Yuko Hirohata

Properties of silicon nitride films prepared by magnetron sputtering

Abstract Numerous silicon nitride films were prepared using different target materials (Si 3 N 4 or Si) and different discharge gases (Ar or N 2 ). In the case with an Si 4 N 4 target and Ar gas pressure (1 Pa), the N-to-Si atomic ratio of the film was stoichiometric, 1.33, even when the substrate temperature was changed considerably. When the As gas pressure was increased from 1 7 Pa, the nitrogen content increased while the deposition rate decreased. In the case with an Si 3 N 4 or Si target and N 2 gas, the nitrogen content increased while the deposition rate decreased with the increase in N 2 pressure. The change in deposition rate due to the pressure was investigated using optical emission spectroscopy. X-ray photoelectron spectroscopy analysis showed that the binding energy is the same if the N-to-Si ratio was in the range from 1.33 to 1.70. It was found by X-ray diffraction analysis that crystallization occured when the substrate temperature was higher than about 1173 K.

Preparation of B4C-mixed graphite by pressureless sintering and its air oxidation behavior

Abstract B4C-mixed graphites with a boron concentration ranging from 3 to 32 mass% were prepared by pressureless sintering. For fabrications of these materials, a mixture of mesocarbon-microbeads and boron carbide powders was used. Final heat treatment temperature employed was around 2000 °C. The bulk densities of the materials were from 1.78 to 1.85 g/cm3, and the flexural strength was in the range of 44–87 MPa. These materials have a good machinability with shore hardness less than 71. Oxidation loss by air of this material with a total boron concentration more than approx. 10 mass% appeared to be almost completely suppressed at temperatures below 800 °C, where a little weight gain was observed due to the formation of boron oxide. In the temperature range from 900 to 1300 °C, appreciable oxidation loss was observed and the rate decreased as boron concentration increased.

Gas permeability of SiC/SiC composites as fusion reactor material

Permeability of helium gas in SiC/SiC composites material, which is one of the most important properties in application of SiC/SiC composite for first wall and blanket of fusion reactors, was studied by using a vacuum apparatus. Three tubular and two flat plate SiC/SiC composites were prepared by different preparation processes. The measurement of permeability coefficient of helium gas was carried out with pressure ranging from 10 2 to 10 5 Pa at room temperature. The permeability coefficient of the SiC/SiC composite largely depended on the preparation method. In three tubular materials, the SiC/SiC composite made by both polymer impregnation and pyrolysis (PIP) and melt infiltration (MI) methods showed the lowest permeability, 9.1 x 10 -7 m 2 /s, which was approximately two orders of magnitude smaller than one of the material made only by PIP method. The permeability of the flat plate SiC/SiC composites made by both liquid phase sintering (LPS) and hot pressing (HP) was approximately 1.5 x 10 -9 -4.0 x 10 -11 m 2 /s. The difference of permeability was related to the microscopic structure, i.e. pores and cracks.

Deuterium retention of DIII-D DiMES sample

Abstract The deuterium retention property of B4C converted graphite and isotropic graphite exposed to DIII-D deuterium plasma was examined by using a technique of thermal desorption spectroscopy. Major outgassing species were HD, D2 and CD4 in both the graphite and the B4C. In the case of the graphite, the ratios of deuterium desorbed in the forms of HD, D2 and CD4 to the total desorption amount of deuterium were 40%, 27% and 33%, respectively. In the case of the B4C, which was covered by carbon due to redeposition, these ratios were similar to those of the graphite. In a thermal desorption spectrum of deuterium, three desorption peaks appeared in both the graphite and the B4C covered by the redeposition layer. At low temperature region, the desorption rate of deuterium for the B4C covered by the redeposition layer was larger than that of the graphite. From two dimensional distribution of deuterium retention, it was seen that the retained amount at the electron drift side was quite large. The amount at the ion drift side and the edge of inward major radius was also observed to be large. The average retained amount of the graphite was almost the same as that of the B4C covered by the redeposition layer.

Hydrogen and helium retention properties of B4C and SiC converted graphites

Abstract The hydrogen or helium retention properties of B 4 C converted graphite, SiC converted graphite and isotropic graphite were examined, by the irradiation of a hydrogen or helium ion, followed by a thermal desorption spectroscopy measurement. The energy of the hydrogen or helium ion was 1.7 or 5.0 keV, respectively. For both B 4 C and SiC, two peaks appeared in the H 2 desorption spectra. The lower temperature peaks of B 4 C and SiC correspond to detrappings of B–H and Si–H bonds, respectively. It is considered that the higher peak is due to detrapping of the C–H bond. The amounts of retained hydrogen at room temperature (RT), obtained from the H 2 desorption spectra, were 6.71×10 17 , 1.0×10 18 and 5.8×10 17 H cm −2 for isotropic graphite, B 4 C and SiC, respectively. For the He desorption spectrum in the case of B 4 C, two peaks appeared at 300 and 950°C. For SiC, a sharp desorption peak was observed at 850°C, in addition to the low temperature peak at ≈300°C. For graphite, the spectrum had a single peak at 300°C. When the helium fluence was increased, this peak temperature shifted to the higher temperature region. The amount of retained helium in B 4 C, SiC and isotropic graphite at RT were 2.7×10 17 , 4.2×10 17 and 3.4×10 17 He cm −2 , respectively. It is seen that the trapped amount of helium is not negligibly small.

