Haru-Hisa Uchida

Effect of oxygen sorption layers on the kinetics of hydrogen absorption by tantalum at 77– 700 K

Abstract Hydrogen absorption from the gas phase by tantalum is strongly impeded by oxygen sorption layers at temperatures below 700 K. The removal of these layers increases the absorption rate by several orders of magnitude. Experiments performed between 77 and 700 K using.specimens with and without such oxygen surface layers enabled us to identify the processes which determine the rate of hydrogen absorption at low temperature.

Surface phenomena in hydrogen absorption kinetics of metals and intermetallic compounds

Abstract Oxide layers on metal surfaces impede or prevent hydrogen absorption at temperatures below 400 °C. Passivation can be caused by slow dissociation of hydrogen molecules on the surface, slow transfer of chemisorbed hydrogen atoms into the oxide or slow permeation of hydrogen atoms through the oxide layer. Experiments on the hydrogen absorption kinetics of film samples with known oxygen precoverage yield direct information on reaction models if relevant results of other techniques are taken into account together with inherent thermochemical and atomistic limits. Typical examples are discussed to elucidate this approach for the investigation of complex reaction mechanisms in hydrogen absorption kinetics at room temperature. They indicate that hydrogen dissociation on the surface is a very important partial step in the reaction and plays a substantial role in activation and passivation of hydrogen absorption reactions at room temperature.

Kinetics of hydrogen absorption by titanium, tantalum, tungsten, iron and palladium films with and without oxygen preabsorption at 300 K

The hydrogen absorption rates of titanium, tantalum, tungsten, iron and palladium films 15–20 nm thick were measured at room temperature and hydrogen pressures between 10−9 and 10−4 mbar using the volumetric method. Films with clean surfaces absorb gas amounts in the concentration ranges corresponding to hydride formation for titanium and tantalum and equivalent to about one monolayer of hydrogen for tungsten, iron and palladium with reaction probabilities in the range 1–10−4. The reaction rates are strongly reduced if the film surface is coated with oxygen at thicknesses equivalent to several monolayers. Iron and palladium films precoated with oxygen show an H2O partial pressure peak which indicates that the oxygen sorption layer is reduced by the formation of H2O during exposure to hydrogen.

Degradation of gold-aluminium ball bonds by aging and contamination

The degradation of gold aluminium ball bonds has been studied by shear tests, resistivity measurements, micrographs, EDX and AES as a function of aging time at elevated temperatures and after various contamination treatments before and after bonding. Samples were subjected to ion etching treatment immediately before bonding in order to guarantee a comparable initial state. Incorporation of large amounts of Cl, Br, and F into the surface layer of the pad and other contamination treatments before bonding did not show detectable effects in subsequent annealing treatments as long as the bonding process itself has not been impeded by thick surface layers. The degradation process during annealing can be described as a three stage mechanism. Each stage can be clearly distinguished by the fracture behavior in the shear test, the morphology of the bond shown by the micrographs and by the change of contact resistivity.

The phase equilibria of Ce-H and Sm-H systems☆

Abstract The phase equilibria of the Ce-H and Sm-H systems were investigated. Hydrogen equilibrium pressures were measured in the range 1 × 104–5 × 106 Pa as a function of hydrogen concentration for [H] [M] in the range 2.2–2.9 and temperatures between 423 and 823 K. The measured isotherms for both systems are presented together with the isotherms previously measured for their dihydrides. The existence of cerium trihydride was not confirmed. The heat of solution of hydrogen in cerium dihydride shows a sudden change at [H] [Ce] = 2.7 which seems to indicate a change in the chemical bonding state of the dissolved hydrogen atoms. The isotherms of the Sm-H system indicate the existence of the samarium trihydride phase. A large hysteresis was found in the measured isotherms of this system. The value of the heat of formation derived for the trihydride is about half of that found for samarium dihydride.

