It has been well documented that the reaction rate during the nitridation of a silicon powder compact has typically two maxima, the first lying around 1200 °C and the second around the melting temperature of silicon (1410 °C), depending, for example, on impurities in the system [1-3]. The most likely mechanism to explain the onset of the first abrupt reaction maximum is the devitrification of the native silica on the powder surface and exposure of fresh silicon surfaces, thus initiating the nucleation and growth of S13N4. Here the devitrification means that the silica film is removed for example by being dissolved in an eutectic liquid formed with impurities such as Fe, Al [4,5] or by reduction, particularly in the presence of hydrogen [6], The explanations for the second reaction maximum has been considered to be that at higher temperature the diffusion flux becomes higher [2] or the vaporization of silicon increases [7]. The melting of silicon giving a faster rate of nitrogen diffusion [8] or creating space for reaction product [9] are also considered to the explanations. However, there are also some conflicts in the literature. The intention of this work is to study the nitridation mechanisms at the melting temperature of silicon.
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