Influence of YbP on the thermoelectric properties of n-type P doped Si95Ge5 alloy

Abstract

Since the report of high zT in Si95Ge5 there has been significant interest in low Ge alloy compositions for thermoelectric applications. The application of YbP was explored as a means to lower thermal conductivity. A series of 3% phosphorus (P) doped n-type Si95Ge5 (SiGe) alloy was reacted with YbH2 (0, 1, 2%). YbP was formed in the SiGe alloy matrix from the reaction between YbH2 and P during the Spark Plasma Sintering (SPS) process. Thermoelectric property measurements were performed on sintered pellets from room temperature to 1273 K. X-ray diffraction patterns were collected from the ground powder samples and confirmed the main phase possessed diamond structured Si95Ge5 (space group: Fd3¯m) as well as the presence of YbP (space group: Fm3¯m). The carrier concentration of the sample was controlled by the amount of YbH2 added, removing some of the phosphorus to form YbP. n-type Si95Ge5 alloy samples with higher YbP amounts showed higher electrical resistivity and lower thermal conductivity attributed to loss of the P dopant. Another composite series of n-type Si95Ge5 with YbP were synthesized with additional P compositions (1, 2%). The thermoelectric properties were characterized from room temperature to 1273 K, and the samples possess electrical resistivity, carrier concentrations, and thermal conductivity as expected from the additional P dopant. The presence of YbP lowered lattice thermal conductivity when the sample was appropriately doped. The Seebeck coefficients were measured with both off-axis and uniaxial axis experimental configurations. These results show that the off-axis measurements overestimate the Seebeck coefficients of the Si95Ge5 alloy samples. This is attributed to a cold finger effect and therefore only the uniaxial data are combined for zT calculations. The n-type Si95Ge5 samples with YbP and less than 3% P dopant show similar zT compared with the Si95Ge5 sample with no YbP inclusions and 3% P dopant with a peak zT of 0.6 at 1200 K.