Thermoelectric n-type silicon germanium synthesized by unidirectional solidification in microgravity

Abstract

Thermoelectric n-type Si0.2Ge0.8 and Si0.7Ge0.3 added 1at%P for Si–Ge were synthesized by unidirectional solidification in microgravity. Microgravity with ±10−2g for 0.46s was obtained in free fall using a 2m-drop tower. The microstructure of the sample solidified in microgravity and 1g was dendrite on the surface that contacted a Cu chill block. The microstructure of the cross section along the cooling direction was dendrite and the columnar dendrite structure was mainly aligned along the solidification direction. The dendrite became larger with depth from the surface that contacted the Cu chill block, and the width of the primary dendrite arm solidified in microgravity exceeded that in 1g. Ge was segregated to the secondary arm of columnar dendrite. The Si/Ge atomic ratio in the primary and secondary arm of Si0.7Ge0.3-1at%P solidified in 1g and microgravity was slightly higher than that of solidified Si0.8Ge0.2-1at%P in 1g and microgravity. The secondary arm of solidified Si0.7Ge0.3-1at%P was wider than that of solidified Si0.8Ge0.2-1at%P. P was distributed uniformly in Si–Ge solidified in microgravity. The electrical conductivities of Si0.8Ge0.2-1at%P and Si0.7Ge0.3-1at%P were anisotropic in directions along and perpendicular to the solidification direction. The dimensionless Figure of Merit, ZT, of the direction along the solidification direction at 1000K was estimated from thermal conductivity, electrical conductivity adjusted by the anisotropy, and the Seebeck coefficient in 1g and microgravity. The ZT of the sample solidified in microgravity at 1000K along the solidification direction was 1.19.