Effect of magnetic field on macrosegregation in (Bi, Sb)₂Te₃

A thermoelectric material (Bi_0.25Sb_0.75)₂Te₃ was grown by a vertical gradient freezing technique in high magnetic fields of up to 8 T. The radial distribution of composition showed macrosegregation when the specimens were solidified in the magnetic field. The radial segregation increased with increasing intensity of the magnetic field. On the other hand, the longitudinal distribution for both the specimens under 0 T and 8 T had a variation at the initial transient, but no effect of the magnetic field on the composition distribution was observed. A numerical calculation of the solid/liquid interface shape, thermal and solutal fields suggested that the difference in thermal conductivity between the solid and the liquid phases would cause the concave interface shape and a driving force of the convection around the s/l interface. It was concluded that the interaction of the convection mass transfer reduced by the magnetic field interacts with the diffusion mass transfer, causing the radial macrosegregation.