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A5: Nanoscale and low dimensional effects
Semiconducting β-FeSi2 is composed of nontoxic elements, which exist in great abundance on earth. β-FeSi2 is a potentially useful material in thermoelectrics. The purpose of this work is to improve the dimensionless figure of merit, ZT value by decreasing the thermal conductivity, which can be made by decreasing the grain size of FeSi2 bulk by processing of FeSi2 powders with planetary ball-mill, followed by sintering with electric discharge plasma activated sintering system(Ed-PAS, Elenix, Inc.). α-FeSi2 powders (purity 99.9%) specially produced for us by Fukuda Metal Foil & Powder Co., Ltd. were used as starting materials. The α-FeSi2 powders were annealed for 5hrs at 900oC by an electrical furnace in N2 gas flow in order to convert the α phase to β phase. Subsequently, these powders were planetary ball-milled with a PlanetM2-3F(Nagao System, Inc., Japan) at 1260rpm for 5hrs in liquid ethanol(C2H6O) or hexane(C6H14), which was immersed into Al2O3 crucibles in the air and Ar, respectively. After the milling, the grain sizes of powders were decreased to approximately 400nm. Then, the FeSi2 powders were sintered with Ed-PAS at 780-1000oC for 5-10min in vacuum. XRD analysis showed that after sintering the phase of bulk transformed from β-FeSi2 to α-FeSi2 with small amount of ε-FeSi. α-FeSi2 can convert to β-FeSi2 by subsequent annealing at 900 oC. The density of bulk with milling in ethanol was 81%, while that in hexane increased to 91.2%. EDS analysis showed that the oxygen composition ratio included in FeSi2 powders with milling in ethanol was 16.7%, whereas that with milling in hexane decreased to 4.4%. These results indicate that the oxidation of FeSi2 powders can be suppressed by milling in hexane and a formation of oxides which surround the FeSi2 powders prevents sintering of FeSi2. The Hall effect measurements and thermoelectric properties of these bulks are further investigated.