V. S. Kiiko

Electronic structure and properties of beryllium oxide

This review focuses on computer simulation studies of the nature of chemical bonding in BeO and its electronic structure and physicochemical properties. The capabilities of modern quantum-chemical methods are analyzed with application to various structural defects in BeO, phase equilibria in the Be-O system, pressure-induced polymorphic transformations of BeO, and its mechanical, thermal, and spectroscopic properties.

Luminescence of a Transparent Alumina Ceramic Doped with Chromium and Titanium

Specimens of transparent alumina ceramic (TAC) are prepared by slip casting. An alumina spinel, formed during sintering under vacuum or under reducing conditions, as well as individual phases, produced by thermochemical doping with chromium and titanium, are identified. The spectra of steady-state x-ray luminescence (SXL) and pulsed cathodoluminescence (PCL) and the PCL decay kinetics are studied. The highest STL and PCL intensity were observed in specimens that were prepared by thermal diffusion using a carbon + TiO2 pack. Ti-doped TAC, characterized by a rapid decay of luminescence in the UV region, can be recommended as a promising material for the detector of a scintillation dosimeter. The luminescent chromium ions (isomorphic substituents of aluminum ions in the crystal lattice) are unsuitable for that purpose because of their long luminescence de-excitation time.

MAGNETIZATION OF BERYLLIUM MONOXIDE (BeO) WITHOUT MAGNETIC IMPURITIES: A FIRST-PRINCIPLES STUDY

Using the ab initio VASP-PAW method with the generalized gradient approximation, the magnetic behavior of wurtzite-like beryllium oxide (BeO) without magnetic impurities has been predicted. Our results indicate the transformation of non-magnetic insulator BeO into magnetic half-metal at the substitution of oxygen atoms by carbon or in the presence of beryllium vacancy.

Correlation Between Thermally Stimulated Luminescence and Ceramic Properties of Beryllium Oxide (A Review)

Thermally stimulated luminescence (TSL) of BeO powders and ceramic samples of density 2.10 – 2.93 g/cm3 made of the same batch of powder is investigated. It is established that with increasing density of BeO samples (and, accordingly, increasing mean size of microcrystals), as well as with increasing vacuum in a cryostat chamber, the positions, number, and intensity of low- and high-temperature TSL peaks of adsorption and non-adsorption origin are modified. It is assumed that impurity and adsorption centers of volume and surface origin, as well as internal lattice defects, are responsible for the formation of TSL centers in BeO powders.

Effect of heat treatment on the electronic spectrum and mechanical properties of BeO ceramics

The full-potential linear muffin-tin orbital method with a generalized gradient approximation, in conjunction with experimental structural data obtained for BeO ceramics during heating-cooling cycles, was used to analyze the effect of heat treatment on the electronic structure of BeO and to evaluate the thermal expansion coefficients and bulk modulus of BeO ceramics. According to the calculation results, BeO remains a widegap semiconductor in the range 80–300 K, and its band gap varies by no more than ∼0.02 eV. The bulk modulus of BeO ceramics (∼170 GPa) is 18–25% lower than that of single-crystal BeO.

Temperature Dependence of Luminescence in Beryllium Ceramics

A relationship is established between x-ray luminescence and variation of the elementary cell parameters of BeO. The lattice deformation and pyroelectric fields lead to the formation of point defects (which are luminescence centers), on which excitons, as well as holes and conduction electrons, are presumably localized.

Paramagnetic Defects in Aluminum Oxide Nanopowders and Ceramics Based on These Powders

The defective structure of aluminum oxide nanopowders and ceramics based on these powders is analyzed using the electron paramagnetic resonance method. Inherent structural point defects, which are oxygen vacancies having a trapped electron (F+ centers), are identified in the nanopowders. Among the impurity defects, the signals of Fe3+, Cr3+, and Ti3+, which occupy different crystallographic positions in Al2O3 or exist as separate phases, are identified in the nanopowders and ceramics.

Defects in beryllium ceramics

Point defects and possible reasons for defect formation in beryllium ceramics with B3+, Al3+, Zn2+, Li+, and rare-earth element impurities produced by slip casting, dry-press molding, and high-temperature compression are investigated using electron paramagnetic resonance. A certain number of impurity centers are identified which determine the optical, luminescent, diffusion, and other physicochemical properties of beryllium ceramics.

Ultrasound velocity and absorption in BeO, Al2O3, ZrO2, and SiO2 ceramics

We have measured the ultrasound velocity and absorption in BeO, Al2O3, ZrO2, and SiO2 ceramics. The results indicate that the ultrasound velocity in oxide ceramics depends on the nature of the basic oxide component, the density of the material, and the preferential alignment of the grains. The ultrasound velocity in ceramics is shown to correlate with their thermal conductivity: with increasing thermal conductivity, the ultrasound velocity increases. The ultrasound absorption in oxide ceramics decreases with decreasing temperature, and vice versa, with increasing temperature, the ultrasound attenuation coefficient increases.

REACTION OF BERYLLIUM CERAMIC CRUCIBLES WITH RARE-EARTH METAL MELTS

Intense chemical reaction of beryllium ceramic crucibles with alloys of metals containing f-elements is established using optical absorption spectra, PCL, x-ray diffraction, x-ray-photoelectron spectroscopy, and electron probe in inert and reducing atmospheres (at high temperatures). Beryllium oxide in the diffusion zone is occasionally reduced to the metallic state, which leads to the destruction of ceramic crucible walls. This restricts the use of the specified crucibles in melting alloys containing rare-earth metals.