from being obtained. The lack of scattering was attributed to insufficient exposure to air, which produced insufficient hydrolysis. For the Ba,Cu3 DIK sol, however, quite different behaviour was observed (Fig. 4). With the addition of the barium precursor in a stoichiometric ratio of 2:3 to the copper(r1) solution a colour change from dark-green to blue resulted. The log-log slope for this sol was 3.9, or almost colloidal.Thus, the barium methoxyethox- ide precursor was responsible for initiating a drastic change in the solution structure (originally D =2.2). Sols containing the YBa2Cu3 ratio were produced using MOE-MEK, MOE-MEK-toluene, and the DIK solvent systems. The initial MOE-MEK and MOE-MEK-toluene 123 sols exhibited D values of 0.8 and 1.9, respectively. Aged sols were obtained by stirring 123 sols for a month in capped vials. The measured fractal dimensions for these sols were 2.9 and 2.6 for MOE-MEK and MOE-MEK-toluene sols, re- spectively. MOE-MEK and MOE-MEK-toluene gel samples were produced after stirring the sols in air for 1 week.The measured fractal dimensions were 2.4 and 2.2, respectively. These values compare well to the standard 123 sol and gel values obtained using SAXS, 2.39-2.19. The gels produced from the 123 aged sols did not show differences from their freshly prepared counterparts, indicating a consistent gelation process. Sol and gel samples containing pyridine and lutidine were produced by 10 wt.% additions directly to 123 MOE-MEK- toluene sols. The lutidine sol and corresponding gel showed fractal dimensions of 2.3 and 2.4, respectively. There are indications, from both the fractal dimension and the intensity data, that lutidine gel structure shows maximum organization of 0.02 mol kg-'. The fractal dimension at this concentration is tending towards 4, which would correspond to a structure with sharply defined interfaces between subunits.We plan to investigate this further. These values are consistent with the observed SAXS data of Table 1. The fractal dimension during the sol-to-gel transition also shows the same increasing trend, D =2.4-2.88 for the resulting gel. The pyridine sol exhibited a quite different structure with D = 1.3, which is comparable to the SAXS measured values of 2.41-2.03. The large fractal dimension difference is attributed to an increased exposure to air for the SANS sample. For the 123 DIK sol a non-linear slope of the log-log plot was observed (Fig. 4). In the larger Q region, 0.010-0.017 A-(representative of 370-630 A structural features), a slope of 3.6 is observed, whereas in the lower Q region, 0.005-0.010 A-' (representative of 630-1250 A structure), a D value of 1.5 was observed.These data suggest that as a result of the intermediate Ba2Cu3 colloidal structure, a variable dimen- sionality structure is formed in the final sol. This behaviour is similar to that observed for colloidal silica aggregation which produces a fractal dimension of l.2.24 In this silica system, aggregation of colloidal silica elements proceeds in an almost one-dimensional fashion. The colloidal aggregation process has been modelled using the diffusion-limited cluster- cluster growth in two dimensions which predicts a value of D of 1.45 k0.03. It is apparent from the SAXS and SANS data that the hydrolysis and condensation reactions of this system produce very complex network structures, both in the individual precursor sols and in the three-component alkoxide systems.We have found the fractal-dimension data useful in charac- terizing the sol-to-gel network structure development, as well as for understanding the solution behaviour, i.e. substrate wettability. 4.3 Contact Angle A 5 x lop6dm3 drop of 0.01 mol kg- ' standard sol completely wet the sapphire test substrate (Table 3). However, the more concentrated sol, 0.1 mol kg-', showed a 22.5" contact angle. The wettabilities of the base-modified sols were drastically different than for the standard sols. For example, the contact angle of the pyridine-containing 0.1 mol kg- ' sol was three times less than that of the standard sol contact angle of 6.5", a very large increase in wettability.On the other hand, the corresponding lutidine sol droplet gelled before a contact angle could be measured, ca. 30 s. These large differences in the contact angle are in part related to differences in the polymeric network structure of the sols. In the case of the pyridine-containing sols, a lower network dimensionality should allow easier diffusion of the sol components across the sol/substrate interface. The reduced wettability of the lutidine sol, however, is believed to be related to the increased air sensitivity of the sol, which promotes an increased viscosity and rapid gelation. 