View source for Physics:Application of silicon-germanium thermoelectrics in space exploration

[[File:SiGe RTG.png|thumb|Essential components of a SiGe radioisotope thermoelectric generator]]
[[Engineering:Silicon-germanium|Silicon-germanium]] (SiGe) thermoelectrics have been used for converting heat into electrical power in [[Engineering:Spacecraft|spacecraft]] designed for deep-space [[Organization:NASA|NASA]] missions since 1976. This material is used in the radioisotope thermoelectric generators (RTGs) that power ''[[Astronomy:Voyager 1|Voyager 1]]'', ''[[Astronomy:Voyager 2|Voyager 2]]'', ''[[Engineering:Galileo (spacecraft)|Galileo]]'', ''[[Engineering:Ulysses (spacecraft)|Ulysses]]'', ''Cassini'', and ''[[Astronomy:New Horizons|New Horizons]]'' spacecraft. SiGe thermoelectric material converts enough radiated heat into electrical power to fully meet the power demands of each spacecraft. The properties of the material and the remaining components of the RTG contribute towards the efficiency of this thermoelectric conversion.

== Properties ==
[[File:SiGe Unicouple Assembly.png|text-bottom|thumb|Components of the SiGe unicouple]]
Heavily [[Physics:Doping (semiconductor)|doped]] semiconductors, such as [[Engineering:Silicon-germanium|silicon-germanium]] (SiGe) thermoelectric couples (also called thermocouples or unicouples), are used in space exploration.<ref>{{cite journal|last1=Tiwari|first1=Pratibha|last2=Gupta|first2=Nishu|last3=Gupta|first3=K.M.|title=Advanced Thermoelectric Materials in Electrical and Electronic Applications|journal=Advanced Materials Research|date=April 2013|volume=685|pages=161–165|doi=10.4028/www.scientific.net/AMR.685.161|bibcode=2012AdMaR.443.1587Z|s2cid=111227236}}</ref><ref>{{Cite book|last1=Böttner|first1=H.|title=Twenty-First International Conference on Thermoelectrics, 2002. Proceedings ICT '02 |chapter=Thermoelectric micro devices: Current state, recent developments and future aspects for technological progress and applications |date=August 2002|pages=511–518|doi=10.1109/ICT.2002.1190368|isbn=978-0-7803-7683-0|s2cid=195862812 }}</ref>

SiGe alloys present good thermoelectric properties. Their performance in thermoelectric [[Physics:Power|power]] production is characterized by high [[Physics:Dimensionless quantity|dimensionless]] [[Engineering:Figure of merit|figures-of-merit]] (ZT) under high temperatures, which has been shown to be near 2 in some nanostructured-SiGe models.<ref name=dingwall3>{{cite journal|last1=Dingwall|first1=F.|title=Optimization of Silicon-Germanium Thermoelectric Modules for Transportation Corps Silent Boat Design|journal=Radio Corporation of America|date=May 1963|volume=TRECOM Technical Report 63-17|url=http://www.dtic.mil/dtic/tr/fulltext/u2/412341.pdf|archive-url=https://web.archive.org/web/20160304050345/http://www.dtic.mil/dtic/tr/fulltext/u2/412341.pdf|url-status=dead|archive-date=March 4, 2016|id=Accession Number: AD0412341}}</ref><ref>{{cite journal|last1=Lee|first1=Eun Kyung|last2=Yin|first2=Liang|last3=Lee|first3=Yongjin|last4=Lee|first4=Jong Woon|last5=Lee|first5=Sang Jin|last6=Lee|first6=Junho|last7=Cha|first7=Seung Nam|last8=Whang|first8=Dongmok|last9=Hwang|first9=Gyeong S.|last10=Hippalgaonkar|first10=Kedar|last11=Majumdar|first11=Arun|last12=Yu|first12=Choongho|last13=Choi|first13=Byoung Lyong|last14=Kim|first14=Jong Min|last15=Kim|first15=Kinam|title=Large Thermoelectric Figure-of-Merits from SiGe Nanowires by Simultaneously Measuring Electrical and Thermal Transport Properties|journal=Nano Letters|date=13 June 2012|volume=12|issue=6|pages=2918–2923|doi=10.1021/nl300587u|pmid=22548377|bibcode = 2012NanoL..12.2918L |s2cid=20551131 }}</ref><ref>{{cite journal|last1=Joshi|first1=Giri|last2=Lee|first2=Hohyun|last3=Lan|first3=Yucheng|last4=Wang|first4=Xiaowei|last5=Zhu|first5=Gaohua|last6=Wang|first6=Dezhi|last7=Gould|first7=Ryan W.|last8=Cuff|first8=Diana C.|last9=Tang|first9=Ming Y.|last10=Dresselhaus|first10=Mildred S.|last11=Chen|first11=Gang|author-link11=Gang Chen (engineer)|last12=Ren|first12=Zhifeng|title=Enhanced Thermoelectric Figure-of-Merit in Nanostructured p-type Silicon Germanium Bulk Alloys|journal=Nano Letters|date=10 December 2008|volume=8|issue=12|pages=4670–4674|doi=10.1021/nl8026795|pmid=19367858|bibcode=2008NanoL...8.4670J}}</ref>

