Enhancement of Thermoelectric Properties of Bulk Si
by Dispersing Size-Controlled VSi2
The efficiency of a thermoelectric (TE) device is determined by the temperature difference applied across it and the dimensionless figure␣of merit (ZT) of its TE materials. High ZT value means high performance, which requires high carrier mobility (μ) as well as low lattice thermal conductivity (κlat); i.e., ZT is roughly proportional to μ/κlat. Si exhibits good electrical properties, but its κlat is remarkably high, leading to low ZT. To enhance the ZT, we adopted nanostructuring via dispersion of nanoscale silicide precipitates in bulk Si, which would lead to κlat reduction while retaining good electrical properties. Here, we focused on VSi2 as the dispersed precipitates, because Si and VSi2 have a eutectic point located in the Si-rich part. The eutectic composition is Si:VSi2 = 91:3, i.e., Si:V = 97:3 in atomic ratio. In the present study, we successfully fabricated Si–VSi2 composite bulks with composition (Si100P3)97V3 and various morphologies by a method combining melt spinning (MS) and spark plasma sintering (SPS). By controlling mainly the MS cooling rate and SPS temperature, the size and dispersion morphology of the VSi2 precipitates were optimized. It was revealed that, on dispersing size-controlled VSi2 precipitates, κlat decreased while μ did not change significantly, thus μ/κlat increased. A maximum ZT value of 0.23 at 1070 K was obtained, approximately 37% higher than that of optimized bulk Si.