K Niihara

Nano-sized Particles Formed by Pulsed Discharge of Powders Sealed in Heat-shrinkable Tubes

Pulsed wire discharge has been developed as a method to form nano-sized particles, wherein a large electrical current pulse is passed through a thin metal wire. Various wire materials have been used to form nano-sized particles by this method so far. However, materials which are not sufficiently ductile to form thin wires are unsuitable for this method. We have developed a new method for powders to be discharged to form nano-scale particles instead of wires. The powders were placed in heat-shrinkable tubes and sealed by heating. Short metal wires were set at both the ends of each tube for electrical connection to the electrodes of the discharge equipment. These tubes were set in a chamber whose atmosphere could be controlled. A large current pulse was then applied to the powders in the tubes. SiC particles with a cubic crystalline structure were synthesized in N2 gas from mixed powders of Si and C with a C/Si molar ratio of unity. However, mostly the Si phase remained in the particles. Then, mixed powders with a C/Si ratio of 2 were discharged. The obtained particles were mainly composed of the SiC phase, although Si and graphite phases remained mingled.

Influence of oxygen content on hardness of Cr(N,O) thin films deposited by an RF sputtering method

Cr(N,O) thin films were deposited by radio frequency reactive unbalanced magnetron sputtering on Si(100) or glassy carbon substrates. In this paper, the influence of oxygen content on hardness of Cr(N,O) thin films was investigated. The compositional analysis was carried out by Rutherford backscattering spectroscopy. It was found that these thin films contained up to 44at.% of oxygen. Phases in the samples were determined by X-ray diffraction. Cr(N,O) thin films show only peaks based on CrN. The microstructure was observed by utilizing a transmission electron microscope. At <1at.% of oxygen, crystallite size was approximately 100nm. Then, in accordance with increasing of oxygen content, crystallite size was decreased. The hardness of thin films was measured by using a nanoindenter. The micro hardness was changed with varying the oxygen content and the microstructure such as crystallite size. Thus, it was thought that the hardening on Cr(N,O) thin films was caused by solution hardening and/or Hall-Petch relationship.

Fine-structured TiO2 ceramic patterns on the ceramic surface fabricated by replication

The ability to fabricate high precision micro- to nanoscale structure in a wide variety of materials is of crucial importance for the advancement of microtechnology, nanotechnology and nanoscience. Also, the ability to create micrometer and sub-micrometer architecture for functional ceramics is a prerequisite of exploring the rich field of ceramic nanotechnology. In this work we fabricated three-dimensional oxide ceramic materials with fine-structure over multiple length scales by combining replication patterning technique, polyvinyl alcohol (PVA), oxide ceramic material (TiO2) nano-sized particles. Our study is based on the idea that PVA can be easily detached from a mold by peeling. We confirmed that micron and sub-micron-sized fine-structured oxide ceramic patterns containing nano-sized pores could be fabricated using this procedure. The results presented demonstrate the compositional and structural diversities that are possible with a facile approach and simple method.

Microstructures of nanotubes reinforced alumina fabricated by two fast-sintering methods

Spark plasma sintering (SPS) and Self-propagating High-temperature Synthesis/ quick pressing (SHS/QP) methods were used to fabricate nanotubes reinforced alumina. The difference in microstructure was discussed. In the SHS/QP process, the maximum densification temperature is 1660°C and the heating rate is about 1600°C /min. The whole densification process in SHS/QP is very short, which is much beneficial to protect the nanotubes and restrain the growing of grains. The fracture toughness of the sample prepared by SHS/QP is up to 4.9MPam½ for 1mass% CNTs/Al2O3 composites, which shows excellent toughening effects of nanotubes.

Particle size control of silver nanoparticles prepared by pulsed wire discharge in liquid media

Silver nanoparticles were prepared by pulsed wire discharge (PWD) using silver wire in deionized water at various relative energy (K) from 10 to 98, which is ratio of the charged energy of the capacitor in the electrical circuit to the vaporization energy of the wire. From energy deposition calculated by the measured voltage and current waveforms, deposited energy of the wire was increased with increasing K. From X-ray diffraction (XRD) analysis, prepared nanoparticles were phase identified as silver. From transmission electron microscopy observations, the shape of prepared silver nanoparticles were spherical and the median particle diameter (D50) and the geometric standard deviation (σg) were calculated from the particle distribution. D50 was decreased from 34 to 19 nm with increasing K. The particle size in prepared by PWD in liquid media can be controlled by K.

Preparation of Alumina Nanoparticles by Pulsed Wire Discharge in Water

Alumina nanoparticles were prepared by pulsed wire discharge (PWD) using aluminium wire in deionized water at relative energy of 2, which is the ratio of the charged energy of the capacitor and the vaporization energy of the wire. From voltage and current waveforms during PWD, calculated deposited energy of the wire was larger than the vaporization energy of the wire. Scanning electron microscopy images showed that the prepared Al2O3 nanoparticles were spherical particles with the median particle diameter of 103 nm and the geometric standard deviation of 2.3. The X-ray diffraction analyses indicated that the prepared nanoparticles were identified as γ-Al2O3. From these results, pure γ-Al2O3 with particle size of around 100 nm was successfully synthesized by PWD in water.