Electron microscopic study of the dehydration of diaspore

The thermal transformation of natural, single-crystal diaspore (D) to corun dum (C) and water has been analyzed in detail by means of transmission elec tron microscopy (TEM). Pieces of single- crystal diaspore were partially de hydrated at temperatures ranging from 400 to 600 degreesC and were prepared in cross-section for TEM studies. The transformation of diaspore was studi ed at the faces (100)(D), (010)(D), and (001)(D), with special attention to the microstructural details at the reaction interface. Irrespective of the transformation direction, the dehydration product is hi ghly porous. Nanometer-sized pores form two-dimensional arrays arranged par allel to the basal plane of corundum forming lamellae that are separated by thin regions of dense corundum. The periodicity of the lamellar structure is 3.7 nm, which causes distinct satellite reflections in diffraction patte rns in the case of dehydration at moderate temperatures (<450 <degrees>C). At higher temperatures, the degree of regularity, as well as the satellite reflection intensities, decrease. During transformation, the diaspore crystal cleaves parallel to (010)(D), a nd micro-cracks are developed parallel to (100)(D) and to (001)(D) in the c orundum due to the misfit of the lattices. The entire crack system as well as the lamellar pore system play an important role in the dehydration proce ss because they serve as effective transport paths for the water vapor. The rapid dehydration at the (010)(D) faces is attributed to fast-propagating cracks along [010](D). The slow transformation processes along [100](D) and [001](D) are discussed in terms of difficulties in crack propagation and i n generating suitable crack systems, respectively.