Heavily doped semiconductors, such as silicon-germanium (SiGe) thermoelectric couples (also called thermocouples or unicouples), are used in space exploration.
SiGe alloys present good thermoelectric properties. Their performance in thermoelectric power production is characterized by high dimensionless figures-of-merit (ZT) under high temperatures, which has been shown to be near 2 in some nanostructured-SiGe models.
SiGe alloy devices are mechanically rugged and can withstand severe shock and vibration due to its high tensile strength (i.e. >7000 psi) and low dislocation density. SiGe material is malleable with standard metallurgical equipment and bonds easily to construct components. SiGe alloy devices can operate under high temperatures (i.e. >1300 ˚C) without degradation due to their electronic stability, low thermal expansion coefficient and high oxidation resistance.
Near the Sun, solar cell performance deteriorates from high incident particle flux and high temperatures from heat flux. However, thermoelectric energy conversion systems that use thermoelectric materials (e.g. SiGe alloys) as a supplemental source of power for missions near the Sun can operate unprotected in vacuum and air environments under high temperatures due to their low sensitivity to radiation damage. Such properties have made SiGe thermoelectrics convenient for power generation in space.
The multifoil cold stack assembly, composed of molybdenum, tungsten, stainless steel, copper, and alumina materials, provides the insulation between the electrical and thermal currents of the system. The SiGe n-leg doped with boron and SiGe p-leg doped with phosphorus act as the intermediary between the heat source and electrical assembly.