Etching Studies on the Spinel Layer Formed by Solid State Reaction between Periclase and Sapphire

Abstract

The spinel layer formed by solid state reaction between periclase and sapphire was investigated with an etching technique. The aims of this study are listed as follows; (1) the determination of the relative orientation of spinel to sapphire (2) the examination of the three-dimensional microstructure of spinel to conjecture the mechanism of crystal growth.Grinding, polishing, etching and observations with an optical microscope are repeated in this order. The spinel was etched for 40-120 min in conc. H2SO4 at 200°C.According to the results of this experiment on the periclase-grown spinel, the spinel layer consisted of the grains elongated to the growing direction. This is the evidence of the epitaxial growth of this layer.The sapphire-grown spinel had two kinds of texture; one was observed in the central part of the layer, and another in the surrounding part of the layer.On the etched face of the central part of the sapphire-grown spinel, triangular etch pits were observed and relative orientation of the spinel layer to sapphire was determined on the basis of the direction of etch pits. The results of the orientation study were in agreement with those of the other authors. The sapphire-grown spinel had the twinning structure, and at the boundary of twinning, deep grooves were formed by etching. These grooves extended along the direction ‹112›, and tended to run parallel with each other. This phenomena were explained from the mechanism of rearrangement of the oxygen atoms at spinel/sapphire interface. At this interface, the oxygen must change the stacking sequence from the hexagonal closed packing of sapphire (…ABABAB…) to the cubic closed packing of spinel (…ABCABC…). This rearrangement is probably performed by the mechanism proposed by Schmalzried et al., i. e. a movement of the bundles of Shockley partial dislocations along the glide planes.The surrounding part of the sapphire-grown spinel had little tendency to orientate to sapphire, and had a little degree of epitaxy. It was assumed that the sapphire-grown spinel in this region had grown by a sort of exsolution mechanism.