Tsuyoshi Hagio

Studies on X-ray diffraction and Raman spectra of B-doped natural graphite

Abstract The structure of B-doped natural graphite has been investigated using X-ray diffraction and Raman scattering methods. The B-doped graphite has crystal lattice parameters different from those of pristine graphite. The a-axis is elongated and the c-axis contracted by B doping. Furthermore, B doping induces both an increase in intensity of the 1360 cm−1 Raman band and the appearance of a new Raman band at 1620 cm−1, accompanied by a gradual shift of the 1580 cm−1 Raman band to a higher frequency. It appeared that boron doping introduces a local distortion around substitutional B atoms within the graphite layer planes.

Microstructural development with crystallization of hexagonal boron nitride

Abstracts are not published in this journal

Bulk Photovoltaic Effect in Reduced/Oxidized Lead Lanthanum Titanate Zirconate Ceramics

The dependence of the bulk photovoltaic effect (BPE) on illumination intensity and wavelength has been studied in (Pb0.97, La0.03)(Zr0.52, Ti0.48)1-0.03/4O3 ceramics normally sintered at 1200° C for 2 h and then heat-treated in various atmospheres such as nitrogen, air and oxygen at 900° C for 1 h to investigate the enhancement of the BPE in the nitrogen-treated sample. The values of the Glass constant and the photoconductivity increased with decreasing oxygen partial pressure (p O2) during the heat treatments, whereas that of the dark conductivity decreased, indicating the creation of oxygen vacancies. No significant change was observed in ion oxidation states in the heat-treated samples by X-ray photoelectron spectroscopic analysis. The wavelength showing the maximum value of the photovoltaic current became shorter with decreasing p O2 during the heat treatments and simultaneously the value of the maximum current notably increased. At the same time broadening of the current curves was also noted in the spectral responses. It is proposed that some of the electrons originating from the formation of oxygen vacancies can be expected to contribute to the enhancement of the BPE.

Graphite and boron carbide composites made by hot-pressing

Composites consisting of graphite and boron carbide were made by hot-pressing mixed powders of coke carbon and boron carbide. The change of relative density, mechanical strength and electrical resistivity of the composites and the X-ray parameters of coke carbon were investigated with increase of boron carbide content and hot-pressing temperature. From these experiments, it was found that boron carbide powder has a remarkable effect on sintering and graphitization of coke carbon powder above the hot-pressing temperature of 2000° C. At 2200° C, electrical resistivity of the composite and d(002) spacing of coke carbon once showed minimum values at about 5 to 10 wt% boron carbide and then increased. The strength of the composite increased with increase of boron carbide content. It was considered that some boron from boron carbide began to diffuse substitutionally into the graphite structure above 2000° C and densification and graphitization were promoted with the diffusion of boron. Improvements could be made to the mechanical strength, density, oxidation resistance and manufacturing methods by comparing with the properties and processes of conventional graphites.

Carbon/ceramics composites — Preparation and properties

Abstract Fabrication methods for carbon/ceramics composites were established by using two different processes of hot-pressing and pressureless sintering without any binder phase. In the hot pressing method, some boron compounds were found to be an effective aid for sintering and graphitization of coke powder above 2000°C under some pressure. When the content of boron compound such as B4C was high, graphite/B4C composites could be fabricated. If some other ceramic powder such as NbC, TiC or TaC was mixed in addition to the B4C, three component composites with graphite matrix could be obtained. In pressureless sintering method, raw coke carbon powder was ground for a long time to be transformed in to a sinterable and non-graphitizing-type carbon powder. From a mix of ceramic powders such as SiC or B4C with the ground coke powder, the composites of carbon/SiC or carbon/SiC/B4C systems could be fabricated by heat-treatment under normal pressure. Some properties of the graphite samples and carbon/ceramic composites were investigated. It was found that their mechanical properties were much better than those of conventional graphite samples and the resistance to oxidation and corrosion was also excellent. It is suggested that the composites could be applied as bearing or mechanical seals both for use in high temperature environments and as machine parts in contact with some molten metals.

