Thermoelectric materials for space applications

TL;DR: In this article, the authors review the knowledge acquired over the last years on several families of thermoelectric materials, the performances of which are close or even higher than those conventionally used in RTGs to date.

Abstract: Solid-state energy conversion through thermoelectric effects remains the technology of choice for space applications for which, their low energy conversion efficiency is largely outweighed by the reliability and technical requirements of the mission. Radioisotope thermoelectric generators (RTGs) enable the direct conversion of the heat released by nuclear fuel into the electrical power required to energize the scientific instruments. The optimization of the conversion efficiency is intimately connected to the performances of the thermoelectric materials integrated which are governed by the transport properties of these materials. Recent advances in the design of highly efficient thermoelectric materials raise interesting prospects to further enhance the performances of RTGs for future exploratory missions in the Solar system. Here, we briefly review the knowledge acquired over the last years on several families of thermoelectric materials, the performances of which are close or even higher than those conventionally used in RTGs to date. Issues that remain to be solved are further discussed.

Summary

1. Introduction

• Both the n- and p-type legs are brazed on the metallic plates to ensure low electrical contact resistances (too high contact resistances are detrimental to high output performances of the device).
• One of the major drawback of RTGs is their low conversion efficiency 𝜂𝑅𝑇𝐺 , which remains on the order of 6 – 10% [1,14-16], although various non-conventional designs of the thermoelectric legs or of the TEG itself have been studied.
• In addition, achieving extremely low values of 𝜅𝑝ℎ is usually obtained in highly-disordered or amorphous compounds [1,3], the nature of which prevents high mobility of the charge carriers, necessary to maintain 𝜌 to low values, from being achieved.
• Their main physical properties and advantages compared to other thermoelectric compounds will be discussed before highlighting the challenges that remain to be overcome.

2. State-of-the-art thermoelectric materials in RTGs

• Historically, chalcogenide semiconductors have been the materials of choice for thermoelectric applications in power generation [1,43,44].
• Using 241Am as the fuel source results in lower temperatures at the hot side compared to 238Pu-based sources, making the well-mastered Bi2Te3-based TE modules a viable strategy to power European deep-space probes from the mid 2020s onwards.
• This peculiarity is important regarding their integration in RTGs.
• As the authors will see below, the thermal stability of optimized thermoelectric materials should be also ensured to be potential candidates for integration in RTGs.

3. Novel thermoelectric materials for RTGs

• 1 SnX (X = Se and Te) compounds for mid-temperature range Significant efforts are currently being devoted to the identification, synthesis and optimization of novel materials with superior thermoelectric properties that could replace the state-of-the-art n-type and p-type thermoelectric compounds that have been used in RTGs for decades (Fig. 5).
• In particular, both SnSe and SnTe have been extensively investigated due to their favorable Ac electronic properties, low lattice thermal conductivity and the high number of elements that can act as effective hole-like or electron-like dopants [56,57,66-79].
• The VBs are mainly composed of two maxima at the L and points of the Brillouin zone, giving rise to light holes (L) and heavy holes .
• Due to this band-shape-modification effect, the thermopower values are strongly enhanced, yielding large power factors in samples with optimized composition [90,91].

3.2 Skutterudites

• Among the novel thermoelectric materials candidates that emerge over the last two decades, skutterudites, named after the Norwegian small mining town Skutterud where a CoAs3-based mineral has been identified in 1845, are probably the closest to a qualification into an advanced RTG.
• This interesting ability of the structure to host various elements in these cages is the key crystallographic characteristic of skutterudites, which shapes their thermal transport [29,30].
• The presence of these guest atoms has two important consequences on the transport properties of CoSb3.
• The charge balance achieved between the filling element R and the complexes T4X12 yields diamagnetic semiconductors in agreement with the Zintl-Klemm formalism.
• On the space application side, n-type skutterudites remain the leading candidates for integration into RTGs and, after more than two decades of intense research endeavor, will likely integrate the 48-couple PbTe/TAGS RTGs currently powering the Mars Curiosity rover.

3.3 Novel materials operating above 1000 K

• While many families of thermoelectric materials exhibit their maximum thermoelectr ic performances below 800 K, only few are known to be able to operate at temperatures up to 1300 K while, concomitantly, surpassing the thermoelectric properties of the traditionally- used Ac c ted m an us cr pt Si1-xGex alloys [47].
• The complexity and diversity of their crystal structure, along with charge carrier mobilities that remains sufficient ly high, are important ingredients to design novel efficient thermoelectric materials.
• Compared to p-type Zintl phases, only few n-type analogues have been investigated to date [143-145].
• For each types of clusters, an optimal MEC can be predicted either from simple electron counting rules or by electronic band structure calculations [154].

3.4 Beyond thermoelectric properties

• They should nevertheless meet several other important requirements for integration into RTGs and space qualification.
• The diffusion of elements into the thermoelectric materials can act as dopants, potentially degrading the thermoelectr ic performances.
• The high stress levels that can develop within each legs can result in their breakage, thereby strongly limiting the lifetime of the module.
• While all these aspects are common to TEGs developed for terrestrial applications in power generation at high temperatures, the tolerance of the thermoelectric materials to radiations is a specific, yet critical, facet of space applications [9,174,175].
• While the dose received from external sources is strongly mission dependent, the interna l bombardment can be estimated.

