Nano-Crystalline Powders and Suspensions Generated Using A Flow-Through Hydrothermal Process, Part I: Characterization
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JOHN G. DARAB, M. F. BUEHLER, J. C. LINEHAN, AND D. W. MATSON
Pacific Northwest Laboratory [1], Richland, WA 99352 ABSTRACT A wide range of ultra-fine, nano-crystalline powders and suspensions have been produced using the Rapid Thermal Decomposition of precursors in Solution (RTDS) technology. These materials include single and multi-component iron-, zirconium-, titanium-, nickel-, and chromium-oxide/oxyhydroxide powders. RTDS, which was developed at Pacific Northwest Laboratory, is a flow-through hydrothermal process capable of producing nano-crystalline particulate material at rates of up to 100 grams of solid per hour. We present the results of characterization efforts on RTDS iron oxyhydroxide and zirconium oxide systems. As-collected RTDS suspensions were characterized using optical light scattering. Separated RTDS powder$ were evaluated using X-ray diffraction, electron microscopy, gas adsorption analysis, thermal gravimetric analysis, and chemical analysis. INTRODUCTION Particulate materials having crystallite sizes in the nanometer range are of interest from both research and industrial standpoints due to the fact that the physical and chemical properties of these particles are overwhelmingly dominated by the characteristics of their interfaces, i.e., free surfaces or grain boundaries. Researchers have estimated that for a five-nanometer-diameter particle, roughly 50% of the atoms reside at interfacial regions [2]. The fact that much of the material in "nano-crystalline" powders lies either on particle surfaces or in intergranular boundaries makes them attractive for use in processes where interfacial interactions play an important role. Such applications may include their use as heterogeneous catalysts, abrasives, and as precursors for the production of materials with special mechanical, magnetic or electronic properties. In the literature, terms such as "ultra-fine", "nano-crystalline", "nanoscale", "nanophase", "nano-particle", etc. are often used (or misused), interchangeably to describe powders or particles produced by various technologies. In this paper particulate materials are considered "nano-crystalline" if they contain crystallites of diameters less than 100 nanometers. In most instances, isolated nano-crystals are not encountered as "nano-particles", but rather form the building blocks of larger structured assemblies held together by strong or weak crystal-crystal interactions [3]. Thus, "nano-crystalline" powders may be composed of porous aggregates or fully dense polycrystalline particles having dimensions well into the micrometer range. Particles, in any form, which are less than ten microns in diameter are defined here as "ultra-fine". The extent and form of nano-crystal aggregation/agglomeration may have a significant impact on the properties of nano-crystalline powders. 499 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
Consequently, it is important to go beyond analysis techniques which probe only crystallite size (e.g., powder X-ray diffraction, Mossbauer spectr
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