J O U R N A L O F C H E M I S T R Y Materials An oxalate–peroxide complex used in the preparation of doped barium titanate Sven van der Gijp,* Louis Winnubst and Henk Verweij L aboratory for Inorganic Materials Science, Department of Chemical T echnology, University of T wente, P.O.Box 217, 7500 AE Enschede, T he Netherlands A method is described for the preparation of homogeneously doped barium titanate, which can be applied in non-linear dielectric elements.Ba and Ti salts are dissolved, mixed with hydrogen peroxide and added to a solution of ammonium oxalate, resulting in the formation of an insoluble peroxo–oxalate complex. The presence of oxalate ions and a high pH are necessary for the formation after calcination of a stoichiometric and sinterable powder.To characterise the structure of the precipitating complex, the thermal decomposition of the complex is studied by means of XRD, d-TGA and FTIR. It is found that the precipitating complex is BaTi0.91Zr0.09O2(C2O4) 2H2O. The calcined powder prepared with the peroxo–oxalate method contains no second phase, in contrast to powders prepared with the oxalate method and the peroxide method.of oxalate, which changes the complexation properties of the Introduction titanium peroxo complex. The diVerence between the oxalate Doped barium titanate is used in a broad range of electro- process and the peroxo process is that the oxalate process is ceramic devices. One example is a pulse-generating device, conducted at high temperatures, low pH and in the absence which can be applied in lamp starters in a fluorescent lamp.1 of hydrogen peroxide.Hydrogen peroxide changes the solu- For this type of application non-linear dielectric behaviour is bility of titanium peroxo complex drastically.7 However, needed. The required material properties for this type of neither the peroxide nor the oxalate method have been used application are: a high relative permittivity, a steep gradient up till now for Zr-doped BaTiO3.of the polarisation versus electric field hysteresis curve and In order to obtain dielectric properties next to a singlestable non-linear characteristics. These specifications can be phase polycrystalline material, a dense ceramic is required. met when the ceramic devices are made with a sinterable, Therefore, no aggregates in the powder may be present. The homogeneous Zr-doped barium titanate powder.1–3 grain size of the final ceramic must be large, because large Nowadays, doped barium titanate powders are mainly pro- grains reduce internal stress and low values of internal stress duced commercially by the mixed oxide process.The powders result in a higher value for the dielectric maximum.12 made by this route have certain disadvantages, like a large In this paper the eVect of oxalate on the powder properties and non-uniform particle size and a bad chemical homogeneity.is described. The precipitating complex in the peroxo–oxalate However, the mixed oxide process is easy to perform and when process and the thermal decomposition of the precipitating no evaporation occurs, stoichiometric powders can easily peroxo–oxalate complex are characterised by means of thermobe obtained.gravimetric analysis (d-TGA) and infrared spectroscopy However, an improvement in homogeneity and morphology (FTIR). The peroxo–oxalate method should result in a sinof the powder can be obtained when wet-chemical routes are terable, homogenous and single-phase polycrystalline powder.utilised. This development will lead to better dielectric behaviour. Unfortunately, during wet-chemical preparation all kinds of undesired side-reactions may occur, which makes control of Experimental procedure the stoichiometry more complicated. Powder and ceramic preparation An already existing commercial wet-chemical preparation route is the oxalate process.4–6 In this process pure, undoped Peroxo–oxalate method.Titanium oxychloride (0.15 mol; barium titanate is produced stoichiometrically by the formation 0.15 M) and zirconium chloride (0.015 mol; 0.015 M) were added and precipitation at low pH and 60 °C of a barium titanium to concentrated nitric acid (25 ml ) and water. The total volume oxalate complex. Another process for the preparation of barium of the solution was 1 l.This solution was mixed with an titanate is the so-called peroxo process based on the formation aqueous solution of barium nitrate, prepared from barium and precipitation of titanium peroxide complexes at high pH carbonate (0.17 mol; 0.085 M) and nitric acid (50 ml; 0.25 M). and room temperature.7 This process is described not only for The total volume of the mixed solution was 3 l.Finally, to this BaTiO3, but also for the production of titanium-rich materials mixture hydrogen peroxide (30%, 100 ml) was