APPLIED ORGANOMETALLIC CHEMISTRY Appl. Organometal. Chem. 13, 383–397 (1999) Preparation of Barium Strontium Titanate Powder from Citrate Precursor Chen-Feng Kao* and Wein-Duo Yang Department of Chemical Engineering, National Cheng Kung University, Tainan, 70101, Taiwan TiCl4 or titanium isopropoxide reacted with INTRODUCTION citric acid to form a titanyl citrate precipitate. Barium strontium citrate solutions were then BaTiO3 is ferroelectric and piezoelectric and has added to the titanyl citrate reaction to form gels. extensive applications as an electronic material. It These gels were dried and calcined to (Ba,Sr)- can be used as a capacitor, thermistor, transducer, TiO3 powders. The gels and powders were accelerometer or degausser of colour television. characterized by DSC/TGA, IR, SEM and BaTiO3 doped with strontium retains its original XRD analyses. These results showed that, at characteristics but has a lower Curie temperature 500 °C, the gels decomposed to Ba,Sr carbonate for positive temperature coefficient devices under and TiO2, followed by the formation of (Ba,Sr)- various conditions. TiO3. The onset of perovskite formation oc- Besides solid-state reactions, chemical reactions curred at 600 °C, and was nearly complete at have also been used to prepare BaTiO3 powder. 1 1000 °C. Traces of SrCO3 were still present. Among them the hydrolysis of metal alkoxide , The cation ratios of the titanate powder oxalate precipitation in ethanol2, and alcoholic prepared in the pH range 5–6 were closest to dehydration of citrate solution3 are among the more the original stoichiometry. Only 0.1 mol% of the attractive methods. In 1956 Clabaugh et al.4 free cations remained in solution. The titanyl described the preparation of barium titanyl oxalate citrates were precipitated in either ethanol or tetrahydrate for conversion to high-purity barium acetone. The acetone-derived precipitates were titanate. Kudaka et al.5 also prepared stoichiometric always viscous, but those with a sufficient barium titanyl oxalate tetrahydrate. Many elements quantity of alcohol were powdery. are quite soluble in citric acid,6 so a citrate The specific surface areas of the ceramic precursor appeared to be useful for preparing powders obtained by air- , vacuum- and freeze- titanate powders. Indeed, barium titanyl citrate drying methods were 8.3 Â 103, 10.2 Â 103 and precursors have become a very important route to 3 2 1 12.5 Â 10 m kg , respectively. The powder prepare BaTiO3. obtained by freeze-drying had the lowest degree In this study, barium strontium titanyl citrate gels of agglomeration. The precipitated powders of were used to produce (Ba,Sr)TiO3 powders. Be- titanyl citrate which were freeze-dried and sides investigating the effect of the water content calcined at 1100 °C were compacted and sin- and pH of the starting solutions, and the Ba/Sr tered at 1300 °C to obtain dense ceramic bodies stoichiometry needed to obtain optimum reaction with 95% of the theoretical density. Copyright conditions, we also studied the chemistry and # 1999 John Wiley & Sons, Ltd. thermogravimetric behaviour of the precursors during calcining, in order to understand the crystal Keywords: barium strontium titanate; citrate and microstructure changes. method; titanyl citrate precursor 7 The BaTiO3 phase transformations are shown in Scheme 1. 120°C0°C90°C cubic ! tetragonal ! monoclinic ! rhombohedral Scheme 1 BaTiO3 phase transformations. * Correspondence to: Chen-Feng Kao, Department of Chemical The addition of Ca, Sr and Pb dopant permits one Engineering, National Cheng Kung University, Tainan, 70101, 8 to change the Curie temperature of BaTiO3; the Taiwan. change can be calculated, depending on how much Contract/grant sponsor: National Science Council; Contract/grant 9 number: NSC 85-2214-E006-003. dopant is added. Andrich reported that each Copyright # 1999 John Wiley & Sons, Ltd. CCC 0268–2605/99/050383–15 $17.50 384 C.-F. KAO AND W.-D.. YANG mole% SrTiO3 added lowered the Curie tempera- Metal alkoxide hydrolysis (sol–gel method) ture of BaTiO3 from 125 °Cby3% and each As there are many alkoxides which can be used in mole% PbTiO3 added raised the Curie temperature sol–gel methods, controlling the powder unifor- by 5%. Hence (Ba1xSrx)TiO3 and (Ba1xPbx)- mity, better chemical stoichiometry and high purity TiO3 can be tailored so that a given positive can be attained than when using conventional temperature coefficient device can operate either ceramic powder processing.11 The method involves above or below 125 °C (Eqns [1] and [2]). dissolving alkoxides of titanium and barium in ! organic solvent and adding water at appropriate BaTiO3 SrTiO3 Ba1xSrx TiO3 temperatures to hydrolyse these alkoxides to Tc < 125 C 1 precipitate the mixed oxides. The reaction can be described by Eqn [4]. ! BaTiO3 PbTiO3 Ba1xPbx TiO3 ! Ba(OC3H7 2 Ti(OC5H11 4 3H2O Tc > 125 C 2 BaTiO3 #2C3H7OH 4C5H11OH 4 The shortcomings of this approach are chemical Synthesis of (Ba,Sr)TiO powder instability of the metal alkoxides, the complexity of 3 the process and the high cost. Therefore this method Solid-state reaction is unsuitable for mass production. BaCO3, SrCO3 and TiO2 can be mixed and calcined at 1100–1200 °C to give (Ba,Sr)TiO3 powders (Eqn [3]). Hydrothermal synthesis This method can be used at very low temperatures ! 