4.4 Thin Films The ability to produce uniform thin films using the spin- casting technique relies heavily on a sol's ability to wet the substrate uniformly, such that when a substrate is accelerated rapidly, uniform removal of excess sol occurs.'' For each method the sol concentration is a critical controlling variable, which affects the film uniformity, microstructure and thickness.188 If a sol is too dilute, a film thickness of less than 100OA results, and multiple coats may be required. Multicoat appli- cations are less desirable than a single coating because of greater exposure to particulate contamination in both the sol and the air. These particles hinder the uniform removal of excess sol. Thus, it is desirable to use a relatively concentrated sol, which wets the substrate completely and forms a uniform film 1-2 pm thick in one application.It is interesting to note that the pyridine-containing sol, having a SANS-determined fractal dimension of 1.3, wets the sapphire substrate better than a standard sol five times more dilute. One explanation for this behaviour is that the more linear polymeric species in pyridine sol can more easily diffuse across the solid/liquid interface than the more three-dimen- sional standard sol elements.25 One desirable feature of a consolidated YBa2Cu307 -x thin film is crystallographic orientation, epitaxy. Highly oriented thin films exhibit critical current values 1000 times those observed for sintered compact specimens.26 For single crystals such as Si or GaAs a mosaic spread of 0.05" in 6 is usually exhibited.Our analysis of the oriented film sample showed a mosaic spread of 0.28" in 6 for the 001 reflection (Fig. 7). This measurement indicates that with respect to the c axis, the film appears as a single crystal. Our 123 film orientation was greater than that reported by researchers using molecular- beam epitaxy techniques, 0.8" in 6 FWHM for the 100 film peak.26A Huber four-circle diffractometer was used to deter- mine whether the a and b axes of the film were aligned with those of the cubic substrate. The results indicate the film to be coupled three-dimensionally with the (001) SrTiO, sub- strate. However, no distinguishable difference in the a and b lattice parameters of the film was observed.Thus, it was concluded that the film was composed mainly of the 123 tetragonal phase. It was determined that the spin-casting technique allowed formation of the most uniform thin films, producing film thickuesses of 0.1-2.0 pm. The fractal dimension of a given sol could be used to determine whether spin casting or spray coating would yield the best films. For example, the intermedi- ate colloidal-like DIK sols (D=3.9) were more easily spray coated than the MOE-MEK sols which had fractal dimen- sions near 2.4. The MOE-MEK system was found to be best suited to the spin-casting technique. It was observed that the quality of the spray coatings produced using the transparent MOE-MEK-toluene was not as good as that produced using spin-casting or dip-coating methods.Thicknesses of 1-10 pm could be produced; however, most films cracked prior to removal from the hot plate. When DIK sol was used, favourable adhesion to the substrate was observed and initial cracking could be avoided for films ca. 1 pm thick. An optimum application temperature of 200-220 "C was found for the DIK spray-coated films. Conversion of the tetragonal films to superconducting YBazCu307-x films during annealing in oxygen was incom- plete. Thus, resistivity measurements were not performed. Incomplete tetragonal-to-orthorhombic phase conversion has been reported previously by Manthiram and Gooden-o~gh.~'*~*They report that the presence of 0 2p holes (paired oxygen) allowed the intercalation of oxygen to greater than 06.5without conversion to the orthorhombic phase.We observed similar behaviour in bulk powder samples produced using the alkoxide process." 