The Thermoelectric Performance of Silicon Carbide Semiconductor made from Rice Hull
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ABSTRACTS Thermoelectric properties (Thermoelectromotive force and electrical conductivity) of porous silicon carbide ceramics are evaluated. Porous silicon carbide ceramics are made from rice hull by reacting silica and carbon, both of which are included in rice hull, using hotpressing method[1]. When nitrogen gas is used as enviromental gas for hotpressing, this material becomes n-type semiconductor. It shows comparably high themoelectromotive force. Thermoelectric properties of this material change with the processing conditions.The relationship between thermoelectric properties and microstructure is also evaluated. INTRODUCTION It is known that the electrical conductivity of SiC increases with temperature [2], i.e. SiC is a semiconductor. It also shows comparably high thermoelectromotive force [3]. So it can be used as a thermoelectric material. Rice hulls are thrown away as by-product everywhere in Japan, every year. Rice hulls are mostly composed of silica and some organics, so they are a candidate material for the synthesis of silicon carbide. To utilize rice hulls, a new method to make porous R -silicon carbide ceramics from the rice hull was developed [1]. Both silica and carbon in rice hulls will react directly at high temperatures Rice Hull using hotpressing, when impregnated with Fe as reaction catalyst and sintering aid [4]. Crashing The silicon carbide ceramics, made from rice hulls with addition of nitrogen gas gives an n-type semiconductor with high thermoelectromotive force. The properties of I mpregnat ion of these silicon carbide ceramics and the relationship Fe catalyst between microstructure and properties are evaluated here.
Drying
EXPERIMENT
I Carbon izat ion
Preparation of porous silicon carbide
Preoxidation Fig.1 shows how the porous silicon carbide ceramics are prepared. Fe catalyst is impregnated into the rice hulls with the weight ratio of Fe203 / SiO2 = 0.043. Preoxidation is conducted at 570 K to make the C / Si0 2 weight ratio about 0.45. (This value was obtained after treated at 1273 K in nitrogen atmosphere.) This ratio is deficient in carbon compared to stoichiometric mass ratio 0.6 in equation 1.
Hotpressing Fig.I The Procedure of silicon carbide Ceramics made from Rice Hull
77 Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
SiO + 3C - SiC + 2CO (1) But when reaction occured, a part of SiOa was evaporated as SiO, so the ratio became adequate[l]. T Furnace This powder is then compacted in To Thermometer \ \ , \\\ a graphite mold (size; 30mm Thermo X 30mm) and hotpressed at -couple various conditions.The sintered Spring Pressure / Sample bodies obtained have 3 to 5mm Heater 1 wire To Measurement Equipment thickness and bulk density of 0.5 to 1.7g/cm (they vary with hotpressing condition). To measure thermoelectric properties, it was cut into Fig.2 The outline of Measuring Appartus rod-like shapes,(size; (3-5mm) X (3-5mm) X 20mm). Hotpressing conditions are as follows; enviromental nitrogen gas pressure are 1 X 10 ' Pa to 6 X 10 ' Pa, compacting pressure
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