α-Si3N4 and Si2N2O whiskers from rice husk and industrial rice husk ash

Rice industrialization worldwide generates significant amounts of rice husk as a by-product. When rice husk is burned to obtain energy, a relatively common practice, a substantial portion of the husk turns into ash, and both constitute environmental liabilities. Using rice husk and ash as starting materials to produce high-value products could help in mitigating the environmental impact while providing economic revenue. Rice husk and rice husk ash as produced in a local cogeneration plant without any pretreatments were evaluated as feasible sources for silicon nitride (Si3N4) and silicon oxynitride (Si2N2O) whiskers by carbothermal reduction and nitridation. Rice husk and the ash were held at temperatures between 1200 and 1400 °C for 3 h under flowing nitrogen. Increasing soaking temperature values led to higher whisker development for both starting materials, with the best results observed at 1400 °C. Whereas α-silicon nitride whiskers were obtained when rice husk was employed, the graphite surface-to-ash ratio dictated whisker composition for the ash. Treatment of the ash at the soaking temperature value of 1400 °C led to silicon oxynitride for lower graphite surface-to-ash ratios, but when this ratio was increased, α-silicon nitride predominated. α-silicon nitride whiskers had cross sections ranging from about 100 nm to 1 µm in width, whereas the silicon oxynitride whiskers had cross sections ranging from approx. 100 to 500 nm in diameter. Both types of whiskers were observed to be in the millimeter length range.