D. V. Kostomarov

Chemistry and Kinetics of ZnO Growth from Alkaline Hydrothermal Solutions

Special features (polar growth and nonstoichiometry) of ZnO single crystals grown on seed plates of various crystallographic orientations in the ZnO–KOH–H2O and ZnO–KOH–LiOH–H2O hydrothermal systems are analyzed. The growth proceeds via the interaction of ZnO2-2 anions with crystal surfaces, and its rate depends on the atomic structure and electric charge of the surface. The mechanism underlying the influence of Li+ ions on polar ZnO growth is considered. Partial replacement of Zn2+ by Li+ decreases the positive charge on the (0001) face and hinders the attachment of ZnO2-2 anions. The incorporation of Li ions into the (0001¯) face decreases its negative charge and accelerates growth in the [0001¯] direction.

The chemical action of oxygen on a heater during growth of refractory oxide crystals from melt

The behavior of the W-O2 system has been investigated at 2400 K in the pressure range from 1 to 1 × 10−5 bar. The chemical composition of the solid and vapor phases for the ratio W: O2 = 1: 1 was calculated by minimizing the Gibbs free energy. It is shown that the only solid phase in the system is metallic tungsten (0.333–0.355 mol), whereas trioxide WO3 dominates in the vapor phase; its concentration may reach 99%. It is concluded that providing an inert atmosphere in the growth chamber with a pressure of 1 bar decreases the concentration of atomic and molecular oxygen in the vapor phase and decreases its effect on the tungsten heater.

Chemical Processes in Al2O3-Mo and Al2O3-W systems in a weakly reducing atmosphere

Al2O3-Mo and Al2O3-W systems in a controlled Ar(95%) + H2(5%) atmosphere at T = 2400 K and P = 1 bar have been calculated by the Monte Carlo method. It is established that the presence of hydrogen in these systems leads to the occurrence of OH, H2O2, HO2, H2O, AlOH, AlOOH, AlH, AlH2, and AlH3 components in the gas phase; aluminum hydrides are formed only through the interaction of hydrogen with melt evaporation products. The presence of reducing medium leads to a decrease in the free oxygen concentration by one to two orders of magnitude, which is expected to improve the quality of sapphire crystals.

Oxidation of tungsten in the W-Al2O3 system at temperatures from 2350 to 2500 K and pressures from 1 to 105 Pa

We examine chemical reactions that lead to the oxidation of tungsten in the W-Al2O3 system at T = 2350–2500 K and p = 1–105 Pa. The results indicate that, for p ≥ 10 Pa, tungsten oxidizes through reactions with both Al2O3 vapor and dissociation products (Al2O2, Al2O, AlO2, and AlO). For p ≤ 10 Pa, oxidation is due to direct reaction of tungsten with O and/or O2. For p ≤ 2 Pa, tungsten may react with molten Al2O3. A detailed analysis of tungsten oxidation processes is intended to optimize parameters of the melt growth of corundum crystals.

Possibility of Mo2O6 formation at high temperatures in the range of pressures from 1 to 1 × 10−5 bar

Gibbs free energy minimization was used to consider the formation of complex molybdenum oxide (Mo2O6) at 2400 K in the range of pressures from 1 to 1 to 1 × 10−5 bar for the basic component ratio Mo: O2 = 1: 1. Several ways are shown to lead to Mo2O6 formation: when P = 1 bar, a synthesis reaction involving simple molybdenum oxides (MoO, MoO2, MoO3) is the main way; when P = 1 × 10−3 bar or lower, reactions of (MoO3)n(n = 3−5) complex oxides with metallic molybdenum and molybdenum monoxide (MoO) are.

Chemical reactions in the W-Al2O3 system near the melting point of aluminum oxide under low vacuum

The W-Al2O3 system is thermodynamically analyzed at T = 2400 K and P = 1 × 10−3 bar, and principal chemical reactions and their directions are determined from the sign of ΔGreact. Although solid tungsten does not directly react with Al2O3 melts, its oxidation (dominantly, to WO3) is established to occur via the reaction with aluminum-containing dissociative evaporation products. This reaction is shown to be a multistage process, in which both lower tungsten and aluminum oxides and gaseous W and Al2 may act as intermediates.

Oxidation–reduction processes upon interaction of aluminum oxide melt with molybdenum and tungsten in a hydrogen-containing atmosphere

A thermodynamic analysis of the processes occurring in the Мо–W–Al2O3 system at T = 2400 K and a total pressure of 1 bar, set by controlled reducing Ar + H2 atmosphere, has been performed. It is found that the basic components of the system do not interact directly, although may be actively involved in chemical reactions with participation of other components to undergo numerous cyclic oxidation–reduction processes. Particular attention is paid to the processes involving such chemically active reagents as Н2O2, HO2, H2 (H), gaseous Al, and its hydrides (AlH, AlH2, AlH3).

Behavior of the Mo–Al2O3 system in a controlled reducing atmosphere

Stochastic simulation (Monte Carlo method) has been used to evaluate the Mo–Al2O3 system at T = 2400 K and p = 1 × 105 Pa in a controlled Ar + H2 atmosphere. The results demonstrate that the qualitative and quantitative compositions of the system differ markedly from those in an inert (Ar) atmosphere: the presence of hydrogen in the system leads to the formation of hydrogen-containing vapor species (OH, H2O, AlOH, AlOOH, AlH, AlH2, and smaller amounts of H2O2, HO2, and AlH3). Increasing the hydrogen concentration in a controlled atmosphere leads to a reduction in the total concentration of oxygen and molybdenum oxides, accompanied by an increase in the concentration of elemental Al in the vapor phase. We have identified the main chemical processes that take place in the system and have shown that such processes have a cyclic nature and involve repeated interactions with the participation of the basic components of the system.

Peculiarities of the behavior of the W–Al2O3 system in a controlled reducing atmosphere

The W–Al2O3 system at T = 2400 K and standard pressure (controlled Ar + H2 atmosphere) has been calculated by stochastic simulation. It is shown that the presence of hydrogen leads to the formation of aluminum hydrides, hydrogen oxides, and aluminum hydroxides; the compounds from the two latter groups (except for water) can interact directly with tungsten. The main chemical reactions occurring in the system are determined, based on which a conclusion about the cyclic character of the processes is drawn. Some recommendations on the composition and pressure of controlled atmosphere for growing sapphire crystals are given.

Formation of complex oxides in the W-O2 system at temperatures from 2000 to 2500 K and a pressure of 105 Pa

We examine the main processes that lead to the formation of (WO3)n (n = 2–5) complex tungsten oxides at T = 2000–2500 K and p = 105 Pa. Our results demonstrate that the (WO3)n oxides form not only through WO3 polymerization and WO2 disproportionation but also presumably through tungsten oxidation or reactions of the oxides WO, WO2, and WO3 with each other. We show that WO has a dual nature, participating in both (WO3)n formation and decomposition. We determine the (WO3)n concentrations and interpret the decrease in total (WO3)n concentration with increasing temperature and the increase in W2O6 concentration relative to the other (WO3)n oxides.