The influence of an electric field on the mechanism of combustion synthesis of tungsten silicides
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The synthesis of tungsten silicides by self-propagating combustion has been successfully accomplished under the influence of an electric field. Materials with starting composition ranging from 6 to 30 wt. % Si were investigated by the method of field-activated combustion synthesis (FACS). A threshold field value was required to initiate a self-sustaining wave; the threshold value depended on composition. It was shown that the level of the applied field can influence the mechanism of silicide formation. The silicide \V5Si3 could be formed only at relatively high field values while WSi2 can be formed at any field. The effect of the field on the silicide formation is discussed in terms of its role in liquid phase formation.
I. INTRODUCTION It is low resistivity, high thermal stability, and high oxidation resistance make tungsten silicide (WSi2) an attractive material for a variety of applications.'~3 Tungsten silicides are used (or have potential for use) as ohmic contact, diffusion barriers, interconnects, gates, adhesion promoters, diffusion sources, and Schottky contacts in silicon and gallium arsenide integrated circuit technology.4-5 As in the case of MoSi 2 , the silicides of tungsten form a protective silica surface layer. Thus a silicide layer on metallic tungsten makes it possible to use the latter as a heating element in oxidizing environments up to 2000 °C.6 Silicides of tungsten have been prepared by a variety of methods. In one of the earliest reported accounts, these silicides were synthesized by aluminothermic reactions.7 More recently, other methods have been used to prepare them. These include reactive hot-pressing,8 discharge treatment,9 laser irradiation,10 mechanical alloying,11 low pressure chemical vapor deposition,12 and W-ion implantation.13 Although the self-propagating hightemperature synthesis (SHS) method has been used to prepare other refractory metal silicides,14 apparently no investigation using this method has been reported for the synthesis of tungsten silicides. In part this is due to the relatively low adiabatic combustion temperature, Ta, of these silicides. For example, for the two silicides of tungsten W 5 Si 3 and WSi 2 , Ta is 954 and 1459 K, respectively. As has been empirically determined, the synthesis of materials with Ta =s 1800 K is not possible by SHS.14-15 In addition to the two
silicides, the interaction between tungsten and silicon can result in the formation of a solid solution of silicon in tungsten, as shown in W - S i phase diagram, Fig. 1. It has recently been demonstrated that the synthesis of energetically unfavorable systems (Ta =£ 1800 K) can be activated by the application of an electric field.16 Field-activated combustion synthesis, FACS, has been successfully used to prepare a variety of materials, including SiC, MoSi 2 -SiC, MoSi 2 -Nb, B 4 C-TiB 2 , and others. 17~22 In this method reactants are ignited while under the influence of an electric field. Electric parameter profiles obtained during combustion wave propagation, as well as microstructural obse
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