J. MATER. CHEM., 1991, 1(6), 1031-1034 Low-temperature Synthesis of YBa,Cu,O,-, Films by the Solution Process using Y-Ba-Cu Heterometallic Alkoxide Shingo Katayama and Masahiro Sekine Colloid Research Institute, 350-1 Ogura, Yahata-higashi-ku, Kitakyushu 805, Japan Superconducting YBa,Cu,O,-, films have been synthesized successfully at a low temperature of 600 "C by the solution process using the Y-Ba-Cu heterometallic alkoxide. The heterometallic alkoxide was synthesized by chemical modification and partial hydrolysis of component alkoxides. In gel films prepared from a heterometallic alkoxide solution, YBa,Cu,O,_, was obtained as a single phase at 600 "C under an argon atmosphere, whereas YBa,Cu,O,-, together with other crystalline phases was obtained from mixed alkoxide solutions.The heterometal- lic alkoxide precursor and the firing process under argon both proved effective for the low-temperature synthesis of single-phase YBa,Cu,O,._, films. The film fired at 800 "C for 6 h under argon followed by annealing at 450 "C for 24 h under oxygen, showed zero resistance at 74 K. Keywords: Oxide superconductor ; Y-Ba-Cu-0 system ; Metal alkoxide precursor Since the discovery of the high-T, superconductivity of YBa,Cu,O, -x,l various preparation processes of YBa2Cu307-x films have been developed for the application to electronic devices and magnetic shields. The solution process is an ideal route for preparing YB~,CU,O,-~ films since shaped ceramics can be made at relatively low tempera- ture, giving pure and homogeneous products, and because of the simplicity of the preparation. Therefore, many efforts have been made to prepare YB~,CU,O,-~ films by the solution process using starting materials such as acetate^,^ organic and metal alk~xides.~-" In particular, metal alkoxides have potential as precursors in the solution process since hydrolysis and condensation of metal alkoxides provide a polymeric precursor processing metalloxiane bonds, by which it should be possible to obtained YBa2C~307-x at low firing temperature. Lowering the synthesis temperature in the fabrication of YBa2Cu,07 -x films helps to avoid reactions of the film with substrates.Most reported solution processes using metal alkoxides for the preparation of YBa2Cu307 -x powders'' and lead to the formation of BaC03, Y203, and CuO as intermedi- ate phases, which react with each other to yield YBa2Cu307 -x.This is similar to conventional solid-state reactions. Therefore the advantages of the solution process have not been evident because the firing temperature is reduced only to 800 "C, although crystalline particles of the intermediate phases pre- pared by the solution process are small and homogeneously mixed with each other. The stability of BaCO, prevents further lowering of the synthesis temperature. The complete reaction of BaCO, requires a heating temperature of 800 "C or more. Only a few reports are currently available on the solution process which avoids the formation of BaCO, and gives YBa2Cu307-x powders at low temperature.Murakami et a/.', reported the synthesis of YB~,CU~O,-~at 650°C by the solution process using Y-butoxide, Ba-ethoxide, and Cu(N03),. However, (NO,)-was introduced into the system, consequently leading to the formation of Ba(NO,), as an intermediate phase. This formation suppresses that of stable BaCO, to yield YB~,CU~O,-~ at low temperature. In the solution process using metal alkoxides as Y, Ba, and Cu metal sources, Hirano et a1." found ozone gas to suppress BaCO, formation effectively during firing and YBa2Cu307 -x films to be obtained at 650 "C. Horowitz et all3 reported that, when an inert atmosphere was used, a powdery precipitate from the hydrolysis of metal alkoxides led to YBa2Cu307-, formation at 650 "C, but with other phases.The authors prepared YBa,Cu,O, -x fibres using Y-Ba- Cu heterometallic alkoxide. l4 This heterometallic