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The properties of piezoelectric ceramics are enhanced by the development of crystallographic texture. One of the most convenient preparation methods for textured ceramics is the sintering of green compacts containing aligned anisometric particles. This review deals with the mechanisms of texture development by examining microstructure development in the compacts. The characteristics of microstructure development are summarized separately, depending on the preparation methods, which are divided into four groups: oriented consolidation of anisometric particles, templated grain growth, reactive-templated grain growth, and heterotemplated grain growth.

Needlelike Sr0.5Ba0.5Nb2O6 powder particles were prepared from SrCO3, BaCO3, and Nb2O5 in the presence of a mixture of molten SrCl2 and BaCl2. When the composition of the mixture of oxide and carbonate was fixed to be SrCO3:BaCO3:Nb2O5 = 1:1:2, the phase and morphology of the product particles were dependent on the composition of molten salt. The exchange reaction, BaO + SrCl2 = BaCl2 + SrO, enriched Sr in the oxide phase. The relation between the compositions of the salt mixture and the reaction product was determined, and needlelike Sr0.5Ba0.5Nb2O6 particles were obtained by using the molten salt containing 18 mol % SrCl2.

LaNiO3(100)/CeO2(100)/Si(100) as a metal/insulator/semiconductor heterostructure was prepared by a sol-gel method with a spin-coating technique. The structure of each oxide layer was investigated by out-of-plane and in-plane X-ray diffraction analyses. The high degree of {100} -orientation of the CeO2 layers was obtained with higher heat treatment temperatures, 600-700°C. The in-plane measurements revealed that the CeO2 layers had a nonepitaxial structure in spite of the deposition on the Si(100) substrates with good lattice compatibility. The LaNiO3 layers on the CeO2(100)/Si(100) also exhibited the preferential {100} -orientation without epitaxy. The crystallinity of the {100} -oriented grains in the LaNiO3 layers was higher than that in the CeO2 layers. The occurrence of the orientation was discussed in view of the interfacial energies at each interface.

A polycrystalline BaTiO3 bulk ceramic with a preferred ‹111› orientation was fabricated by the reactive templated grain growth method. A green compact was made by tape-casting of slurry containing plate-like Ba5Ti17O40 template particles and BaCO3 powder. Single phase BaTiO3 was obtained by calcining the green compact at 1000°C. The calcined compacts were sintered between 1250 and 1400°C. The degree of orientation increased with an increase in the sintering temperature. Growth of ‹111›-oriented BaTiO3 grains originated from the Ba6Ti17O40 particles is responsible for the increase in the degree of orientation.

The products of reaction between Pb(Zr0.53Ti0.47)O3 (PZT) and glasses in the PbO-SiO2 and PbO-B2O3-SiO2 systems at 850°C were studied using powder and bulk PZT samples. A reaction zone was formed between bulk PZT and the glass. The microstructure of the reaction zone changed with the amount of PZT dissolved in the glass phase, which was dependent on the activity of PbO in both the glass phase and PZT. When the amount of dissolved PZT was small, as in the reaction between PZT and 60mol% PbO⋅40mol% SiO2 glass, the glass phase penetrated into grain boundaries of PZT, and no solid reaction product was formed. When the amount of dissolved PZT was intermediate, as in that between PZT and 40mol% PbO⋅60mol% SiO2 glass, the glass phase penetrated into the PZT pellet and ZrO2 particles were precipitated in the glass phase. When the amount of dissolved PZT was large, as in those between PZT and 60mol% PbO⋅20mol% B2O3⋅20mol% SiO2 glass and 40mol% PbO⋅30mol% B2O3⋅30mol% SiO2 glass, the reaction zone was formed, which was composed of Ti-rich PZT and ZrO2 particles dispersed in the glass phase.

Densification behavior and microstructural characteristics were examined for BaTiO 3 containing 0.25-2.00 mass% Li 2 Si 2 O 5 . Li 2 Si 2 O 5 lowered the sintering temperature of BaTiO 3 . The optimum concentration to obtain high density was 1 mass%. A concentration smaller than the optimum value did not effectively lower the sintering temperature, while a larger concentration resulted in the formation of inhomogeneous microstructure. There was an optimum sintering temperature to obtain compacts with maximum density. Extensive grain growth occurred above the optimum temperature. The grain boundary phase, which was a liquid phase at the sintering temperature, contained TiO 2 -rich particles, whose distribution was non-uniform. The formation mechanisms of such microstructure were discussed.

