Among modern ceramic materials, silicon carbide (SiC) and silicon nitride (Si3N4) are successfully being used in several high-tech applications. SiC offers a useful combination of mechanical properties. It is extensively used as abrasives 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 under severely high temperature service conditions such as seals and valves, rocket nozzles and wire dies etc. Its applications as bearings and extrusion dies utilise its excellent wear and erosion resistance. Thermal and creep resistance properties of SiC find its uses in high temperature electronics and heat exchanger tubes. Heating elements are also made of SiC. They can generate temperatures up to 1650 °C and offer appreciable life under air or inert media. However, any contact with moisture or hydrocarbon gases can adversely affect their age.
Silicon nitride has comparatively lower oxidation resistance and higher thermal conductivity than SiC. Major applications of silicon nitride are as automotive and gas turbine engine parts. It has high strength, fracture toughness and refractoriness which are required properties for ball bearings, anti-friction rollers. It performs remarkably 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 silicon nitride matrix. This Si3N4-bonded silicon carbide is used for some critical applications where very high thermal shock resistance is required. For instance, in particular case of flame-out engine start-up, temperature reaches from ambient to 1600 °C in few seconds followed by an abrupt decrement to 900 °C in less than one second. Si3N4-bonded silicon carbide exclusively endures these conditions.
Traditional methods to produce these ceramic materials are energy intensive and hence expensive. For example, the Acheson process, which is the most widely adapted method to produce commercial-grade SiC, essentially takes 6 – 12 kWh to yield one kg of SiC. An inexpensive method, that uses low cost agro-industrial byproduct, is the pyrolysis of rice husks, first carried out by Lee and Cutler in 1975. Since then many researchers have discussed and used various process routes and modifications to obtain silicon carbide and/or silicon nitride, either in particulate or in whisker form, from rice husks.
Morphological studies on RH reveal that micron size silica particles are distributed in cellulosic part of RH. When these silica particles are made to react with carbon in biomass part of RH under specific experimental conditions, silicon carbide can result. Moreover, besides silicon carbide, modifications in process mechanism lead to formation of some other industrially useful products, viz. silicon nitride, silicon oxynitride (Si2N2O), ultra-fine silica, and solar-cell grade silicon.
Formation of silicon carbide and some other products can be generalised by following simplified equations of chemical reactions taking 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 metallothermic reduction of pure silica with magnesium (99% pure, as reducing agent) takes place in a temperature range 500 – 950 °C in Ar atmosphere.
In the present work, pulverised RH was subjected to TG (from ambient to 800 ºC) and raw RH to pyrolysis at higher temperatures (1350 – 1400 ºC) under nitrogen and argon atmospheres. Main objectives include comprehension of thermal degradation of RH and synthesis of SiC. Comparative studies on gravimetric thermogrammes and effect of heating rate on thermal stability of RH were carried out along with characterisation of products by means of FT-IR, XRD and optical microscopy. The practical approach to get maximum possible yield (i.e. optimised production) was empasised in an easy to understand language, even for the persons not having scientific background.
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