Kan Ashida

Thermal desorption of hydrogen, deuterium and tritium from pyrolytic graphite

Abstract Thermal release of hydrogen, deuterium and tritium implanted into a pyrolytic graphite was studied by means of mass analyzed thermal desorption spectroscopy along with surface characterization by X-ray photoelectron spectroscopy. Hydrogen (or its isotopes) ions were implanted into the sample at room temperature with an applied voltage of 5 kV using a conventional ion gun. Subsequently, the sample was heated to 900 ° C with various temperature ramps to measure the thermal desorption spectra. The implanted hydrogen (or its isotopes) was predominantly desorbed as H 2 and in small amount as CH 4 . The desorption spectra of H 2 changed gradually while repeating the implantation-desorption cycles and became reproducible after the total dose amounting to 1 × 10 19 ion/cm 2 , indicating that the virgin graphite is modified due to formation/accumulation of radiation damage. For the modified graphite, three desorption peaks were observed. The first peak is attributed to the desorption of hydrogen atoms trapped on the carbon atoms in the normal graphite lattice. The others correspond to differently trapped hydrogen atoms in the graphite. The desorption of the first peak obeyed the second order kinetics with respect to the amount of the implantation, indicating that the rate determining step is the surface association reaction of the hydrogen atoms. The activation energy was estimated as 44 kcal/mol for three hydrogen isotopes. However, the isotope effect appeared on the frequency factor: their ratio was estimated as H 2 :D 2 :T 2 = 3:1.5:1 . The desorption of methane obeyed the pseudo-first order kinetics with an activation energy of 38 kcal/mol.

Trapped states of deuterium implanted into graphite and the thermal desorption

Abstract Chemical sputtering of graphite and the retention of hydrogen are important subjects for thermonuclear fusion devices. We measured thermal desorption processes of deuterium implanted into graphite at room temperature by means of the flash desorption spectroscopy along with surface characterization by XPS and SIMS. Chemical shift of the Cls peak was observed by XPS due to the deuterium-ion implantation. In SIMS spectra, CD± and C2D− signals appeared after the implantation. Based on these observations, it was concluded that C-D and/or C2-D species were formed on the surface due to the implantation. The implanted deuterium desorbed as D2 and CD4 by flashing the sample above 500°C. The fraction of the desorbed amount of CD4 strongly depended on the flashing mode. Moreover, no methane appeared after the irradiated sample was annealed above 500°C. It was concluded that a part of the C-D and/or C2-D species associated each other in the graphite during the flash desorption to form molecular deuterium which presumably adsorbed on carbon atoms.

TWO DIFFERENT SPECIES OF DEUTERIUM IMPLANTED INTO A PYROLYTIC GRAPHITE OBSERVED BY XPS-SIMS.

Trapped states of deuterium implanted into a pyrolytic graphite with 5 keV were investigated by means of the x‐ray photoelectron spectroscopy and the secondary ion mass spectrometry. Chemical shift and broadening of C 1s peak were observed in the XPS spectra due to deuterium‐ion implantation at room temperature. The shift and the broadening increased with the deuterium fluence to reach constant values. In the SIMS spectra, CD− and C2D− peaks appeared due to the deuterium implantation. The intensity of the CD− peak increased with the fluence to reach a constant value, whereas that of the C2D− peak passed through a maximum to decrease to a constant value. A linear relation was observed between the chemical shift and the CD− peak. The chemical shift and the CD− peak disappeared after annealing the sample at 600 °C for 5 min, whereas the broadening of the C 1s peak and the C2D− peak disappeared after annealing the sample at 900 °C for 5 min. On the basis of these observations, it is concluded that at least tw...

Catalytic inactivities of Ni alloys expressed by surface and bulk compositions

The surface compositions of three kinds of powdered alloys with different structures, CuNi (fcc-fcc), FeNi (bcc-fcc), and CoNi (hcp-fcc), were determined by the penetration of β-rays of 63Ni mixed previously in the alloy, then the effect of the respective alloy structure both of the surface and of the bulk on the catalytic hydrogenation of ethylene was studied. The surfaces of these alloys were found to be Ni-poor for the wide range of alloy composition. The catalytic activity expressed by each surface composition was difficult to explain by the physicochemical properties of the surface. The activity expressed by the bulk composition of CuNi of fcc structure was high in the region of unfilled d-band, and so were that of FeNi and that of CoNi in the region of fcc structure. The activity of FeNi for the dimerization of 14C-ethylene was studied, of whose activity the similar tendency was observed as found in the hydrogenation reaction. These could be interpreted as the results of the positive contribution of the dissolved hydrogen in bulk to the catalytic reactions.

Surface characterization of a Zr–V–Fe getter by XPS‐SIMS—activation process and D2O exposure

To apply hydrogen storage materials (getters) to tritium recovery and storage, it is important to understand the activation process of the getters and their pumping characteristics not only for tritium gas, but also impurity gases such as HTO. From this viewpoint, the activation process of the Zr–V–Fe getter (St‐707, SAES Getters) and absorption process of D2O were investigated with XPS‐SIMS. An as‐received getter surface was observed to be covered with H2O, CO, O2, and hydrocarbons. In addition, the getter components formed respective oxides on the surface. The activation, which consisted of vacuum heating above 500 °C, caused the disappearance of the adsorbed impurities from the surface, forming a metallic surface consisting of Zr(87 at.%) and V(13 at.%). Exposure to D2O vapor at 25 °C resulted in the adsorption of D2O as D2O(a) and/or OD(a). In addition, a part of the surface was oxidized. Elevation of the exposure temperature to 300 °C caused the disappearance of the D2O(a) and/or OD(a). Consequently ...

