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PROPERTIES 73 Densification and Sintering has recently been carried out using the gas-condensation method, and the material was subsequentl The synthesis of nanophase TiO2 (rutile)y studied by a variety of techniques as a function of sintering temperature (Siegel et al., 1988). Nanophase ceramics, with their high grain-boundary purity and small particle size, leading to higher reactivity among grains and shorter and more effective diffusion paths, are expected to sinter at lower temperatures than normally available ceramics. They are also expected to exhibit considerably improved sinterability and mechanical properties, owing to a lack of brittle second phases in their interfaces and more effective crack-energy dissipation via their ultrafine grain-boundary networks. Small Ti particles were first produced in a He-gas atmosphere and then oxidized on the cold-finger of the production chamber prior to in situ compaction at about 1.4 GPa at room temperature. The resulting TiO2 nanophase compact had a mean grain diameter of 12 nm. Grain-size distributions were determined by TEM, and Vickers microhardness was measured at room temperature on the as-compacted sample and as a function of sintering temperature up to 1400°C. The results of these microhardness measurements are shown in Figure 19; they are compared Figure 19 Vickers microhardness (in units of kgf/mm2) of TiO2 (rutile) measured at room temperature as a function of one-half hour sintering at successively increased temperatures. Results for a nanophase sample with a 12-nm initial mean grain size (squares) are compared with those for a coarser-grained compact with a 1.3-µm initial mean grain size (diamonds) (Siegel and Hahn, 1987).