SiGe alloy devices are mechanically rugged and can withstand severe shock and vibration due to its high [[Physics:Ultimate tensile strength|tensile strength]] (i.e. >7000 psi) and low dislocation density.<ref name=dingwall3 /><ref name=xie6>{{cite journal|last1=Xie|first1=Ming|last2=Gruen|first2=Dieter M.|title=Potential Impact of ZT = 4 Thermoelectric Materials on Solar Thermal Energy Conversion Technologies|journal=The Journal of Physical Chemistry B|date=18 November 2010|volume=114|issue=45|pages=14339–14342|doi=10.1021/jp9117387|pmid=20196558}}</ref> SiGe material is malleable with standard metallurgical equipment and bonds easily to construct components.<ref name=dingwall3 /> 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.<ref name=dingwall3 /><ref name=xie6 /><ref>{{cite web|last1=Jurgensmeyer|first1=Austin Lee|title=High efficiency thermoelectric devices fabricated using quantum well confinement techniques|url=https://projects-web.engr.colostate.edu/ionstand/graduates/papers/Jurgensmeyer%20Thesis.pdf|website=Colorado State University Libraries|date=Summer 2011|access-date=March 9, 2023}}</ref>

Near the [[Astronomy:Sun|Sun]], [[Physics:Solar cell|solar cell]] performance deteriorates from high incident [[Physics:Flux|particle flux]] and high temperatures from [[Physics:Heat flux|heat flux]].<ref name=berlin9>{{cite journal|last1=Raag|first1=V.|last2=Berlin|first2=R.E.|title=A silicon-germanium solar thermoelectric generator|journal=Energy Conversion|date=December 1968|volume=8|issue=4|pages=161–168|doi=10.1016/0013-7480(68)90033-8}}</ref> However, thermoelectric energy conversion systems that use [[Physics:Thermoelectric materials|thermoelectric materials]] (e.g. SiGe alloys) as a supplemental source of power for missions near the Sun can operate unprotected in [[Physics:Vacuum|vacuum]] and air environments under high temperatures due to their low sensitivity to [[Physics:Radiation damage|radiation damage]].<ref name=berlin9 /> Such properties have made SiGe thermoelectrics convenient for power generation in [[Space|space]].
The multifoil cold stack assembly, composed of [[Chemistry:Molybdenum|molybdenum]], [[Chemistry:Tungsten|tungsten]], [[Chemistry:Stainless steel|stainless steel]], [[Chemistry:Copper|copper]], and alumina materials, provides the insulation between the [[Physics:Electric current|electrical]] and [[Physics:Heat current|thermal currents]] of the system. The SiGe n-leg doped with [[Chemistry:Boron|boron]] and SiGe p-leg doped with [[Chemistry:Phosphorus|phosphorus]] act as the intermediary between the heat source and electrical assembly.

