Papers by Author: Koichi Niihara

Abstract: Copper nanoparticles have been prepared by pulsed wire discharge (PWD) using copper wire in deionized water at various relative energy (K) from 0.8 to 5.5, which is ratio of the charged energy of capacitor in the electrical circuit to the vaporization energy of the wire. From the X-ray diffraction analysis of the prepared nanoparticles in deionized water at various K, the nanoparticles prepared at all conditions were identified as Cu and Cu₂O. The Cu content in prepared nanoparticles was increased with the decrease in K, and changed from 77 to 95 [%]. Additionally, the deposited energy in the arc discharge after the wire heating was decreased with decreasing K. From these results, we considered that the Cu content was increased with decreasing the deposited energy in the arc discharge during particle formation.

Abstract: The addition of h- BN to a polycrystalline Si3N4 was to increase the fracture toughness and other mechanical properties such as flexural strength and hardness of the material. The hot pressed samples were prepared from the mixture of α-Si3N4, AlN, MgO and h-BN. The composite contained from 0 to 2 wt.% BN powder with sintering aids (9% AlN + 3% MgO). The transparency, mechanical properties and microstructure of hot pressed polycrystalline Si3N4-BN composite materials were investigated by UV/VIS spectrophotometer, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The transparency decreased with increasing the content of h-BN into Si3N4.

Abstract: Nano-technology is a super microscopic technology to deal with the structures of 100 nm or smaller. This technology also involves the developing of TiO2 materials or TiO2 devices within that size. The aim of the present paper is to synthesize the silver doped nano-TiO2 by Sonochemistry method and to evaluate the effect of different percentages (0.2-1.5%) of silver load on TiO2 in the algae inactivation, The sample were characterized by using different techniques as XPS, TEM, SEM. Photo-catalytic activity of Silver doped TiO2 were evaluated through the inactivation of red tide (Cochlodinium) using UV-irradiation (315-400nm). The best result was found with silver doped TiO2.

Abstract: Dense nanocrystalline Y2O3 ceramics without grain growth have been successfully obtained by a new method, which is based on the self-propagating high temperature synthesis and quick pressing. A suitable self-propagating system with a maximum combustion temperature of 1350 °C and a heating rate of 1300 °C/min was chosen as a chemical furnace to supply the heat to densify nanocrystalline Y2O3. Dense samples without grain growth were obtained when the applied pressure was 120 MPa.

Abstract: Cr-Me-N-O (Me; Ni, Cu and Mg) thin films have been designed and successfully prepared by the pulsed laser deposition (PLD) method. It was found that Me, which form the monoxide MeO, are effective for hardening the Cr(N,O) thin films.

Abstract: In order to give machinability to hard and brittle AlN ceramics, the homogeneous dispersion of fine BN particles into AlN matrix was investigated. The AlN/BN nanocomposite was fabricated by hot-pressing AlN-BN composite powder, which was prepared by reducing and heating AlN particles containing a mixture of boric acid, urea and carbon. The nanocomposite containing 20 vol.% BN showed high strength, machinability and relatively high thermal conductivity.

Abstract: A novel organic/inorganic composite material which can work as a pressure sensor has been synthesized. This composite basically contains nano-sized carbon particles as conductive filler into an elastomer matrix to achieve pressure sensitive function. The superior pressure sensitive effect was showed at 0.88vol% of carbon particles while in the previous study the micro-sized carbon particle was required above 30vol%. In addition, nano-sized silica particles were added to reinforce electrical insulation properties and to improve the pressure sensing reliability of the composite. The addition of the surface modified silica improved better both the sensing performance and the mechanical strength than the composite containing non-modified silica particles. These polymer-based sensors have economical advantage due to low cost production regardless of the high performance.

Abstract: Based on low-temperature hot-press sintering and rapid thermit reaction, heat-resistant metal matrix composites with nano-ceramic reinforcement were prepared via reactive hot pressing. According to XRD, the composites comprised predominantly of (fcc) Cr0.19Fe0.7Ni0.11, (fcc) Fe-Cr and alumina at 700°C through the highly-exothermic thermit reaction between the starting powders. Three-point bending strength, fracture toughness, Vickers hardness and relative density increased with the increase of hot-press sintering temperature and holding time. The improving mechanical properties may be explained by increasing of content of (fcc) Cr0.19Fe0.7Ni0.11. SEM analysis showed a microstructure consisting of equiaxial granules at 700°C for 1 h and a uniformly dispersed network of very fine grains at 700°C for 2 h. It is considered that, in the reactive hot-pressing process, Al atoms diffused into the metal matrix (Fe2O3, Cr, Ni) sites and formed Al2O3 and Fe-Cr-Ni matrix. Such a technique offers the possibility of synthesizing heat-resistant metal matrix composites with nano-ceramic reinforcement materials at considerably lower temperature.

Abstract: Multi-wall carbon nanotubes (MWCNTs) with a diameter of 20-30 nm were dispersed as a conductive phase into yttria stabilized tetragonal zirconia polycrystalline (3Y-TZP) to add electrical conductivity. The 3Y-TZP/MWCNT nanocomposites were fabricated by pressureless sintering under inert atmosphere. Electrical conductive function was successfully introduced by small amount of CNT addition. Critical volume fraction of the conductive phase for the percolation was analyzed and was found to be 0.390 vol% of CNT, which was much smaller than that for nano-sized carbon black dispersed 3Y-TZP (2.55 vol%). Microstructural investigation revealed that dispersed CNTs formed continuous 3-dimensional nano-network within the 3Y-TZP matrix, that contributed to the excellent conductive properties. Fracture strength was not improved much, while fracture toughness was increased by the CNT addition, due mainly to its crack bridging and/or pull-out mechanisms. It was considered that the use of anisotropic nano-sized conductive phase is advantageous to obtain electrically functionalized nanocomposite ceramics.

Abstract: A pulsed wire discharge (PWD) apparatus for mass production of nanosized powders was developed. The apparatus had a wire feeder, and could prepare 1.5 g of Cu powder in 200 sec. The mean surface diameter of Cu powder was 86 nm. The particle size distribution of the powder prepared by 100 discharges was increased than that by one discharge. In addition, the median diameter of the powder after the 100 discharges was larger than that by one discharge. A part of the nanosized powders in production chamber of the apparatus would be grown by the deposition of plasma or vapor formed by the next discharge.