Application of silicon-germanium thermoelectrics in space exploration

<h2 id="properties">Properties</h2>

Heavily <a href="/facts/Doping_(semiconductor)/WrGdtWnm">doped</a> <a href="/facts/Semiconductors/zFf3mGTE">semiconductors</a>, such as <a href="/facts/Silicon-germanium/eKH6o6Uz">silicon-germanium</a> (<a href="/facts/SiGe/eKH6o6Uz">SiGe</a>) thermoelectric couples (also called <a href="/facts/Thermocouples/FW9cYU5D">thermocouples</a> or unicouples), are used in space exploration.<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a><a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>
SiGe <a href="/facts/Alloys/I7w7WQWR">alloys</a> present good <a href="/facts/Thermoelectric/GbRYmwIG">thermoelectric</a> properties. Their performance in thermoelectric <a href="/facts/Power_(physics)/mkaEyRnn">power</a> production is characterized by high <a href="/facts/Dimensionless_quantity/TINrpQIq">dimensionless</a> <a href="/facts/Figure_of_merit/kdSJ3Cpy">figures-of-merit</a> (ZT) under high <a href="/facts/Temperatures/QyGe4gmj">temperatures</a>, which has been shown to be near 2 in some <a href="/facts/Nanostructured/rkeQGqt7">nanostructured</a>-SiGe models.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a><a class="footnote-ref" id="fnref:4" href="#fn:4">4</a><a class="footnote-ref" id="fnref:5" href="#fn:5">5</a>
SiGe alloy devices are mechanically rugged and can withstand severe shock and vibration due to its high <a href="/facts/Ultimate_tensile_strength/Cv2jPplR">tensile strength</a> (i.e. >7000 psi) and low <a href="/facts/Dislocation/m1tkJYIq">dislocation density</a>.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a><a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> SiGe material is <a href="/facts/Malleable/SBJsahEk">malleable</a> with standard <a href="/facts/Metallurgical/1SANyxX4">metallurgical</a> equipment and bonds easily to construct components.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> SiGe alloy devices can operate under high <a href="/facts/Temperatures/QyGe4gmj">temperatures</a> (i.e. >1300 ˚C) without degradation due to their electronic stability, low <a href="/facts/Thermal_expansion_coefficient/BP2vjX0N">thermal expansion coefficient</a> and high <a href="/facts/Oxidation/Pyd5RVcj">oxidation</a> resistance.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a><a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>
Near the <a href="/facts/Sun/CadPCVHh">Sun</a>, <a href="/facts/Solar_cell/3L814UCC">solar cell</a> performance deteriorates from high incident <a href="/facts/Flux/Aw3P5Bvb">particle flux</a> and high temperatures from <a href="/facts/Heat_flux/YjF54Tcf">heat flux</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> However, thermoelectric energy conversion systems that use <a href="/facts/Thermoelectric_materials/p8FKD76U">thermoelectric materials</a> (e.g. SiGe alloys) as a supplemental source of power for missions near the Sun can operate unprotected in <a href="/facts/Vacuum/BTk3goyV">vacuum</a> and air environments under high temperatures due to their low sensitivity to <a href="/facts/Radiation_damage/xTPC6Xg3">radiation damage</a>.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> Such properties have made SiGe thermoelectrics convenient for power generation in <a href="/facts/Space/tq6LUM3C">space</a>.
The multifoil cold stack assembly, composed of <a href="/facts/Molybdenum/RX6vp2bF">molybdenum</a>, <a href="/facts/Tungsten/WYwbNDYX">tungsten</a>, <a href="/facts/Stainless_steel/IDdo8bEx">stainless steel</a>, <a href="/facts/Copper/I8PEE8oX">copper</a>, and alumina materials, provides the insulation between the <a href="/facts/Electric_current/Dc1xdnUX">electrical</a> and <a href="/facts/Heat_current/FQEdnFVf">thermal currents</a> of the system. The SiGe n-leg doped with <a href="/facts/Boron/Pmwgd7Mf">boron</a> and SiGe p-leg doped with <a href="/facts/Phosphorus/27xCeIqs">phosphorus</a> act as the intermediary between the heat source and electrical assembly.