12 SrCO3 BaCO3 TiO2 Ba,Sr TiO3 to prepare multicomponent perovskite powders. ° CO2 "3 For example, at 120–130 C and 5–50 bar, one can synthesize BaTiO3 and SrTiO3 powders from The advantages are simplicity and low cost, but titanium hydroxide and barium and strontium the disadvantages are that the high calcining 12 solutions. BaTiO3 can be easily obtained from temperature results in very large grain sizes and TiOCl2 and BaCl2 in NaOH solution to prepare therefore cannot be used to obtain materials with a 10 uniform-size, spherical grains of ultrafine powder. high dielectric constant; if we want fine powder, In hydrothermal synthesis, however, incomplete grinding is required, leading to pollution problems; reactions and inappropriate Ba/Ti ratios make it moreover, the mixing of large amounts of starting necessary to supplement the mixture with an excess materials is not easy and can produce mixtures of barium or titanium ions to maintain chemical Ba2TiO4,Ba6Ti17O10 and BaTi4O9 phases as side- stoichiometry, as precipitation leads to loss of products. metal ions. Chemical reaction methods To obtain BaTiO3 particles smaller than 0.7 m, Synthesis from a complex precursor with dielectric constants of 5000–6000, chemical Clabaugh4 prepared barium titanyl oxalate tetra- reaction methods have been developed to control hydrate for conversion to high-purity barium both phase formation and grain growth, leading to titanate, but the disadvantage is that barium titanate uniform fine particles. does not exist in neutral solution. To obtain The chemical reaction methods currently used to appropriate cation ratios, one must control the 10 5 prepare BaTiO3 can be classified as follows. precipitation process carefully. Kudaka et al. 2 4 2- indicated that the Ba /Ti /C2O4 ratio should be Spray drying and roasting techniques 1.05:1:2.2 to obtain a product with Ba/Ti = 1.0. 2 2 4 Ba ,Sr and Ti solutions are mixed at high Because BaCO3 is very stable and not easily temperature and then converted to (Ba,Sr)TiO3 decomposed, BaTiO(C2O4)2Á4H2O is calcined in 10 through thermal decomposition. The advantage of air, not to BaTiO3, but to the intermediate this process is the low deviation from stoichiome- Ba2Ti2O5ÁCO3, and only over 700 °C does the 13 try. The disadvantages are: (a) agglomeration intermediate decompose to form BaTiO3 powder. which requires grinding and (b) hundreds of parts Consequently, atomically mixed BaTiO3 from this per million of Cl that remain to occupy the oxalate complex precipitate precursor is not positions of the oxygen atoms in BaTiO3. obtained easily. Copyright # 1999 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 13, 383–397 (1999) PREPARATION OF BARIUM STRONTIUM TITANATE 385 Table 2. Conditions for precursor coprecipitation at various solvent ratios Solvent/precursor Solvent Ba/Sr/Ti (mol) (volume ratio) Ethanol 1:1:2 8:1 1:1:2 10:1 1:1:2 13:1 1:1:2 15:1 1:1:2 20:1 Acetone 1:1:2 8:1 1:1:2 10:1 1:1:2 13:1 1:1:2 15:1 (BaTi)/(citric acid ethylene glycol). The pre- cursor after heat treatment in air produces BaTiO3, obtained via a solid-state reaction between inter- mediate species.12 It is noteworthy that the Ba/Ti ratio in solution should be 1:1 at pH < 2.6, and 2:1 at pH > 3.2. This liquid mixing process has been used to synthesize over one hundred different oxides,14 Figure 1 Flowchart for the preparation of (Ba,Sr)TiO3 powders from citrate. including titanates, zirconates and niobates. The advantages of this process are: good control of chemical stoichiometry, low processing tempera- tures (<800 °C); and easy dopant addition. The disadvantages are the very large mass losses during Citrate process calcination and agglomeration. In this method, tetrabutyl titanate, citric acid and ethylene glycol are mixed to give a solution and then barium carbonate, dissolved in formic acid 10 and water, is added; then the pH is adjusted EXPERIMENTAL to induce coprecipitation at Ba/Ti = 1:1 of 14 BaTi(C6H6O7)3Á4H2O. Under optimum condi- tions, one should consider the concentrations of Chemicals citric acid and ethylene glycol, and the mole ratio of Barium chloride (BaCl2Á2H2O, >99% purity; Ferak), strontium chloride (SrCl2Á6H2O, EP;Haya- shi), anhydrous titanium (IV) chloride (TiCl4, EP, >99.5% purity; Shimakyu) and titanium (IV) Table 1. Conditions for precursor coprecipitation > with C H OH/citrate (13:1) at various pH values isopropoxide {[(CH3)2CHO]4Ti, 98% purity; 2 5 Janssen} were used as the starting reagents for Ba(II), Sr(II) and Ti(IV), respectively.