== Power generation ==
SiGe thermocouples in an RTG convert [[Physics:Heat|heat]] directly into [[Physics:Electricity|electricity]]. 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 closed circuit electric current without extra circuitry or external power sources.<ref name=dingwall3 /><ref name=furlong12>{{cite news|last1=Furlong|first1=Richard R.|last2=Wahlquist|first2=Earl J.|title=U.S. Space Missions Using Radioisotope Power Systems.|url=http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf|agency=American Nuclear Society|publisher=Nuclear News|date=April 1999|access-date=2015-03-17|archive-date=2018-10-16|archive-url=https://web.archive.org/web/20181016011258/http://www3.ans.org/pubs/magazines/nn/pdfs/1999-4-2.pdf|url-status=dead}}</ref>

A large array of SiGe thermocouples/unicouples form a [[Physics:Thermopile|thermopile]] that was incorporated into the design of radioisotope thermoelectric generators (RTGs) used in the missions ''[[Astronomy:Voyager program|Voyager]]'', ''[[Engineering:Galileo (spacecraft)|Galileo]]'', ''[[Engineering:Ulysses (spacecraft)|Ulysses]]'', ''Cassini'', and ''[[Astronomy:New Horizons|New Horizons]]''.<ref name=fleurial10 >{{cite web|last1=Fleurial|first1=Jean-Pierre|last2=Caillat|first2=Thierry|last3=Nesmith|first3=Bill J.|last4=Ewell|first4=Richard C.|last5=Woerner|first5=David F.|last6=Carr|first6=Gregory C.|last7=Jones|first7=Loren E.|id=Jet Propulsion Laboratory/California Institute of Technology (2011)| title=Thermoelectrics: From Space Power Systems to Terrestrial Waste Heat Recovery Applications|url=http://energy.gov/sites/prod/files/2014/03/f13/fluerial.pdf|website=U.S. Department of Energy}}</ref> On these spacecraft, Pu-238 dioxide fuel undergoes [[Physics:Radioactive decay|natural decay]]. The SiGe thermocouples/unicouples convert this heat to hundreds of [[Watt]]s of electrical power.<ref name=furlong12 />

== Thermocouple/unicouple assembly ==
[[File:SiGe Thermocouple Function.png|thumb|Conceptual diagram of a thermocouple (unicouple)]]
The thermocouples/unicouples attached to the outer shell consist of a SiGe alloy [[Physics:Doping (semiconductor)|n-leg doped]] with boron and a SiGe [[Physics:Doping (semiconductor)|p-leg doped]] with phosphorus to provide thermoelectric polarity to the couple.<ref name=xie6 /><ref name=IECEC >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. [http://www.fas.org/nuke/space/gphs.pdf 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)</ref> The electrical and thermal currents of the system are separated by bonding the SiGe alloy thermocouple to a multifoil cold stack assembly of [[Chemistry:Molybdenum|molybdenum]], [[Chemistry:Tungsten|tungsten]], [[Chemistry:Stainless steel|stainless steel]], [[Chemistry:Copper|copper]], and alumina components.<ref name=IECEC /> Several layers of [https://web.archive.org/web/20150219015544/http://jpsglass.com/aerospace.html Astroquartz] silica fiber yarn electrically insulate the legs of the SiGe thermocouples. In between the inner insulation system and the outer shell, copper connectors form the electrical circuit, which uses a two-string, [[Engineering:Series and parallel circuits|series-parallel wiring]] design to connect the unicouples.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}  The circuit loop arrangement minimizes the net [[Physics:Magnetic field|magnetic field]] of the [[Physics:Thermoelectric generator|generator]].<ref name=IECEC />

== Application history ==
[[File:RTG Space Exploration Timeline.png|upright|thumb|RTG Space Exploration Timeline]]
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.

