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Nitridation Mechanisms of Silicon Powder Compacts
Book chapter

Nitridation Mechanisms of Silicon Powder Compacts

M Barsoum, P Kangutar and M. J Koczak
A Collection of Papers Presented at the 13th Annual Conference on Composites and Advanced Ceramic Materials: Ceramic Engineering and Science Proceedings, pp 794-806
01 Jan 1989
DOI: https://doi.org/10.1002/9780470310557.ch30

Abstract

chemical vapor infiltration nitridation kinetics reaction bonded silicon nitride si powder compacts thermogravimetry
The nitridation of Si powder compacts was examined by measuring the flow rate dependence of nitrogen and N 2 /H 2 gas mixtures during slow heating of the compacts while simultaneously monitoring the oxygen partial pressure of the exiting gas. The reaction was found to occur in two stages: an initial stage, the temperature (1215°C) of which was found to be independent of oxygen partial pressure and gas composition, and a second higher temperature stage. Small (4%) additions of hydrogen gas greatly enhanced the extent of reaction at lower temperature. It is believed that the first stage is initiated as a result of devitrification of the native SiO 2 layer, catalyzed by Fe impurities, and the exposure of the underlying Si. The reaction sequence at that point has been unequivocally shown to be: [equations] as evidenced by a large increase in the oxygen partial pressure simultaneously with the onset of the initial stage. In the absence of hydrogen, both reactions are rapidly suppressed as the liberated oxygen competes with the nitrogen for the exposed Si surface and reoxidizes it. In the presence of hydrogen, however, this stage is quite pronounced and the role of hydrogen appears to be, in addition to aiding in the formation of SiO, to react with the oxygen and prevent reoxidation of the Si allowing for more extensive nitridation at lower temperatures. The main nitridation reaction at 1300°–1400°C appears to be a time dependent diffusion controlled process that depends on the nature of the passivating layer that forms during the initial stage, which in turn depends on gas composition, i.e., vol% H 2 .

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