Gravitational influences on the liquid-state homogenization and solidification of aluminum antimonide
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THE
III-V semiconducting compound aluminum antimonide (AISb), with an energy gap of 1.62 eV, is potentially a high-efficiency solar cell material.I For the conversion of solar energy outside the atmosphere, it can be shown2 that AiSb would have a conversion efficiency of 25 pct as compared to a maximum of about 17 pct for silicon. Unfortunately, this compound, as indicated by the AI-Sb phase diagrams in Fig. i, has for many years presented problems to investigators in their attempt to synthesize either polycrystalline or single crystal materials4-7 for property determination or device development. Heretofore, the reported laboratory attempts to achieve microstructural and compositional homogeneity of AISb include prolonged heating at 100 to 150°C above the liquidus, rotating and vibrating the crucible, and in-process composition adjustments as well as paddling to remove oxides during crystal growth. In addition to the reported sluggish homogenization of the two liquids and the uncertainty in melting temperature of the compound, 3 difficulties were also encountered in generating reproducible physical and semiconducting properties of the laboratory samples.%s Commercially, only the polycrystalline material is available. As shownin Table I, commercial samples have the correct stoichiometric composition as determined by wet chemical analysis. The macrostructure of a typical grain of this starting material can be seen in Fig. 2. Although stoichiometrically correct, this polycrystalline material is microstructurally CHOH-YI ANG (deceased)was Former Member of Technical Staff, Ivan A. GettingLaboratories,The Aerospace Corporation, Los Angeles, CA. LEWISL. LACYis Chief, Solid State Branch, Space Processing Division, Space Sciences Laboratory, NASA/Marshall Space Flight Center, Marshall Space Flight Center, AL35812. Manuscript submitted February 15, 1978. METALLURGICAL TRANSACTIONS A
inhomogeneous, showing the presence of light A1- and S b - r i c h phases in addition to the d a r k e r compound phase. The high reactivity of the m a t e r i a l to a humid atmosphere could be due in part t o the microinhomogeneity in composition. The l a r g e density difference between A1 (2.7 g/cm s) and Sb (6.62 g/cm 3) could be an important contributor t o inhomogeneity in m i x i n g of the two elements in the liquid state. Furthermore, gravity induced convective perturbations d u r i n g the onset of solidification and constitutional supercooling d u r i n g crystal growth could conceivably c a u s e localized segregation of phases which might be rich in one element or the other. A r e c e n t paper by Carruthers 9 reviews crystal growth processes in a low gravity environment and s u m m a r i z e s some of the results from previous e x p e r iments. T o date, four a r e a s have been identified which define the consequences of low gravity for m a t e r i a l s processing: 9 1) Containerless handling of molten liquids to suppress container reaction which can lead t o a reduction of contamination d u r i n g crystal growth, the thermod
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