Kinetic study on the production of silicon nitride by direct nitridation of silicon in a fluidized bed: Experiment and modeling.

摘要

Direct nitridation of porous silicon pellets {dollar}(dsb{lcub}p{rcub}{dollar} {dollar}sim{dollar} 400 {dollar}murm m){dollar} composed of fine grains {dollar}(dsb{lcub}g,m{rcub}{dollar} {dollar}sim{dollar} 2 {dollar}murm m){dollar} was performed in a fluidized bed reactor (55 mm ID) using nitrogen (30-90%) - hydrogen (5-50%) - argon mixtures as the nitriding gas in the temperature range 1200-{dollar}1390spcircrm C.{dollar} The effects of reaction temperature, hydrogen, nitrogen and pretreatment of raw materials on the nitridation of silicon and the yields of {dollar}alpha{dollar}- and {dollar}beta{dollar}-form were investigated. It was shown that a high silicon conversion (99%) and a high {dollar}alpha /beta{dollar} ratio ({dollar}sim{dollar}10) in the produced silicon nitride can be achieved by controlling the reaction temperature and the content of nitriding atmosphere.; Nitridation is initiated after an induction period which becomes shorter with an increase in reaction temperature and/or nitrogen concentration, but is unaffected by hydrogen. Both the final conversion of silicon and {dollar}alpha /beta{dollar} ratio increase with an increase in reaction temperature and/or with a decrease in nitrogen concentration, but remain essentially unaffected by hydrogen.; The mechanism of nitridation remains unclear. The most reasonable assumption for it, based on TEM photos of reacting pellets, is that the process is controlled by the nitrogen transport through the crackling, polycrystalline nitride layer, with nitride crystallites detaching from the silicon surface after reaching a critical thickness, so fresh silicon surface is exposed. A mathematical model developed on the basis of this assumption, applied to the conversion of a single grain, predicted the critical nitride layer thickness in agreement with experimental observations.; Modeling was also applied to nitridation of grains having a wide size distribution, in which case the effective silicon surface area was shown to reasonably linearly decrease with an increase in the overall conversion. The assumption of a constant average silicon consumption rate per this area led to a correlation which predicts the progress of nitridation in a wide range of experimental conditions. The results show that the average nitridation rate per the effective surface area obeys a first order rate law with respect to nitrogen and that the process has an apparent activation energy of {dollar}sim{dollar}340 kJ/mol in the temperature range 1200-{dollar}1300spcircrm C.{dollar}