=== Multi-hundred-watt (MHW) applications ===
''[[Astronomy:Voyager 1|Voyager 1]]'' and ''[[Astronomy:Voyager 2|Voyager 2]]'' [[Engineering:Spacecraft|spacecraft]] launched in August and September 1977 required [[Astronomy:MHW-RTG|multi-hundred-watt]] ([[Astronomy:MHW-RTG|MHW]]) RTG containing plutonium oxide fuel spheres for an operational life appropriate for exploration of [[Astronomy:Jupiter|Jupiter]], [[Astronomy:Saturn|Saturn]], [[Astronomy:Uranus|Uranus]], and [[Astronomy:Neptune|Neptune]].<ref name=furlong12 /> Conversion of the [[Physics:Radioactive decay|decay]] heat of the plutonium to electrical power was accomplished through 312 silicon-germanium (SiGe) thermoelectric couples. A hot junction temperature of 1273 [[Kelvin (unit)|K]] (1832 °F) with a cold junction temperature of 573 K (572&nbsp;°F) compose the temperature gradient in the thermoelectric couple in the RTG.<ref name=furlong12 /> 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.<ref name=furlong12 /> Similar MHW-RTG models are also used on the two U.S. Air Force communications Lincoln Experimental Satellites 8 and 9 (''LES-8/9'').<ref name=fleurial10 />

=== General purpose heat source (GPHS) applications ===
The ''[[Engineering:Galileo (spacecraft)|Galileo]]'' spacecraft launched on October 18, 1989, the ''[[Engineering:Ulysses (spacecraft)|Ulysses]]'' on October 6, 1990, the ''Cassini'' on October 15, 1997, and the ''[[Astronomy:New Horizons|New Horizons]]'' on January 19, 2006. All of these spacecraft contain the [[Engineering:GPHS-RTG|general purpose heat source (GPHS)]] RTG commissioned by the U.S. Department of Energy.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}  The GPHS-RTG employs identical heat-to-electrical conversion technology used in the [[Astronomy:MHW-RTG|MHW-RTG]]s from the ''Voyager'' missions, using SiGe thermocouples/unicouples and the Pu-238–fueled GPHS.<ref name=furlong12 /> ''New Horizons'' made its historic flyby  past [[Astronomy:Pluto|Pluto]] and its moons on July 14, 2015 ([http://pluto.jhuapl.edu/ see JHU Applied Physics website]). The spacecraft's next destination will be a small Kuiper Belt object (KBO) known as [[Astronomy:486958 Arrokoth|486958 Arrokoth]] that orbits nearly a billion miles beyond [[Astronomy:Pluto|Pluto]].<ref>{{Cite web|url=https://www.nasa.gov/feature/nasa-s-new-horizons-team-selects-potential-kuiper-belt-flyby-target|title=NASA's New Horizons Team Selects Potential Kuiper Belt Flyby Target|date=28 August 2015}}</ref> 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.<ref name=dingwall3 />

== RTG alternative ==
Missions after 2010 requiring RTGs will instead use the [[Astronomy:Multi-mission radioisotope thermoelectric generator|multi-mission radioisotope thermoelectric generator]] (MMRTG) containing [[Chemistry:Lead telluride|lead telluride]] (PbTe) thermocouples and Pu-238 dioxide for spacecraft power applications.{{Citation needed|date=June 2016}}

== See also ==
* [[Physics:Thermoelectric cooling|Thermoelectric cooling]]
* Advanced Stirling Radioisotope Generator
* Radioisotope heater units

== References ==
{{reflist}}

[[Category:Nuclear power in space]]
[[Category:Thermoelectricity]]
[[Category:Nuclear technology]]
[[Category:Spacecraft components]]
[[Category:Inorganic chemistry]]

{{Sourceattribution|Application of silicon-germanium thermoelectrics in space exploration}}