Toshiro Yamashina

The deposition rate of metallic thin films in the reactive sputtering process

Abstract A model is presented giving the deposition rate during the reactive sputtering of metals in atmospheres of oxygen and nitrogen in an argon plasma. On the basis of the difference between the sputtering rates of the metal and its chemical compound, the surface coverage, the sticking probability and the incident flux of reactive gas atoms, a semi-empirical equation has been derived. Computer calculations have been made to determine the specific constant values for several reactive sputtering processes such as MoO2, MoN2, TiO2 and TiN2. In each experimental observation an abrupt decrease of the sputtering rate was found at a particular pressure of reactive gas. The model might be applied successfully for sputtering processes over a wide pressure range of the reactive gas.

A quantitative analysis of surface segregation and in-depth profile of copper-nickel alloys

Abstract By utilizing the difference in escape length of the Auger electrons with different energies, a calculation of the in-depth distribution of atomic concentration at the surface is presented on the basis of Palmbergs physical mechanism. Experimental results on clean surfaces of Cu-Ni alloys over the entire composition region with Auger spectroscopy were performed to make the in-depth profile of surface composition caused by annealing. The alloy composition of the first atomic layer at the surface was plotted against the bulk composition, showing significant enric enrichment of Cu atoms in that layer. The results indicated that the segregation takes place in about four atomic layers at the surface.

An AES study of surface segregation of AgAu alloys with ion bombardment and annealing

Abstract Equilibrium segregation and selective sputtering in the surface of AgAu alloys have been investigated systematically with argon ion bombardment and with annealing by means of AES measurements. Slight enrichment of Ag was observed on the alloy surfaces after the annealing of the alloys at 550°C, while relatively large enrichment of Au was observed on the ion-bombarded surfaces with the use of Au (240 eV) and Ag (300 eV) Auger electrons. With the aid of other Auger electrons with different escape lengths, it was found that the concentration varies with distance from the surface within the sampling depth of the Auger electrons. On the basis of the above facts, the depth profiles were proposed for the annealed and the ion-bombarded surfaces. The uppermost surface layer is enriched more with Ag than the apparent AES observations on both the ion-bombarded and the annealed surfaces. The proposed depth profiles on both the surface layers were compared with previous results by different authors.

SiC coatings for first-wall candidate materials by R.F. sputtering

Abstract The r.f. sputtering technique was applied to form SiC coating films on first-wall candidate materials such as molybdenum, stainless steel and pyrolytic carbon at various temperatures. The coating films were examined by means of scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy and roughness factor (RF) measurements. It was found that the coating films consisted of α-SiC and grew at relatively low temperatures, namely 300 °C, 600 °C and 800 °C on molybdenum, 304 stainless steel and pyrolytic carbon surfaces, respectively. The surfaces of α-SiC films grown above these temperatures were relatively smooth with small RFs in comparison with those prepared at lower temperatures. Lower temperature deposition gave rise to amorphous and rough coating films with considerably larger RFs.

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.

Sputtering process of a silicon carbide surface with energetic ions by means of an AES-SIMS-FDS combined system

Abstract Surface phenomena on silicon carbide following interaction with energetic hydrogen ions and argon ions have been studied by means of simultaneous, in situ measurements with a combined system of AES-SIMS-FDS (Flash Desorption Spectroscopy). Bombardment by 0.7 and 1.5 keV argon ions was observed to sputter the surface atoms, both silicon and carbon, with the same sputtering yields. In the case of bombardment by hydrogen ions, on the other hand, silicon atoms were sputtered out preferentially through chemical sputtering to form silicon hydrides at room temperature. In-depth composition profiles of silicon carbide irradiated by 100-keV D + ions were also examined by the combined system.