5. Conclusions The network structure of the sols and gels studied was effectively investigated using SAXS and SANS. The fractal dimensions of MOE-MEK-toluene-processed sols and gels J. MATER. CHEM., 1991, VOL. 1 indicate a network structure (DMtypically in the range 2-2.5). The network structure development was modified effectively using the organic base additives lutidine and pyridine. Pyri- dine-containing sol exhibited decreased network dimensional- ity and drastically increased substrate wettability; these measurements were supported with the contact-angle measurements. The spin- and dip-coating techniques produce the most uniform thin films when a transparent stable sol is used, and the spray-coating technique is best for more colloidal sols, e.g.those produced using the diisopropyl ketone solvent system. Three-dimensionally oriented YBa2Cu307-x thin films, 0.1 mm thick, were produced on single-crystal (001) SrTiO, substrates using spin casting. The degree of film orientation was greater than that achieved for MBE-deposited films. The films were not, however, superconducting, owing to incomplete tetragonal-to-orthorhombic phase conversion; X-ray diffraction data indicated the films to be composed of essentially single-phase tetragonal YBa2Cu307 -x. The authors would like to thank the following people for their constructive advice and suggestions as well as assistance in performing the reported work: Mark Teepe, David Kenzer and Julie Twaddle, Joyce McMillian, Irena Drumler, Hayden Chen, Phil Giel, Michael Elvis Buckley, and the BOIC.Small- angle X-ray scattering was performed on equipment partially supported by DEFG05-85ER7523 1. The staff at NIST, specifically Charlie Glinka, performed the SANS experiments. Funding at the University of Illinois, Materials Research Laboratory was provided by NSF-DMR 86-12860, that at the Science and Technology Center for Superconductivity, was provided by NSF (National Science Foundation) STC (Science and Technology Center) 88-09854, and that for the Center for Microanalysis of Materials by DOE (Department of Energy) DE-AC 02-76ER 01198.Oak Ridge National Laboratory is operated by Martin Merietta Energy Systems Inc., for the United States Department of Energy under Contract No. DE-AC05-840R2 1400. References 1 M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett., 1987, 58, 908. 2 K. E. Easterling, C. C. Sorrel], A. J. Bourdillon, S. X. Dou, G. J. Sioggett and J. C. Macfarlane, Muter. Forum, 1988, 11, 30. 3 J. D. Mackenzie, SPIE, 1988, 878, 128. 4 J. D. Mackenzie, in Ceramic Chemical Processing, ed. L. L. Hench and D. R. Ulrich, Wiley-Interscience, New York, 1986, pp. 113-122. 5 D. C. Bradley, in Metal Alkoxides, Academic Press, New York, 1978, pp. 303-361. 6 C.H. Brubaker Jr. and M. Wicholas, J. Inorg. Nucl. Chem., 1965, 27, 59. 7 J. V. Singh, B. P. Baranwal and R. C. Mehrotra, Z. Anorg. Allg. Chem., 1981, 477, 235. 8 M. A. Accibal, J. W. Draxton, A. H. Gabor, W. L. Gladfelter, E. Hassler and M. Mecartney, University of Minnesota Research Paper, 1988. 9 G.M. Whitesides, J. S. Sadowski and J. Lilburn, J. Am. Chem. SOC.,1974, 96, 2829. 10 K. S. Mazdiyazni, R. T. Dolloff and J. S. Smith 11, J. Am. Ceramic. Soc., 1969, 52, 523. 11 H. S. Horowitz, S. J. McLain, A. W. Sleight, J. D. Druliner, P. L. Gai, M. J. VanKavelaar, J. L. Wagner, D. D. Biggs and S. J. Poon, Science, 1989, 243, 66. 12 S. Hirano, T. Hayashi, M. Miura and Hiroyuki Tomonaga, Bull. Chem. Soc. Jpn., 1989, 62, 888. 13 H. Kozuka, T. Umeda, J.S. Jin and S. Sakka, Muter. Res. Soc. Symp. Proc., 1988, 121. 14 E. J. Pope and J. D. Mackenzie, J. Nun-Cryst. Solids, 1988, 101, 198. J. MATER. CHEM., 1991, VOL. 1 189 15 C. J. Brinker, A. J. Hurd and K. J. Ward, Fundamentals of Sol-J. Zarzyscki, Fourth International Workshop on Glasses and Gel Thin Film Formation, Sandia National Lab, 1986. Glass Ceramics from Gels, July 13-15, 1987, Kyoto, Japan. 16 D. W. Schaefer, MRS Bull., February 1988, 22. 23 K. D. Keefer, MRS Bull., October l/November 15, 1987, 29. 17 L. A. Feigin and D. I. Svergun, Structure Analysis by Small-Angle 24 M.K. Kotvanova, M. A. Fedotov, L. P. Kazanskii and E.A. X-Ray and Neutron Scattering, Plenum Press, New York, 1987. Torchenkova, Koord. Khim., 1984, 10, 1062. 18 S. A. Kramer, G. Kordas, J. McMillan, G. C. Hilton and D. J. 25 M. J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New VanHarligen, Appl. Phys. Lett., 1989, 53,156. York, 1989, pp. 240-275. 19 G. Moore, S. Kramer and G. Kordas, Muter. Lett., 1989, 7, 415. 26 M. Hong, J. Kwo and C. H. Chen, IEEE Comp., 1988, 11, 407. 20 G. A. Moore, G. Kordas, J. D. Jorgensen, J. Faber, F. Rotella, 27 A. Manthiram and J. B. Goodenough, Nature (London), 1987, R. L. Hitterman and K. J. Volin, J. Muter. Chem., 1991, 1, 175. 329, 701. 21 T. Lours, J. Zarzycki, A. Craievich, D. I. Dos Santos and 28 J. B. Goodenough and A. Manthiram, in Chemistry of Oxide M. Aigerter, lnternational Workshop on Glasses and Glass Cer- Superconductors, ed. C. N. R. Rao, Blackwell, Oxford, 1988, amicsfrom Gels, July 13-15, 1987, Kyoto, Japan. pp. 101-112. 22 A. Craievich, D. I. Dos Santos, M. Aegerter, T. Lours and Paper 0/03047C; Received 6th July, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100181

