We attempted to fabricate metal/dielectrics composite capacitors exhibiting high effective dielectric constant by a lowtemperature wet chemical approach. The green compacts consisting of Ti metal particles, BaTiO 3 fillers, and TiO 2 precursor nanoparticles were successfully converted into Ti/BaTiO 3 composite compacts by the hydrothermal method at 160°C. The effective dielectric constant of these composites tends to increase with the Ti metal content and jumped up to over 10 3 near the percolation threshold. When we used the Ti(core)BaTiO 3 (shell) particles as the metal component, the thin BaTiO 3 shell layers covering on the Ti particles prevented the direct contact among metal particles and increases the percolation threshold. Since the effective dielectric constant of these composites depends largely on the dielectric properties of the BaTiO 3 layers, it is important to control the microstructure of these composites to improve the dielectric properties of such composites.

Silver (Ag) nanoparticle-loaded strontium titanate (SrTiO3) nanoparticles were attempted to be synthesized by a sol-gel-hydrothermal method. We prepared the titanium oxide precursor gels incorporated with Ag+ and Sr2+ ions with various molar ratios, and they were successfully converted into the Ag-SrTiO3 hybrid nanoparticles by the hydrothermal treatment at 230 °C in strontium hydroxide aqueous solutions. The morphology of the SrTiO3 nanoparticles is dendritic in the presence and absence of Ag+ ions. The precursor gels, which act as the high reactive precursor, give rise to high nucleation and growth rates under the hydrothermal conditions, and the resultant diffusion-limited aggregation phenomena facilitate the dendritic growth of SrTiO3. From the field-emission transmission electron microscope observation of these Ag-SrTiO3 hybrid nanoparticles, the Ag nanoparticles with a size of a few tens of nanometers are distributed without severe agglomeration, owing to the competitive formation reactions of Ag and SrTiO3.

LaNiO 3 cuboid particles were successfully synthesized by the solgel method in the presence of molten chlorides. The precursor gels derived from the La(NO 3 ) 3 and Ni(CH 3 COO) 2 solution were heated with molten chloride, NaCl or KCl, above the melting point of respective molten salts. The formation reaction of LaNiO 3 was completed below the melting point of these chlorides, and then crystal growth process might be facilitated in liquid phase provided by the molten chlorides due to an increase in the masstransportation rate. The formation of the submicron to micron-sized LaNiO 3 cuboid particles, which are enclosed by {100} facets of pseudocubic perovskite, is confirmed by the electron microscope observations and the electron diffraction analysis.

To develop the high-volume energy storage device based on ceramic capacitors, we attempted that Ti metal particles working as internal electrodes were distributed in barium titanate (BT) layers. We compared two kinds of the metal particles; uncoated Ti metal particles and Ti-BT core-shell particles (Ti metal particles with thin BT coating layers). The green compacts consisting of the metal particles, titanium oxide precursor particles, and barium titanate fillers were successfully converted into the Ti/BT composite compacts by the hydrothermal method without heating procedure. The effective dielectric constant of these composites tends to increase with a metal content, and then drastically increases up to over 10 3 near the percolation threshold (insulator-metal transition point). These dielectric constant behaviors can be explained by the percolation theory. By using the core-shell particles, the percolation threshold increases from 0.609 to 0.734, suggesting that the BT shell layers were effective to suppress the current leakage.

To develop ceramic capacitors with a high effective dielectric constant, we attempted to fabricate BaTiO 3 (BT) complexes with embedded Ag nanoparticles by wet chemical processes. Ag nanoparticle-adsorbed dendritic BT particles, Ag-BT hybrid particles, were synthesized from the solgel-derived precursor gel powders containing Ag, Ba, and Ti by hydrothermal treatment. These particles were pressed with BT fillers and TiO 2 precursor nanoparticles into green compacts, and then, the green compacts were chemically converted into the Ag/BT nanocomplex compacts in Ba(OH) 2 aqueous solution under the hydrothermal condition at 160 °C. The effective dielectric constant of the resultant Ag/BT nanocomplexes increases with an increase in Ag content. The maximal effective dielectric constant of approximately 900 was recorded for the nanocomplex with the Ag content of 10.7 vol %.

