Simultaneous Synthesis and Consolidation of Nanostructured MoSi 2
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´ric Bernardb) LRRS, UMR 5613 CNRS, University of Burgundy, “Fine Grained Materials” Group, BP47870, F-21078 Dijon, France
Fre´de´ric Charlot LRRS, UMR 5613 CNRS, University of Burgundy, “Fine Grained Materials” Group, BP47870, F-21078 Dijon, France and UMR 5060 CNRS, “Nanomaterials: Far From Equilibrium Phase Transitions” Group, F-90010 Belfort, France
Eric Gaffetc) UMR 5060 CNRS, “Nanomaterials: Far From Equilibrium Phase Transitions” Group, F-90010 Belfort, France
Zuhair A. Munird) Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 (Received 29 August 2001; accepted 10 December 2001)
A new process combining electric field activation and the imposition of pressure from mechanically activated powder mixtures is demonstrated as a means to simultaneously synthesize and densify nano-MoSi2 in one step. Nanophase reactants (Mo + 2Si) produced by mechanical activation are reacted by field activation with the simultaneous application of a uniaxial pressure. Mo + 2Si powders were comilled in a specially designed planetary mill to obtain nanometric reactants but to avoid formation of any product phases. These were then subjected to high alternating currents (1600 A) and pressures of 106 MPa. Under these conditions, a reaction is initiated and completed within a short period of time (3–6 min). The relative density of the product ranged from 82 to 93%. The crystallite size of the MoSi2 compound was determined by x-ray diffraction line-broadening analysis using the Langford method. The size ranged from 58 to 75 nm.
I. INTRODUCTION
Due to their attractive high-temperature properties, transition metal silicides have been the focus of numerous investigations.1– 4 In particular, the MoSi2 compound has received considerable attention recently as a material for high-temperature applications. Its properties provide a desirable combination of a high melting temperature (2293 K), high Young’s modulus (440 GPa),3,5 high oxidation resistance in air 6 or in a combustion gas environment,7 and a relatively low density (6.25 g cm−3). a)
Present address: Dept. of Materials, Oxford University, Parks Rd, OX1 3PH, U.K. e-mail: [email protected] b) e-mail: [email protected] c) e-mail: [email protected] d) e-mail: [email protected] 542
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J. Mater. Res., Vol. 17, No. 3, Mar 2002 Downloaded: 16 Apr 2015
However, like many similar high-temperature compounds, dense MoSi2 is usually prepared by an arc melting or powder metallurgical method; the latter involves a multistep process of synthesis and subsequent densification through sintering or hot pressing.8–10 However, none of these processing routes leads to nanostructured molybdenum disilicide. Such materials, which possess phase structures modulated on a scale of less than 100 nm, have properties that are not only different from but also superior to those with a larger grain size.11–13 The relatively limited number of experimental data on mechanical properties on nanomaterials has been
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