Activation process and absorption/desorption of D2O for Zr-V-Fe getter

Abstract Nonevaporable getters have wide applicability for tritium handling systems. From this view point, the activation process of the Zr-V-Fe getter (St-707) and absorption/desorption of D 2 O on the getter surface were investigated, by means of XPS-SIMS and mass analyzed thermal desorption spectroscopy. XPS-SIMS measurements revealed that the getter surface exposed to air was covered with adsorbed H 2 O, CO and small amounts of hydrocarbons and that the getter components are oxidized. Upon heating of the getter above 500 °C, the adsorbed species disappeared from the surface, partly due to desorption and partly due to migration into the bulk. Consequently, metallic Zr and V appeared on the surface, whereas Fe disappeared. The surface composition was evaluated to be 87 at% Zr-13 at% V. After the activation, water (D 2 O ) was readily absorbed into the getter at 300 °C in the form of deuterium atoms. The absorption rate was proportional to the partial pressure of water, indicating that the rate determining step for the absorption is the dissociation of water molecules on the surface. The absorption rate constant was 0.009 and 0.24 cc/s/cm 2 (net surface area) [or 1.5 and 39 cc/s/cm 2 (projected area)] at 25 and 300°C, respectively. Only D 2 was desorbed from the getter exposed to D 2 O at 25 and 300 °C. The rate determining step for the desorption is association of deuterium atoms on the surface diffused from the bulk.

Surface characterization of various graphites by x-ray photoelectron, secondary ion mass, and Raman spectroscopies

Graphite is the primary candidate for the first wall of magnetically confined fusion devices. For this purpose, it is important to know the surface properties of graphite to understand the plasma–surface interactions as well as vacuum properties of graphite. From this viewpoint, we examined the binding states of carbon atoms, inherent hydrogen content, and crystallinity of the surfaces of isotropic graphites prepared by several Japanese companies as well as anisotropic ones with x‐ray photoelectron (XPS), secondary ion mass (SIMS), and Raman (RS) spectroscopies. Although no measurable difference in the binding state of carbon atoms was detected among the isotropic and anisotropic graphites with XPS, RS revealed that their crystallite sizes differed from each other. Namely, the crystallite sizes of the isotropic graphites were in the range from 100 to 300 A, whereas those of the anisotropic graphites were more than ∼1000 A. In addition, nongraphitized carbon which was not observed for the anisotropic graph...

Permeation, diffusion, and solution of hydrogen isotopes, methane, and inert gases in/through tetrafluoroethylene and polyethylene

Permeation of three hydrogen isotopes, methane, and inert gases were measured for a tetrafluoroethylene (TFE) and a polyethylene (PE) film at room temperature by means of the time‐lag method in order to establish physicochemical properties which determine the solubility and the diffusivity of these gases. It was observed that the diffusion constant of the inert gases changed exponentially with the heat of vaporization, and their solubility constant was an exponential function of the Lennard‐Jones force constant of the gases for both films. On the other hand, hydrogen isotopes and methane deviated from these relations. It is concluded that the dispersive force plays an important role for the solution and the diffusion of the inert gases, whereas chemical interactions between the solute and the solvent play an important role.

Gettering of Hydrogen Isotopes by Zr-Ni Alloys

Activation process and absorption/desorption of hydrogen isotopes were studied for Zr-Ni alloys by means of XPS-SIMS and thermal desorption spectroscopy. Alloying of Ni to Zr gave rise to considerable modification of the getter properties: it caused the changes in the activation temperature, the activation energy for hydrogen absorption, the heat of absorption, selective pumping property, and the kinetic isotope effect. The results indicate that one can develop Zr-Ni getters applicable to the various unit processes in the fuel handling systems of thermonuclear reactors.

Diffusion constants of tritium in graphites and compensation effect

The diffusion constants of tritium and/or its isotopes for graphite so far reported scatter considerably in the Arrhenius plot diagram. This, however, has not been seriously considered, despite that the understanding of the scattering is indispensable to estimate the tritium inventory and permeation in/through the graphite first walls of D-T burning experimental deviees. We show in the present paper that there is regularity in the scattering: namely, the presence of a linear relation between the logarithms of the pre-exponential factors and the activation energies (the compensation effect). The temperature dependence of the diffusion constants and the compensation effect were analyzed by assuming the presence of three distinct diffusion channels in graphite: channels along the a- and c-axis in a graphite grain and along the grain boundary. Computational simulations could reproduce fairly well the temperature dependence of the experimental data and the compensation effect by assuming proper effective pre-exponential factors for the respective channels, where the activation energies for the respective channels were chosen from the observed values. Namely, the scattering of the diffusion constants and the compensation effect can be interpreted as a consequence of the multi-diffusion channels.