added. This so- (BaTi2O5) as well as barium zirconates and calcium and called ‘precursor’ solution was added dropwise to a solution magnesium titanates.7–11 of aqueous ammonia (0.5 mol) and diVerent amounts of oxalic The peroxo–oxalate process described in this paper is a acid with a total volume of 5 l.During the addition of the combination of the oxalate process and the peroxide process. precursor solution the pH was kept constant by the addition This powder preparation method is suitable for the production of small amounts of aqueous ammonia. of Zr-doped barium titanate (BaTi0.91Zr0.09O3). Doping is In this paper the ratio of oxalate ion concentration over the possible due to the simultaneous presence of hydrogen peroxide concentration of all the metal ions (Ba+Ti+Zr) present in and ammonium oxalate at high pH.solution is called x (0x0.5); note that when x=0 this Note that the only diVerence between the peroxide process process is the peroxide process. All experiments were performed and the peroxo–oxalate process described here is the presence at room temperature. After two hours stirring, the formed precipitate was filtered, washed with ethyl acetate, dried at 150 °C and calcined at †E-mail: s.vandergijp@ct.utwente.nl J.Mater. Chem., 1998, 8(5), 1251–1254 1251900 °C. Powders were first pre-pressed uniaxially at 80 MPa and subsequently isostatically pressed at 400 MPa.All compacts were sintered at 1400 °C for 5 h; heating rate 2 °C min-1, cooling rate 4 °C min-1. Oxalate method. Barium carbonate (0.1 mol) was carefully added to an aqueous solution of nitric acid (50 ml ). To this solution titanium oxychloride (0.091 mol) and zirconium chloride (0.009 mol) were added. This solution had a volume of 2 l. This solution was added dropwise to a solution of oxalate (0.25 mol) in water (1 l ) at a temperature of 60 °C.4 After 2 h stirring the precipitated complex was separated by filtration and calcined at 900 °C.Characterisation The decomposition of the dried precipitate was studied by Fig. 1 XRD of calcined powder prepared with increasing amounts of using TGA (Stanton Redcraft STA 625, heating rate 5 °C min-1 oxalate, x, as indicated on the right; *denotes the (cubic) perovskite to 1000 °C), and Fourier-transform infrared spectroscopy structure (FTIR).FTIR measurements were performed in situ at temperatures from 200–800 °C at temperature intervals of 20 °C Apparently, the presence of oxalate during complex forma- (with a holding [analysis] time at each temperature interval of tion results in a diVerent structure of the final precipitated a few minutes).complex. Doping with Zr seems possible due to the simul- XRD measurements were performed using a Philips PW taneous presence of oxalate and hydrogen peroxide. 1710 diVractometer with filtered Cu-Ka1-radiation, l= When a Zr-doped BaTiO3 calcined powder is prepared 1.4508 A ° . The chemical composition was measured with X-ray according to the oxalate method (in the absence of hydrogen fluorescence spectroscopy (XRF, X-ray spectrometer, Philips peroxide and in acidic environment), secondary phases are PW 1480/10).detected too, mainly BaZrO3, indicating that the oxalate Particle size distributions were measured with a Microtrac method is not suitable for the preparation of BaTi0.91Zr0.09O3.X-500 (Leeds and Northrup). The morphology of the powder and the microstructure of the ceramic were studied with Identification of the precipitating complex scanning electron microscopy (JEOL, JSM 35CF at 15 kV). The microstructure of the ceramics was revealed by etching at The weight loss found from TGA, after calcination to a a temperature 30 °C below the sintering temperature in a temperature of 1000 °C, for the dried precipitates prepared nitrogen atmosphere.To study the influence of oxalate on the with the peroxide method (x=0) is 20%. This corresponds to ligand structure of the complex, the diVerence in absorption the theoretical weight loss found for the thermal decomposition maximum was measured for a solution at pH=4, containing of BaTiO2(O2) 2H2O, the complex formed with the peroxide titanium and hydrogen peroxide, before and after the addition method,7 to BaTiO3.Note that both oxygen ligands in of oxalic acid. UV–VIS measurements were carried out with a BaTi(O2)O2 3H2O are diVerent. Philips PU 8740 spectrophotometer. When oxalate is introduced in the process, the weight loss Non-isothermal densification was studied on a Netzsch 410 increases to 31% for x=0.5.Also the thermal decomposition dilatometer; heating rate 2 °C min-1, cooling rate 4 °C min-1, behaviour changes when more oxalate is added in the process, holding time 3 h. Density measurements were performed with as can be seen in Fig. 3 ( later). In this figure the derivatives of