Shizuo Naito

Oxidation of Porous Silicon in Dry and Wet Environments under Mild Temperature Conditions

Oxidation behavior of porous silicon under various environments of dry and wet air, and solution with and without appropriate oxidant at mild temperatures has been investigated. The progress of oxidation was followed by infrared spectroscopy. The presence of water vapor greatly accelerates the oxidation rate in comparison with the rate in dry air. The oxidized states are clarified with the help of oxidation experiments of partially hydrogen-desorbed porous silicon, which does not contain SiH2 and SiH3 as the hydride species. An oxidation mechanism is proposed to explain that oxidation is accelerated in the presence of water vapor and at the partially hydrogen-desorbed porous silicon. Further, oxidation behavior of porous silicon in solution containing appropriate oxidant is also investigated. The rate is very rapid and the oxidation does not produce the back-bond oxidized state of OySiHx in contrast to the oxidation in air.

Kinetics of hydrogen absorption by α‐zirconium

The kinetics of hydrogen absorption by polycrystalline α‐zirconium metals has been investigated by the use of spherical specimens in the pressure range 2×10−5–1×101 Pa and in the temperature range 673–1073 K. The experimental results are reasonably explained on the basis of the model in which the process is assumed to be composed of three successive steps: dissociative adsorption with lateral interaction between adsorbed hydrogen atoms; jump of the adsorbed hydrogen atom into the interstitial site; and finally its diffusion into the metal bulk. Both the first and third steps are rate controlling under the conditions of low pressures or low temperatures within the pressure and temperature ranges of this investigation. At low temperatures the absorption rate is significantly affected by the lateral interaction as the pressure is increased. Some parameters which characterize the derived rate equations have been determined: the activation energies are (1.7±0.5)×10−21 J/H atom for dissociative adsorption and (...

Theoretical study of nitrogen adsorption on Zr(0001) surface

Abstract The chemisorption of nitrogen on the zirconium (0001) surface has been investigated by first-principles total-energy and force calculations. Our results show that nitrogen adsorption at subsurface octahedral sites is energetically more favorable than at surface sites. The calculated binding energy, surface relaxation, work function and electronic structure are in good agreement with available experimental results.

Diffusivity and solubility of hydrogen and deuterium in yttrium

The diffusivity and solubility of hydrogen and deuterium in polycrystalline α-yttrium have been measured in the temperature range 1073–1373 K. The ratios of the measured diffusion coefficient of deuterium to that of hydrogen showed the classical value 1/√2. The activation energy of diffusion has been found to be 510 meV for both hydrogen and deuterium. The differences in the solubility and diffusivity between hydrogen and deuterium have been explained by using rate theory and assuming that hydrogen and deuterium atoms in yttrium are slightly anharmonic oscillators.

Structural Changes of Porous Silicon Surface with Thermal Annealing Studied by 29Si Nuclear Magnetic Resonance Spectroscopy and Fourier Transform Infrared Spectroscopy

Abstract29Si nuclear magnetic resonance (NMR) spectra and infrared spectra were measured for as-prepared and annealed porous silicon (PS) samples to characterize the change of PS structure. Annealing changed the infrared spectra remarkably: after 4-h annealing, the signals due to SiH2 disappeared and the intensity of the signals due to SiH decreased. On the other hand, the 29Si NMR spectra with magic-angle spinning (MAS) were not much affected by the annealing. The linewidth of spectra without MAS, however, increased with annealing time with the peak location unchanged. Annealing caused hydrogen on the PS surface to be desorbed, especially in the case of SiH2 species, and (SiH)2 dimer structure was produced during the annealing.

