Molybdenum-oxygen interaction during growth of refractory oxide crystals from the melt in vacuum
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TAL GROWTH
Molybdenum–Oxygen Interaction during Growth of Refractory Oxide Crystals from the Melt in Vacuum 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 December 19, 2008
Abstract—The behavior of the Mo : O2 system in a temperature range of 2350–2500 K under a pressure of 1 × 10–5 bar has been investigated. The compositions of gas and solid phases and the main chemical reactions describing molybdenum oxidation are determined for different ratios of the basic components. The thermo dynamic calculations made it possible to recommend a applying a specific atmosphere during the growth of leucosapphire and aluminum yttrium garnet crystals from the melt by the Bagdasarov method. PACS numbers: 05.70.Ce, 82.60.s DOI: 10.1134/S1063774509040178
INTRODUCTION
The total increment in Gibbs’ free energy of an individual material was calculated as
Molybdenum is widely used in growing oxide crys tals from the melt [1]. When using the Bagdasarov method to grow, Mo serves both as a material for the container with the melt and for protective screens of the heating unit (active elements) and screen blocks installed before the container with a charge and behind it (passive elements) [2]. This growth tech nique obviously suggests that the material of the con tainer with the melt has a temperature that is some what higher than that of the protective screen material [3]. Hence, it is necessary to consider the behavior of the Mo:O2 system not at one fixed temperature but in a certain temperature range; for the Al2O3 and Y3Al5O12 oxides considered here, this range is from 2350 to 2500 K (i.e., near the optimal growth temper ature). A thermodynamic analysis of the Mo : O2 sys tem at 2400 K was performed in [4]; however, the state of the system in high vacuum (~1 × 10–5 bar) was not considered. The purpose of this study was to analyze these thermodynamic conditions in detail and deter mine the main chemical processes occurring in the Mo:O2 system at these parameters.
P
∫
∫
∆G = ∆G ∆T + ∆G ∆P = – ∆S dT + ∆V dP. T0
(1)
P0
Expressing the temperature dependence of entropy in terms of the specific heat and using the Kelley equa tion [10], –2
C p = a + bT + cT ,
(2)
we obtain T
T
T
∫
∫ ∫
–2
∆G = – ∆S 0 dT – dT ( a + bT + cT ) d ln T T0
T0
T0
(3)
P
dP + RT , P
∫
P0
where T0 and P0 are the standard temperature and pressure (in our case, 298.15 K and 1 bar, respectively); T = 2350, 2400, and 2500 K; and P = 1 × 10–5 bar. Then, Eq. (3) takes the final form ∆G = – [ ∆S 298.15 ( T – 298.15 ) T
+a THEORETICAL ANALYSIS
T
T dT + b ln 298.15
∫
298.15
To determine the composition of the gas and solid phases in the Mo:O2 system (liquid phases are not formed in this case [5]), we used the standard tech nique for calculating the thermodynamic functions [6] while taking into account the reference data [7–9].
T
T
c + 2
∫
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