<h2 id="power-generation">Power generation</h2>
SiGe thermocouples in an RTG convert <a href="/facts/Heat/FjDGssCs">heat</a> directly into <a href="/facts/Electricity/hNC8D65W">electricity</a>. Thermoelectric power generation requires a constantly maintained temperature difference among the junctions of the two dissimilar metals (i.e. Si and Ge) to produce a low power <a href="/facts/Electric_circuit/zt94kTW0">closed circuit</a> electric current without extra <a href="/facts/Circuitry/IRfQ6Cc2">circuitry</a> or external power sources.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a><a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>
A large array of SiGe thermocouples/unicouples form a <a href="/facts/Thermopile/cTuF62oI">thermopile</a> that was incorporated into the design of <a href="/facts/Radioisotope_thermoelectric_generators/Tsjjvp5h">radioisotope thermoelectric generators</a> (RTGs) used in the missions <a href="/facts/Voyager_program/cPm6qnNx">Voyager</a>, <a href="/facts/Galileo_(spacecraft)/H57agAgz">Galileo</a>, <a href="/facts/Ulysses_(spacecraft)/0nEMdFvj">Ulysses</a>, <a href="/facts/Cassini_(spacecraft)/PWknrDeY">Cassini</a>, and <a href="/facts/New_Horizons/G5XTU5DK">New Horizons</a>.<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a> On these spacecraft, <a href="/facts/Pu-238/JzY7TPFE">Pu-238</a> dioxide fuel undergoes <a href="/facts/Radioactive_decay/Ya6FVjt4">natural decay</a>. The SiGe thermocouples/unicouples convert this heat to hundreds of <a href="/facts/Watt/VK8oqRrm">Watts</a> of electrical power.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a>

<h2 id="thermocoupleunicouple-assembly">Thermocouple/unicouple assembly</h2>

The <a href="/facts/Thermocouples/FW9cYU5D">thermocouples/unicouples</a> attached to the outer shell consist of a SiGe alloy <a href="/facts/Doping_(semiconductor)/WrGdtWnm">n-leg doped</a> with boron and a SiGe <a href="/facts/Doping_(semiconductor)/WrGdtWnm">p-leg doped</a> with phosphorus to provide thermoelectric polarity to the couple.<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a><a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> The electrical and thermal currents of the system are separated by bonding the SiGe alloy thermocouple to a multifoil cold stack assembly of <a href="/facts/Molybdenum/RX6vp2bF">molybdenum</a>, <a href="/facts/Tungsten/WYwbNDYX">tungsten</a>, <a href="/facts/Stainless_steel/IDdo8bEx">stainless steel</a>, <a href="/facts/Copper/I8PEE8oX">copper</a>, and alumina components.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a> Several layers of <a href="https://web.archive.org/web/20150219015544/http://jpsglass.com/aerospace.html">Astroquartz</a> silica fiber yarn electrically <a href="/facts/Insulator_(electrical)/SfSXkrh5">insulate</a> the legs of the SiGe thermocouples. In between the inner insulation system and the outer shell, copper connectors form the electrical <a href="/facts/Circuit_(electronic)/IRfQ6Cc2">circuit</a>, which uses a two-string, <a href="/facts/Series_and_parallel_circuits/007qC7j8">series-parallel wiring</a> design to connect the unicouples. The circuit loop arrangement minimizes the net <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> of the <a href="/facts/Thermoelectric_generator/ISMDGaUD">generator</a>.<a class="footnote-ref" id="fnref:21" href="#fn:21">21</a>

<h2 id="application-history">Application history</h2>

SiGe has been used as a material in RTGs since 1976. Each mission that has used RTG technology involves exploration of far-reaching regions of the solar system. The most recent mission, New Horizons (2005), was originally set for a 3-year exploration, but was extended to 17 years.