the Archimedes technique using mercury.the TGA (d-TGA) results are given for three precipitates prepared with x=0.125, x=0.25 and x=0.50. It can be seen Results and Discussion that with an increasing amount of oxalate in the reaction mixture the d-TGA signals change, especially at approximately The complexation starts with the addition of a red-coloured 250 and 700 °C. Therefore, the addition of oxalate in the aqueous solution of titanium, zirconium, barium and hydrogen process results in the formation of a diVerent precipitating peroxide to a solution of ammonium oxalate.This results in complex as compared to the precipitating complex in the case the direct formation of a yellow precipitate. During the reaction of the peroxide method (x=0). some gas formation is observed, probably oxygen formed by Confirmation that such a change in ligand structure does the partial decomposition of hydrogen peroxide in the basic occur in the presence of oxalate ions (x>0), is provided by aqueous environment.the shift of the absorption maximum of the complex. This shift takes place from 356 nm in the absence of oxalate to 392 nm Influence of oxalate on powder properties in the presence of oxalate, as measured with UV–VIS spectroscopy in acidic aqueous environment.Fig. 1 shows XRD results of the calcined powders prepared with increasing oxalate content. It is clear that the amount of To characterise the structure of the precipitating complex in the case of x=0.5 FTIR measurements and TGA were per- second phase in the calcined powder as determined by XRD depends on the oxalate concentration. The powder prepared formed in situ as a function of temperature in the range 1000–4000 cm-1.The spectra for x=0.5 are given in Fig. 2. according to the peroxide method (x=0) contains secondary phases. The secondary phases present are mainly BaTi2O5 and IR signal 1 (at 3500 cm-1) corresponds to water. It is clear that water is still present at temperatures up to 600 °C.Most BaCO3. For increasing amounts of oxalate, the amount of these secondary phases decreases. At 0.50 molar equivalent of the water is removed at temperatures between 400 and 500 °C. Signal 2 (1700 cm-1, a characteristic signal13 at oxalate no secondary phases are present and only the cubic perovskite phase is present. 1300 cm-1 is present but is however not visible in this plot) correspond to CO vibrations of an oxalate ligand.14 These In spite of the change in phase composition of the powders, XRF measurements indicate that there is no diVerence in vibrations have disappeared at 500 °C.This means that this initial complex is no longer present above 500 °C. Peaks composition between the powders prepared according to the peroxide method (x=0) and the oxalate method (x=0.5).marked 3 (2500, 1750, 1500 and 1050 cm-1) are attributed to 1252 J. Mater. Chem., 1998, 8(5), 1251–1254Fig. 3 d-TGA of dried precipitate prepared with increasing amounts of oxalate, x (0.25, 0.5 and 1) theoretical weight loss for the proposed complex corresponds to the weight loss found with TGA. The temperatures of the above reaction mechanism steps correspond to the three maxima in the d-TGA spectrum for x=0.5 in Fig. 3. Considering the FTIR and XRD data these temperatures can be regarded as onset temperatures for these reactions. The maxima in Fig. 3 found for lower concentrations of oxalate are a mixture of the maxima found for the decomposition of the peroxo–oxalate structure and those maxima found in the thermal decomposition of the peroxo complex as pro- Fig. 2 Infrared spectra of the decomposition of the peroxo–oxalate complex (temperature intervals 20 °C), x=0.5. 1: water, 2: complex, 3: duced with the peroxide method (x=0) and described below.7 BaCO3, 4: Ti(OH)x BaTiO2(O2) 3H2O CA 300 °C BaTiO2(O2) H2O+2H2O CO vibrations in (barium) carbonate.13,14 The intensity of these signals first increases and reaches a maximum at 600 °C, BaTiO2(O2) H2O CA 500 °C BaTiO2(O2) H2O after which the intensity decreases again.This indicates that the intermediate formed after decomposition of the initial complex consist at least partially of BaCO3. The vibrations BaTiO2(O2) CA 750 °C BaO2+TiO2 (rutile)�BaTiO3+1/2O2 corresponding to BaCO3 are still present at 800 °C. Finally, signal 4 (3550 cm-1) corresponds to OH vibrations of Powder morphology and densification Ti(OH)x.15 This signal arises at temperatures above 700 °C. XRD analyses indicate the onset of crystalline perovskite The presence of ammonium oxalate also has an influence on formation at 800 °C, therefore the intermediate containing the the powder morphology.An increase in the oxalate concen- BaCO3 formed from the initial complex starts to decompose tration leds to a decrease in particle size (see Table 1).The at this