The products of reaction between Pb(Zr0.53Ti0.47)O3 (PZT) and PbO-B2O3 glasses at 850°C were studied using powder and bulk PZT samples. PZT dissolved in glass phase and solid products were precipitated. The reaction zone composed of glass phase and solid products was formed between bulk PZT and glass. As the B2O3 content in the glass increased, the amount of dissolved PZT and the thickness of reaction zone increased. When glasses with 40 and 60mol% B2O3 were used, the solid products were Ti-rich PZT and ZrO2 (ZrO2-TiO2 solid solution), which were expected from the phase diagram of the PbO-ZrO2-TiO2 system. When a glass with 80mol% B2O3 was used, the solid products were ZrO2 and TiO2, which were supposed to be formed by non-equilibrium reaction sequences. The microstructure of the reaction zone depended on the chemical composition of glass, indicating that the solubility of constituent oxides of PZT was dependent on glass composition.

Ag-doped NdBa2Cu3O7-δ superconducting ceramics were prepared by a sol-gel method. Ag was distributed uniformly in the sintered samples. The incongruent melting temperature of the NdBa2Cu3O7-δ samples prepared by our sol-gel method was lower than that of the samples prepared by the conventional powder method. The incongruent melting temperature was further decreased by doping with Ag. The authors consider that the incongruent melting is induced by reaction between NdBa2Cu3O7-δ and Ag. The superconducting transition width ΔT(ΔT=Tc(onset)-Tc(end)) was decreased by doping with Ag. The electrical resistivity of a 10mass% Ag-doped sample dropped sharply from 93K (Tc(onset)) and showed zero resistivity at 86K (Tc(end)).

Grain-oriented KSr2Nb5O15 solid solutions were prepared by hot-pressing. The compositions examined were 0.333 KNbO3-0.667 SrNb2O6 (stoichiometric KSr2Nb5O15, abbreviated as KSN 33/67) and 0.420 KNbO3-0.580 SrNb2O6 (abbreviated as KSN 42/58). The maximum degree of orientation for KSN 33/67 was 0.21 as measured by the Lotgering method, whereas that for KSN 42/58 was 0.86. KSN 42/58 forms a liquid phase at the hot-pressing temperatures. The liquid phase promoted grain boundary sliding and the formation of rod-like grains with large aspect ratios during hotpressing, resulting in the large degree of orientation. The Curie temperature was dependent on the degree of orientation. Samples with larger degree of orientations had the lower Curie temperatures for KSN 33/67, for which the larger grain size was responsible. For KSN 42/58, on the other hand, samples with larger degree of orientation had higher Curie temperatures. The shift of the Curie temperature is caused by internal stress generated at the ferroelectric to paraelectric phase transition. The neighboring grains with similar orientation reduce the magnitude of the internal stress.

Superconducting NdBa2Cu3O7-δ (Nd123) thin films were prepared by a sol-gel method. Nd and Ba-alkoxides and Cu(OH)2 were used as metal sources, and isopropanol and dimethylaminoethanol were used as solvents. Sols were spin-coated on polycrystalline Ag metal substrates, dried at 80°C and finally heated at 800°C, resulting in the formation of single-phase Nd123. The films exhibited preferred orientation along the c-axis after coating two times. However, the orientation was disturbed when the number of coatings increased. This behavior is attributable to the interfacial reaction between the film and the substrate to form liquid phases and/or structural relaxation of the gel film upon heating. Superconducting transition was observed at 90K after annealing the films in O2 with superconducting quantum interference device (SQUID) measurements.

Lanthanum fluoride thin films were prepared on silica glass substrates by a sol-gel method using trifluoroacetic acid (TFA) as a fluorine source. A coating solution was produced from lanthanum acetate and trifluoroacetic acid dissolved in 2-propanol. Triuoroacetate gel films were formed by spin coating the sol, which were converted to LaF3 thin films by heat-treatment at 300-500°C for 10min. The formation of LaF3 is considered to result from the reaction between lanthanum ions and the fluorine species generated by the decomposition of trifluoroacetate ions at elevated temperatures. The film fired at 300°C had a smooth surface with a grain size of 30nm. Cracks and voids were only observed in the films fired at 400 and 500°C. The films fired at 300 and 400°C exhibited transmittance of greater than 90% in the visible region.

フラックス法により板状Bi4Ti3O12粉体を合成し, テープ成形して粉体粒子の配向した成形体を作り, 普通焼結により配向性Bi4Ti3O12セラミックスを作製した. 焼結体の密度と結晶粒子配向度に及ぼす成形体の成形条件と焼成条件の影響を調べた. スラリー中の粉体量が増加するとテープ中の粉体粒子配向度が増加したが, 粉体量が多すぎるとテープの連続性が失われた. 焼成密度は焼成温度の上昇と共に増加したが, 結晶粒子配向度は1000℃までは一定で1050℃以上で増加した. 結晶粒子配向度の増加は粒成長による. テープ成形の際のブレードの開きを狭くすることにより, 成形体中の粒子配向の均一性は増加した.