Among modern ceramic materials, silicon carbide (SiC) and silicon nitride (Si3N4) are successfully used in various high-tech applications. SiC offers a useful combination of mechanical properties. It is widely used as an abrasive and structural material. Its high hardness, chemical inertness, resistance to abrasion and oxidation at temperatures above the melting point of steel qualify it for use in extremely high temperature service conditions, such as seals and valves, rocket nozzles and wire dies, etc. Its applications such as bearings and extrusion dies take advantage of its excellent resistance to wear and erosion. The creep and thermal resistance properties of SiC find their use in high temperature electronics and heat exchanger tubes. The heating elements are also made of SiC. They can generate temperatures up to 1650°C and offer appreciable service life under air or inert media. However, any contact with moisture or hydrocarbon gases can negatively affect its age.
Silicon nitride has comparatively lower oxidation resistance and higher thermal conductivity than SiC. The main applications of silicon nitride are car parts and gas turbine engines. It has high strength, fracture toughness and refractoriness, which are required properties for ball bearings, anti-friction rollers. It has remarkable performance when exposed to molten metal and/or slag.
A combined form of silicon carbide and nitride has been developed as silicon carbide grains bonded in a silicon nitride matrix. This Si3N4 bonded silicon carbide is used for some critical applications where very high thermal shock resistance is required. For example, in the particular case of flameless motor starting, the temperature rises from room temperature to 1600°C in a few seconds, followed by an abrupt drop to 900°C in less than a second. Silicon carbide bonded to Si3N4 exclusively resists these conditions.
Traditional methods for producing these ceramic materials are energy intensive and therefore expensive. For example, the Acheson process, which is the most widely adapted method for producing commercial grade SiC, requires essentially 6 to 12 kWh to produce one kg of SiC. An economical method, using low-cost agro-industrial by-products, is the pyrolysis of rice hulls, first performed by Lee and Cutler in 1975. Since then, many researchers have discussed and used various process routes and modifications to obtain carbon carbide. silicon and/or silicon nitride, either in particulate or whisker form, from rice hulls.
Morphological studies on the RH reveal that micron-sized silica particles are distributed in the cellulose part of the RH. When these silica particles are made to react with carbon in the biomass part of HR under specific experimental conditions, silicon carbide can result. Furthermore, in addition to silicon carbide, modifications in the process mechanism lead to the formation of some other industrially useful products, viz. silicon nitride, silicon oxynitride (Si2N2O), ultrafine silica, and solar cell grade silicon.
The formation of silicon carbide and some other products can be generalized by following simplified equations of chemical reactions that take place at higher temperatures:
For silicon carbide:
SiO2 + 2C → SiC + CO2
SiO2 + 3C → SiC + 2CO
2Si + 2CO → 2SiC + O2
For silicon nitride and oxynitride:
3Si + 2N2 → Si3N4
3SiO2 + 6C + 2N2 → Si3N4 + 6CO
3SiO2 + 2N2 → Si3N4 + 3O2
Si3N4 + O2 → Si2N2O + SiN2O
SiN2O + Si → Si2N2O
For silicon:
SiO2 + 2Mg → 2MgO + Si
This metalothermal reduction of pure silica with magnesium (99% pure, as reducing agent) takes place in a temperature range of 500 – 950 °C in an Ar atmosphere.
In the present work, the pulverized HR was subjected to TG (from ambient to 800 ºC) and the crude HR to pyrolysis at higher temperatures (1350 – 1400 ºC) in nitrogen and argon atmospheres. The main objectives include the understanding of the thermal degradation of RH and the synthesis of SiC. Comparative studies of gravimetric thermograms and the effect of heating rate on the thermal stability of RH and product characterization by FT-IR, XRD and optical microscopy were carried out. The practical approach to getting the best possible yield (ie, optimized production) was emphasized in language that was easy to understand, even for people with no scientific background.