Thermodynamic analysis of the W-Al 2 O 3 system near the melting temperature of Al 2 O 3 : II. Chemical interactions at
- PDF / 218,896 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 41 Downloads / 152 Views
TAL GROWTH
Thermodynamic Analysis of the W–Al2O3 System near the Melting Temperature of Al2O3: II. Chemical Interactions at 1 × 10–5 bar D. V. Kostomarov, Kh. S. Bagdasarov, S. A. Kobzareva, and E. V. Antonov Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiі pr. 59, Moscow, 119333 Russia email: [email protected] Received June 3, 2009
Abstract—The W–Al2O3 system is considered at a basic component ratio of 1 : 1. The composition and com ponent concentrations in the closed system under isobaric–isothermal conditions and a residual pressure of 1 × 10–5 bar are determined by stochastic simulation and the minimization of the Gibbs free energy. The basic chemical reactions leading to tungsten oxidation near the Al2O3 melting temperature are determined, and the possibilities of their occurrence are calculated. Understanding the behavior of the system under these condi tions allows one to stabilize the conditions for growing leucosapphire crystals from melt. DOI: 10.1134/S1063774510030235
INTRODUCTION The growth of refractory oxide crystals (in particu lar, Al2O3) from melt by horizontal directed crystalli zation is generally performed in one of two ways: growth in either an inert or weakly reducing medium or growth in vacuum (pressure of ~1 × 10–5 bar) [1]. The previous study of the Mo–Al2O3 system at T = 2400 K in the pressure range from 1 to 1 × 10–5 bar revealed that a decrease in pressure leads to an increase in the fraction of metallic molybdenum involved in a chemical interaction with the products of melt evapo ration [2]. A decrease in pressure in the W–Al2O3 system at the same fixed temperature leads to similar conse quences: the chemical interaction involves the mate rial of not only the boat and heatresistant screens (Mo) but also the heater (W) [3], especially as a decrease in pressure by several orders of magnitude significantly changes the character of evaporation of the material crystallized (Al2O3) [4]. The purpose of this study is to analyze the changes in the behavior of the system and reveal the main chemical reactions determining the interaction of the W–O2 type, which can be useful for applications (optimizing the growth conditions). PRELIMINARY NOTES Since Gibbs developed the theory of chemical equilibrium in multicomponent systems [5], it has become possible to completely estimate the phase and quantitative ratios between the components in the final state of a chemical system without analyzing all the reactions that occur as the chemical equilibrium is established. When analyzing the system as applied to
crystal growth, this approach is obviously insufficient because it does not take into account the possibility of multiple phase transitions from one aggregate state to another. As a result of multiple transitions (for exam ple, for tungsten) according to the solid gas scheme, the W heater may lose its initial configura tion, which obviously may drastically deteriorate the operation of the entire crystallization unit. Therefore, it is important to analy
Data Loading...