alkoxide appeared to be a precursor having component metals in molecules. The use of heterometallic alkoxide should therefore lead to the production of a homogeneous gel film and reduction in the formation temperature of YBa2Cu307 -x. In this study, the firing atmosphere and improvement of the metal alkoxide precursor were examined to find means for obtaining YBa,Cu30, -x films at low temperature. Experimental Y(O-iC3H7),, Ba(OC2H5),, and Cu(OCH,), were used as starting alkoxides (where 0-iC3H7 is the isopropoxy group). Y(O-iC3H7), was commercially obtained (High Purity Chemi- cals).Ba(OC,H,), was synthesized by the addition of 2.4~10~~mol of Ba metal to 50 cm3 of ethanol and the obtained solution was used. Cu(OCH3), was synthesized by reacting CuCl, in methanol with KOCH3.15 Insoluble Y(O-iC3H7), and CU(OCH,)~ were modified so that they would dissolve in a solvent. Y(O-iC3H7), (1.2x lo-, mol) was dissolved in 50 cm3 of 2-methoxyethanol molwith 1.2~10~~of ethyl acetoacetate (EAA) or 3.6 x lop2mol of 2-dimethylaminoethanol. Cu(OCH3), (3.6 x lov2mol) was dissolved in 50 cm3 of 2-methoxyethanol with 7.2 x lo-, mol of ethylenediamine or 2-dimethylamino- ethanol. In modification with 2-dimethylaminoethanol, a sub- stitution reaction of OR groups with the modifier formed M[OCH2CH2N(CH3),],." In a modification with EAA, a substitution reaction of isopropoxy groups in Y(O-iC3H7), gave a chelate compound.14 In a modification with ethylene- diamine, co-ordination to Cu(OCH3), occurred without an OR-substitution reaction to give a chelate compound having methoxy gro~ps.'~ The three alkoxide solutions shown in Table 1 were used to dip-coat films on substrates. The hetero- metallic alkoxide solution A was prepared as follows: Cu(OCH3), modified with ethylenediamine was partially hydrolysed with equimolar water and mixed with Y-alkoxide modified with EAA and Ba-alkoxide solutions. Mixed alkox- ide solutions B and C were prepared as follows: Y(0-iC,H,), modified with EAA, Ba(OC2H5),, and CU(OCH,)~ modified with ethylenediamine were mixed to give an alkoxide solution B.Y(O-iC3H7)3 modified with 2-dimethylaminoethanol, 1032 Table 1 Chemical modifiers used in the experiments solution Y -alkoxide Ba-alkoxide Cu-alkoxide -A" EAAb enc B EAA -en C DMAEd -DMAE a Heterometallic alkoxide solution; ethyl acetoacetate; ethylene-diamine; 2-dimethylaminoethanol. Ba(OC2H5)2, and CU(OCH,)~ modified with 2-dimethylamin- oethanol were mixed to give an alkoxide solution C. Films were formed on partially stabilized Y20,-Zr02 (PSZ) substrates by dip-coating at a draw-up speed of 3 mm s-' in an N2 atmosphere using the three solutions. The size of PSZ substrates used in this experiment was 30 mm x 15 mm x 1 mm. The coated substrates were sub-sequently heated at 200 "C for 5 min in air. This procedure was repeated 15 times so as to increase film thickness.Finally, the films were fired at 500-800 "C for 6 h under argon, after which they were annealed at 450 "C for 24 h under oxygen. The crystal structures of films were examined by X-ray diffraction (Rigaku) using Cu-Ka radiation with a mono-chromator. To determine film morphology, a scanning electron microscope (SEM) was used on JSM-840A (JEOL). Electrical resistance of the films was measured by the conventional four- probe method. Results and Discussion Influence of Firing Atmosphere on Formation of YBa2Cu,0, -x in Films prepared from Heterometallic Alkoxide Solution A Before the firing process, gel films were shown to be amorph- ous by X-ray diffraction. They were fired at various tempera- tures under air or argon to study the formation of YBa2Cu,07 -,.The X-ray diffraction patterns of films fired at 600, 700, and 800 "C for 6 h under air are shown in Fig. 1. Films fired at 600 and 700 "C had BaCO,, Y203, and CuO. In the film fired at 800 "C, YBa2Cu3O7_, was observed as a single phase, as was also noted by