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(2), 191-195 Depth Profiles and Electrochemical Properties of IrO, Electrocatalysts Stabilized with TiO, Achille De Battisti,*" Andrea Barbieri," Anna Giatti," Giancarlo Battaglin,b Sandro Daolio" and Angelo Boscolo Boscolettod aDipartimento di Chimica dell'Universita and Centro CNR per lo Studio della Fotochimica e Reattivita S.E.C.C.,via L. Borsari 46, 1-44 100 Ferrara, Italy bUnita CIFM, Dipartimento di Chimica Fisica, Universita di Venezia, Calle Larga S. Marta 2137, 30 123 Venezia, Italy "lstituto di Polarogra fia ed Elettrochimica Preparativa del CNR, C.so Stati Uniti 4, 35100 Padova, Italy dMONTEDIPE, via della Chimica, 5,Porto Marghera (Ve), Italy lrO,/TiO, mixed-oxide electrocatalysts of different composition have been studied by Rutherford backscattering spectrometry (RBS) and cyclic voltammetry. Obtained results indicate that enrichment of titanium oxide in the outermost part of the coatings occurs.Comparison with data for the RuOJTiO, system suggests that the enrichment is partially controlled by the concentration of the noble-metal ion in the precursor salt mixture and by the nature of the anions present in it, while it is practically independent of the nature of the noble-metal ion itself. Plots of voltammetric charge against iridium concentration in the coatings exhibit maxima for a sample with 30 mol% lr02. An interpretation of the electrochemical behaviour on the basis of the shape of composition depth profiles has been attempted. Keywords: Electrocatalyst; Depth profile; Iridium dioxide; Titanium dioxide Electrocatalysts based on complex mixtures of Group 8 metal oxides (eg.Ru02, IrO,) and metal oxides of Groups 4A and 5A, have found wide application in industrial preparative electrochemistry. 1-5 They are generally obtained by thermal decomposition of suitable precursor salt mixtures supported on corrosion-resistant metals such as Ti and some of its alloys. Group 8 oxides give good electronic conductivity and catalytic properties to the materia1,'*2*4-7 while the other components are added in order to increase its corrosion resistance under industrial cell conditions. Many different compositions of mixed-oxide electrocatalysts have been pro- posed by the patent literature. They are generally quite complex, and any rationalization in this field is a difficult task. In fact, most of the existing works concern pure active components: RuO,, Ir02, and their mixtures.The role of the stabilizing component is examined in a limited number of papers, essentially devoted to the electrochemical behav-1*13714iOur8,10.12 or bulk microstru~ture~~' of Ru02/Ti02 electrodes. In the present work the investigation is extended to Ir0,-based materials, for which only few results are avail- able.15 The surface morphology of the samples and their microstructures have been studied by scanning electron microscopy (SEM), and by wide-angle X-ray scattering (WAXS), respectively. Considering that the electrochemical charging process involves not only their surface but also the outermost part of the bulk ~olume,'~-~~ concentration depth profiling of the metal ions and oxygen in the films has been carried out.For this purpose, Rutherford backscattering spec- trometry (RBS)26,27 has been used. For comparison, data for the analogous Ru02/Ti02 system have also been reported. The results of the ex situ characterization have been correlated with the electrochemical results, obtained by cyclic voltamme- try (CV). Experimental The compositions of the investigated layers, expressed in IrO, mol% were: 20, 30, 50, 70 and 80. Layer thicknesses were in the range 200-300 nm. Each composition consisted of four samples, belonging to the same preparation batch. They were prepared by a thermal decomposition method, outlined else- here.