Thermoelectric properties of BiSbTe-type alloys prepared by chill-casting and cryo-milling

Highlights: • Cryomilling introduced fragmentations of layered eutectics leading to high ZT in the sample having Bi to Sb ratio 1:2. • Milling initially introduced high density of anti-site defects that lead to p-type behavior. • Antisites and vacancies interact on longer milling to produce donor-like n-type defect leading to a p-to n-type transition. • A comparative study of the thermoelectric behavior of bulk and Cryomilled BiSbTe₃ with or without excess tellurium was done. • The results are compared with all available studies on BiSbTe₃ and indicate no systematic trend. A need for further studies. The thermoelectric properties of Bi_xSb_1-xTe₃ processed through milling have been extensively studied with varied results that needs to be rationalized. Since cryo-milling is a relatively cleaner milling process compared to other milling processes, we explore this technique to reduce the particle size prior to compaction to get an insight into the thermoelectric properties. We first report the results of cryomilled BiSbTe₃ compacted by spark plasmas sintering (SPS) process and show that milling leads to a change in transport behavior from 'p' to 'n' type similar to that reported for Bi₂Te₃. It is shown that the defects induced by milling is responsible for this change in materials behavior. We further study the effect of change in Bi to Sb ratio and excess tellurium on the thermoelectric properties when processed through cryo-milling as well as casting using a chilled copper block. Excess tellurium leads to a microstructure containing a fraction of layered eutectic that gets further fragmented during cryo-milling. Although the Seebeck coefficient does not change significantly, reflecting the cleanliness of the process, the thermal conductivity changes during hot compaction leading to an improvement of ZT. However, a comparison of all the reported results does not reveal any systematics in thermoelectric properties reflecting the difficulties with these materials.