Analysis of Particle and Crystallite Size in Tungsten Nanopowder Synthesis
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INTRODUCTION
EXTENSIVE research over the last two decades on both metals and ceramics has shown that the sintering of highly agglomerated nanocrystalline powders usually results in bulk compacts with grain sizes in the micrometer range.[1–3] In many cases, this is due to the long sintering times and high temperatures associated with the specific sintering technique, which inevitably results in significant grain growth. However, in cases where the sintering time is kept purposefully short and the processing temperatures are lowered, such as in spark plasma sintering (SPS),[4] grain growth can still be significant due to the agglomerated nature of the powders. Indeed, it has been found that for some materials, the particle (i.e., agglomerate) size of powders tends to define the final grain size of the dense compact under typical SPS conditions.[5,6] Thus, characterizing the crystallite size of a powder from X-ray diffraction or transmission electron microscopy is necessary, but not sufficient, in order to determine the sintering behavior of OLIVIA A. GRAEVE, Associate Professor, and RAGHUNATH KANAKALA, Postdoctoral Researcher, are with the Kazuo Inamori School of Engineering, Alfred University, Alfred, NY 14802. Contact e-mail: [email protected] ABHIRAM MADADI, Graduate Research Assistant, and KAUSTAV SINHA, Graduate Research Assistant, are with the Department of Chemical and Metallurgical Engineering, University of Nevada, Reno, NV 89557. Manuscript submitted December 30, 2008. Article published online June 24, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
a powder and the final grain size after completion of sintering. We report here the synthesis and characterization of tungsten nanopowders of varying degrees of agglomeration. The motive behind this work is twofold: (1) to develop a precipitation process for the synthesis of tungsten nanopowders and (2) to determine the effect of processing conditions on both the crystallite size and the particle (agglomerate) size of the powders. Our interest in this material is justified both from an applications point of view[7] and as a model metallic material to elucidate behavior of high-temperature metals during postsynthesis heat treatments of powders prepared by precipitation. Thermal treatments are usually necessary when using precipitation processes for nanopowder preparation, and they can affect the levels of agglomeration very dramatically, depending on the particular material, in turn affecting the sintering behavior of the powders. Several techniques have been used for the preparation of nanocrystalline powders of tungsten and tungsten oxides, with varying degrees of success. These include self-propagating high-temperature synthesis,[8] reverse micelle synthesis,[9] thermal methods,[10–13] mechanical milling,[14] and electrochemical techniques.[15] Most of the resulting powders from these processes are very likely heavily agglomerated, although a determination of the levels of agglomeration have not been reported in most cases. In this study, we endeavor to present a full
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