Ultramicrotomy of industrial alumina

Industrial materials, such as alumina, often pose difficulties in their preparation for TEM examination. Composite and particulate materials are particularly difficult to prepare using conventional thinning techniques, i.e., ion beam and chemical jet thinning. Ultramicrotomy (UM) can be used to produce TEM specimens with a uniform thickness and an unaltered composition. Some crystalline materials, i.e., alumina hydrate, were difficult to section due to conflict between the cutting direction and cleavage planes. Sectioning was successful when these two directions were mutually parallel or perpendicular. At other orientations shattering occurred. Microcrystalline particulate materials, i.e., calcined alumina, were sectioned successfully in particles < 70 microns in diameter. The phases found in industrial alumina particles were gamma, delta, theta, and alpha alumina. Gamma alumina consisted of fine-grained, equiaxed crystallites. The selected area electron diffraction (SAED) patterns indicated poor crystallinity with a distinct hexagonal texture. Delta and theta alumina appeared as an undifferentiated intermediary microstructure of elongated grains. The SAED patterns indicated poor crystallinity, but without a distinct texture. Alpha alumina was found to be a coarse-grained crystalline phase with high diffraction contrast. SAED patterns consisted of fine, randomly oriented spots. Considerable variation was observed in the distribution of phases. In some specimens, discrete particles of gamma and alpha predominated. In others, particles were a mixture of phases representative of the bulk composition. To characterise these samples, TEM of numerous whole particles was required. Ultramicrotomy was the only preparation technique capable of producing such samples.