^^,,^ Both precursor salts were dissolved in isopropyl alcohol (Fluka purissimum).The concentration of IrC13 3Hz0 was 1% wjv. The solution of the titanium salt was prepared at the same molar concentration. Solutions were mixed in convenient ratios, then 'painted' on the titanium foil (1 cm x 1 cm plates). After solvent evaporation and short age- ing, the precursor salt deposits were thermolysed at a tempera- ture of 400 "C under oxygen. Supports were polished mechanically with emery paper, then with diamond pastes of different coarseness (7-1 pm) and, finally, chemically polished. Ru02/Ti02 films (70 mol% of RuO,) were prepared, using the couples Ru(NO)(N03),/TiC14 and RuCl, 3H,0/TiC14 as precursor salts.The thermolysis temperature was the same as for the preparation of Ir0,-based materials. Nominal IrO, concentrations in the oxide coatings were evaluated on the basis of data relative to the original chemicals used in the preparation, and may consequently be affected by a certain degree of uncertainty, in the range of a few mol%. Ir, Ti and 0 concentration-depth profiles were determined by RBS using a 4He' beam with energies of 1.0 or 2.2 MeV.? Scattered particles were detected at 160" by a surface-barrier silicon detector. Depth resolution in the very-near-surface region was 15-20 nm. Depth profiles were obtained by a fitting procedure of the experimental spectra. This procedure makes use of a computer code developed in the University of Padova by A. Carnera,30 based on the principles of backscattering analy- sis,26*27which synthesizes spectra from given concentration profiles.Profiles were represented as segmented lines, since the depth resolution of RBS broadens edges, hindering a detailed study near the points of sharp change in slope. The assumed concentration profiles were varied until a satisfactory fit of the synthesized spectrum to the experimental one was obtained. The estimated uncertainties in Ir, Ru and Ti relative t 1 eVz1.602 x 1O-l9J. concentrations were ca. 1-2% in the range of maximum concentration values. The uncertainty relative to oxygen was larger, owing to the low elastic scattering cross-section, and to the overlap of the oxygen signal with that of the titanium substrate.The depth scale is expressed in atomscm-,, the natural units of RBS analysis. Conversion of this scale to that in usual length units was carried out by dividing by the molecular density of the oxide film. This was assumed to be the weighted average of the densities of the noble-metal oxide and TiO, (rutile). This procedure may introduce systematic errors in the reported depth values, which we estimate to be not larger than 10%. Much larger uncertainties affect the compositions at the interface region between oxide layer and support, owing to the impossibility of evaluating separately the effect of interdiffusion of species, variations in thickness of the layer and roughness of surfaces on which they are formed.At this stage of the research, however, detailed features of the oxide layer/metal support interface are not strictly needed, surface properties of relatively thick coating being the main target of the present work. Electrochemical experiments were performed with a Solartron 1286 electrochemical interface. Automatic data acquisition and elaboration were performed with in-house ~oftware.~' Cyclic voltammetry measurements were carried out in 1 mol dmP3 perchloric acid solutions, following widely adopted procedures. The potential range explored was 0.00-1.20 V (us. SCE) and potential sweep rates were chosen between 0.005 and 0.1OOV s-'. The anodic charges were obtained by integration of the anodic part of the voltammo- grams performed at a potential sweep rate of 0.050 V s-'.The estimated uncertainty for voltammetric charges was ca. 1Yo. Results A qualitative indication of the surface morphology of the supported layers has been obtained by SEM. Results for samples with IrO, concentrations of 30 and 70 mol% are given in Fig. 1. In both cases larger-sized particles can be seen within a structure which essentially consists of very small grains. An examination of SEM images of all the investigated samples shows the number of larger particles decreasing with IrO, content. X-Ray diffraction data indicate that only one phase, an Ir0,/Ti02 solid solution with rutile structure, exists in the investigated composition range. Therefore, the forma- tion of the phase can be reasonably assumed to take place according to the mechanism proposed for pure TI-O,.