In-Situ Formation and Characterization of a-Si 3 N 4 Whiskers from Nano Amorphous Si-N-C Powders
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ABSTRACT a-Si3N4 whiskers were formed from laser-synthesized nanoscale amorphous Si-N-C powders at 1873K under 1 atm N2. The as-formed whiskers were characterized by TEM, STEM, XRD techniques and the process conditions for the whisker growth were studied. The whiskers exhibit various morphologies such as the long thick straight, the prismatic, the ribbon-like, and knuckled whiskers. The gas phase reaction among N2, SiO, and CO gases leads to Si3N4 whisker growth on the pre-crystallized o-Si3N4 grains by the Vapor-Solid (VS) mechanism along specific crystal planes such as {1101}., which ensures an in-situ formation. No addition of other catalyst and the atomic combination of the elements in the Si-N-C powders ensure a high purity of the whiskers.
1 INTRODUCTION Si3N4 exhibits superior mechanical performance being considered as one of the most promising high temperature structural materials[l]. Si 3N4 whiskers, since they are near single crystals, have mechanical performance approaching their theoretical values, and have become important reinforcing components in ceramic-matrix composites, metal-matrix composites and polymer-matrix composites[2]. Due to these important applications of the whiskers, the synthesis of Si 3N4 whiskers has attracted widespread attentions in the past and various methods havw been developed such as carbothermal reduction [3], silicon nitridation [4] and synthesis from silicon halide [5]. A common method for synthesis of Si 3N4 whiskers is carbothermal reaction. In the method, silica-containing compounds and carbon are used as reactants[2-3]. This method has the merit of low reactant cost for commercial productions. However, the whisker purity is low due to the retention of unreacted silica-containing compounds or carbon which generally are needed to be removed by post-processing heat treatment. Also, the requirement of the addition of metallic catalysts (such as Fe and Co) for whisker growth by the Vapor-Liquid-
275 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society
Solid (VLS) process leads to the presence of spherical impurity droplets on the whisker tips degrading whisker purity [6]. The whiskers formed by the other methods also contain AI-Si or Fe-Si spherical alloy droplets on the whisker tips[4]. In the present paper, we report an in-situ formation of a-Si 3N4 whiskers from the pyrolysis of nano-amorphous Si-N-C powders. In this process, no catalyst was added to the reactants and the whisker formed by a vapor-solid condensation process, ensuring a high purity of the whiskers formed. The whiskers were characterized by TEM. The whisker growth mechanism is discussed.
2 EXPERIMENTAL Whisker synthesis experiment was conducted in a graphite resistance furnace. The raw materials, Si-N-C powders, were synthesized from CO 2 laser pyrolysis of hexamethyldisilazane vapor [7]. The powders sized smaller than 30nm in diameter, were amorphous as identified by XRD, and had chemical compositions of Si:50wt%, N:28wt% C:15wt% and O:5wt%. The powders (1g) were slightly pressed into
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