Synthesis of dense nanometric MoSi 2 through mechanical and field activation
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J. Woolman Facility for Advanced Combusion Synthesis, Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616
G. Cao Dipartimento di Ingegneria Chimica e Materiali, Universita` di Cagliari, Piazza d’Armi, 09123, Cagliari, Italy
Z.A. Munir a) Facility for Advanced Combustion Synthesis, Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 (Received 5 September 2000; accepted 27 February 2001)
The effect of mechanical and field activation on the synthesis of dense nanometric MoSi2 was investigated. Powders of Mo and Si, milled separately or comilled in a planetary ball mill, were reacted in a spark plasma synthesis (SPS) apparatus under different electric current conditions. Milled powders reacted faster and required less current than unmilled powders. Mixtures of powders which were milled separately (to nanometric size) reacted in the SPS to produce micrometric ␣–MoSi2. Similar results were obtained for samples comilled to produce nanometric reactants which did not contain detectable amounts of the product phase. When products form during milling, they contain both the ␣ and  modifications of MoSi2. The product after the SPS reaction was nanometric MoSi2 with a crystallite size of 140 nm.
I. INTRODUCTION
The synthesis of molybdenum disilicide has been accomplished by a variety of methods, including arc melting, combusion synthesis (SHS), and mechanical alloying. The product of mechanical alloying is typically an agglomerated powder while that of the SHS process is usually a porous solid. In a more recent study, Gras et al.1 combined these two approaches by milling powders of Mo and Si an then reacting them by the SHS method. They reported the product to be nanometric ␣–MoSi2. However, the synthesized silicide was highly porous, as is the case in the majority of materials prepared by this method. Obviously in many cases the practical advantages of nanomaterials can only be realized when highly dense products can be made. The process of making dense nanomaterials, however, is not simple, as will be discussed subsequently. It has been suggested that the scarcity of adequate mechanical properties on
a)
Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 5, May 2001
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nanomaterials is the consequence of the difficulty of preparing high-density samples.2 The feasibility of a new approach for the simultaneous synthesis and densification of nanomaterials has been recently demonstrated.3–5 In this paper we report the results of an investigation on the synthesis of dense nanometric MoSi2 By far the majority of investigations on the synthesis of this silicide were those focused on the effect of milling on the formation of a nanometric product. In these investigations, two types of mills were employed: SPEX and planetary mills. Using the former type, Schwarz et al.6 reported the formation of MoSi2 after 20 h of milling under an unspecified cond
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