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To achieve high thermal-to-electric energy conversion efficiency, it is desirable to operate thermoelectric generator devices over large temperature gradients and also to maximize the thermoelectric performance of the materials used to build the devices. However, no single thermoelectric material is suitable for use over a very wide range of temperatures (~300-1000K). It is therefore necessary to use different materials in each temperature range where they possess optimum performance. This can be achieved in two ways: 1) multistage thermoelectric generators where each stage operates over a fixed temperature difference and is electrically insulated but thermally in contact with the other stages 2) segmented generators where the p- and n-legs are formed of different segments joined in series. The concept of integrating new thermoelectric materials developed at the Jet Propulsion Laboratory into a segmented thermoelectric unicouple has been introduced in earlier publications.

The Cassini spacecraft, NASA's mission to investigate the Saturn system, has undergone a system-level thermal balance test program to permit verification of the engineering subsystem thermal designs in the simulated worst-case environments. Additionally, other objectives such as functional checkouts, collection of thermal data for analytical model adjustment, vacuum drying of propellant tanks, and flight temperature transducer verification were also completed. In the interest of cost and schedule, transient off-Sunpoint conditions were not tested. The testing demonstrated that the required system resources such as heater power and radiator area were adequate for all engineering subsystems. The only changes required from the results were related to the operation of some of the subsystems. In the instance of the thruster cluster assemblies, allowable flight temperature limits were exceeded for the assumed operational environment.

Although important efforts are actually devoted to improve Si-Ge materials, their thermoelectric energy conversion efficiency remains relatively low and the adimensional ZT value does not exceed 1. Higher values can be obtained by investigating new materials. A search for new high temperature thermoelectric materials identified a certain number of compounds between transition metals and bismuth, antimony and germanium as potential candidates. Results of the preliminary synthesis of samples by a variety of techniques (Bridgman, mechanical alloying…) are presented as well as some electrical measurements. Some compounds showed interesting properties and need to be investigated in more details.