Fluidized-bed nitridation of silicon : direct use of very fine powder for [α]-silicon nitride production Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0z7091568

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  • 2 μm average sized silicon powder was nitrided with 90% N₂/10% H₂ in a fluidized-bed reactor, operated at 1200°C, 1250°C and 1300°C. To fluidize silicon powder, alumina particles with an average size of 300 μm were used as an inert fluidizing conditioner. The feasibility and operating conditions of the fluidization were studied at room temperature. The effects of silicon content and operating temperature on the nitridation of silicon as well as on the formation of α- and β-silicon nitride were investigated in batch and semi-continuous operations. The effects of the average residence time of silicon/alumina mixtures in the fluidized-bed reactor on the nitridation process were studied in semi-continuous operations. In batch operations, a maximum mass fraction of 15 wt% silicon powder could be added to alumina particles at temperatures in the range of 1200 to 1300°C without changing the fluidization quality. When the silicon fraction was increased to 20 wt%, fluidization failed immediately. With a mass fraction of 5% silicon powder, almost 100% α-silicon nitride, which was preferred in applications, was found in the product. β-silicon nitride was facilitated with an increase in silicon fractions in silicon/alumina mixtures. The nitridation process was strongly affected by the reaction temperature. The overall conversion of silicon increased with an increase in reaction temperature. Higher temperature also promoted the formation of β-silicon nitride. The overall conversion of silicon into silicon nitride was also enhanced by hydrogen concentrations. An increase in hydrogen concentration facilitated the formation of α-form silicon nitride. In the semi-continuous operation, the nitridation of 30 wt% silicon/70 wt% alumina mixtures could be achieved without changing the fluidizing quality. Almost 100% α-silicon nitride was found in the product when a 20 wt% silicon/80 wt% alumina mixture was nitrided at 1250°C for an average residence time of up to 4 hours. However, β-silicon nitride was formed when the mixture was nitrided at 1300°C for an average residence time of 3 hours. A mathematical model incorporating kinetic data and carryover of silicon powder was developed to described the total conversion of silicon in batch operation. A semi-continuous model was also proposed, which successfully predicted the overall conversion of silicon powder.
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