A Steady Approach

editorial A steady approach From the discovery of the Seebeck efect in the nineteenth century to its application in the latest space probes, thermoelectrics have carved out a niche for reliable applications. t was the so-called seven minutes of in heating or cooling. A Perspective by terror. The NASA Perseverance rover Zhifeng Ren and colleagues discusses recent Isuccessfully completed atmospheric progress in materials for thermoelectric entry and a controlled soft landing on Mars. cooling. In comparison with thermoelectric Its mission is to use its large suite of tools power generation, which relies on a large to assess not only the past habitability of temperature gradient with the hot side the Jezero Crater but also to select which several hundred kelvin hotter than the cool mineral samples to cache for a future side, thermoelectric coolers are typically sample-return mission. This will require An artist’s impression of the Perseverance rover used near ambient temperature, reducing a sizable and stable energy source for a on Mars. Credit: Neko / Alamy Stock Photo thermal stress on the thermocouple. The long-range, long-duration drive, and this researchers note that the low-temperature energy supply is provided by the heat thermoelectric properties of many materials emitted by PuO2 pellets during radioactive are yet to be comprehensively investigated decay in a radioisotope thermoelectric In this issue, a Letter by Yuya Sakuraba and so have been overlooked for cooling generator. This uses a thermocouple, and colleagues outlines an alternative applications. They also note that a large composed of two different thermoelectric methodology for large transverse mobility difference ratio between electrons materials, to generate voltage. The usage thermopower. They make a closed circuit and holes is important for generating of thermoelectrics in space exploration is composed of two materials, doped silicon (a large Seebeck coefficients, which are also probably the highest-profile application of thermoelectric), and Co2MnGa (a magnetic relevant for larger cooling power. This can these materials, which rely on an effect that material). Upon application of heat the compensate for the bipolar conduction was first observed in the nineteenth century. thermoelectric generates a current, which inherent in very narrow semiconductors, The discovery of thermoelectricity then flows through the magnetic material. and suggests that exploration of these is commonly associated with Thomas The current driven through Co2MnGa materials as well as semimetals could be a Seebeck’s observation in 1821 that generates a transverse voltage, but via the promising strategy for the development of heating a metallic junction could deflect anomalous Hall effect (AHE) that exists in better low-temperature thermoelectrics. a magnetic compass needle, although it the absence of an applied magnetic field. To have a larger impact for everyday life was the work of Hans Ørsted in 1823 that This leads to a transverse thermopower of on Earth, research on thermoelectrics is being noted generation of electric currents and 82.3 μV K–1, an order of magnitude larger pursued on several fronts. Typically, the best coined the term thermoelectricity1. The than other transverse thermopowers. The room-temperature inorganic thermoelectrics basis of thermoelectric power generation researchers also present a phenomenological are Bi2Te 3 alloys, but the high cost of tellurium is the Seebeck effect, where a gradient in theory for the prediction of the transverse can limit applications, and thus researchers temperature along a material can generate voltage in their device, which, as noted have considered alternatives such as Mg3BiSb a voltage differential, with the Seebeck by Andrew May and Brian Sales in their alloys3,4. Typically, thermoelectric materials coefficient (or thermopower) controlling News & Views, can be used to look for are brittle, whereas better flexibility could the size of this voltage difference: the better material combinations: researchers allow close contact between device and larger the Seebeck coefficient the greater should search for materials with a large heat source5. And there is an ongoing the induced voltage. In longitudinal AHE. May and Sales also propose a simpler search to improve figures of merit for thermoelectric materials both the voltage hybrid geometry with a single large-area power generation, for applications such and temperature gradients run along the thermoelectric leg that reduces compatibility as heat harvesting at the microscale or in same axis. The decoupling of electrical issues, and note that although the voltage solar thermoelectric generators6. With the conductivity (which should be large for generated is less than that generated current need for improving energy efficiency, efficient power generation) and thermal by longitudinal devices, the ease of the usage of these materials for waste heat conductivity (which should be small to implementation in power generation could harvesting or energy-efficient coolers will maintain the temperature differential) be a significant advantage. surely continue. ❐ represents a significant materials and device Thermoelectrics have been design problem. An alternative would commercialized for several decades as Published online: 26 March 2021 be to generate thermoelectricity using a temperatures sensors, but also as small-scale https://doi.org/10.1038/s41563-021-00982-9 ‘transverse’ approach, where the temperature coolers where the heat load is 25 W or and voltage gradients are perpendicular, less and they can out-compete vapour References such as in the Nernst effect, simplifying compression cooling. This is due to another 1. Beretta, D. et al. Mat. Sci. Eng. R 138, 100501 (2019). device design2. However, the Nernst aspect of thermoelectricity, the Peltier 2. Sakai, A. et al. Nature 581, 53–57 (2020). thermopower is typically much smaller than effect. As observed by Jean Peltier in 1834, 3. Zhang, J. et al. Nat. Commun. 8, 13901 (2017). 4. Mao, J. et al. Science 365, 495–498 (2019). the equivalent Seebeck coefficient, limiting the passage of electrical current through 5. Jin, Q. et al. Nat. Mater. 18, 62–68 (2019). transverse thermoelectric applications. a junction of two materials can result 6. Kraemer, D. et al. Nat. Mater. 10, 532–538 (2011). NATURE MATERIALS | VOL 20 | APRIL 2021 | 437 | www.nature.com/naturematerials 437.

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