Abstract
Si-Ge alloys has been used for many years in the thermoelectric modules in the NASA space probes in which they convert heat produced by the radioactive decay of a heat source into electricity. This alloy is effective at high temperature (from 700 °C), so it is also a strong interest in the automotive industry. The strong incentive in this area to reduce fuel consumption leads researchers to develop thermoelectric modules that can operate at high temperatures. The composition at which SiGe alloys are the most thermoelectrically efficient is Si0.8Ge0.2. Their figure of merit (ZT) is generally close to 0.75 and 0.45 at 700 °C for type n and p respectively. As Germanium is very expensive, this study aims to develop a Si0.92Ge0.08 alloy that can compare to the existing Si0.8Ge0.2 alloys. To get to a higher level of performance, the thermal conductivity of the chosen composition has to be decreased, which is done by incorporating molybdenum silicides in the Si0.92Ge0.08 alloys.The Si-Ge alloy was synthesized by mechanical alloying, and sintered by SPS. The dopants used are phosphorous and boron for the n and p types respectively. The optimal doping level is 0.7%. ZT obtained for Si0.92Ge0.08 base alloys at 700 °C are equal to 0.7 and 0.5 for n and p types respectively. The nature of stable inclusions in the matrix has been determined by the CALPHAD method to obtain the ternary diagram Mo-Si-Ge. Then, the MoSi2 phase appears to be the only stable phase in the matrix Si0.92Ge0.08. The optimum volume fraction of molybdenum was 1.3% when the materials are sintered at 1280 °C. Therefore, the ZT obtained is higher than 1 at 700 °C for n-type and close to 0.8 for p-type. Adding nanoinclusions has increased performance by 43% (n-type) and 60% (p-type) at 700 °C.