We propose the solvothermal solidification method to prepare dense ceramics at lower temperature. The procedure of the solvothermal solidification method are mainly composed of a preparation of green compact containing precursor particles and a conversion from the precursors into the desired compound via the solvothermal reaction inside the compact. This solvothermal reaction is accompanied by a densification of the ceramics; synthesized particles fill the pores inside the compact and are connected with each other by crystal growth. To prepare dense BaTiO 3 (BT) ceramics, the green compacts consisting of BT and TiO 2 mixed nanoparticles, or TiO 2 nanoparticles were prepared and were converted into BaTiO 3 by the solvothermal treatment below 200 o C. The relative density of the compacts significantly increases by the solvothermal reaction because the conversion reaction of TiO 2 into BT is accompanied by a volume increase. BT ceramics with a high relative density up to approximately 90% could be obtained by optimizing the solvothermal reaction conditions. However, the dielectric constant of these BT dense ceramics prepared by the solvothermal solidification method is much smaller than that of BT ceramics prepared by the solid-phase method because of the defect in the synthesized BT crystals and/or a size effect.

Crystal structures of KNbO 3 (KN)/BaZr x Ti 1%x O 3 [BZT, (0.1 : x : 1)] nanocomposite ceramics have been investigated using synchrotron radiation X-ray powder diffraction, where BZT nanoparticles thinly coated with KN crystals through the heteroepitaxial interface are sintered. The Rietveld analysis based on the multicomponent model reveals that the ceramic grain has the core/multishell structure consisting of a BZT core, distorted BZT and KN multishells. The variations of the volume ratio of the distorted BZT shell region corresponding to the structure gradient interface region and the dielectric property of the ceramics show similar trends as a function of x. From these results, we propose that the structure gradient region is electrically soft, and provides a crucial contribution to the dielectric properties of the nanocomposite ceramics.

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To enhance the dielectric properties of barium titanate (BaTiO 3 , BT)/bismuth ferrite (BiFeO 3 , BF) nano-complex ceramics by a polarization rotation mechanism like a lead zirconate titanate, we tried to introduce structure-gradient region (SGR) in the BT/BF nano-complexes by constructing epitaxial interface between BT and BF layers. The green compacts consisting of TiO 2 and BF nano-particles were hydrothermally converted into the BT/BF complexes compacts with various BT/BF molar ratios. It can be observed that the epitaxial BT layers were formed on the BF particles in the BT/BF complex with a molar ratio BT/BF=0.50 by the high-resolution transmission electron microscope. The dielectric properties of these nano-complexes tend to increase monotonically with the BT/BF molar ratio and the remarkable enhancement cannot be found despite of the formation of the BT/BF epitaxial interface. It can be considered that the displacement of Ti ions might be limited by the compressive stress on BT lattices due to the lattice mismatch at the epitaxial interface, and the dielectric properties did not enhance even if the SGR formed.