Enhanced Photovoltaic Response in Lead Lanthanum Zirconate-Titanate Ceramics with A-Site Deficient Composition for Photostrictor Application

Samples of (Pb0.97, La0.03)(Zr0.52, Ti0.48)1-0.03/4O3 (PLZT) ceramics with and without an A-site deficient composition of 1 mol% in ABO3 perovskite structures have been prepared by sintering at 1200°C for 2 h to investigate the photovoltaic response upon UV illumination, as well as the ferroelectric and piezoelectric parameters such as the polarization-electric field (P–E) hysteresis curve, strain constant, and dielectric constant. Results confirmed that the A-site deficient composition generates a photovoltaic current twofold higher than that in the sample without the deficient composition. In the ferroelectric and piezoelectric parameters, no significant difference was measured between the two samples. These results indicate that the photostrictor response is mostly determined by the photovoltaic properties and that the overall response speed of photostriction can be markedly improved by enhanced photovoltaic current.

Thermal conductivities and Raman spectra of boron-doped carbon materials

Abstract Boron-doped carbons were prepared at temperatures from 1000 to 2800°C. The effect of boron doping on the thermal conductivity of carbons has been studied and discussed with the results from Raman scattering. Boron doping above 2200°C depressed the thermal conductivity of carbons and increased the intensity of the 1360-cm −1 Raman band. It appeared that lowering the thermal conductivity is mainly caused by a distortion of the graphite lattice due to boron doping.

Effect of Multiple Impurity Doping on the Photovoltaic Properties of Lead Zirconate-Titanate Ceramics

Abstract Pb(Zr0.5, Ti0.5)O3 (PZT) ceramics singly doped with 0-3 mol% of Ta and those multiply doped with 1 mol% of Na and 0-4 mol% of Ta have been sintered at 1050-1300 °C for 2h to investigate the change in photovoltaic properties with changes in the concentration of Pb vacancies, and also grain size, upon ultraviolet irradiation. Both photovoltaic current and photovoltage were markedly increased by the doping of Ta in the case of single doping, whereas in the multiply doped samples they showed no significant change up to 1 mol% of doped Ta. Beyond this amount, however, they were found to significantly increase. The average grain size in the singly doped samples decreased with increasing amounts of doped Ta, while that in the multiply doped samples revealed a maximum at around 1 mol% Ta, below which grain size was smaller than that of the nondoped sample. It is concluded from these results that the formation of Pb vacancies onto the perovskite lattice, rather than the existence of doped Ta, has a fundam...

Influence of sputtering atmosphere on crystallinity and crystal orientation of AlN thin films deposited on polycrystalline MoSi2 substrates

Highly oriented AlN thin films have been deposited on polycrystalline MoSi2 substrates by r.f. magnetron sputtering. The total sputtering pressure and the nitrogen concentration in the sputtering gas had a significant influence on the crystallinity and crystal orientation of the films. The film deposited at a sputtering pressure of 0.6 Pa and a nitrogen concentration of 20% indicated high crystallinity, high c-axis orientation (σ=3.1°) and very low surface roughness (Ra=0.7 nm). The crystallinity, crystal orientation, composition and morphology of the films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy. The nitrogen concentration hardly had an effect on the composition of the films; however, it had a great influence on the shape of the fine grains constituting the films. The shape of the grains drastically changed from triangular pyramids of various sizes to uniform fine grains with increasing nitrogen concentration.

Effects of boron addition on some properties of hard-type carbons

The boron-containing hard-type (HT) carbons were prepared by heating the raw coke compacts with 1.6 wt% boron at temperatures ranging from 1000 to 2800° C. Some physical and mechanical properties of boron-doped HT carbons have been measured and compared with those for boron-free materials. It was confirmed that the boron enters the HT carbon at a relatively low temperature of 1400° C and enhances the densification process of compacts during heat-treatment above 1800° C. The addition of boron caused increases in Youngs modulus and thermal conductivity, and decreases in hardness and electrical conductivity of HT carbons. The effects are discussed, and compared with those for graphitizable carbons.