Plasma-surface interactions of graphite as nuclear fusion material

Abstract As nuclear fusion devices are being scaled to larger size with substantially increased plasma stored energy, the engineering tasks associated with plasma-surface interactions of the plasma facing material referred to as the first wall have become increasingly important. In large tokamaks, the energy confinement of the plasma has been substantially improved by the use of graphite or carbon material. Graphite, however, has several disadvantages, e.g. large capacity to absorb gases due to its porous structure and a large erosion rate due to bombardment by hydrogen ions. Therefore, various problems must be solved before graphite can be used as the first wall material of next generation fusion devices. In this paper, we first explain plasma-surface interactions. In particular, the problem associated with impurities is described. We than show the results of our characterization study for several kinds of graphite as the first wall material. These include (1) surface and vacuum properties of graphites, i.e. gas desorption and micro surface area (effective surface area), and (2) interactions of graphite with hydrogen ions, i.e. chemical sputtering, radiation-enhanced sublimation and hydrogen retention. Required conditioning of, and modifications to graphite intended for use as the first wall material are also discussed.

Desorption processes of hydrogen and methane from clean and metal-deposited graphite irradiated by hydrogen ions

Abstract Thermal desorption processes of H 2 and CH 4 from clean graphite surfaces and metal-deposited graphite surfaces have been investigated. Thin metal layers of Ti, Cr and Fe on isotropic graphites were prepared by an electron-beam evaporation method. For these specimens, the irradiation and the desorption experiments were performed with the use of an ECR ion irradiation apparatus. The desorption spectra of H 2 and CH 4 from the clean surface had only a single peak at 700 and 800 °C. These peaks were found to originate from the diffusion limited process and to be attributed to the intrinsic nature of graphite. The metal depositions on the graphite surface caused a new desorption peak for H 2 and CH 4 besides the intrinsic peak. The temperature and intensity of the new peak depended on the metal species. The presence of the metals increased the amount of desorbed H 2 for fluences of more than 1 × 10 18 H / cm 2 but decreased the amount of desorbed CH 4 in the whole fluence range of this study. Fe was most effective to reduce the CH 4 desorption among the three metals.

Overall evaluation study for isotropic graphite as fusion first wall material in japan

Abstract Isotropie graphite has been widely used as first wall material in present large fusion devices. For isotropic graphites with different properties, however, overall evaluations with respect to vacuum engineering properties, thermal-mechanical properties and interations with plasmas have not been performed systematically. In 1986, under the support of the Ministry of Education, Science and Culture, the “Graphite Project Team” was organized. Fifteen institutions participated in this project and eighteen isotropic graphites supplied from seven graphite manufactures of Japan were studied as “common samples”. From each company, both high- and low-density graphites were supplied since it was presumed that the vacuum engineering and thermal-mechanical properties depended on the density. During an approximately two years research period, we have obtained several interesting results on surface roughness, gas desorption, hydrogen permeation, failure due to heat load and fracture toughness. It was found that the vacuum engineering properties such as the surface area, the gas desorption and the hydrogen permeation depended significanly on the pore structure of the graphite. The surface area increased with the bulk density and the hydrogen permeation rate decreased with the bulk density. The gas desorption was very small for the graphite baked in vacuum. Treated in the same way, the amount of gas released from low-density graphite was smaller. The ash content of the graphite could be reduced to ppm levels by halogen gas treatment. The heat load experiments showed that most of the graphites failed at roughly the same heat load. The measured value of the fracture toughness was approximately the same. The change of the surface morphology by hydrogen ion irradiation and the desorption of trapped ions are also discussed.

Thermal desorption process and surface roughness of POCO graphite irradiated by hydrogen ion beam

POCO graphite samples are irradiated by H+3 ion beams with an energy of 9 keV, and the thermal desorption of methane and hydrogen are investigated by a thermal desorption spectroscopy (TDS). Surface roughness factors of the samples are also measured by the BET method. Most of the incident hydrogen ion is trapped in the graphite. The trapped hydrogen is desorbed in the form of H2 and CH4, and the ratio of the trapped hydrogen desorbed in the form of CH4 is (0.4–3)%. An activation energy of the methane is estimated to be 2.3 eV. In the TDS spectra of H2, two peaks of the desorption rate are observed. From the profile of the desorption rate, the surface density of hydrogen is estimated. The estimated activation energy is 1.2 eV. The heating temperatures corresponding to the peaks increase in the low fluence range and decrease in the high fluence range. The surface roughness is reduced with increasing hydrogen fluence. The relation of the surface roughness with the desorption rate is also discussed.