Kh. S. Bagdasarov

Chemical processes and composition of the gas and solid phases in the Al2O3-Mo system in the temperature range 2327–2500 K

The possibility of chemical reactions has been calculated and the composition of the gas and solid phases that are in equilibrium with an aluminum oxide melt is determined. It is shown that, for the Al2O3-Mo system in the pressure range 1−1 × 10−5 bar, evaporation of Al2O3 is incongruent, and the fraction of this component in the gas phase decreases from 51.5 mol % (P = 1 bar) to 0.01 mol % (P = 1 × 10−5 bar). The presence of molybdenum-containing compounds in the gas phase changes the balance of oxygen and aluminum in favor of the latter (the aluminum partial pressure increases by a factor of 1.5–3), as a result of which there may be an aluminum deficit in the solid phase of Al2O3 during crystal growth from a melt. The thermodynamic characteristics (Kp, ΔG, Ptot) of dominant chemical reactions have been calculated for the temperature range 2327–2500 K. Understanding of the chemical processes makes it possible to optimize the growth parameters of leucosapphire single crystals.

Fundamentals of high-temperature crystallization

The fundamental problems of high-temperature crystallization are considered. It is shown that, unlike low-temperature crystallization, high-temperature crystallization proceeds under nonequilibrium conditions, which complicates consideration of related problems.

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.

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.

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.

Chemical reactions in the Mo-W-Al2O3 system at 2400 K and 100 Pa

We examine the behavior of the Mo-W-Al2O3 system at T = 2400 K and p = 100 Pa and identify the main chemical reactions that lead to the oxidation of Mo and W. The results indicate that, under these thermodynamic conditions, direct reaction between Mo, W, and molten Al2O3 is impossible, and oxidation processes are indirect and involve vapor species. For a Mo: W: Al2O3 molar ratio of 1: 1: 1, we calculate the gas-phase composition and demonstrate that the predominant vapor species are Al, AlO, Al2O, MoO, MoO2, and WO3.

Chemical processes in the Mo-W-Al2O3 system in vacuum (1 × 10−5 bar)

The behavior of the Mo-W-Al2O3 system at T = 2400 K and P = 1 × 10−5 bar is considered, and the main chemical reactions that implement the oxidation of Mo and W by the products of dissociative evaporation of the melt are determined. The gas-phase composition is calculated for the equimolar ratio of the components of the system, both for the presence and absence of direct contact of W with Al2O3. It is established that in the first case the dominant components of the gas phase are Al and WO3. In the second case, Al and Mo dominate, whereas the WO3 concentration decreases by a factor of about 2.5.

Molybdenum and Tungsten Combined Oxidation by Dissociative Evaporation of Products during Refractory Oxides Crystal Growth from the Melt

The main chemical reactions and composition of gas and solid phases have been determined for the equimolar ratio Mo: W: O2 = 1: 1: 2 at T = 2400 K in the pressure range of 1-1 x 10-5 bar. It is established that the character of the main processes of combined oxidation depends significantly on the pressure and state of the oxidant (oxygen): at P > 7.52 x 10-5 bar, oxidation reactions involve mainly molecular oxygen, whereas atomic oxygen dominates at lower pressures. At P ≥ 0.424 bar, the solid phase contains not only Mo but also MoO2. At P = 1 x 10-5 bar, the concentration of lower Mo and W oxides and elementary Mo and W in the gas phase sharply increases, which can negatively affect the main crystallization units.