ICT2014 Abstracts

Over the past decade the widely used p-type (Bi_2-xSb_x)Te₃ bulk thermoelectric materials have been subject to various nanostructuring processes for higher thermoelectric performance. However, these nanostructuring processing were conducted on compositions optimized for bulk materials (x ~1.52-1.55).  This leads to the question of whether the optimal composition for bulk materials is the same for their nanoscale counterparts. In this work we hydrothermally grew Bi_2-xSb_xTe₃ nanopowders (nominally, x = 1.46,1.48,1.52 and 1.55) and measured their thermoelectric properties on cold-pressed vacuum-sintered pellets (74-78% of the theoretical density) below 300 K. The measurements were conducted 18 months apart to probe the aging phenomena, with the samples stored in ambient conditions. We have found that (i) the peak of thermopower shifts to lower temperatures upon nanostructuring but it shifts back to higher temperatures upon aging; (ii) the electrical conductivity degrades by a factor of 1.5-2.3 upon aging while the temperature dependence is largely retained; and (iii) the ZT of freshly made samples is sensitive to the x value, a maximum ZT ~1.25(~ 0.62) at~ 270 K (~ 255 K) was attained in the freshly made sample x = 1.55(x= 1.46), respectively; while the ZT of aged samples is significantly lowered by a factor of 2-4 but lesser x-dependent. These observations have been discussed in the context of charge buildup and compensation at grain boundaries.