Conclusions

• The authors have surveyed several families of materials that exhibit transport properties relevant for thermoelectric applications in power generation, making them prime candidates for being integrated in the next generation of RTGs.
• A central aspect of these materials is their high 𝑍𝑇 values that can be optimized through proper doping strategies.
• The wide interest in these materials is testified by the significant, ever-growing amount of literature data available for these families.
• While significant advances have been achieved on the material side, several issues regarding their integration in RTGs remain to be solved, notably regarding their thermal stability over long Further investigations on these materials and on other related families might uncover novel, highly-efficient thermoelectric materials that will enable further enhancing the output performances of RTGs.
• The successful integration of these materials into RTGs may be also beneficial for the development of TEGs and their more widespread use in terrestrial applications, thereby contributing to mitigate mankind’s fingerprint on the global climate.

Figures

Fig. 3. a) Schematic representation of the variations in the thermopower 𝛼 (in absolute value for n-type compounds) as a function of the carrier concentration 𝑛𝐻/𝑝𝐻 called the IoffePisarenko curve. For conventional impurities (solid blue curve), 𝛼 monotonically decreases with increasing the carrier concentration. For specific impurities inducing the formation of a resonant level, that is, a local distortion of the valence or conduction bands, a clear departure

Fig. 5. Temperature dependence of the dimensionless thermoelectric figure of merit ZT of selected novel families of a) n-type and b) p-type thermoelectric materials that have emerged over the last two decades [1,29-31,54-65].

Fig. 4. Temperature dependence of the dimensionless thermoelectric figure of merit ZT of various state-of-the-art n-type and p-type thermoelectric materials [1,45-52].

Fig. 8. Examples of Mo-X clusters (X = S, Se or Te) of various sizes. Several basic units Mo6X8X6 can be condensed to form longer chain-like clusters that are then arranged to form a three-dimensional framework. In the limit of infinite length, compounds Mo6X6 are obtained.

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Submitted on 18 May 2021
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Thermoelectric materials for space applications
Christophe Candol, Souane El Oualid, Dorra Ibrahim, Shantanu Misra,
Oussama El Hamouli, Adèle Léon, Anne Dauscher, Philippe Masschelein,
Philippe Gall, Patrick Gougeon, et al.
To cite this version:
Christophe Candol, Souane El Oualid, Dorra Ibrahim, Shantanu Misra, Oussama El Hamouli, et
al.. Thermoelectric materials for space applications. CEAS Space Journal, Springer, 2021, 13 (3),
pp.325-340. �10.1007/s12567-021-00351-x�. �hal-03190535�

1
Thermoelectric materials for space applications
Christophe Candolfi
1,*
, Soufiane El Oualid
1
, Dorra Ibrahim
1
, Shantanu Misra
1
, Oussama El
Hamouli
1
, Adèle Léon
1
, Anne Dauscher
1
, Philippe Masschelein
1
, Philippe Gall
2
, Patrick
Gougeon
2
, Christopher Semprimoschnig
3,†
, Bertrand Lenoir
1,*
1
Institut Jean Lamour, UMR 7198 CNRS – Université de Lorraine, Campus ARTEM, 2 allée
André Guinier, BP 50840, 54011 Nancy, France
2
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS – Université de Rennes 1 –
INSA de Rennes – Ecole Nationale Supérieure de Chimie de Rennes, 11 allée de Beaulieu, CS
50837, 35708 Rennes Cedex, France
3
European Space Agency, ESTEC, P.O. Box 299, Keplerlaan 1, 2200 AG Noordwijk, The
Netherlands
*
Corresponding Authors: christophe.candolfi@univ-lorraine.fr; bertrand.lenoir@uni v-
lorraine.fr
†
C. S. passed away in 2020
Abstract
Solid-state energy conversion through thermoelectric effects remains the technology of choice
for space applications for which, their low energy conversion efficiency is largely outweighed
by the reliability and technical requirements of the mission. Radioisotope thermoelectric
generators (RTGs) enables the direct conversion of the heat released by nuclear fuel into the
electrical power required to energize the scientific instruments. The optimization of the
conversion efficiency is intimately connected to the performances of the thermoelectric
Accepted manuscript

2
materials integrated which are governed by the transport properties of these materials. Recent
advances in the design of highly-efficient thermoelectric materials raise interesting prospects
to further enhance the performances of RTGs for future exploratory missions in the Solar
system. Here, we briefly review the knowledge acquired over the last years on several families
of thermoelectric materials, the performances of which are close or even higher than those
conventionally used in RTGs to date. Issues that remain to be solved are further discussed.
Keywords: Thermoelectric, RTG, Semiconductors, Space mission
Declarations
Funding
European Space Agency (ESA/ESTEC)
Conflicts of Interest
The authors declare no competing financial interest.
Availability of data and material
Not applicable
Code availability
Not applicable
Accepted manuscript

3
1. Introduction
Thermoelectric materials provide an elegant and versatile way to convert a temperature
difference into electrical power (Seebeck effect) or vice versa (Peltier effect) [1-3].
Thermoelectric generators (TEGs, see Fig. 1), in which these materials are integrated, possess
important advantages over other energy conversion technologies. In particular, the TEGs does
not exhibit any moving parts and are thus noise- and vibration-free during operation, conferring
high mechanical reliability with low maintenance levels and hence, long lifetime. These
properties make TEGs fully autonomous and particularly well-suited for operating in isolated
areas on Earth and in the extreme environments of space and other planetary surfaces. These
TEGs can be either scaled up or downsized, offering a high adaptability for a plethora of
applications ranging from waste-heat recovery in various industrial processes to the powering
of autonomous micro-sensors for Internet-of-things (IoT) applications [4-8].
Accepted manuscript

4
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Accepted manuscript

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Thermoelectric materials for space applications" ?

In this paper, a thermoelectric generator with segmented legs is presented, where the n-and p-type legs are brazed on the metallic plates to ensure low electrical contact resistances.