Yoshifumi Shinzato

Chapter 12 – New Expression of the Chemical Bond in Hydrides Using Atomization Energies

Abstract Atomization energy diagram is proposed for analyzing the chemical bond in the hydrides including perovskite-type hydrides, metal hydrides and complex hydrides. The atomization energies of hydrogen and metal atoms in them are evaluated theoretically by the energy density analysis (EDA) of the total energy, and used for the construction of the atomization energy diagram. Every hydride can be located on such an energy scaled diagram, although there are significant differences in the nature of the chemical bond among various hydrides. When the hydrides have a resemblance in the chemical bond, their locations are close to each other on the diagram. This diagram reflects characteristics of the chemical bond, since the atomization energy of hydrogen changes largely with the covalent or ionic interactions. Also, the roles of constituent elements in the hydrides are understood well with the aid of the atomization energy diagram.

Energetics of Native Defects in Al2O3 and SiO2

Formation energies of various defects in Al2O3 and SiO2 are calculated by using the plane-wave pseudopotential method. Also, the formation energies of Schottky defects and Frenkel defects are evaluated on the basis of these calculations. It is shown that formation energies of these defects are higher in SiO2 than in Al2O3. In other words, less defects are formed in SiO2 than in Al2O3. It is also found that the principal defect is the cation Frenkel defect in Al2O3 but the anion Frenkel defect in SiO2. These results agree with the experimental results that Al ions diffuse preferably in Al2O3 but oxygen ions diffuse in SiO2 at high temperatures.

New expression of the chemical bond in perovskite-type oxides

A new expression for the chemical bond in perovskite-type oxides is proposed based on the atomization energy concept. The atomization energies, Eo for oxygen atom and EM for metal atom in each oxide are evaluated theoretically using the energy density analysis of the total energy, and the 9EO vs. 9EM diagram called atomization energy diagram, is constructed. Every oxide can be located on the atomization energy diagram, although there are significant differences in the nature of the chemical bond among the oxides.

Alloying Effects on Hydrogen Permeability of Niobium

Niobium metal is one of the promising material for hydrogen purification because of its high hydrogen permeability. In order to design and develop a new palladium-free hydrogen permeable membrane, it is important to understand the effects of alloying elements on the hydrogen permeability through metals. However, to the niobium metal, the alloying effects still remain unclear. In the present study, using a DC-polarization technique under the diffusion limiting condition, the hydrogen permeability of Nb-5mol%M alloys were investigated in high precision at 573K. Here, M’s were 4d transition metals, Zr, Mo, Ru and Pd. The permeability of niobium is found to be varied with the addition of a small amount of alloying element. For example, the hydrogen permeability of niobium increases by the addition of Zr but decreases by the addition of Ru.