Tadashi Maruyama

Neutron irradiation effect on the thermal conductivity and dimensional change of graphite materials

Abstract Several kinds of graphite materials, such as fine-grained isotropic graphites, a carbon/carbon composite material, glassy carbon and bulk-boronized graphites, were neutron irradiated at temperatures from 200 to 400°C to fluences of 0.01 to 0.82 dpa in JMTR. It was found that the neutron irradiation markedly reduced the thermal conductivity of the graphite materials, and that the materials irradiated under the same irradiation condition had the same value and the same temperature dependence of the thermal conductivity irrespective of the original values. When the graphite materials were irradiated to over 0.1 dpa, the annealing at temperatures up to 1500°C was necessary to reach 50% of unirradiated value. The effect of boron in the bulk-boronized graphites on the thermal conductivity and dimensional change are also discussed.

Mechanism of nanoblister formation in Ga + self-ion implanted GaN nanowires

The formation of voids and bubbles during ion implantation is an important area of material research. Void and bubble formation can result in swelling and embrittlement of metallic or semiconducting materials, and increase catalytic effects in the nanopores of the bubble. Here, we report the observation of metallic nanoblister formation in GaN nanowires under self-ion implantation using a Ga+ focused ion beam. The mechanism of the blister formation was resolved using high-resolution transmission electron microscopy equipped with electron energy loss spectroscopy and plasmon imaging.

Spectrochemical Analysis of Metal Elements Electrodeposited from Water Samples by Laser-Induced Shock Wave Plasma Spectroscopy

We have succeeded in applying laser-induced shockwave plasma spectroscopy (LISPS) to the problem of the detection and analysis of metal elements deposited from water samples by means of electrolysis. It is shown that metal elements are generally deposited in the form of a thin film on the electrode surface, while the electrode also conveniently serves as a subtarget for the relatively soft metal film, thereby providing the necessary conditions for the generation of shockwave plasma, which is favorable for highly sensitive spectrochemical analysis. It is shown that the detection sensitivity of this method reaches its highest value at low surrounding air pressure of around 1 torr. The lowest detection limit attained for various metal elements investigated in this experiment varies from around ten to a few tens of ppb. This limit can be readily improved upon by incorporating an optical multichannel analyzer into the detection system. We have thus presented a promising method for the realization of a compact mobile monitoring system for the accurate control of water and soil quality.

Fabrication and Thermal Conductivity of Boron Carbide/Copper Cermet

Studies on fabrication and thermal conductivity of B4C/Cu cermet were made to obtain high performance neutron absorber materials for Liquid Metal-cooled Fast Breeder Reactor (LMFBR). A mixed powder of B4C and Cu was mechanically blended at high speed thereby a coating layer of Cu was formed on the surface of B4C powder. Then the B4C powder with Cu coating was hot pressed at temperatures from 950 to 1,050°C to form a B4C cermet. A high density B4C/CU cermet with 70 vol% of B4C and relative density higher than 90% was successfully fabricated. In spite of the low volume fraction of Cu. the B4C/Cu cermet exhibited high thermal conductivity which originated from the existence of continuous metallic phase Cu in B4C/Cu cermet.

Effects of neutron irradiation and subsequent annealing on strength and toughness of SiC ceramics

Abstract Three kinds of SiC fabricated with different sintering aids were neutron-irradiated to fluences from 8.0 × 1023 to 6.0 × 1024 n/m2 (E > 1 MeV) in the Japan Materials Testing Reactor (JMTR). Neutron irradiation increased bending strength of hot-pressed SiC containing about 1% BeO by 10%, while it decreased that of reaction-bonded (about 9% free Si) and pressureless-sintered (about 1% B and C) SiC by 30 and 20%, respectively. Correspondingly, fracture toughness of hot-pressed SiC measured by the Vickers indentation method was raised by irradiation. The subsequent annealing led to a decrease in fracture toughness and a macroscopic length with increasing annealing temperature. These results are discussed in terms of localized plastic deformation, through the dislocation channeling around tips of cracks.

