Transformation Mechanism From Sepiolite to Silicon Nitride (Si_3N_4)


Abstract: Powder metallurgy production of Si_3N_4 materials has been used in different fields of production such as ceramic cutting tools, turbine components, automotive valves and ball-bearing components. This study was undertaken to determine the transformation mechanisms of sepiolite type magnesium hydrosilicate clay mineral to Si_3N_4 by carbothermal reduction and nitridation (CTRN). A sepiolite and carbon black powders mixture with a C/SiO_2 molar ratio of 3 was used for the CTRN process. The CTRN process was conducted in a graphite sample holder in an atmosphere-controlled tube furnace under nitrogen flow of 4.5 cm^3/min, and 1300, 1400°C for 2 and 4 hours exposure. Cristobalite and tridimite structures inside sepiolite were reduced to SiO by carbon black and nitridated by environmental nitrogen gas flow. As a result of the transformation, white fibre-like micromorphologies were formed at the edges inside the sample holder and ceramic tube. This product was identified as pure \alpha-Si_3N_4 by XRD and SEM-EDX analysis and occurred during the gas phase reaction. The second tranformation mechanism occurred between the solid sepiolite and nitrogen gas in the sample holder. Analysis showed that this product was \beta-Si_3N_4.

Keywords: Sepiolite, Carbothermal, Reduction, Nitridation, Fibre Si_3N_4

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