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R. Gronsky National Centerfor Electron Microscopy, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received 14 April 1988; accepted 28 July 1988) Ultrafine-grained, nanophase samples of TiO 2 (rutile) were synthesized by the gascondensation method and subsequent in situ compaction. The samples were studied by a number of techniques, including transmission electron microscopy, Vickers microharness measurements, and positron annihilation spectroscopy, as a function of sintering temperature. The nanophase compacts with average initial grain sizes of 12 nm were found to densify rapidly above 500 °C, with only a small increase in grain size. The hardness values obtained by this method are comparable to or greater than those for coarser-grained compacts, but are achieved at temperatures 400 to 600 °C lower than conventional sintering temperatures and without the need for sintering aids.

I. INTRODUCTION The gas-condensation method1"3 for the production of small particles in the size range of 2 to 100 nm has recently enabled the synthesis of a new class of ultrafinegrained materials by the in situ compaction and sintering of these particles.4 The resulting nanophase materials, which may contain crystalline, quasicrystalline, or amorphous phases, can be metals, ceramics, or composites with rather different and improved properties than normal coarse-grained polycrystalline materials. Much of the research completed to date on the synthesis, characterization, and properties of these new materials has been recently reviewed,56 and some of their potential for the future has been discussed. A number of advantages of nanophase material over traditional materials have been suggested.6 Some of these advantages particular to nanophase ceramics should be as follows, (i) Their small particle size during synthesis allows increased sinterability at lower temperatures and smaller residual pore sizes, resulting from a combination of high driving forces and short diffusion distances, avoiding the need for sintering aids, (ii) The exceptional physical and chemical control available in the gas-condensation method lets the particle surfaces be maintained clean during synthesis, allowing subsequent high grain-boundary purity and thus negligible Permanent address: Department of Nuclear Physics, University of Madras, Madras, India. b> Present address: Materials Research Laboratory, University of Illinois, Urbana, Illinois. c) Permanent address: Institute of Solid State Physics, Academia Sinica, Hefei, People's Republic of China. d) Permanent address: Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing, People's Republic of China. a)

J. Mater. Res. 3 (6), Nov/Dec 1988

http://journals.cambridge.org

interfacial phase formation, (iii) The large fraction of atoms residing in interfaces, almost one-half in the case of a 5 nm grain size, may allow for new atomic arrangements in the interfaces and thus novel and possibly improved ceramic properties. Such properties may include, for example,