the solution process described previously." When fired under air, YBa2Cu,07 -x film is formed at 800 "C through intermediate phases of BaCO,, Y203, and CuO. Although YBa2C~307-x film is formed at a relatively low temperature owing to the fine mixture state of the intermediate phases, the formation of stable BaC0, prevents further lowering of the synthesis 800°Cla a1111 S 600"C S S 30 26i" 40 50 Fig.1 X-Ray diffraction patterns of films fired at 600,700, and 800 "C for 6 h under air. a, YBa,Cu,O,-x; b, BaCO,; c, CuO; s, substrate J. MATER. CHEM., 1991, VOL. 1 temperature. BaC0, may possibly be formed by reaction of Y-Ba-Cu-0 amorphous film with C02 evolved by combus- tion of residual organics presented in films owing to firing in the presence of oxygen. To suppress the evolution of C02and avoid the formation of BaCO,, firing under argon as reported by Horowitz et ~1.'~was conducted. X-Ray diffraction patterns of films fired at 550, 600, and 700 "C for 6 h under argon are shown in Fig. 2. Although X-ray diffraction peaks attributable to BaCO, and CuO were noted for the film fired at 550"C, YBa2Cu307-, precipitated at a temperature as low as 600 "C and crystallinity improved with increase in firing temperature up to 700°C.Firing under argon did not lead to BaC0, formation, as reported.', YBa2C~307-xis thus shown to be obtainable at low temperature. Despite firing under argon, BaCO, was formed at 550 "C. A possible explanation is as follows. In the formation of YBa2Cu,07 -x through the inter- mediate phase of BaCO,, the formation temperature is 800 "C Inor abo~e.~-~*" gel films prepared by this method, YBa2C~307-xwas formed at 800°C or above when firing under air. Consequently, BaCO, in the film fired at 550°C under argon was formed by reaction of the fired film with C02 following removal of the film from the furnace, owing to the high activity. YBa2Cu307-, at 600 "C may thus be formed directly from gel films prepared from heterometallic alkoxide when firing under argon.Influence of Precursors on Formation of YBa2Cu30, -x Firing under inert atmosphere was shown above to be essential to the low-temperature synthesis of YBa2Cu3O7-,from heterometallic alkoxide. To assess the influence of precursors, films were dip-coated on a PSZ substrate using alkoxide solutions A, B, and C, followed by firing at 600 "C for 6 h under argon. Solution A contained heterometallic alkoxide possibly formed as follow^:'^ CU(OCH,),(~~)~+H20+Cu(OCH3)(en)2(OH)+CH30H Cu(OCH,)(en),(OH)+ M(OR), +(CH,0)(en)2Cu-O-M(OR) + ROH here en =ethylenediamine and M = Y(EAA), Ba. The final alkoxide derivative should have the structure of heterometallic alkoxide.Solution B contains mixed alkoxides such as Y-al- koxide modified with EAA, Ba-alkoxide and Cu-alkoxide modified with ethylenediamine. The same chemical modifiers a 600°C In, S 550c -11 I I I I 20 30 40 50 261" Fig. 2 X-Ray diffraction patterns of films fired at 550,600 and 700 "C for 6 h under argon. a, YBa,Cu,O, -x; b, BaCO,; c, CuO; s, substrate J. MATER. CHEM., 1991, VOL. 1 as those of solution A were used. Solution C contains mixed alkoxides such as Y-alkoxide modified with 2-dimethylamino-ethanol, Ba-alkoxide and Cu-alkoxide modified with 2-dimethylaminoethanol. X-Ray diffraction patterns of films prepared from solutions A, B, and C are shown in Fig. 3. In all cases, YBa,Cu,O,-, was formed at 600 "C under argon, but in films prepared from solutions B and C, BaCO, was also observed with YBa2Cu307-x.By firing under inert atmosphere, YBa2Cu,07-x was readily obtained from amorphous films prepared from the metal alkoxide solutions. The homogenous dispersion of Y, Ba, and Cu atoms in the amorphous gel films thus appears possible before firing and homogeneity must be improved by heterometallic alkoxide solution A. Y, Ba, and Cu species are considered to be deposited simultaneously on substrates from heterometallic