Effects of reaction parameters on the carbothermal reduction and nitridation of rice husk ash for silicon nitride synthesis

Abstract

The synthesis of silicon nitride, one of the most promising structural materials for high-temperature and high mechanical-stress applications, from rice husk, which is an abundant agricultural waste was investigated. Rice husk was first pyrolyzed at 600 degree celcius for 3 h to produce rice husk ash (RHA). The RHA was then subjected to the carbothermal reduction and nitridation process at the temperature in the range of 1400-1470 degree celcius. The product obtained could be categorized into three forms, i.e. carbon containing dark gray powder at the bottom of the sample holder, white fibrous material on top and long fibers on the edge of sample holder. X-ray diffraction analysis confirms that silicon nitride is the major crystalline constituent in all product types. Transmission electron micrographs reveal that the long fiber is polycrystalline while the fibrous top layer consists of the collection of silicon nitride single crystals. The temperatureprogrammed oxidation of the dark gray product suggests that the dark gray powder contains residual carbon, which can be removed. Mass fraction of dark gray powder decreases, while the fraction of both white long fibers and white fibrous top layer increase when either reaction temperature or reaction duration is increased. By changing the overall flow of the gas mixture during the reaction, it is suggested that the process involves in the generation of siliceous species. Hydrogen addition is found to be essential for generation of silicon monoxide vapor and formation of fibers. Gas-solid reaction between nitrogen, carbon and silica is enhanced in the carbothermal reduction and nitridation of the acid-treated rice husk ash. Mechanism of the process is also investigated by comparing with the carbothermal reduction and nitridation of pure silica mixed with excess carbon.