Epitaxial growth of magnesia and spinel on sapphire during incongruent reduction in molten magnesium
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Epitaxial growth of magnesia and spinel on sapphire during incongruent reduction in molten magnesium P. Kumar, S.A. Dregia, and K.H. Sandhage Department of Materials Science and Engineering, 477 Watts Hall, 116 West 19th Avenue, The Ohio State University, Columbus, OH 43210 (Received 21 December 1998; accepted 30 April 1999)
The types and structures of oxide phases produced during the incongruent reduction of sapphire (single-crystal Al2O3) by molten magnesium were examined. Polished {1010} faces of sapphire were exposed to molten magnesium at 1000 °C for 100 h. Such exposure resulted in the formation of a continuous, epitaxial layer of spinel (MgAl2O4) on sapphire and a continuous, epitaxial layer of magnesia (MgO) on the spinel. X-ray pole figure analyses indicated that two variants of spinel and magnesia had formed in a manner consistent with the following orientation relationships: (1010)Sapphire㛳(110)Spinel㛳(110)Magnesia/[0001]Sapphire㛳[111]Spinel㛳[111]Magnesia.
I. INTRODUCTION
A solid phase that transforms on heating into one or more new solid phases and a liquid is said to undergo incongruent melting. Examples of other incongruent reactions that can occur between solid and fluid reactants include incongruent dissolution (i.e., where one or more solid products form at or near the surface of a solid that is dissolving into a liquid)1–11 and incongruent vaporization (i.e., where one or more new solid phases form at or near the surface of a solid that is undergoing reactive vaporization).12–14 Although not usually recognized as such, displacement reactions between a solid oxide and a molten metal can also be considered to be incongruent if a new solid oxide is formed at or near the interface between the solid and liquid reactants. Several incongruent reduction processes have recently been developed for the fabrication of dense, near netshaped Al 2 O 3 /metal composites. 15–24 In the cocontinuous ceramic composite (C4) process, Al2O3/ Al(Si) composites have been produced by allowing dense preforms of amorphous SiO2 to undergo the following incongruent reduction reaction with molten Al15,16: 4{Al} + 3SiO2 ⳱ 2Al2O3 + 3{Si}
(1)
where the brackets {} denote a species dissolved in a liquid solution. In the alumina aluminide alloy (3A)17–19 and reactive metal penetration (RMP)20–24 processes, composites of Al2O3 with Al-bearing metal alloys (i.e., solid solutions or intermetallic compounds) have been produced by allowing a variety of crystalline oxides (e.g., aluminum titanium oxide, magnetite, mullite, nickel aluminate, niobia, silica, titania) to undergo incongruent reduction reactions with molten aluminum. For the 3312
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reactions reported to date in the C4, 3A, and RMP processes, the volume of the oxide produced (alumina) was less than the volume of oxide consumed.25 Kumar et al.26–28 have recently reported the use of incongruent reduction reactions that generate a larger
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