<h3>Multi-hundred-watt (MHW) applications</h3>
<a href="/facts/Voyager_1/39jCxR9I">Voyager 1</a> and <a href="/facts/Voyager_2/9FAEaHUr">Voyager 2</a> <a href="/facts/Spacecraft/YQqQaUw0">spacecraft</a> launched in August and September 1977 required <a href="/facts/MHW-RTG/JLopBPjA">multi-hundred-watt</a> (<a href="/facts/MHW-RTG/JLopBPjA">MHW</a>) RTG containing <a href="/facts/Plutonium_oxide/xHwo6rSx">plutonium oxide</a> fuel spheres for an operational life appropriate for exploration of <a href="/facts/Jupiter/95m7XjMT">Jupiter</a>, <a href="/facts/Saturn/D54LcGJ9">Saturn</a>, <a href="/facts/Uranus/x7YwT1ar">Uranus</a>, and <a href="/facts/Neptune/1eucwHIC">Neptune</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a> Conversion of the <a href="/facts/Radioactive_decay/Ya6FVjt4">decay</a> heat of the plutonium to electrical power was accomplished through 312 silicon-germanium (SiGe) thermoelectric couples. A hot junction temperature of 1273 <a href="/facts/Kelvin_(unit)/E3trzC4C">K</a> (1832 <a href="/facts/Fahrenheit_(degree)/2TVmyJod">°F</a>) with a cold junction temperature of 573 K (572 °F) compose the temperature gradient in the thermoelectric couple in the RTG.<a class="footnote-ref" id="fnref:23" href="#fn:23">23</a> This mechanism provided the total electrical power to operate the spacecraft's instruments, communications and other power demands. The RTG on Voyager will produce adequate electrical power for spacecraft operation until about the year 2020.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a> Similar MHW-RTG models are also used on the two U.S. Air Force communications <a href="/facts/Lincoln_Experimental_Satellite/u356UIM2">Lincoln Experimental Satellites 8 and 9</a> (<a href="/facts/Lincoln_Experimental_Satellite/u356UIM2">LES-8/9</a>).<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a>

<h3>General purpose heat source (GPHS) applications</h3>
The <a href="/facts/Galileo_(spacecraft)/H57agAgz">Galileo</a> spacecraft launched on October 18, 1989, the <a href="/facts/Ulysses_(spacecraft)/0nEMdFvj">Ulysses</a> on October 6, 1990, the <a href="/facts/Cassini_(spacecraft)/PWknrDeY">Cassini</a> on October 15, 1997, and the <a href="/facts/New_Horizons/G5XTU5DK">New Horizons</a> on January 19, 2006. All of these spacecraft contain the <a href="/facts/GPHS-RTG/pQnxnjE3">general purpose heat source (GPHS)</a> RTG commissioned by the <a href="/facts/U.S._Department_of_Energy/PRp6Cz8g">U.S. Department of Energy</a>. The GPHS-RTG employs identical heat-to-electrical conversion technology used in the <a href="/facts/MHW-RTG/JLopBPjA">MHW-RTGs</a> from the Voyager missions, using SiGe thermocouples/unicouples and the Pu-238–fueled GPHS.<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a> New Horizons made its historic flyby past <a href="/facts/Pluto/egq7MBeR">Pluto</a> and its moons on July 14, 2015 (<a href="http://pluto.jhuapl.edu/">see JHU Applied Physics website</a>). The spacecraft's next destination will be a small <a href="/facts/Kuiper_Belt/IvLzYp5P">Kuiper Belt</a> object (KBO) known as <a href="/facts/486958_Arrokoth/7HQPCVNA">486958 Arrokoth</a> that orbits nearly a billion miles beyond <a href="/facts/Pluto/egq7MBeR">Pluto</a>.<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a> Based on performance, data and modeling for the SiGe alloy RTGs, the GPHS-RTGs on Ulysses, Cassini and New Horizons are expected to meet or exceed the remaining power performance requirements for their deep-space missions.<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a>

<h2 id="rtg-alternative">RTG alternative</h2>
Missions after 2010 requiring RTGs will instead use the <a href="/facts/Multi-mission_radioisotope_thermoelectric_generator/W4C0Y5bX">multi-mission radioisotope thermoelectric generator</a> (MMRTG) containing <a href="/facts/Lead_telluride/xTQCNgRu">lead telluride</a> (PbTe) thermocouples and Pu-238 dioxide for spacecraft power applications.

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Thermoelectric_cooling/fzQLmYjh">Thermoelectric cooling</a></li>
<li><a href="/facts/Advanced_Stirling_Radioisotope_Generator/DrXMMseG">Advanced Stirling Radioisotope Generator</a></li>
<li><a href="/facts/Radioisotope_heater_units/6IoyhN7Q">Radioisotope heater units</a></li></ul>

