A flame process for synthesis of unagglomerated, low-oxygen nanoparticles: Application to Ti and TiB 2
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I.
INTRODUCTION
A. Background
AS the field of nanophase materials matures, methods of producing large quantities of materials will be required. The gas condensation method, though extensively studied and a valuable research tool, has inherently low production rates compared with traditional commercial routes for powder synthesis. Additionally, the production of multicomponent materials can be limited by incompatible thermodynamic properties of the constituent materials.[1] Other possible methods that have been considered include solution phase processing, high-energy ball milling, and various aerosol processes, including furnace reactors, sputtering, plasma reactors, laser ablation, and gas-phase combustion (flame) synthesis. Of these approaches, flame synthesis is well established as a commercial process for large-scale production of high-purity powders such as titania, silica, alumina, and carbon black.[2,3] Flame synthesis is particularly attractive because powder purity and production rates are high, while energy costs are low. Although nanometer-sized primary particles are produced in flames, a severe weakness of flame synthesis is that these particles typically leave the flame as long-chain hard agglomerates.[4] This is not a particular problem for current commercial applications because these applications do not require that the powders be free from hard agglomerates. Nonetheless, in R.L. AXELBAUM, Associate Professor, and S.M.L. SASTRY, Professor, are with the Department of Mechanical Engineering, Washington University, St. Louis, MO 63130. D.P. DUFAUX, formerly Research Assistant with the Department of Mechanical Engineering, Washington University, is Senior Engineer, National Environmental Technologies, Inc., Charlotte, NC 28273. C.A. FREY, formerly Research Assistant with the Department of Mechanical Engineering, Washington University, is Staff Scientist, MEMC, St. Peters, MO 63376. Manuscript submitted December 13, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
advanced applications where powders of high melting point materials are to be consolidated into parts having near-theoretical densities, agglomeration must be minimal. For structural applications, three critical issues need to be addressed before a method of synthesizing nanopowders can be considered to be commercially viable: (1) production rates must be scaleable to the order of those for traditional powder production, (2) particles must be unagglomerated and less than 100 nm in diameter; and (3) the powders, which possess extremely high surface areas, must have high purity and be resistant to surface oxidation or hydrolysis during handling. The objectives of this research are to establish a flame-synthesis process that satisfies these criteria and is applicable to a wide range of materials. Titanium and titanium boride are selected as representative metal and ceramic materials for synthesis. Titanium is selected, in part, because of its high affinity for oxygen. Identifying a means of handling nanoparticles of titanium without excessive
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