Ultrafine Barium Titanate Powders via Microemulsion Processing Routes John Wang, * ,† Jiye Fang, † Ser-Choon Ng, ‡ Leong-Ming Gan, §,¶ Chwee-Har Chew, § Xianbin Wang, ‡ and Zexiang Shen ‡ Department of Materials Science, Department of Physics, Department of Chemistry, and Institute of Materials Research and Engineering, National University of Singapore, Singapore 119260 Three processing routes have been used to prepare barium titanate powders, namely conventional coprecipitation, single-microemulsion coprecipitation using diether oxalate as the precipitant, and double-microemulsion coprecipita- tion using oxalic acid as the precipitant. A single-phase perovskite barium titanate was obtained when the double- microemulsion-derived oxalate precursor was calcined for 2 h at a temperature of as low as 550°C, compared to 600°C required by the single-microemulsion-derived precursor. A calcination for 2 h at >700°C was required for the conven- tionally coprecipitated precursor in order to develop a pre- dominant barium titanate phase. It was, however, impos- sible to eliminate the residual TiO 2 impurity phase by raising the calcination temperature, up to 1000°C. The mi- croemulsion-derived barium titanate powders also demon- strated much better powder characteristics, such as more refined crystallite and particle sizes and a much lower de- gree of particle agglomeration, than those of the conven- tionally coprecipitated powder, although they contained ∼0.2 wt% BaCO 3 as the impurity phase. I. Introduction B ARIUM TITANATE (BaTiO 3 ) is among the most important electroceramics for applications in electronics and micro- electronics owing to its excellent ferroelectric, piezoelectric, and dielectric properties. 1,2 It is widely used as the main con- stituent in many types of electroceramic devices such as mul- tilayer capacitors and positive temperature coefficient resistors (PTCR). Solid-state reaction between BaCO 3 and TiO 2 in an equimolar ratio at temperatures >1200°C has often been used to prepare BaTiO 3 powders. 3 Unfortunately, the conventional solid reaction is associated with many disadvantages including the high impurity and poor powder characteristics, represented by a coarse particle size, wide particle size distribution, irregu- lar particle morphology, and a high degree of particle agglom- eration. It is therefore not surprising to note that a large number of chemistry-based novel processing routes have been devel- oped for the production of fine and homogeneous BaTiO 3 pow- ders. These include coprecipitation, 4–6 sol–gel processing, 7–9 hydrothermal synthesis, 10,11 reactions in molten salts, 12,13 pro- cessing from polymeric precursors, 14,15 and oxalate 16–18 and citrate 19–21 routes, as have been reviewed by Phule and Risbud 1 and Chaput et al. 22 Some of these novel processing routes have demonstrated many apparent advantages over the conventional solid reaction in producing a fine and homogeneous BaTiO 3 powder, although the degree of success varies considerably from one technique to another. Several technologically important ceramic systems have re- cently been synthesized from water-in-oil microemulsions. 23,24 The microemulsion-derived ceramic powders are much finer in particle size, narrower in particle size distribution, and higher in both composition homogeneity and sinterability than those prepared via many other chemistry-based processing routes. 24,25 A water-in-oil microemulsion, which consists of an oil phase, a surfactant, and an aqueous phase, is a thermodynamically stable isotropic dispersion of the aqueous phase in the continu- ous oil phase. 26 The size of the aqueous droplets is in the range of 5 to 20 nm, rendering the microemulsions optically trans- parent. A precipitation/coprecipitation reaction will be brought about in the nanosized aqueous domains when droplets con- taining appropriate reactants collide with each other. Each of these aqueous droplets will be acting as a nanosized reactor for forming nanosized precursor particles. It is both scientifically interesting and technologically chal- lenging to synthesize an ultrafine, preferably nanosized, barium titanate powder. Microemulsions offer the feasibility of refin- ing the particle sizes to nanometer scale, although they are associated with such disadvantages as a low production yield and high production cost when the oil and surfactant phases are washed off. It is, however, possible to recycle them when the microemulsion processing technique is fully developed and matured for industrial applications. Schlag and co-workers 27 have recently tried without success to synthesize fine barium titanate particles of high purity from an inverse microemulsion consisting of decane (oil phase), a nonionic surfactant (Ge- napol OX30, Hoechst, Switzerland), and an aqueous phase containing barium and titanium chlorides. Oxalate precipitates were formed in the nanosized microemulsion domains; how- ever, they were unable to obtain a single-phase BaTiO 3 when the precursor was calcined at various temperatures ranging from 400° to 1200°C. They attributed the failure to the inho- mogeneous dispersion of titanium in microemulsion droplets and the adverse effects of residual surfactant and chlorine counterions left in the precursor. To study the feasibility of deriving ultrafine BaTiO 3 powders from microemulsions con- taining no chlorine ions in the aqueous phase, both single- microemulsion and double-microemulsion processing routes were employed in the present work. In the double- microemulsion processing route, oxalic acid was used as the precipitant. Diethyl oxalate is sparingly soluble in water (aque- ous droplets) and slowly releases oxalic acid when decom- posed. It was chosen as the precipitant in the single- microemulsion route, in order to avoid a very rapid coprecipitation reaction which may result in the formation of a highly agglomerated and chemically heterogeneous BaTiO 3 powder. The microemulsion-derived BaTiO 3 powders were characterized in a close comparison with the one derived from conventional coprecipitation of oxalates. P. P. Phule—contributing editor Manuscript No. 190743. Received September 2, 1997; approved August 27, 1998. Supported by Research Grant No. RP960692 from the National University of Singapore. * Member, American Ceramic Society. ² Department of Materials Science. ‡ Department of Physics. § Department of Chemistry. ¶ Institute of Materials Research and Engineering. J. Am. Ceram. Soc., 82 [4] 873–81 (1999) J ournal 873