Development of High Efficiency Thermoelectric Devices for In-Space Propulsion

The employment of nuclear power for the future long-duration exploration missions requires thermoelectric devices to convert heat to electric energy with radioisotope thermoelectric generators(RTGs). Recently, NASA/HQ Game Changing Technology Development program has awarded the "Ride the Light" project with concept seeking to use power sources, such as lasers and microwave energy, to explore multiple technologies to function as receiving elements of the beamed power. 1.Production of high efficiency PbAgSbTe material. Since the performance of thermoelectric materials is rated by the dimensionless figure of merit, ZT = a2¿T/¿, the first objective of the project is to produce PbAgSbTe material with the highest possible value of ZT. The crystal growth will be performed under different Pb, Sb or Te over-pressures so as to maximize the electrical conductivity, by controlling the stoichiometry as well as the dopants of the grown crystals. 2.Investigation on high efficiency Si-Ge thermoelectric material. In addition to maximize ZT for the PbAgSbTe system, the operating conditions for thermoelectric device also needed to be considered to maximize its efficiency. A higher energy conversion efficiency of thermoelectric devices can be obtained by adopting two (or more) different thermoelectric materials to cover the whole temperature range. 3.Production and testing of high efficiency cascaded thermoelectric devices. A cascaded thermoelectric device will be assembled using n-type PbAgSbTe and p-type PbTe materials at the low temperature range and n- and p-type SiGe materials at the high temperature range. The conversion efficiency will be determined by the power output measured from the current and the resistances of the two electrical circuits and the power input estimated from the furnace heater. Dr. Su will be responsible for the first and the second objectives, i.e., the experimental growth of both the n and p-type PbAgSbTe and SiGe crystals by directional solidification under various conditions and the measurements of ZT as functions of temperature. All of the experimental facilities required for carrying out the investigation exist at Materials and Processing Laboratory in MSFC. These facilities include equipment for crystal growth, such as the vacuum pumps, rocking furnaces, and three directional solidification furnaces (with different capabilities).

Because highly efficient thermoelectric materials are required to produce advanced RTGs for future long-duration exploration, the project is aligned with "Advanced nuclear studies with emphasis on connectivity to future in-space stage requirements", "Advanced in-space propulsion research", as well as "Advanced manufacturing processes".

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