To develop metal/insulator composite capacitors with a high effective dielectric constant, we focus on the microstructure of boundary layer (BL) capacitors. Titanium metal/barium titanate (Ti/BT) composites consisting of Ti metal grains and BT boundary nanolayers were successfully prepared from pressed Ti-BT core-shell particle compacts by a hydrothermal method below 230 °C. In this hydrothermal method, BT polycrystalline shell layers connected to each other by crystal growth via a dissolution and reprecipitation mechanism in barium hydroxide [Ba(OH) 2 ] aqueous solutions. The thickness of BT boundary layers may increase with an increase in the Ba(OH) 2 concentration and determine electrical properties. These Ti/BT composites prepared in 250 and 500 mmol/dm 3 Ba(OH) 2 solutions exhibit effective dielectric constants of over 5000 and 3000, and loss tangents below 0.075 and 0.060 in the frequency range of 100 Hz-100 kHz at room temperature, respectively.

We attempted to fabricate the high-capacitance insulator/conductor composite ceramics with embedded conductive oxide particles insulated by insulator-oxide epitaxial layers to enhance their dielectric breakdown strength. We selected BaTiO 3 (BT)/LaNiO 3 (LN) composite ceramics because perovskite-type BT is possible to grow epitaxially on the perovskite-type LN due to the similarity of their lattice constants. The LN nanoparticles prepared by the sol-gel method were mixed with TiO 2 precursor particles and pressed into green compacts. The BT/LN composite ceramics were fabricated from these green compacts by the conversion reaction of TiO 2 into BT in the hydrothermal condition at 175 o C. The effective dielectric constant of the BT/LN composite ceramics with the LN content of 17 mol% is slightly enhanced as compared to the BT ceramics prepared by the hydrothermal treatment because the LN particles acted as the internal electrodes. However, the low resistivity of the fabricated BT/LN composite ceramics suggests that the LN particles were not completely insulated by the insulator epitaxial layer. The decomposition of LN in the hydrothermal treatment and the agglomeration of the LN nanoparticles gave rise to current leakage.

A series of metal/insulator composite capacitors with embedded metal particles in an insulator layer has attracted attention because of their high effective dielectric constant. We tried to improve dielectric breakdown strength of these metal/insulator composite capacitors by covering individual metal particles with insulator ceramics nanolayers. Micrometer-sized Ti metal particles were homogeneously covered by BaTiO 3 (BT) nanolayers with different thicknesses by the hydrothermal method, and these Ti (core)-BT (shell) particles were used for the fabrication of Ti/BT composites. The dielectric constant of the resultant Ti/BT composite capacitors with a Ti metal content of approximately 22 vol% is ranged around 800-1000, and the dielectric breakdown strength (E b ) is slightly enhanced by using the Ti-BT core-shell particles as compared to the Ti/BT composite prepared by using uncoated Ti metal particles. However, the E b of the Ti/BT composites is independent of the BT shell-layer thickness of the core-shell particles. It can be considered that the BT shell layers are helpful for an enhancement in homogeneity of the metal-particle distribution, but cannot suppress the current leakage at high voltage due possibly to their low-crystallinity and/or porous structure.Key words: composite capacitors, core-shell particles, hydrothermal method, low-temperature process, dielectric properties INTRODUCTIONA ceramic capacitor is one of the promising candidates for high-performance energy storage devices because of a high charge and discharge rate. However, an energy density of ceramic capacitors is insufficient for energy storage applications. On the other hand, a series of conductor/insulator ceramic composite capacitors such as metal/insulator ceramic composite capacitors exhibit an extremely high effective dielectric constant. There are many reports for the dielectric properties of the metal/insulator composite capacitors, and the effective dielectric constant over 10 4 was obtained [1][2][3][4]. In general, these metal/insulator ceramic composite capacitors are fabricated by sintering a mixture of metal powders and insulator ceramic powders at a high temperature, and thus most kinds of metals are not available except for novel metals. Moreover, for the case of using low-melting-point metals, an unexpected drastic change of microstructures of metal/insulator composite materials is possible to be caused by grain growth, melting, or evaporation of metals and to lead to deteriorate electrical properties of the composites [5][6][7]. Thus, low-temperature fabrication processes of such composite materials are required.We have previously reported the low-temperature fabrication of the Ti/BaTiO3 (BT) composite materials with embedded Ti metal particles in the BT layer by the hydrothermal method [8,9]. The effective dielectric constant of the Ti/BT composite capacitors increases with the Ti metal content up to around 8000, and such dielectric constant behavior can be explained by the percolation theory [10,11]. On the other hand,...