Hydrogen emission by Nd-YAG laser-induced shock wave plasma and its application to the quantitative analysis of zircalloy

An experiment was carried out to demonstrate the detection of a hydrogen emission line, HI656.2nm (Hα), in a plasma induced by a Q-switched Nd-YAG (YAG, yttrium aluminium garnet) laser in a low pressure gas on various types of samples, such as zinc, a glass slide, and a zircalloy tube. Contribution by surface water could be suppressed by a laser cleaning treatment and the resulting calibration curve obtained for zircalloy tube samples doped with various concentrations of hydrogen (0, 200, 540, and 960) suggest potential applications to the quantitative analysis of hydrogen. A study of the dynamic process represented by the time profiles of the hydrogen emission, in comparison with those for zinc atomic emission, revealed a specific feature that is related to the small mass of hydrogen. This specific feature can be explained by the shock wave excitation mechanism in terms of new hypothetical process, namely, a mismatch between the movement of ablated hydrogen atoms and the formation of the shock wave.

Characteristics of Hydrogen Emission in Laser Plasma Induced by Focusing Fundamental Q-sw YAG Laser on Solid Samples

Hydrogen emission has been studied in laser plasma by focusing a Nd-YAG laser (1,064 nm, 50 mJ, 8 ns) on various types of samples, such as copper plate, zinc plate and glass plate. Several parameters influencing the emission were varied, such as the type of gas (air, nitrogen and helium), gas pressures (ranging from 2 up to 760 Torr) and laser power density. It was found that Hα emission with a narrow spectral width occurs with high efficiency when the laser plasma is produced in the low-pressure region. It was also confirmed that the conventional well-known laser-induced breakdown spectroscopy (LIBS), which usually carried out at atmospheric air pressure, cannot be applied for the analysis of hydrogen as impurity. This specific characteristic of the pressure dependence of hydrogen is interpreted based on our shock wave model, taking account of the fact that the hydrogen mass is extremely light compared to that of the host elements.

Hydrogen analysis in solid samples using laser-induced helium plasma at atmospheric pressure

A special technique for the modification of laser-induced breakdown spectroscopy (LIBS) has been developed to improve the spectral quality of hydrogen emission from a solid sample in helium gas at atmospheric pressure. In this technique, the plasma was generated by focusing a fundamental Nd-YAG (yttrium aluminum garnet) laser into a surrounding helium gas. The helium atoms excited to their metastable states would then serve to excite the atoms of the solid material vaporized by using another Nd-YAG laser. When properly synchronized, the resulting hydrogen emission line of H I 656.2 nm shows a dramatic improvement of the emission intensity and the spectral quality over what was obtained by conventional LIBS technique. This study further reveals that this improvement is mainly due to the role of the metastable excited state in a helium atom, which allows the delayed detection to be performed at a favorable moment when the charged particles responsible for the strong Stark broadening effect in the plasma hav...

Atomic Hydrogen Emission Induced by TEA CO2 Laser Bombardment on Solid Samples at Low Pressure and its Analytical Application

Hydrogen emission has been studied in laser plasmas by focusing a TEA CO2 laser (10.6 μm, 500 mJ, 200 ns) on various types of samples, such as glass, quartz, black plastic sheet, and oil on copper plate sub-target. It was found that Hα emission with a narrow spectral width occurs with high efficiency when the laser plasma is produced in the low-pressure region. On the contrary, the conventional well-known laser-induced breakdown spectroscopy (LIBS), which is usually carried out at atmospheric air pressure, cannot be applied to the analysis of hydrogen as an impurity. By combining low-pressure laser-induced plasma spectroscopy with laser surface cleaning, a preliminary quantitative analysis was made on zircaloy pipe samples intentionally doped with hydrogen. As a result, a good linear relationship was obtained between Hα emission intensity and its concentration.

Hydrogen analysis of zircaloy tube used in nuclear power station using laser plasma technique

It is shown that remarkable improvements essential to a quantitative spectrochemical analysis of hydrogen emissions from the zircaloy samples were achieved when the low-pressure surrounding air used in the previous experiment of Nd-YAG laser-induced shockwave plasma was replaced by an inert gas. Using the high-purity (99.999%) nitrogen gas at 1.5 Torr, a linear calibration curve of the HI 656.2 nm emission line was obtained with a zero intercept from the zircaloy samples prepared with various hydrogen concentrations. Further, when the surrounding nitrogen gas was replaced by a helium gas, more than an order of magnitude enhancement was obtained on the signal-to-noise ratio, yielding a detection limit of less than 5 ppm.