Optimising the Yield of Silicon Carbide Synthesised from Indigenous Biomass Husk using Different Catalysts

Silicon carbide offers a wide spectrum of electrical, chemical and mechanical properties, for which it is used in a variety of modern applications [1]. Silicon Carbide has high density, good thermal conductivity, extreme hardness, excellent corrosion and thermal shock resistance.These versatile properties enable it to beused in high temperature and structural applications. Due to high energy band gap and saturated drift velocity, it is also used for high temperature semiconductor material as compared to silicon based materials [2]. High purity silicon carbide can be synthesised from rice husk (RH) which generally contains 71 – 87 wt% organic components such as cellulose, lignin and sugar [3], and 13-29 wt% inorganic components. Oxidation of RH vaporizes the organic part and residual inorganic component is termed as rice husk ash (RHA) [4,5] more than 95% of which comprises ultrafine silica [6]. RHA is being used for some specific purposes such asceramic glaze, insulator, roofing shingles, waterproofing chemicals, oil spill absorbent specialty paints, flame retardants, carrier for pesticides and insecticides, fertilizer conditioner and zeolites synthesis [7]. The formation of SiC from RH pyrolysis is accomplished in a single or two stages. In single stage method, RH is pyrolysed at 1300-1500°C [8]. The amorphous silica in the RH can produce silicon carbide, silicon nitride, silicon oxynitride, pure silicon and other silicon based compounds depending upon the composition, atmosphere, reaction temperature and pre-pyrolysis treatment. However, the following chemical reactions occur during the carbothermal reduction of RH in the inert or reducing atmosphere at elevated temperatures [9]:


Introduction
Silicon carbide offers a wide spectrum of electrical, chemical and mechanical properties, for which it is used in a variety of modern applications [1]. Silicon Carbide has high density, good thermal conductivity, extreme hardness, excellent corrosion and thermal shock resistance.These versatile properties enable it to beused in high temperature and structural applications. Due to high energy band gap and saturated drift velocity, it is also used for high temperature semiconductor material as compared to silicon based materials [2]. High purity silicon carbide can be synthesised from rice husk (RH) which generally contains 71 -87 wt% organic components such as cellulose, lignin and sugar [3], and 13-29 wt% inorganic components. Oxidation of RH vaporizes the organic part and residual inorganic component is termed as rice husk ash (RHA) [4,5] more than 95% of which comprises ultrafine silica [6]. RHA is being used for some specific purposes such asceramic glaze, insulator, roofing shingles, waterproofing chemicals, oil spill absorbent specialty paints, flame retardants, carrier for pesticides and insecticides, fertilizer conditioner and zeolites synthesis [7]. The formation of SiC from RH pyrolysis is accomplished in a single or two stages. In single stage method, RH is pyrolysed at 1300-1500°C [8]. The amorphous silica in the RH can produce silicon carbide, silicon nitride, silicon oxynitride, pure silicon and other silicon based compounds depending upon the composition, atmosphere, reaction temperature and pre-pyrolysis treatment. However, the following chemical reactions occur during the carbothermal reduction of RH in the inert or reducing atmosphere at elevated temperatures [9]: SiO 2 (s) + 3 C (s) →SiC (s) + 2 CO (g) (i) SiO 2 (s) + 2 C (s)→Si (s) + 2 CO (g) (ii) And overall reaction is: SiO 2(amorphous) + 3C (amorphous) → SiC (crystalline) + 2CO (gas) Prior to pyrolysis, RH can be subjected to various catalytic treatments. Pyrolysis of HCl treated RH led to improved yield of SiC as compared that obtained from untreated raw RH [10]. RH can be treated with alkaline solution such as that of sodium hydroxide.Sodium silicatecan also be used as catalyst and has been proved beneficial in order to get enhanced yield of the products [11]. Using hydrated cobalt chloride and hydrated iron chloridealong with ammonium hydroxideas catalysts produce silicon carbide at comparatively low temperatures [12]. A study on pyrolysis of RHin the plasma arc reactor under argon atmosphere inferred that considerable product yield can be obtained in very short time [13]. Some other useful catalytic treatments include the use of iron chloride, cobalt chloride and nickel chloride [9].

