Papers by Author: Keiichi Katayama

Abstract: Lithium nickel manganese oxide, LiNi_0.5Mn_1.5O₄, a cathode material for lithium-ion batteries was synthesized by a microwave heating method. Synthesized samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical properties. The results revealed that spinel LiNi_0.5Mn_1.5O₄ powders can be directly synthesized by microwave heating. The precursor prepared using 48 to 64 g of PVA solution would be best to synthesize LiNi_0.5Mn_1.5O₄ successfully by microwave heating.

Abstract: The synthesis of Ba_1-xSr_xTiO₃ (BST) powders was attempted using KOH-KNO3 molten salt. The starting materials, Ba(NO3)2, Sr(NO3)2 ,TiO2,and the mixed salt of KOH-KNO3, were well-mixed. The mixture was heated at 300 to 500°C for 1 hour, and the product was thoroughly washed with de- ionized hot water. When the mixture containing excess TiO2 was used, single-phase BST powders with a particle size of 10~50nm were obtained.

Abstract: Alpha- or beta-Si3N4 powder with larger grain size was uses as starting material, and the effect of heat-treatment on thermal conductivity of Si3N4 ceramics using MgO, Y2O3 and SiO2 as sintering additives was investigated in terms of their microstructure and the amount of grain boundary phase. Most of the components derived from sintering additives existed as glassy phase in sintered Si3N4. After heat-treatment at 1950oC for 8 h, the amount of glassy phase significantly decreased, and then small amount of glassy phase existed in Si3N4 ceramics was crystallized as Y2O3 and Y2Si3N4O3. In the case of Si3N4 ceramics using SN-7 powder, thermal conductivity of heat-treated Si3N4 was around twice of the value of sintered Si3N4, and the thermal conductivity was increased from 41.4 to 87.2 W/m•K due to not only the reduction of grain boundary phase but also the grain growth. In the case of Si3N4 using SN-F1 powder, thermal conductivity of Si3N4 ceramics was also significantly increased from 36.0 to 73.2 W/m•K after heat-treatment. In this case, the reduction of grain boundary phase mainly affected the thermal conductivity of Si3N4 ceramics because the grain size of heat-treated Si3N4 was nearly the same as that of sintered Si3N4. The reduction of grain boundary phase from Si3N4 was effective for the improvement of their thermal conductivity in addition to grain growth of Si3N4.

Abstract: Cathode materials for a lithium-ion battery were prepared using an industrial microwave furnace. The prepared materials were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical properties. Single-phase LiMn2O4 powders could be prepared in a few minutes using an industrial microwave furnace. The obtained results showed that this microwave heating method is a promising industrial technique for efficient preparation of cathode materials.

Abstract: Barium strontium titanate (BST), which has a typical perovskite-type structure, shows excellent electrical properties. BST powders were synthesized by heating mixtures of commercial reagents with an excess of TiO2 in molten salts of KOH and KNO3, and the sintering behavior of the resulting powders was evaluated. The sample with the highest density of 95% was obtained by sintering of the BST powders.

Abstract: TiO2-based ceramic humidity sensors containing small amounts of Li2O and V2O5 were prepared by a solid-reaction method. The relationship between the pore structures of the sensors and one of the most important humidity properties, hysteresis in humidity sensitivity, was studied. The humidity sensitivity was investigated by measuring the impedance of the sensors under various conditions of humidity, and the pore-size distribution was measured by mercury-intrusion porosimetry and by the nitrogen-adsorption method. The hysteresis property of the sensors was shown to be significantly dependent on their microstructure, particularly the volume ratio of mesopores to all pores in the sensor elements. Nitrogen adsorption was found to be more useful than mercury porosimetry for determining the proportion of mesopores in the samples.

Abstract: The synthesis of Ba0.5Sr0.5TiO3 (BST) powders was attempted using KOH-KNO3 molten salt. Single-phase BST powders could not be obtained when stoichiometric powders were regardless of the molten salt composition. Only when powders with excessive TiO2 were used, single-phase BST powders were obtained. The shape and aggregation state of the single-phase BST powder changed according to the heating temperature and the molten salt composition.

Abstract: Superlattice-structured Bi4Ti3O12- PbBi4Ti4O15 single crystals were grown, and their properties of polarization hysteresis and leakage current along the a axis were investigated. Oxidation annealing led to a marked increase in leakage current, while annealing in N2 atmosphere yielded a marked decrease in leakage current at room temperature. These results show that electron hole is the dominant carrier for the leakage current. A well-saturated polarization hysteresis with a remanent polarization of 41 μC/cm2 was observed, which is suggested to originate from the peculiar ferroelectric displacement of Bi in the Bi2O2 layers.