Thermal expansion of bulk nanostructured n-type SiGe nanocomposite from 300 to 1400 K

The use of the nanostructured nanocomposites aims to improve the peak figure of merit (ZT) values of thermoelectric alloys. In addition to ZT, the coefficients of the thermal linear expansion (CTLE) of these alloys are equally important for estimating stresses imposed by the thermal cycling inherent to waste heat recovery operations. In this study, we report the calculated “technical alpha”, i.e., the experimental mean CTLE values for the bulk nanostructured n-type Si₈₀Ge₂₀ alloy doped with phosphorus at 298–1220 K. The elemental composition of the spark plasma sintered (SPS)-specimen after thermal cycling in the infrared furnace is characterized by means of AES spectroscopy in combination with ion sputtering. Small amounts of contaminants, such as Fe and O, and large losses of P are detected. Elongations have been measured using ULVAC DL-1500-RH high-speed dilatometer with various rates of heating and cooling. Anomalous thermal behavior of the n-type Si₈₀Ge₂₀(P_2.2) composite is observed at high temperatures. Previously, the same anomaly was detected in the temperature dependence of the elongation of pure silicon. There are some discrepancies between our measurements of the thermal expansion of nanostructured n-SiGe composites and those reported in the literature. Moreover, in this study, SiGe solid solution is also studied to verify the additive scheme and to explore deviations from it for predicting CTLE values at higher temperature on the basis of the properties of the pure components. These calculations of the CTLE of undoped SiGe alloys match with the results of the dilatometric and X-ray experiments conducted during the period 1964–1987 and the calculations based on other theoretical approaches.