Silicon–Germanium: The Legacy Lives On

Abstract

Alloy systems comprised of silicon with germanium, lead with tellurium, and bismuth with antimony have constituted a majority of thermoelectric applications during the last half-century. These legacy materials are primarily covalently bonded with a maximum ZT near one. Silicon–germanium alloys have provided the thermal to electrical conversion for many of NASA’s radioisotope thermoelectric generator (RTG) configurations and for nearly all of its deep space and outer planetary flights, such as Pioneer I and II, Voyager I and 11, Ulysses, Galileo, and Cassini. The remarkable success of these materials and their respective devices is evidenced by the fact that there has never been a failure of the RTG systems even after over 1 billion cumulative mission-hours. The history of this alloy system as a thermoelectric conversion material spans over six decades and research to further improve its performance continues to this day. Si-Ge alloys have long been a mainstay of thermoelectric research because of a fortuitous combination of a sufficiently high melting temperature, reasonable energy band gap, high solubility for both n- and p-type dopants, and the fact that this alloy system exhibits complete miscibility in the solid state, which enable tuning of both electrical and thermal properties. This article reviews the history of silicon–germanium as a thermoelectric material and its use in NASA’s RTG programs. Since the device technology is also a critical operational consideration, a brief review of some of the unique challenges imposed by the use in an RTG is also discussed.

Advancing very high temperature thermoelectric performance of Yb4Sb3 through dual-substitutions: a combined experimental and theoretical study

“…4,5 The conversion efficiency of such modules mostly depends on the performance of the integrated thermoelectric components, which is evaluated by the adimensional figure of merit zT = S 2 T/(rk), S being the Seebeck coefficient, T the absolute temperature, r the resistivity and k the thermal conductivity of the material, respectively. Currently, viable solutions within the realm of very high temperatures remain rather constrained, owing to the scarcity of materials presenting suitable transport properties above 800 K. [6][7][8][9] Both n-and p-type SiGe have been used for decades as the main solution by NASA, 10 especially for radio-isotope thermoelectric generators (RTGs), 11 complemented by the Zintl phase Yb 14 MnSb 11 which serves as a p-type alternative. 12 Recent investigations have highlighted the potential candidacy of Yb 21 Mn 4 Sb 18 as well within this temperature range, exhibiting a zT value of 0.8 at 800 K. 13 Furthermore, Half-Heusler compounds have also emerged as promising candidates for the development of high temperature thermoelectric modules up to 1100 K. 14 While the fabrication of thermoelectric devices for power generation necessitates meticulous considerations encompassing low contact resistances and harmonious thermal expansion, 15 the paramount factor governing net power generation indisputably remains the figure of merit exhibited by both n-type and p-type constituents.…”

Determination of the Seebeck Coefficient and Other Thermoelectric Parametersusing Specific Resistivity and Concentration of Charge Carriers of N-Si0.96Ge0.04 Alloy Irradiated by 60Co γ-photons

“…SixGe1-x alloys are materials widely used in many fields of science and technology [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Nuclear radiation detectors, pressure sensors, thermistors, thermal neutron monochromators and X-ray diffractometry devices are also created on their basis [16][17][18][19].…”

Sputtering Codeposition and Metal-Induced Crystallization to Enhance the Power Factor of Nanocrystalline Silicon

“…Materialwise, Bi 2 Te 3 , , PbTe, , and SiGe − can be considered the classical functional ingredients to constitute a thermoelectric device. The last two have dominated the application area representing the biggest success of thermoelectricity, space exploration, due to its use in radioisotope thermoelectric generators (RTG). − Not being restricted to the three aforementioned compounds, material research has broadly expanded toward selenides, − skutterudites, half-Heuslers-like (Hf,Ti,Zr)­CoSb, or TiNiSn and full Heuslers, including the current development of Fe 2 VAl-related alloys. , …”