Kinetics of hydrogen absorption of tantalum coated with thin films of palladium, iron, nickel, copper and silver

Abstract The effects of coatings of palladium, nickel, iron, copper and silver on the kinetics of hydrogen atom absorption of tantalum wires were measured in the temperature range 300–700 K. Thin palladium coatings were the most effective and yielded absorption rates that were near the theoretical value for a diffusion-controlled reaction mechanism. This corresponds to an enhancement factor of 104 or more at room temperature. Nickel and iron films increased the reaction rate by one to two orders of magnitude but copper and very thin silver films yielded no pronounced improvement of the small absorption rates of hydrogen on tantalum below 500 K.

Catalytic effect of nickel, iron and palladium on hydriding titanium and storage materials

Abstract Results of experiments on hydriding film samples of titanium and other metals, andtitanium films covered with overlayers of various metals, indicate that the slow dissociation of H2 molecules on film surfaces contaminated with O2 is a very important partial reaction step. A remarkable enhancement in H2 dissociation is observed if nickel, iron or palladium acting as catalysts are present on the surface of oxidized titanium or storage materials. This is demonstrated by experiments performed with titanium film samples using the volumetric Wagener technique or a piezoelectric quartz microbalance, and also by high pressure hydriding-dehydriding cycling tests with storage materials containing additions of palladium powder or an aluminium oxide suspended palladium catalyst. The development of storage materials with improved resistance against poisoning gas impurities in H2 seems therefore to be mainly a problem of surface modification and/or catalysis. This aspect of the problem is discussed on the basis of relevant effects which have been measured.

Kinetics of hydrogen absorption of titanium/metal (vanadium, chromium, manganese, iron, nickel) sandwich films with and without oxygen precoverage at 300 K☆

Abstract The hydrogen absorption kinetics of sandwich samples (40 nm of titanium, 3–7 nm metal overlayers) has been investigated with the volumetric Wagener method at 300 K and pressures below 10−4 mbar H2. The reaction rates are found to be proportional to pH which demonstrates that H2 dissocition is the rate-determining step. By stepwise-increased O2 preabsorption the effect of oxide surface layers has been studied. The interrelation between the reduction in the reaction rates for O2 and H2 absorption with increasing O2 preabsorption was found to be the same in all systems.

Effects of Oxidation and Hydroxidation on the Activations of LaNi5-xMx(M = Al,Mn,Co) Alloys by Hydrogen Gas*

The partial replacement of Ni with Al, Co and Mn in the LaNi5 based alloys reduces the incubation period of the initial activation and the reactivation of the oxidized alloy. The alloys containing Al exhibit increasing reactivities with H2 even in the progress of oxidation. Co in the alloys seems to stabilize the surface oxides-metal interfaces, resulting in the inhibition of pulverization. Prolonged oxidation of the alloys containing Mn yielded the markedly reduced the incubation time for activation. The pretreatment of the alloys in a KOH solution was found effective to accelerate the activation, and for which the excess concentrations of La and/or Ni in the surfaces seem to be responsible.

Reaction kinetics of H2 absorption by lanthanum with and without surface oxide layers

Abstract The kinetics of H2 absorption by lanthanum with and without surface oxide layers has been investigated at 77 and 298 K using a volumetric method. Lanthanum film samples carefully prepared under ultrahigh vacuum conditions exhibit an initial reaction probability of unity at 77 and 298 K in the formation of a hydrogen solution in lanthanum. The dihydride formation proceeds at an almost constant rate at 298 K until the sample becomes completely hydrided whereas the reactivity decreases as the hydride layer grows at 77 K. The pressure dependence of the reaction rate indicates that the rate-controlling step in the H2 absorption by lanthanum changes according to the surface conditions of lanthanum: for lanthanum with a clean surface, H2 transport in the gas phase is rate controlling at H2 pressures lower than 10−9 mbar and with increasing H2 pressure or hydrogen surface concentration H2 dissociation as a rate-controlling step is replaced by hydride formation at the hydride-lanthanum interface. For lanthanum with oxide layers, two rate-controlling steps, the H2 dissociation on and the hydrogen permeation through the oxide layers, seem to be important.