~, Accordingly, the formation of larger particles can be due to the existence of specific sites at which nucleation and further growth occur under more favourable conditions.Considering that such sites have to be based on Ir species (Ir polynuclear complexes for instance3,), it seems reasonable that the number of particles in question decreases with decreasing Ir02 content in the layers. Most of the reacting mass, on the other hand, will undergo the transformation to oxide solid solution in a, more or less, aspecific way, creating the morphologically more homogeneous part of the layer. Fig. 2 shows, as an example, the RBS spectrum of the sample containing 30 mol% of IrO, and 70 mol% of Ti02.For the sake of comparison, the spectrum synthesized from the concentration profiles shown in Fig. 3 is also shown in Fig. 2. Profiles of the film containing 70 mol% of IrO,, exemplifying the situation for greater noble-metal content, are presented in Fig. 4.In both Fig. 3 and Fig. 4 the non-uniform- ity of the distribution of metal components across the oxide layers is evident. A quite similar shape of composition depth profiles has been found also for the films with an 11-0, nominal content of 20, 50 and 80 mol%. In particular, enrichment with titanium dioxide is observed in the near-surface region. J. MATER. CHEM., 1991, VOL. 1 Fig. 1 SEM image of the surface of IrO,/TiO, mixed-oxide films.(a) 30 mol% IrO,; (b) 70 mol% IrO,. Magnification of the outer part of the photographs is four times less than in the central part n Ir I I 0.2 0.4 0-6 0.8 energy/MeV Fig. 2 Experimental RBS spectrum (histogram) (4He+, 1.0 MeV, 8= 160"),with superimposed simulation (continuous line) of a IrO,/TiO, coating (30 mol% of IrO,) prepared at 400 "C on a mirror-finished Ti plate Data obtained by secondary-ion mass spectrometry (SIMS) for a film containing 80 mol% of IrO," are in agreement with these results. This feature, found also in the case of Ru0,-based system^,^^-^^ is not restricted to the very-near- surface region of the films, but extends to the order of 10 nm below the surface itself. According to the results in Fig.3 and 4 and to those for the other film compositions studied, the coating thickness across which the compositional change is J. MATER. CHEM., 1991, VOL. 1 0.4 -Ir -/0.2 0.0 I I 0 200 400 600 th ickness/n m Fig. 3 Depth profiles of Ir, Ti and 0, for a Ti-supported IrO,/TiO, with iridium content corresponding to 30 mol% of 11-0, (from data in Fig. 2) 2" 1.4-1 \ 1600 th icknesdn rn Fig. 4 Depth profiles of Ir, Ti and 0,for a Ti-supported IrO,/TiO, coating (70 molo/o of 11-0,) observed depends on the nominal composition, being larger for the sample containing 30 mol% of Ir02. In none of the samples does oxygen stoichiometry appear to show anomalies, the expected value of 2 being found in any case within the experimental uncertainties, independent of the nominal noble- metal content of the supported layers or their depth.At this point it may be useful to discuss the possible effect of the surface morphology of the films on RBS results. The general aspects of this problem have been discussed in the literature, in the case of both supported films36 38 and bulk ~ samples.39 Dramatic effects have been observed, as expected, when the film is discontinuous, broken off into small 'islands', and part of the substrate remains partially uncovered. For bulk rough samples, on the other hand, almost negligible effects are observed when incident particles strike the sample surface in the normal direction, and the backscattered particles are detected at angles typical of RBS analysis (larger than 150').Since the oxide films studied in the present work completely cover the substrate, and only one phase is present, their spectra should correspond to a situation closer to that for bulk rough samples. Variations in thickness should influ- ence the tailing edges of the signals, thus hindering the reliable analysis of the interface between substrate and coating. In order to justify this assumption, we made many attempts to reproduce the experimental spectra by supposing the sample to consist of 'islands' of different thicknesses, covering different percentages of the area of the film. These attempts were successful only if the profiles of all the islands were very similar to each other, and very close to those reported in Fig.3-5. We can conclude that, if we restrict our analysis to the outermost part of the coatings, depth profiles are practi- 0 nominal ___, 100 300 500 thicknesshrn Fig. 5 Depth profiles of Ru, Ti and 0 for Ti-supported RuO,/TiO, coatings. Composition 70 mol% RuO,. Solid