Naofumi Ohtsu

Helium and hydrogen trapping in W and Mo single-crystals irradiated by He ions

Abstract Retention of He and accumulation of H in the near surface layer of W and Mo single-crystals were studied during and after the implantation of He ions with 2–10 keV at 295 and 820 K. The He retention was saturated at a concentration of a He/metal ratio of about 0.25, depending on the implantation temperature. Subsequent He implantation caused H accumulation in the He saturated layer, up to a maximum concentration about equal to that of He. The initial H uptake rate just after the He irradiation was comparable to the impingement rate of the H2 or H2O molecule at the crystal surface from the residual gas. For the He irradiation at 820 K, blisters and exfoliation with large sizes were observed on the crystal surface, where impurities other than H and He were also enriched.

Ion beam analysis of helium and its irradiation effect on hydrogen trapping in W single crystals

Abstract Retention of He implanted into W single crystals and the He irradiation effects on H behavior were studied by ion beam analysis techniques. During implantation of 4 He+ with 2–10 keV at 295 K, an accumulation of H started in the He implanted layer when the retained He concentration saturated. For the crystal irradiated by 10 keV He+ at 820 K, a remarkable increase of H was found in the He saturated layer, after stopping the implantation and cooling down the crystal below 400 K. Though blisters and exfoliation were observed for the surface irradiated at 820 K, less lattice disorder was found in the implanted layer and the thermal release of H occurred at lower temperature, in comparison with the crystal implanted at 295 K.

Hydrogen and deuterium uptake in helium implanted layer of Mo and W

The accumulation of hydrogen isotopes in the helium (He) implanted layer of Mo and W single crystals was studied, focusing on the effects of the high dose irradiation of He ions and on uptake behavior of hydrogen (H) and deuterium (D) in various H 2 and D 2 pressure. A remarkable H uptake occurs in the He saturated layer of the crystals irradiated by 10 kev 4 He ions to a dose about 2 x 10 22 He/m 2 , after the heat treatment up to 850 K. The integrated amount of retained H reaches the same values before the thermal release procedure, though H-uptake rates depend on the gas pressure. The H is preferentially uptaken in the He saturated layer, when D 2 and/or D 2 O gas is introduced in the vacuum chamber. The large difference between the H- and D-uptake rates suggest the oxide formation at the crystal surface where the microstructure is significantly changed due to the He irradiation.

Hydrogen accumulation in the helium implanted surface of Mo single crystals

Abstract The up-take behavior of hydrogen and deuterium was studied for the surface layer of Mo single crystals irradiated by 10 keV He ions with a high dose, at which the blisters and exfoliation were formed on the surface. Hydrogen accumulation was found in the H saturated layer up to a maximum concentration of about 0.2 H/Mo at room temperature. Although the up-taken H concentration profiles and the morphological changes of the irradiated surface depended on irradiation temperatures, thermal release behavior of up-taken H were similar for both specimens irradiated at 295 and 820 K. The higher pressure of H 2 and/or H 2 O caused the quicker accumulation of H, and the hydrogen dissociation seemed to be sensitive to the surface contamination. The up-take rate for H was an order of magnitude larger than that for D, even by letting mainly D 2 and/or D 2 O into the vacuum chamber.

Hard-ceramic layer formed on Ti-29Nb-13Ta-4.6Zr and Ti-6Al-4V ELI during Gas Nitriding

The surface of Ti-29Nb-13Ta-4.6Zr (TNTZ) subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical titanium alloy, Ti-6Al-4V ELI (Ti64). After gas nitriding, the microstructures near the surface of these alloys were observed by optical microscopy, X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. In both alloys, two titanium nitrides (TiN and Ti2N) are formed and the α phase precipitated by gas nitriding. Furthermore, oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO2 is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.

Hydrogen Accumulation in Surface of Perfluorosulfonic Acid Membranes after γ-Ray Irradiation using Elastic Recoil Detection Techniques

Electrical conductivity measurements and elastic recoil detection (ERD) analysis were carried our on perfluorosulfonic acid membranes that were irradiated by 60Co γ-ray. Maximum electrical conductivity of irradiated specimen was obtained at 40–60 °C, though that of unirradiated specimen increased with increasing temperature. And 4000-fold higher conductivity was obtained in irradiated membrane at 40 °C under dry condition. ERD analysis indicated that hydrogen concentration of the irradiated membrane is 1.5-fold higher than that of the unirradiated specimen. It is considered that the γ-ray generates a lot of site that can trap hydrogen and the trapped hydrogen cannot easily move. As the results, irradiated membrane retains more hydrogen than unirradiated one and the electrical conductivity of irradiated membrane is higher than that of unirradiated membrane in any conditions. γ-Ray irradiation is effective for improving the electrical conductivity of proton-conducting membranes.

