Microwave Sintering of Nanocrystalline TiO 2
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MICROWAVE SINTERING OF NANOCRYSTALLINE TiO 2 J. A. EASTMAN 1 , K. E. SICKAFUS 2 , J. D. KATZ 2 , S. G. BOEKE 2 , R. D. BLAKE 2 , C. R. EVANS 2 , R. B. SCHWARZ 2 and Y. X. LIAO1 1)
Argonne National Laboratory, Materials Science Division, Argonne, IL 60439
2)
Los Alamos National Laboratory, Materials Science Division, Los Alamos, NM 87545
ABSTRACT Nanocrystalline TiO 2 compacts having initial approximate mean grain sizes of 14 nm and approximate green densities of 70% of theoretical were sintered by short-time exposure in a 2.45 GHz microwave cavity to maximum temperatures of 800, 1000 or 1200 'C. Sample densities were measured before and after exposure to microwaves using Archimede's method. Transmission electron microscopy and x-ray diffraction were utilized to monitor grain growth and phase changes. Rutherford backscattering was used to monitor any changes in oxygen stoichiometry. The results of this study indicate that enhanced densification behavior is obtained for microwavesintered samples relative to samples sintered using conventional pressureless-sintering techniques. INTRODUCTION The idea that nanocrystalline materials (ultrafine-grained bulk materials having typical grain sizes of 2-20 nm) produced by the gas-condensation technique [1, 2] would have interesting properties was first suggested by Gleiter almost ten years ago [3]. Since then, a number of studies have shown that nanophase metals and ceramics do possess a variety of interesting and sometimes improved properties with respect to their conventional coarser-grained counterparts. For example, it has been shown that nanocrystalline TiO2 has superior sintering properties [4,5], higher strain rate sensitivity [6] and enhanced transport behavior [7] compared to coarser-grained TiO2. An important potential limitation for the use of nanocrystalline materials, particularly ceramics, is the difficulty of sintering the materials to near theoretical density without concomitant undesired grain growth. It has been shown by Hahn and co-workers that nanocrystalline TiO2 undergoes rapid grain growth when sintered to densities greater than 90% of theoretical by annealing in oxygen at 1000 *C for 15-20 hours [5, 8]. The same authors determined that significant improvement could be obtained by either doping the samples with a small amount of yttrium, presumably pinning the grain boundaries, or by employing hot-isostatic pressing instead of pressureless sintering techniques. The present study was undertaken in order to determine whether microwave-sintering could offer an improved means of densifying nanocrystalline ceramics without accompanying serious grain growth. Studies of the sintering behavior of conventional coarser-grained ceramics using microwave energy have shown great promise for this technique as a means of rapidly densifying a Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
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ceramic while maintaining fine grain sizes [9-11]. This improved behavior has been attributed to the rapid heating rates obtainable with microwaves and
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