Oxide layers on GaAs prepared by thermal, anodic and plasma oxidation: In-depth profiles and annealing effects

Abstract The in-depth profiles of oxide layers prepared by thermal, anodic or plasma oxidation were investigated by means of simultaneous Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) measurements. With the AES measurements it was found that the oxides grown by thermal oxidation consisted only of Ga2O3; a large pile-up of arsenic was found near the interface between the oxide layer and the substrate. In contrast, oxides formed by anodic and plasma oxidation contained both Ga2O3 and As2O3 and showed a much smaller pile-up of arsenic. The annealing of as-grown oxide layers at relatively low temperatures (around 300°C) caused the oxide composition to become more uniform with depth and made the interface thinner. Annealing at relatively higher temperatures gave rise to a deficiency of arsenic in the oxide and created a pile-up of arsenic near the interface. Preliminary X-ray photoelectron spectroscopy measurements revealed the presence of unoxidized arsenic in the sputterer surfaces of anodic and plasma oxides. The SIMS measurements revealed that the relative intensity of the secondary ions varied in a complicated way in the oxide and near the interface, where no appreciable changes in composition were observed by AES. The SIMS results suggest the existence of different oxides having different physiochemical properties; these differences probably alter the ionization yield and/or the process of emission of the secondary ions.

Effect of mixing of hydrogen into nitrogen plasma

Abstract In surface nitriding by plasma, a major concern is to enhance the density of reactive species of nitrogen. One method is to mix a gas with an ionization potential lower than that of nitrogen. The hydrogen gas mixing was carried out in an electron cyclotron resonance (ECR) nitrogen plasma. Relative density of molecular ion in the vicinity of a substrate was measured by an optical emission spectroscopy. Under a fixed discharge pressure, the densities of nitrogen molecular ion (N2+) and excited molecular nitrogen (N2∗) were observed: they have a maximum, when a ratio of hydrogen pressure to nitrogen pressure was 0.5. Silicon nitriding was also conducted by using the nitrogen–hydrogen mixed plasma. In the case of maximum densities of reactive species, silicon nitriding was most effective.

Hydrogen retention properties of polycrystalline tungsten and helium irradiated tungsten

Abstract The hydrogen retention properties of a polycrystalline tungsten and tungsten irradiated by helium ions with an energy of 5 keV were examined by using an ECR ion irradiation apparatus and a technique of thermal desorption spectroscopy, TDS. The polycrystalline tungsten was irradiated at RT with energetic hydrogen ions, with a flux of 10 15 H cm −2 s −1 and an energy of 1.7 keV up to a fluence of 5×10 18 H cm −2 . Subsequently, the amount of retained hydrogen was measured by TDS. The heating temperature was increased from RT to 1000°C, and the heating rate was 50°C min −1 . Below 1000°C, two distinct hydrogen desorption peaks were observed at 200°C and 400°C. The retained amount of hydrogen was observed to be five times smaller than that of graphite, but the concentration in the implantation layer was comparable with that of graphite. Also, the polycrystalline tungsten was irradiated with 5 keV helium ions up to a fluence of 1.4×10 18 He cm −2 , and then re-irradiated with 1.7 keV hydrogen ions. The amount of retained hydrogen in this later experiment was close to the value in the case without prior helium ion irradiation. However, the amount of hydrogen which desorbed around the low temperature peak, 200°C, was largely enhanced. The desorption amount at 200°C saturated for the helium fluence of more than 5×10 17 He cm −2 . The present data shows that the trapping state of hydrogen is largely changed by the helium ion irradiation. Additionally, 5 keV helium ion irradiation was conducted on a sample pre-implanted with hydrogen ions to simulate a helium ion impact desorption of hydrogen retained in tungsten. The amount of the hydrogen was reduced as much as 50%.

Material probe analysis for plasma facing surface in the Large Helical Device

Material probes have been installed at the inner walls along the poloidal direction in large helical device (LHD) from the first experimental campaign. After each campaign, the impurity deposition and the gas retention have been examined to study the plasma surface interaction and the degree of wall cleaning. In the 2nd campaign, the entire wall was thoroughly cleaned by glow discharge conditioning and the number of main discharge shots increased. For the 3rd and 4th campaigns, graphite tiles were installed over the entire divertor strike region, and then the wall condition was significantly changed compared with the case of a stainless steel (SS) wall. It was seen that graphite tiles in the divertor were eroded mainly during main discharges, and the SS first wall mainly during glow discharges. During main discharges the eroded carbon was deposited on the entire wall. A reduction of metal impurities in the plasma was observed, which corresponds to the carbonized wall. The deposition thickness was great at the wall far from the plasma. Since the entire wall was carbonized, the amount of discharge gases retained such as H and He became large. In particular, helium retention was large at a position close to the anodes used for helium glow discharge cleanings. One characteristic of the LHD wall is a large retention of helium since the wall temperature is limited to below 368 K. In order to reduce the recycling of the discharge gas, wall heating is necessary.

Hydrogen retention of B4C converted graphite

The hydrogen retention of B 4 C converted graphite was investigated by using ECR ion irradiation apparatus. The retention properties were examined by a technique of thermal desorption spectroscopy (TDS) after the hydrogen ion irradiation. The H 2 desorption peaks appeared both at 350 and 700°C. The former peak was regarded as the detrapping from B-H bondings, and the latter from C-H bondings in the B 4 C layer. The CH 4 desorption peaks appeared both at 250 and 700°C. Compared with the case of graphite, the hydrogen retention of B 4 C rapidly decreased with the temperature, since the detrapping of B-H bondings occurred around at 300°C. The total amount of trapped hydrogens in the B 4 C was 1.5 times larger than that of the graphite. The activation energies of H 2 and CH 4 desorptions of the B 4 C were considerably smaller than those of the graphite. The hydrogen retention of the B 4 C was largely reduced by the He + ion bombardment after the hydrogen ion irradiation. In particular, the reduction ratio for the hydrogens trapped by boron was observed to be large