Experimental Work
RH was pre-treated with three different reagents, viz. Na 2 SiO 3 , HCl and HCl + Na 2 SiO 3 . Firstly, samples were prepared by soaking RH for 24 h in solutions of different concentrations of Na 2 SiO 3 in distilled water ranging from 1-6 g/l(designated as A1-A6). Secondly, RH was boiled in 5 N HCl solution for 1 h. Acid treated rice husk (B1) was thoroughly rinsed with distilled water. For the third category (C1), RH was boiled in 5 N HCl solution for 1 h, thoroughly washed with distilled water, dried at 90°C for sufficient time and then soaked in 2 g/l solution of Na 2 SiO 3 in distilled. All the samples were ground tomesh and stored in a drying oven. Finely ground and dried RHwere subjected to different processes including direct pyrolysis, oxidation of pyrolysed powder to remove residual carbon and finally HF treatment to remove SiO 2 and get pure SiC powder.

Results and Discussion
Two peaks corresponding to SiO 2 and SiC appeared in each diffractogram. The average value of SiC peaks was 2θ = 35.977, belong to (111) planes of β-SiC. All the peaks of SiO 2 appeared approximately at same angles with an average value 2θ = 21.771. During continuous heating, RH powder undergoes various changes such as volatilisation of organic matter, graphitisation of carbon, crystallisation of amorphous silica, formation of silicon carbide whiskers and conversation of SiC whiskers into particles. Presence of silica was in the form of cristobalite. The Figure1 shows a comparison of XRD patterns of the pyrolysed and then allowed to cool at the same rate. Argon gas was purgedduring the whole course of heating and cooling at a rate of 0.1 lmin -1 . Oxidation of pyrolysed products was carried out at 750°C for 3 h in SentroTech (ST-1800) box furnace. The pyrolysed products after removal of unreacted carbon were identified by means of X-Ray diffraction analysis (using PANalytical Diffractometer) using Cu-K α radiation. Following the XRD analysis, oxidised powder wastreated with 40% hydrofluoric acid in order to obtain pure SiC powder. Morphology of the desired product (SiC powder) was examined by means of scanning electron microscope (SEM SU-3500). Energy dispersive spectrometry (EDS) coupled with SEM provided elemental analysis of the products. products from sodium silicate treated RH. The highest peak of SiC was observed from pyrolysis of sample A2. Figure 2a and 2b shows micrographs of pure SiC powder obtained from samples A2 and B1. It is evident that silicate treated sample produced micron size carbide particles of irregular geometry whereas acid treatment fostered whiskers formation.
The EDS spectrum of A2 is shown in Figure 3 and corresponding elemental analysis given in Table 1 illustrate that the product contained Si, C, O and some other impurities present in the form of metallic oxides in the powder.
The effect of concentration of sodium silicate with respect to weight percent of SiC and SiO 2 is shown in Figure 4. As the concentration of Na 2 SiO 3 increased from 2-6 g/l, the high concentration caused a thick coating of silicate on the surface of RHs which promotes the formation of SiO 2 , because it actsas a barrier between the SiO gas in the media and the amorphous C in RH thereby delaying the pyrolysis reactions. Moreover,as the temperature of pyrolysis reactions increases from 1350 to 1550°C the formation of whiskers also decreases.Presence of  Calculated theoretical yield of SiC from RH is 58.3 wt% and practically the yield obtained was 51 wt% [11]. Maximum yield obtained in this study was 41.46 wt% in case of sample A2.

Conclusions
Pre-treatments with sodium silicate proved effective to improve the yield of SiC from rice husks. Pyrolysis of pre-treated rice husks with 2 g/l (Na 2 SiO 3 ) resulted in maximum yield i.e. 41.46 wt%. Pre-treatment with hydrochloric acid had slight effect on increasing the yield of the product.