alkoxide to give homogeneous gel films. In mixed alkoxide solutions B and C, however, slight heterogeneity in gel films is considered to build up owing to differences of hydrolysis rates of metal alkoxides.Although the chemical modifiers of solution A were the same as those of solution B, only solution A having the structure of heterometallic alkoxide gave a single phase of YBa2Cu307-x, thus showing it is possible for heterometallic alkoxide precursors to provide homogeneous gel films. Characterization of YBa2Cu30, -x Films Oxygen deficiency (x) in YBa2Cu307-,influences structural and superconducting properties. In the compo-sition range x~0-0.3gives the so-called ortho-I phase with 90 K T,. In the more oxygen-deficient xzO.3-0.5, the ortho-I1 phase with 60 K T, appears. Above xzO.5, the compound has a tetragonal structure and shows no superconductivity. YBa2Cu3O7_,films fired under argon have high oxygen deficiency and no superconductivity.YBa,Cu,07 -,films prepared using solution A were thus fired under argon, followed by annealing under oxygen to increase the oxygen content. The temperature dependence of electrical resistance for films fired at 600, 700, and 800 "C for 6 h under argon, followed by annealing at 450 "C for 24 h under oxygen, is shown in Fig. 4. Electrical resistance was normalized to resist-ance at 300 K. T,(onset) for the films was ca. 90-95 K. Films fired at 600 and 700°C did not show zero resistance above 40K. That fired at 800 "C showed T,(zero) of 74K. The resistance uersus temperature curve of the film fired at 600 "C showed two drops around 60 and 90 K, possibly correspond-ing to a mixed state of ortho-11 and ortho-I phases.The a I B .I I S Y Rs ;I L I I I 5020 30 261" 40 Fig. 3 X-Ray diffraction patterns of films prepared from solutions A, B, and C, followed by firing at 600°C for 6 h under argon. a,I YBa,Cu,O,-x; b, BaCO,; c, &O; d, Y,O,; s, substrate 1033 I I I 1I 2 Y 0 c2 U--.. h blU 01 I I 0 50 100 150 200 TiK Fig. 4 Temperature dependence of electrical resistance for films fired at (a) 600, (b)700, and (c) 800 "C for 6 h under argon, followed by annealing at 450 "C for 24 h under oxygen. Electrical resistance is normalized to resistance at 300 K Fig. 5 SEM of the morphology of the film fired at (a) 600, (b)700, and (c) 800 "C for 6 h under argon, followed by annealing at 450 "C for 24 h under oxygen transition width decreased with increase in firing temperature.This may possibly have been due to the influence of the degree of crystallization caused by the firing temperature. Fig. 5 shows SEM photographs of the morphology of films fired at (a)600, (b) 700, and (c) 800 "C for 6 h under argon, followed by annealing at 450 "C for 24 h under oxygen. These films had no cracks, and film thickness was ca. 1 pm. In films fired at 600 and 700 "C, grains were too small to be observed. Grain size may have been less than 0.1 pm. In the film fired at 800 "C, they eventually attained submicrometre size. Grain size increased with firing temperature. Using homogeneous gel films prepared from heterometallic alkoxide and firing under argon atmosphere, YBazCu307 -x was possibly formed directly from amorphous gel films and showed high T,(zero) of 74 K in films prepared by the solution process.It is considered that the further improvement of superconducting properties such as T,(zero) and J, requires the investigation of the appropriate combination of firing and annealing conditions. Conclusions YBaZCu3O7-x films were successfully fabricated at low tem- perature by the chemical process using a homogeneous solu- tion of metal alkoxides. (1) When firing under argon, YBa,Cu,O,-, precipitated from gel films at a temperature as low as 600 "C. (2) The heterometallic alkoxide solution gave a more homogeneous gel film than that possible with mixed alkoxide solutions. 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