Ion-induced luminescence of silica glasses and optical fibers

Ion induced luminescence was studied for SiO2 glasses and SiO2 based optical fiber materials with different hydrogen and oxyhydrate concentration. The luminescence of the visible wavelengths was measured during the irradiation of protons and also heavier ions with low (5~10 keV) and high (0.2 ~ 2 MeV)energies, at a temperature rane between 295 and 600 K. Hydrogen concentration profiles were also examined by the ion beam analysis techniques to compare the nominal OH values. In addition to a prominent broad peak of 460 nm, characteristic peaks were detected at around 390 nm and 660 nm, depending on the OH contents. For fused silica specimens with lower OH, however, a peak at 390 nm was found at a small dose and its intensity decreased quickly with an increase of the ion dose. For synthesized silica with higher OH concentration, a small peak was found at 650 nm, corresponding to the non-bonding-oxygen-hole-center, while the 390 peak not appeared. Except for the low-OH synthesized silica, there existed a large amount of hydrogen, which does not form OH. The origin of the luminescence and the damage process will be discussed in connection with the nuclear and electronic energy loss by the penetrating energetic ions.

Re-emission of hydrogen implanted into graphite by helium ion bombardment

Abstract Helium ion-induced re-emission of hydrogen implanted in graphite with 5 keV H2+ ion beam at room temperature has been investigated in the energy range from 60 to 200 keV, by means of a high energy elastic recoil detection method with 16 MeV O5+ ion beam. The hydrogen concentrations in graphite rapidly decrease with increasing the He+ ion fluence and reach a constant value of H/C≅0.2. The re-emission rates of hydrogen from graphite increase with decreasing incident energy of the He+ ion. The He+ ion-induced detrapping cross-sections have been evaluated by analyzing the experimental data with mass balance equations including elementary processes such as detrapping, trapping and local molecular re-combination between an activated hydrogen atom and a trapped one. The elastic displacement collisions with energetic carbon recoils produced by He+ ion bombardments are dominant for the detrapping of hydrogen in the range of the incident energies.

Characterization and dissolution behavior in a physiological solution of heat-treated CaTiO3 thin films with different thicknesses

Characterization of heat-treated CaTiO3 thin films of 10, 20, 30 and 50 nm in thickness and their change after immersion in a simulated body fluid were investigated by grazing incident angle X-ray diffractometry and Auger electron spectroscopy (AES). The CaTiO3 films were prepared on titanium substrate by sputter-deposition of CaTiO3 target followed by heating in an electric furnace at 873 K in air for 7.2 ks. The CaTiO3 films were immersed in 0.8% NaCl solution for 14 d. All the films before heat treatment were non-crystallized films and after heat treatment, only the 50-nm film was crystallized to perovskite-type CaTiO3. In AES in-depth profiles after heating, Ca diffusion was not observed in the 50-nm film, whereas Ca diffusion toward the Ti substrate was observed in the 10-, 20- and 30-nm films. After immersion for 14 d, the vicinity of surface of the 10, 20 and 30 nm thick CaTiO3 films were dissolved into the NaCl solution, while the 50-nm thick CaTiO3 film was scarcely dissolved. Since dissolution from biomaterials in a human body has possibility to harm, the CaTiO3 film should be deposited more than 50 nm in thickness and heat-treated at 873 K.

Studies of behavior of hydrogen in fused silica by ion beam analysis technique

Retention and thermal release behavior of hydrogen isotopes in fused silica, synthesized silica and optical fibers were investigated by ion beam analysis technique. Initially contained H in the interior of the specimens is about 0.1~0.2 at.% at room temperature, irrespective of the nominal value of OH concentration. Besides, H atoms more than 1 x 1016H/cm2 was found at the surface. The thermal release of the H atoms from the interior was affected by re-trapping at the near surface. During 5 keV ion injection, the retained D in the implanted layer was quickly saturated with a concentration of about 1 x 1021D/cm3. Under the subsequent D injection to doses above 1 x 1018D/cm2, D atoms were trapped with a concentration about 1 at.% in the depth far beyond the projected ranges of D ions. Thermal release of D in the injected layer started at lower temperature than that from the larger depth for lower implantation dose, while the two release curves close to each other for the higher dose. Irradiation of 10 keV He ion into the fused silica caused H up-take in the He implanted depth, where no He atoms were retained.