Papers by Author: Koichi Niihara

Abstract: Carbon nanotubes reinforced alumina was fabricated by spark plasma sintering method. When adding 0.2wt% nanotubes, the fracture toughness of the composites prepared increases 19% compared with the pure alumina ceramics. The effect of sintering schedule on microstructure and mechanical properties is investigated systematically. Microstructure studies reveal that at high sintering temperature, the nanotubes tend to gather in the gaps surrounded by three or more grains in a flocculent state, which leads to poor mechanical properties. Raman spectrum indicates that long sintering duration may cause serious nanotubes destruction and lower the mechanical properties.

Abstract: Machinable BN/Si3N4 and electroconductive TiN/Si3N4 nanocomposites were prepared, using powders synthesized through an in-situ nitridation method in flowing ammonia gas. Due to the homogeneous mixing of various components in the powders, nanocomposites with homogeneous second phase distribution in the matrix were obtained. These nanocomposites showed enhanced strengths and distinctive functionalities. BN/Si3N4 nanocomposite with 20-25vol% BN showed a relatively high strength of over 700 MPa and was able to be machined into complicated shapes with diamond bits. Electroconductive TiN/Si3N4 nanocomposite with 25vol% TiN showed a high strength of 1100MPa and low electrical resistivity of 1.1×10-2 ⋅cm, and was promising for electrical discharge machining.

Abstract: Fe/MgO nanocomposites, which are applicable to high frequency electronic devices, were fabricated by ultrasonic spray pyrolysis and selective reduction processes. Transmission electron micrographs showed that nano ferromagnetic Fe particles were isolated by MgO insulating matrix. With the increase of the reduction temperature, the particle size and saturation magnetization of the nanocomposites increased, which resulted in the decrease of the coercive force and the increase of the permeability. Furthermore, the ferromagnetic resonance peak of the nanocomposites was not observed up to 9 GHz.

Abstract: Co-free LaFePdO3 perovskite catalyst with the self-regenerative function of Pd was developed. This technology was named the “intelligent catalyst”. Suppression capacity for Pd particle growth and catalytic activity of the Co-free perovskite LaFePdO3 were compared with those of LaFeCoPdO3. It was confirmed that Pd particles on LaFePdO3 maintained a nano-particle size by the results of XAFS analysis and TEM observation after aging in engine exhaust gas at 900 °C, and LaFePdO3 demonstrated an excellent light-off performance. Further, the design configuration for LaFePdO3 in the washcoat was investigated to maximize the self-regenerative function under practical conditions.

Abstract: 3Y-TZP/TiNiCo composites have been successfully fabricated by three step heating (dehydrogenation, hydrogen reduction, hot-pressing) of 3Y-TZP /NiO/TiH2/CoO powder mixtures. XRD analysis revealed that TiNi-base intermetallic compounds such as Ni4Ti3, NiTi, Ni3Ti had formed. The bending strength of 3Y-TZP/TiNiCo composites (~650 MPa) were much higher than those of 3Y-TZP monolith (
350 MPa) sintered at the same condition. The electrical resistivity characteristics indicated that 3Y-TZP/30 vol%TiNiCo composites were good electrical conductors. Cobalt addition to TiNi phase influenced on electrical properties of final composites, while their fracture strength was unchanged by the Co addition.

Abstract: Multi Wall Carbon Nanotubes (MWCNTs) with a diameter of 20-30 nm were used as a conductive phase to add electric conductivity to yttria stabilized tetragonal zirconia (3Y-TZP). Almost fully dense 3Y-TZP/MWCNTs nanocomposite was obtained by pressureless sintering under inert atmosphere and Hot Isostatic Pressing (HIP) treatment. The conductivity of the nanocomposites increased with increasing content of MWCNTs. Moreover, the fracture toughness increment of the composite was confirmed at 0.5 wt% addition. Scanning electron microscopy and transmission electron microscopy observation of the microstructures showed that MWCNTs were fairly homogeneously dispersed in the 3Y-TZP matrix.

Abstract: The Si3N4/YSiO2N composite in which crystalline YSiO2N was formed as grain boundary phase was fabricated by hot-pressing the mixture of SiO2, Si3N4 and Y2O3. The fracture toughness of this composite was significantly improved, compared to the Si3N4 composites containing Y5Si3O12N or Y2Si3O3N4 as a grain boundary phases. To clarify the toughening mechanism, the microstructure and the crack propagation profiles were observed.

Abstract: Si3N4 ceramics with V2O5 based glasses as sintering additives were successfully fabricated by a powder mixing process and rapid sintering by the PECS method. The fabricated materials by Pulsed Electric Current Sintering (PECS) exhibited very fine microstructure with α and β grains. Electric conductivity for the Si3N4/(V2O5-B2O3) and the Si3N4/(V2O5-B2O3-Al2O3) at room temperature were four and six orders of magnitude higher at room temperature, and two and three orders of magnitude higher at 1000 oC than the conventional Si3N4, respectively

Abstract: Mullite-based iron nanocomposites were prepared by the reduction of a mullite-iron oxide solid solution and successive hot pressing. The solid solution was obtained from the heat treatment of diphasic gel by sol-gel method. Some of the α-iron nanoparticles have an intra-granular structure just after reduction. Mechanical properties are strongly affected by the content of iron. Low iron content is beneficial to strengthening while high iron content can improve the fracture toughness. Furthermore, the nanocomposites also behave ferromagnetic properties at room temperature.

Abstract: Ceramic nanocomposites became nowadays an important ingredient of many structural and electronic ceramics, as well as ceramic coatings. The same applies to chemically processed and environmental related ceramics. The performance and characteristics of ceramic components are considerably influenced by the characteristics of precursor powder. The outstanding properties possessed by advanced nanoceramics are achieved through exceptional composition and microstructure that require very careful control throughout the successive stages of the applied processing.