Sintering characteristics of nanocrystalline TiO 2
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INTRODUCTION
The development of advanced ceramics for use as structural or electronic components faces numerous obstacles. Internal flaws, high temperature processing, and negligible ductility are among the most severe. Several strategies have been followed in attempting to overcome these problems. Sintering aids have been used to lower sintering temperatures from near the melting point to more economically viable temperatures, although the use of sintering aids often has deleterious effects on mechanical and electrical properties. Improved sintering characteristics have also been achieved by increasing green body densities, obtaining more uniform particle sizes, and controlling pore size distributions.1"3 Most promising for advanced ceramic processing, however, is the reduction of the particle size of the starting powder. Small particle sizes have the advantages that (i) densification (in later stages of sintering) depends inversely on the fourth power of particle size,4 (ii) internal flaws associated with large pores can be minimized, and (iii) ductility can be gained through diffusional creep mechanisms.5'6 It is not surprising, therefore, that much current research has focused on obtaining and processing fine particle ceramic powders. Various approaches have been undertaken to prepare ultrafine-grained ceramics. Many of these techniques rely on solution chemistry to precipitate fine particles from chemical precursors.7'8 One drawback of this method is that the resultant powder must be fired at very high temperatures to drive off contaminants before sintering can be carried out. Gleiter et al. have developed a new technique for the synthesis of fine-grained materials, nanocrystal processing, which maintains clean particle surfaces and low temperature processing.9 Nanocrystal processing combines inert gas J. Mater. Res., Vol. 5, No. 3, Mar 1990
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condensation of metal or ceramic vapors and in situ collection and compaction of the ceramic powder. Initial studies of nanocrystalline ceramics have demonstrated interesting and potentially important properties, e.g., low-temperature sintering10 and superplasticity.11 Despite these promising preliminary studies, very little effort has been directed toward characterizing the microstructure of the "as-prepared" or sintered nanocrystalline ceramic samples or relating the microstructure to observed properties. For example, it is possible, as we will show, that the specimens employed for the lowtemperature superplasticity measurements were not fully dense. This would have important implications for the understanding of such experiments as well as for potential applications. We have begun such microstructural characterizations on nanocrystalline TiO2 using high resolution scanning electron microscopy (SEM), x-ray diffraction, gravimetry, and the nitrogen adsorption method of Brunauer, Emmett, and Teller (BET)12; we report here on the specific surface area, total porosity and pore size distribution, grain size, and density
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