Nanostructured Powders: A New Class of Materials for Forming High Performance Particulate Coatings
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GANESH SKANDAN Nanopowder Enterprises Inc., 120 Centennial Ave., Piscataway, NJ 08854-3908
ABSTRACT Nanostructured powders, with particle sizes in the range I to 100 nm, have shown great promise in the forming of advanced films and coatings for a variety of applications including tribological, high surface area supports for catalysts, chemical gas sensors, and optoelectronics. This paper is a summary of two major classes of powder synthesis techniques: (1) precipitation from aqueous salt solutions, and (2) condensation from vapor phase. The solution approach leads to powder particles that are spherical shells consisting of nanograins. The powder is milled with melting point lowering compounds and reagglomerated to make it suitable for thermal spraying. In contrast, the vapor condensation approach yields powder particles that are discrete, and loosely agglomerated. In a variation of the process, the heated clusters or nanoparticles are allowed to sinter on a heated substrate, forming porous or dense coatings. Films/coatings with thicknesses between 0.5 gm and 20.0 ýtm at rates in excess of I pm/min. are formed by the in situ deposition process. The synthesis techniques, powder and coating characteristics, and applications of the technologies, are described in this paper. INTRODUCTION During the past several years, there has been a growing interest in a new class of materials called nanocrystalline, nanophase, nanostructured, or ultrafine-grained materials [1-3], consisting of nanoscale particles or grains (1- 100 nm). As shown in Fig. I, the general term nanostructured, refers to either powders, laminated structures, coatings or bulk materials [4]. In some cases, the individual powder particles are single crystals, while in others each particle may be composed of several nanocrystals. Furthermore, coatings and bulk structures can be made partially dense with nanoscale open porosity. The focus of this paper is on powder production and in situ deposition of nanostructured films/coatings. Nanocrystalline materials, which have a very high surface or interfacial area, exhibit dramatic changes in properties, such as enhanced sinterability at low temperatures [5, 6], improved UV scattering [7], very high hardness and wear resistance [8], enhanced gas sensitivity [9], smaller particle size in colloidal suspensions, superior magnetic and dielectric strength, and enhanced optoelectronic properties [10]. Nanoparticles are ideally suited to form Chemical Mechanical Polishing slurries for global planarization and as heat transfer fluids [11, 12], as well as coatings and bulk structures in applications such as high performance cutting tools, high surface area
351 Mat. Res. Soc. Symp. Proc. Vol. 501 © 1998 Materials Research Society
supportsmaterial for catalysts, sinks, chemicalbygas Fig. technology 2 shows thedescribed range ofinmarkets and specific systemsheat being addressed the sensors. vapor phase this paper. Each of these applications has specific powder requirements in terms of chemical composition, particle morphology
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