Saburo Kose

Electronic structure calculations of transition metal-alumina interfaces

Abstract The electronic structures of the Nb-Al 2 O 3 interfaces have been calculated using the tight-binding electronic theory. It has been shown by using the models of the Al 2 O 3 (0001) slab with Nb layer deposited, that a direct chemical bond of both covalent and ionic characters can be established between the surface O atoms of Al 2 O 3 and the Nb atoms. This result is consistent with recent photoelectron spectroscopy studies. It is possible that the present type of interfacial bond is the origin of adhesion and stabilization of the observed abrupt Nb-Al 2 O 3 interfaces. General trends of the electronic structure and chemical bond at the interfaces between Al 2 O 3 and a series of 4 d transition metals have also been examined using the same theoretical method. We have presented a systematic perspective of the bonding nature and reactivity at the transition metal-Al 2 O 3 interfaces from a viewpoint of both the metal-surface O interaction and the metal-surface Al interaction.

The self-consistent tight-binding method : application to silicon and silicon carbide

The self-consistent tight-binding (SCTB) model proposed by Majewski and Vogl (1987) has been extended to be applicable for calculations of lattice defects in solids or disordered systems with both ionic and covalent characters that cannot be treated using other types of tight-binding theories. The precise formulation of electronic structure, total energy and atomic forces in the supercell technique has been presented. In order to apply this method to lattice defects in SiC, the parameters and functional forms have been examined so as to reproduce the basic properties of Si, SiC and C. The nature of the bonding and the phase stability in Si and SiC have been analysed by the present SCTB method.

A theoretical study of the atomic and electronic structure of a grain boundary in cubic SiC

The atomic and electronic structure of the (122) Sigma =9 grain boundary in cubic SiC has been calculated for the first time using the self-consistent tight-binding (SCTB) method. An atomic model consisting of a zig-zag arrangement of five-membered and seven-membered rings similar to that in the same boundary in Si or Ge has been constructed from a high-resolution electron microscope image, although Si-Si and C-C wrong bonds are repeated alternately at the interface in this model. The authors have also performed calculations of the same boundary in Si using the SCTB method for comparison, and have obtained results similar to those previously obtained by other theoretical schemes. The calculated boundary energy in SiC has shown that the present atomic model can exist stably compared with the two surfaces, and the calculated boundary electronic structure in SiC has no deep states in the gap as well as that in Si. These results indicate the possibility that stable boundary structures can be constructed by arranging structural units in SiC as well as in covalent semiconductors. However, it has been found that the presence of the wrong bonds greatly influences the boundary energy and the boundary electronic structure.

Theoretical study of polar interfaces of the (122) Sigma =9 grain boundary in cubic SiC

The polar and non-polar interfaces of grain boundaries in the zinc blende structure can be defined by the stoichiometry in the interface region, and it is possible to construct two polar interfaces and one non-polar interface for a symmetrical-tilt grain boundary for which the interface comprises the polar surfaces.

The atomic and electronic structure of the {211}/{111} facets in Si

The atomic and electronic structure of the {211}/{111} facets in Si have been examined based on the atomic model of Bourret and Bacmann. The optimum atomic structure has been determined by energy-minimisation calculations in the Keating model, and the electronic structure has been calculated using a supercell technique and a tight-binding model. Relatively large distortions exist at one of the two types of facet junctions between the {211} and {111} interfaces. This distorted region considerably influences the electronic structure at the facet junction compared with that at the other regions. However, there are no states introduced inside the minimum gap.

Effects of Sulfur Addition on the Superconductivity of Y–Ba–Cu–O Compound

Sulfur added Y-Ba-Cu-O compounds with a nominal composition YBa2Cu3OxSy were prepared by two methods; a) using CuS or Cu2S instead of CuO, b) annealing the YBa2Cu3Ox in H2S atmosphere. By the method a), sulfur atoms were not incorporated into the crystal lattice of the 1:2:3 phase and made it multiphasic. Although Tc was lowered with increasing sulfur content, Jc for y=0.03 was about twice as high as that for y=0.00. In the case of the method b), sulfur atoms could be incorporated into the crystal lattice of 1:2:3 phase when the compound was annealed at temperatures up to 200°C. The sulfur-incorporated compounds showed semiconductive behavior.