<h2 id="references">References</h2>

<ol>
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<li id="fn:19">Bennett, G.L; Lombardo, James; Hemler, Richard; Silverman, Gil; Whitmore C.; Amos, Wayne; Johnson, E.; Schock, Alfred; Zocher, Roy; Keenan, Thomas; Hagan, James; and Richard Englehart. Mission of Daring: The General-Purpose Heat Source Radioisotope Thermoelectric Generator, AIAA 2006-4096, 4th International Energy Conversion Engineering Conference and Exhibit (IECEC), 26–29 June 2006, San Diego, California (Accessed 10 February 2015) <a href="http://www.fas.org/nuke/space/gphs.pdf" target="_blank">http://www.fas.org/nuke/space/gphs.pdf</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Bennett, G.L; Lombardo, James; Hemler, Richard; Silverman, Gil; Whitmore C.; Amos, Wayne; Johnson, E.; Schock, Alfred; Zocher, Roy; Keenan, Thomas; Hagan, James; and Richard Englehart. Mission of Daring: The General-Purpose Heat Source Radioisotope Thermoelectric Generator, AIAA 2006-4096, 4th International Energy Conversion Engineering Conference and Exhibit (IECEC), 26–29 June 2006, San Diego, California (Accessed 10 February 2015) <a href="http://www.fas.org/nuke/space/gphs.pdf" target="_blank">http://www.fas.org/nuke/space/gphs.pdf</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Bennett, G.L; Lombardo, James; Hemler, Richard; Silverman, Gil; Whitmore C.; Amos, Wayne; Johnson, E.; Schock, Alfred; Zocher, Roy; Keenan, Thomas; Hagan, James; and Richard Englehart. Mission of Daring: The General-Purpose Heat Source Radioisotope Thermoelectric Generator, AIAA 2006-4096, 4th International Energy Conversion Engineering Conference and Exhibit (IECEC), 26–29 June 2006, San Diego, California (Accessed 10 February 2015) <a href="http://www.fas.org/nuke/space/gphs.pdf" target="_blank">http://www.fas.org/nuke/space/gphs.pdf</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Furlong, Richard R.; Wahlquist, Earl J. (April 1999). "U.S. Space Missions Using Radioisotope Power Systems" (PDF). Nuclear News. American Nuclear Society. Archived from the original (PDF) on 2018-10-16. Retrieved 2015-03-17. <a href="https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf" target="_blank">https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">Furlong, Richard R.; Wahlquist, Earl J. (April 1999). "U.S. Space Missions Using Radioisotope Power Systems" (PDF). Nuclear News. American Nuclear Society. Archived from the original (PDF) on 2018-10-16. Retrieved 2015-03-17. <a href="https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf" target="_blank">https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">Furlong, Richard R.; Wahlquist, Earl J. (April 1999). "U.S. Space Missions Using Radioisotope Power Systems" (PDF). Nuclear News. American Nuclear Society. Archived from the original (PDF) on 2018-10-16. Retrieved 2015-03-17. <a href="https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf" target="_blank">https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
<li id="fn:25">Fleurial, Jean-Pierre; Caillat, Thierry; Nesmith, Bill J.; Ewell, Richard C.; Woerner, David F.; Carr, Gregory C.; Jones, Loren E. "Thermoelectrics: From Space Power Systems to Terrestrial Waste Heat Recovery Applications" (PDF). U.S. Department of Energy. Jet Propulsion Laboratory/California Institute of Technology (2011). <a href="http://energy.gov/sites/prod/files/2014/03/f13/fluerial.pdf" target="_blank">http://energy.gov/sites/prod/files/2014/03/f13/fluerial.pdf</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></li>
<li id="fn:26">Furlong, Richard R.; Wahlquist, Earl J. (April 1999). "U.S. Space Missions Using Radioisotope Power Systems" (PDF). Nuclear News. American Nuclear Society. Archived from the original (PDF) on 2018-10-16. Retrieved 2015-03-17. <a href="https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf" target="_blank">https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></li>
<li id="fn:27">"NASA's New Horizons Team Selects Potential Kuiper Belt Flyby Target". 28 August 2015. <a href="https://www.nasa.gov/feature/nasa-s-new-horizons-team-selects-potential-kuiper-belt-flyby-target" target="_blank">https://www.nasa.gov/feature/nasa-s-new-horizons-team-selects-potential-kuiper-belt-flyby-target</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></li>
<li id="fn:28">Dingwall, F. (May 1963). "Optimization of Silicon-Germanium Thermoelectric Modules for Transportation Corps Silent Boat Design" (PDF). Radio Corporation of America. TRECOM Technical Report 63-17. Accession Number: AD0412341. Archived from the original (PDF) on March 4, 2016. <a href="https://web.archive.org/web/20160304050345/http://www.dtic.mil/dtic/tr/fulltext/u2/412341.pdf" target="_blank">https://web.archive.org/web/20160304050345/http://www.dtic.mil/dtic/tr/fulltext/u2/412341.pdf</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></li>
</ol>

Application of silicon-germanium thermoelectrics in space exploration open-in-new

Application of silicon-germanium thermoelectrics in space exploration