lines refer to a film prepared by thermal decomposition of a RuC1, -3H20/TiC1, salt mixture, while dotted lines refer to a film obtained from Ru(NO)(NO,), and TiCl,. In both cases the pyrolysis of salt mixtures was carried out at 400"C in an oxygen atmosphere, as for the case of IrO,/TiO, coatings cally unaffected by their surface topography. This justifies the conclusions reported above. Complementary to earlier preliminary data,33*3" RBS depth profiles relative to a Ru02/Ti02 coating are shown in Fig.5. Ruthenium concentration is 70 mol%. Solid lines indicate the dependence of Ru, Ti and 0 stoichiometry on depth for a coating obtained by thermolysis of hydrated ruthenium tri- chloride/titanium tetrachloride salt deposit. Dashed lines correspond to the mixture obtained by thermolysis of a Ru(NO)(NO~)~/T~C~~salt deposit. The profiles in Fig. 4 and 5 exhibit similar features. An enrichment with Ti species in the outermost part of the layers is observed in each case and the thickness across which the phenomenon occurs is quite similar in the two cases. The concentration of noble metal at the surface is slightly larger for the case of the layer obtained by thermal decomposition of Ru(NO)(NO~)~.The shapes of the profiles of oxygen stoichiometry in Fig. 4 and 5 differ considerably, oxygen content being larger in the case of Ru0,- based films. By means of nuclear reaction analysis, based on the'H(15N,sry)'2C reaction, maxima in hydrogen concentration us. depth profiles have been dete~ted.~' Furthermore, by thermoanalytical methods it has been shown that the elimin- ation of a chemically bound water from Group 8 metal-oxide films is slow and inc~mplete.~"~~ In agreement with these experimental results, the maxima in the oxygen concentration profiles in Fig. 5, can be tentatively attributed to slow water migration from the underlying part of the coatings during the thermal treatments, needed to bring the overall thickness to the required value. However, further experimental evidence is required to explain both the influence of the nature of the ruthenium salt on the segregation of Ti species and the high level of average oxygen stoichiometries, such as that shown in Fig.5. As mentioned previously, the electrodes characterized by different ex situ techniques were also studied in situ by CV. As described in the Experimental, the CV experiments were performed in 1 mol dmP3 HC104. The voltammograms show a couple of moderately pronounced peaks (anodic, cathodic). Comparison of their potentials with the literature data2 indicates that the peaks are due to the solid-state redox process: IrO,+H++e-+IrOOH By integration of the anodic part of the voltammograms the anodic charge, q* can be obtained, which reflects the micro- structural texture of oxide electrodes,’ and is considered a measure of their catalytic activity.’3l2 In Fig. 6, the dependence of q* on the film composition is shown.A maximum of anodic charge is observed around the IrOz bulk concentration of 30 mol%. More generally, larger charge values are associated with intermediate/low noble-metal contents. Electrodes con- taining more Ir species, exhibit lower charge-storage capacity. Similar results have been obtained for electrode materials J. MATER. CHEM., 1991, VOL. 1 formation of solid solutions. On the other hand, enrichment with one component in the outermost part of several different systems seems to be caused by the different reactivity of precursors. The different stability of the oxides formed in the bulk of the phases and at their surface is not considered to be an important factor in these cases. The thermal oxidation of some binary alloys such as Sn-Pb46 and Cu-Ni4’ results in a surface oxide layer the composition of which depends mainly on the rate of oxidation of the metal components of the alloy.The same has been observed for stainless as well as for anodic oxidation of metal ~ystems.~~.~~ As far as supported coatings are concerned, White and co-workers dernon~trated~l.~~how migration of Ti species takes place in Rh and Pt thin films deposited on flat TiO, surfaces. This causes what the authors define as ‘encapsulation’ of the noble- metal film by titanium oxyspecies.In our case, precursor salts of the elements of Groups 4 and 5 certainly exhibit a larger reactivity towards oxygen, compared with the precursor salts of Ir or Ru and this could explain, according to the above considerations, the observed based on and R~0~/Zr0~.