ELECTRONIC STRUCTURE CALCULATIONS OF METAL/ALUMINA INTERFACES

The electronic structure of the interfaces between Al 2 O 3 (0001) and Nb layers has been calculated using the empirical tight-binding method and the slab model. It has been shown that a direct chemical bond of both covalent and ionic characters can be established at the interface between the surface O atoms of Al 2 O 3 and the Nb atoms, which is consistent with the recent photoelectron spectroscopy study. General trends of the electronic structure and chemical bond at the interfaces between Al 2 O 3 (0001) and a series of 4d transition metals have been examined using the same theoretical method.

Hot-Pressing of Alumina

Hot-pressing experiments were carried out with high purity alumina in graphite dies at 1300°-1600°C under an applied pressure of 200kg/cm2. Densification behaviors of the compacts were measured by a dilatometric method during the hot-pressing runs.A new rate equation for hot-pressing was proposed based on a consideration of effective stress relating to porosity. The equation is expressed as follows:d ρ/dt=ασ(1-ρ)e4(1-ρ), and the integrated form f(ρ)=ασt+Cwhere α includes viscosity coefficient for the plastic flow mechanism or diffusion coefficient for the diffusion mechanism. ρ-f(ρ) relations were given in this paper.The densification data were analyzed with Murrays, Rossis, Fryers, Shimohiras and the new rate equations. Fryers and the new rate equations seemed to be more satisfactory to express the densification behaviors of the compacts. Several activation energies for the processes were obtained with these rate equations.

Creep Deformation of Sintered Zirconium Diboride at High Temperatures

Creep deformation of sintered zirconium diboride were studied at 1690°-1890°C under compressive stress of 160-500kg/cm2 using hot-press furnace. The samples for this experiment were prepared by hot-pressing and grouped into three porosities.Deformation of zirconium diboride was remarkable above 1800°C. The higher load was applied, the more quick deformation occured. Deformation rates also increased with increasing porosity. But its temperature dependences were independent of porosity and load. Activation energy for deformation was 100-120kcal/mole. Sintered zirconium diboride showed plastic behavior and yield loads were 130-160kg/cm2 for samples with 19-20% porosity at the temperature range of this experiment. The effects of porosity and grain size on deformation rates were discussed. Viscosity calculated from this experiment was compared with that from densification experiment.

Dielectric Properties of Hot-pressed Alumina

Hot-pressing technique is very advantageous to form easily a body with high purity and high density without large grain growth. Thus hot-pressed alumina has outstanding mechanical strength and excellent electrical properties.Dielectric properties of alumina depend on purity, density, and surface conditions as well as many other factors. Humidity also influences dielectric properties to a large extent.An effort was made to examine the influence of porosity on dielectric properties of hot-pressed alumina.Four kinds of alumina powder were used, as shown in Table 1. Hot-pressing of alumina was carried out at 1500°-1900°C and 200kg/cm2 for 10minutes in graphite molds heated by high-frequency induction. Disk specimens prepared were 32mm in diameter and about 3.5mm thick, and of total porosity between 0% and 14%. Dielectric property measurements were carried out using Q-meter, at the frequency range from 5×104 cps to 5×107 cps, in atmospheres with the relative humidity of 0% to 60% at a room temperature.The relations between the kind of alumina and the dielectric property were not found clearly in hot-pressed alumina specimens with low porosity. The dielectric loss of hot-pressed alumina was mainly determined by its porosity as well as by relative humidity of atmosphere and was dependent on frequency. At lower frequencies the dielectric loss increased with increasing porosity. At higher frequencies the dielectric loss increased independently of porosity. As the porosity increased, the minimum value of the energy loss occured at higher frequencies. Water adsorbed on open pores may be mainly contributing to the dielectric loss. The effect of relative humidity on the dielectric loss depended on open pore fraction. The dielectric constant of hot-pressed alumina was in the range of values 10 to 13. The effect of relative humidity on the dielectric constant was only found in alumina specimens with high porosity. Although the dielectric loss was influenced considerably by humidity, the dielectric constant was hardly affected at all.