Kenji Ichimura

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.

Absorption and desorption of hydrogen, deuterium, and tritium for Zr–V–Fe getter

Nonevaporable getters have wide applicability for developing the tritium handling techniques for thermonuclear fusion devices. From this viewpoint, mechanisms of the absorption and desorption of hydrogen isotopes and the isotope effects were investigated for a Zr–V–Fe alloy (St‐707) by means of the mass analyzed thermal desorption spectroscopy. It was observed that the absorption rate was proportional to the first power of the pressure, indicating that the rate limiting step is the dissociative adsorption of hydrogen isotopes on the surface. The activation energy was very small, in the order of magnitude of a few tens of calories per mole in a temperature range from −196 to 200 °C. The desorption rate was proportional to the square of the amount of absorption, indicating that the rate limiting step is the associative desorption reaction of hydrogen atoms or ions diffused to the surface from the bulk. The rate constants for hydrogen and deuterium were determined as kd(H2)=(5.3+2.6−1.7)exp[−(28.0±0.7)×103/R...

Alloying effect on the activation processes of Zr‐alloy getters

Gettering materials, most of which are alloys, have wide applicability to tritium processing functions in fusion and fission reactors such as storage, supply, recovery, separation, and so on. However, required getter properties differ depending on unit process conditions. To develop suitable getters for each unit process, it is important to investigate fundamentals of alloying effects on their properties. Therefore, we studied the activation processes of three Zr‐alloy getters (Zr–Al, Zr–Ni, and Zr–V–Fe) by means of x‐ray photoelectron spectroscopy–secondary ion mass spectrometry and thermal desorption spectroscopy. It was observed that the formation of a metallic Zr surface is the principal process for activation, by which the getters show pumping action for various gases. A considerable alloying effect was observed on the activation processes of the three getters. The activation temperature varied with alloying elements: 800 °C for Zr–Al, 700 °C for Zr–Ni, and 400 °C for Zr–V–Fe. It was concluded that t...

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...

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.

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.

Recovery and Storage of Tritium by Zr-V-Fe Getter

The rates of ab/desorption of water vapor for zr-V-Fe getter were investigated by means of mass analyzed thermal desorption spectroscopy. The absorption rate obeyed first order kinetics with respect to the pressure of water vapor. The activation energies for absorption were determined as 1.8 (H/sub 2/O), 2.7 (D/sub 2/O), and 3.2 (T/sub 2/O) kcal/mol. Only hydrogen was desorbed by heating the getter in which water was absorbed. The desorption obeyed second order kinetics with respect to the amount of absorption. The activation energies for desorption were determined as 28.0 (H/sub 2/O), 28.6 (D/sub 2/O), and 29.3 (T/sub 2/O) kcal/mol. It is concluded that the rate determining step for absorption is the dissociation reaction of adsorbed water molecules or hydroxyl groups on the surface. The rate determining step for desorption is the association reaction of hydrogen atoms which diffuse from the bulk to the surface.

Absorption/desorption of hydrogen isotopes and isotopic waters by Zr-alloy getters

Zr‐alloy getters have been applied to tritium handling and vacuum conditioning for fusion devices. Some of their properties, however, should be improved to apply them in future devices. From this viewpoint, we have studied the effects of alloying on the getter properties of Zr alloys. We found that the activation energy of absorption and desorption of hydrogen varied considerably with alloying. The activation energy for hydrogen absorption was 0.74 for Zr61Al39, 0.01 for Zr57V36Fe7, 0.63 for Zr67Ni33, and 2.8 kcal/mol for Zr85Ni15, whereas that for Zr was 2.6 kcal/mol. The heat of hydrogen absorption was 27.8 kcal/mol for Zr: it changed with alloying as 32.0–33.4 (Zr61Al39), 27.8–28.4 (Zr57V36Fe7), 29.0 (Zr67Ni33), and 28.0 (Zr85Ni15). In addition, the ratio of the pumping speed of water vapor to that of hydrogen at room temperature varied with alloying element: for example, 1/40 for Zr57V36Fe7 and 1/4 for Zr67Ni33. The alloying effects mentioned above are considered due to modification of the electronic ...