Anisotropic layered Bi₂Te₃-In₂Te₃ composites: control of interface density for tuning of thermoelectric properties

Layered (Bi_1-xIn_x)₂Te₃-In₂Te₃ (x = 0.075) composites of pronounced anisotropy in structure and thermoelectric properties were produced by zone melting and subsequent coherent precipitation of In₂Te₃ from a (Bi_1-xIn_x)₂Te₃ (x > 0.075) matrix. Employing solid state phase transformation, the Bi₂Te₃/In₂Te₃ interface density was tuned by modifying the driving force for In₂Te₃ precipitation. The structure-property relationship in this strongly anisotropic material is characterized thoroughly and systematically for the first time. Unexpectedly, with increasing Bi₂Te₃/In₂Te₃ interface density, an increase in electrical conductivity and a decrease in the absolute Seebeck coefficient were found. This is likely to be due to electron accumulation layers at the Bi₂Te₃/In₂Te₃ interfaces and the interplay of bipolar transport in Bi₂Te₃. Significantly improved thermoelectric properties of Bi₂Te₃-In₂Te₃ composites as compared to the single phase (Bi_1-xIn_x)₂Te₃ solid solution are obtained.