Grain boundary dislocation interactions in nanocrystalline Al 2 O 3
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To explore the mechanism of grain growth, gas phase synthesized nanopowders of Al2O3 were heated in ambient conditions at elevated temperatures. Transmission electron microscopy and x-ray line broadening studies were performed to determine the microstructural parameters like crystallite size and root-mean-square (rms) strain. Increase in crystallite size with a decrease in dislocation density was observed for annealing the powder at higher temperatures. From a detailed analysis of the dislocation interactions, it was shown that polygonization like interaction of dislocations is the primary cause for such growth. A model for such growth is proposed. From the measured values of the rms strain and crystallite size at different temperatures, the ratio of the bulk to the shear modulus was determined. The measured ratio was found, within experimental uncertainties, to be close to the bulk value.
I. INTRODUCTION
The challenge in processing of nanocrystalline ceramics lies in being able to achieve reasonable density without excessive grain growth. Successful production of high-density nanocrystalline TiO21–3 and ZrO24–6 has not been achieved in gas phase synthesized Al2O3. Although a number of studies on processing this nanopowder have so far been reported,7–16 very little information is available on the sintering of gas phase synthesized nano Al2O3. Misra et al.17 have used plasma activated sintering of ␥–Al2O3 and have reported grain growth at all temperatures in the interval of 1300–1670 °C. Although higher sintering pressure has been recommended to achieve reasonable consolidated properties, no mechanism of grain growth has been proposed. Zhao et al.18 reported a substantial change in the isothermal bulk modulus, compared with that of bulk value in nanophase ␥–Al2O3 and ␣–Al2O3, provided the grain size is below a critical value. Kruger19 observed that the bulk modulus of nanophase ␥–Al2O3 is independent of the crystallite size in the range 21–67 nm. However, available literature on nanocrystalline Al2O3 clearly indicates the need for an understanding of the physical processes responsible for the grain growth of these powders for possible technological applications. The size range normally discussed in nanocrystalline powders is from a few tens of nanometer to hundreds of
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0136 900
J. Mater. Res., Vol. 22, No. 4, Apr 2007 http://journals.cambridge.org Downloaded: 15 Mar 2015
nanometer. Incidentally, this has been the range of sizes of polycrystalline materials and also the thicknesses of majority of thin films studied so far. Conventional techniques like transmission electron microscopy (TEM) and x-ray diffraction (XRD) techniques have been used with great success to explain the grain growth mechanisms in polycrystalline materials. However, the essential difference between the small grain polycrystalline and nanocrystalline materials lies with the process of synthesis. In the synthesis of nano-sized materials, the gr
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