High‐Temperature Diffusion of Hydrogen and Deuterium in Titanium and Ti3Al

Diffusion coefficients (D{sub H} and D{sub D}) of hydrogen (H) and deuterium (D) in {beta}-titanium (Ti) and {beta}-Ti{sub 3}Al have been obtained from the rates of gaseous H and D absorption by samples in the temperature ranges of 1,173 to 1,473 K for {beta}-Ti and 1,423 to 1,523 K for {beta}-Ti{sub 3}Al. Activation energies for diffusion ({epsilon}{sub d}) are found to be 0.26 eV in {beta}-Ti and 0.39 eV in {beta}-Ti{sub 3}Al, which are smaller than those corresponding to {alpha}-Ti and {alpha}-Ti{sub 3}Al. Ratios D{sub D}/D{sub H} in {beta}-Ti and {beta}-Ti{sub 3}Al are found to be larger than the classical value, 1/{radical}2, and those in {alpha}-Ti and {alpha}-Ti{sub 3}Al. The observed smaller {epsilon}{sub d} in {beta}-Ti than in {alpha}-Ti is due to the difference between the body-centered cubic structure of {beta}-Ti and the hexagonal close-packed structure of {alpha}-Ti and the observed larger D{sub D}/D{sub H} in {beta}-Ti than in {alpha}-Ti due to phonon-assisted jumps of diffusing H atoms in {beta}-Ti. The observed distribution of aluminum atoms in it, and the larger D{sub D}/D{sub H} in {beta}-Ti{sub 3}Al than in {alpha}-Ti, {alpha}-Ti{sub 3}Al, and {beta}-Ti by the presence of aluminum atoms as well as by the phonon-assisted H-atom jumps.

Auger electron spectroscopy and electron energy loss spectroscopy study of the adsorption of nitrogen on a polycrystalline zirconium surface

Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) were used to investigate the adsorption of nitrogen gas on a polycrystalline zirconium surface at room temperature. It was found that the adsorption of nitrogen is saturated at an exposure of ∼10 L, the thickness of the nitride formed on the specimen surface is 0.4–0.5 nm at a nitrogen exposure of 100 L, and the surface has the same large electronic density of states 4–5 eV below the Fermi energy as ZrN. The measured AES and EELS spectra are consistent with the electronic structure calculated for the Zr(0001)-(1×1)-N structure.

Auger electron, electron energy loss and secondary electron emission spectroscopic studies on the oxidation of zirconium at high temperatures and room temperature

Auger electron (AES), electron energy loss (EELS) and secondary electron emission spectroscopy (SES) have been used to investigate the surface oxidation of zirconium at room temperature and high temperatures, 773–973 K, under low oxygen pressures 1.3 × 10–5–1.3 × 10–3 Pa. The kinetic energies of the Auger and the secondary electrons and the electron energy losses by single electron excitations are explained by the electronic structure in the core and the valence states of the metal and the oxide of zirconium. The energy loss by the collective excitation of plasmon is also observed in the EELS measurement for the metal and the oxide surface. The increase in the relative peak-to-peak height of the oxygen Auger transition and of the zirconium Auger transition by oxidation at high temperatures does not depend simply on the oxygen exposure represented by the product of oxygen pressure and exposure time, i.e. exposure in Langmuir, because of the dynamic competition between surface processes and the diffusion process of oxygen into the bulk. The rate of oxide growth is found to be parabolic at high temperature (773 K) and at 1.3 × 10–5 Pa.

Measurements of Young’s modulus and the modulus of rigidity of the solid solution of hydrogen in zirconium between 300 and 1300 K

Young’s modulus (E) and the modulus of rigidity (G) have been measured for zirconium between 300 and 1300 K and for zirconium hydrides ZrHx (0<x<0.9) at 941 and 1001 K, using an apparatus capable of making in situ measurements under ultrahigh vacuum. Values of E and G have been obtained from resonance frequencies for bending and torsion vibrations of a polycrystalline zirconium wire. As the temperature increases, E and G of zirconium decrease in the α phase of an hcp structure and in the β phase of a bcc structure with an abrupt decrease at the α→β transition temperature 1135 K. As the hydrogen concentration increases, E and G of ZrHx decrease in the α phase and increase in the β phase.

High-temperature diffusion of hydrogen and deuterium in palladium

The diffusivity of hydrogen and deuterium in palladium has been measured at temperatures between 773 and 1373 K and at hydrogen and deuterium concentrations less than 10–4 H/Pd and D/Pd (atomic ratio). The measured diffusion coefficients for hydrogen (DH) and deuterium (DD) showed Arrhenius behaviour. The dependence of the ratio DDDH on temperature has been explained by a model of diffusion in which a hydrogen atom in an octahedral site of the palladium lattice can jump into the adjacent octahedral site only when local deformation of the palladium lattice assists the jump.