~~’~~ segregation phenomena. As Fig. 5 shows, change in the Ru Discussion and Conclusions The results presented in this work provide details of some general physicochemical features of Ti-supported mixed-oxide coatings based on Ir02. As far as depth profiling is concerned, RBS results confirm that following the preparation method described in the Experimental, surface enrichment with Ti species occurs. Comparison of Fig. 4and 5 also indicates that, for approximately the same bulk noble-metal concentration the extent of Ti surface enrichment is comparable.Reasons for this could be tentatively sought in the surface activity of one of the two oxide components; this type of segregation has been met in oxide systems of a different nature.4s In our case, however, the situation seems to be more complex. In fact, the observed enrichment with titanium dioxide species takes place within a finite thickness, and not only at the surface, in two or three molecular layers (multilayer forma- tion). As previously observed, this enrichment occurs in the case of one-phase system^^^.^' and also for the Ru02/Ta205 system,33 for which microstructural analysis excludes the 0 0 0 0 0 1 I I 1 I I 20406090 mot% IrO, Fig.6 Dependence of the anodic charge density, obtained by inte-gration of the anodic part of cyclic voltammograms (potentialsweep rate: 50 mV s-l), on electrode composition salt from chloride to nitrosyl nitrate is sufficient to cause a less pronounced compositional anisotropy, although the final product is the same oxide mixture in both cases. As far as the electrochemical charge-storage capacity is concerned, we have to consider that the charging process, whatever the direction of the potential sweep, can be con- sidered as due to two separate contributions: (i) the double- layer charging, bound to the capacity of the electrode/solution interface, and (ii) the oxidation-state changes involving noble- metal ions.The first of these should be essentially related to the real electrode surface area, the second to the concentration of sites capable of undergoing the redox process. The former cannot be correlated easily to any simple compositional variable. Even assuming that the maxima of real surface area can be generated by segregation of new phases, which is bound, in turn, to changes in bulk composition,12 we have to bear in mind that, of the four mentioned binary mixtures, it is only in our case and in that of Ru02/Ti02 layers,’ that solid solutions are formed. The number of sites capable of undergoing redox processes, on the other hand, cannot be measured simply by the noble- metal concentration in the electrode surface region.In fact, as previously noted, electron and ion mobility are involved in the complex charging mechanism of this kind of oxide electrode. In the range of iridium concentrations explored in the present work electron conductivity is rather high and, in agreement with the conclusions drawn by other author^;^' the control of the process is due rather to the proton mobility. This, in turn, will be larger with increasing number of defects in the outermost part of the films. The observed changes of the Ir/Ti atom ratio with depth can themselves be considered among the physical causes of defects in local microstructure. Deeper compositionally perturbed regions should therefore imply larger charge-storage capacities. Along these lines the larger charge-storage capacity characteristic of low/intermedi- ate iridium contents can be tentatively allowed for, on the basis of the shape of concentration depth profiles obtained by RBS.The results presented here certainly witness to the com- plexity of a meaningful, exhaustive characterization of mixed- oxide electrocatalysts. Non-electrochemical techniques, however, although raising new questions, supply important information complementary to that available from merely electrochemical approaches. References 1 S. Trasatti and G.Lodi, in Electrodes of Conductive MetaI Oxides, Part A, ed. S. Trasatti, Elsevier, Amsterdam, 1980, p. 301. J. MATER. CHEM., 1991, VOL. 1 195 2 S. Trasatti and G. Lodi, in Electrodes of Conductive Metal Oxides, 28 R.Amadelli, Thesis, Ferrara, 1975. 3 Part B, ed. S. Trasatti, Elsevier, Amsterdam, 1981, p. 521. A. Nidola, in Electrodes of Conductive Metal Oxides, Part B, ed. 29 G. Lodi, C. Bighi and C. De Asmundis, Mater. 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ISSN:0959-9428    

DOI:10.1039/JM9910100191

出版商:RSC    

年代:1991

数据来源: RSC