temperature. smaller agglomerate size as determined from light scattering FTIR analysis at 800 °C still shows the presence of BaCO3. studies after ultrasonic treatment results in an increase of the This was confirmed by room temperature XRD measurements, green density as shown in Table 1.where perovskite as well as BaCO3 signals were found after In Fig. 4 a micrograph is given of a calcined powder prepared heating to 800 °C. XRD analysis of a powder calcined at 900 °C with the peroxo–oxalate method (x=0.5). It is clear that the showed a 100% perovskite crystal structure. Note that the powder consists of agglomerates, which in their turn consist absence of barium carbonate at 900 °C could not be confirmed of aggregates, with a size comparable to the size determined by FTIR measurements because the maximum temperature from light scattering measurements (1 mm).Within these aggrefor the high-temperature FTIR equipment used is 800 °C. gates smaller particles are visible. The average aggregate sizes Using the combined data of TGA, FTIR and XRD measure- of the various powders are given in Table 1.ments the following structure is proposed for the precipitating TEM-EDX measurements are used to study the homogeneity complex (x=0.5): BaTi0.91Zr0.09O2(C2O4) 3H2O. The thermal of the material. TEM-EDX revealed that the composition of decomposition mechanism of this complex is described below, the particles remained constant for 10 selected particles, which a mechanism which is closely related to the decomposition is an important indication that the powder is homogeneous. mechanism for the complex formed with the oxalate method.5 Dilatometer experiments on an isostatically pressed sample revealed a dense (95% rel.density) sample at a temperature of BaTiO2(C2O4) 3H2O+O2 CA 250 °C [BaCO3 TiO2 H2O] 10 °C.The maximum densification rate was at approximately 1180 °C. A sample heated for 10 h sintered at 1400 °C had a +CO2+2H2O grain size of 62 mm. High sintering temperatures are necessary [BaCO3 TiO2 H2O] CA 500 °C [BaCO3 TiO2]+H2O Table 1 EVect of the oxalate concentration on the aggregate size and green density [BaCO3 TiO2] CA 800 °C BaTiO3+CO2 oxalate average aggregate ratio, x size/mm green density (%) In these reactions Zr is left out for reasons of simplicity, it is 0 7 55 assumed that in this case Zr reacts in the same manner as Ti. 0.125 5 58 It can be seen that the complex formed in the peroxo–oxalate 0.25 4 59 complex (x=0.5) is very similar to the complex formed with 0.375 3 60 the peroxide method (x=0), namely BaTi0.91Zr0.09O2- 0.5 1 67 (C2O4) 3H2O instead of BaTi0.91Zr0.09O2(O2) 3H2O.The J. Mater. Chem., 1998, 8(5), 1251–1254 1253amount of oxalate in the process is x=0.5. The peroxide method (x=0) and the oxalate method result in the formation of a second phase. The presence of oxalate during the process ensures complete stoichiometric precipitation and an increasing amount of oxalate results in an increase of the green density. The precipitating complex in the peroxo–oxalate process is BaTi0.91Zr0.09O2(C2O4) 3H2O. The thermal decomposition of this complex is described.Finally, a calcined powder prepared using the peroxo–oxalate method (x=0.5) is homogenous and sinterable. The authors are indebted to Philips Forschungslaboratorien, Aachen, Germany for financial support.Special thanks are due to Dr D. Hennings and Dr O. Steigelmann. References 1 S. Iwaya, H. Masumura, Y. Midori, Y. Oikawa and H. Abe, US Patent, 4,404,029, 1983. 2 D. Hennings and A. Schnel, J. Am. Ceram. Soc., 1982, 65, 539. 3 S. M. Neirman, J.Mater. Sci., 1988, 23, 3973. 4 W. S. Clabaugh, E. M. Swiggard and R. Gilchrist, J. Res. Natl. Bur. Stand., 1956, 56, 289. 5 M. Stockenhuber, H. Mayer and J. A. Lercher, J. Am. Ceram. Soc., 1993, 76, 1185. 6 H. Yamamura, A. Watanabe, S. Shirasaki, Y. Moriyoshi and M. Tananda, Ceram. Int., 1985, 11, 17. 7 G. PfaV, Z. Chem., 1988, 28, 76. Fig. 4 SEM of calcined powder, magnification 25.000, x=0.5 8 G. PfaV, J.Mater. Sci. L ett., 1990, 8, 1145. 9 G. PfaV,Mater. Sci. Eng. B, 1995, 33, 156. 10 G. PfaV,Mater. L ett., 1995, 24, 393. to obtain the large grain sizes. Dielectric measurements show 11 G. PfaV, T hermochim. Acta, 1994, 237, 83. an er value of 27000 at a temperature of 90 °C. At 70 and 12 D. Hennigs, Int. J. High T echnol. Ceram., 1987, 3, 91. 110 °C the relative permittivity has 20% of its maximum value. 13 G. Busca, V. Buscaglia, M. Leoni and P. Nanni, Chem. Mater., 1994, 6, 955. 14 The Sadtler Standard spectra, Sadtler Research Laboratories, Conclusions Philadelphia, USA. The peroxo–oxalate method with suYcient oxalate results in single-phase perovskite, BaTi0.91Zr0.09O3, the optimum Paper 8/01466C; Received 20th February, 1998 1254 J. Mater. Chem., 1998, 8(5), 1251–1254