By tailoring the architecture of a bulk material at several different length scales, the ability of a semiconductor to convert heat into voltage has been optimized to a groundbreaking level of performance. See Letter p.414
The story of thermoelectric materials, which convert heat into electric voltage, began in the early 1950s as part of the scientific plans for the first manned mission to the Moon. An effective, simple and long-lasting energy source was needed to supply the astronauts at their destination. The solution — a thermoelectric generator based on lead telluride — is still working today on the Moon's Mare Tranquillitatis. On page 414 of this issue, Biswas et al.1 describe what is probably the ultimate optimization of the thermoelectric properties of lead telluride, 43 years after that historic Moon landing. They have doubled the efficiency of the material compared with that used in the first generator, a feat that is not only a tremendous step for one group, but also a giant leap for thermoelectrics.
Thermoelectric generators consist of several 'stacks' — devices in which multiple semiconductor blocks are sandwiched between two electrodes. Each stack produces an electric potential difference if there is a stable, long-lasting temperature difference across it. Two types of semiconductor are needed: an n-type semiconductor, in which a material is 'doped' with a small amount of another material to produce an excess of electrons; and a p-type semiconductor, in which doping produces an excess of positively charged voids called holes, which can act as charge carriers.
The semiconductor blocks are arranged so that opposite sides are connected to different electrodes. If a thermoelectric stack is heated on one side, a potential difference is created by the transfer of electrons (or holes) within the device from the hot to the cold end. In this set-up, the device converts thermal energy into electric energy. Alternatively, if a current is supplied to such a device, then the electric energy can be used to generate a temperature difference between the two sides. In other words, the stack acts as a cooling device.
The improvement of existing thermoelectric materials to achieve more effective energy conversion, or the development of new ones, is a demanding task for chemists, materials scientists and engineers. In general, the thermoelectric process within a material and its efficiency are related to three properties: the Seebeck coefficient, which defines the material's ability to generate a potential difference in response to a temperature difference; the electrical conductivity, a measure of the transport of electrons or holes through the material; and the thermal conductivity, which defines how well the material transports or equilibrates heat. Semiconductors that have a reasonably large specific electrical conductivity (in the range of thousands of siemens per centimetre) and a passable Seebeck coefficient (hundreds of microvolts per kelvin) are ideal candidates for efficient thermoelectric power generation, but only if the thermal conductivity is small enough to retain the necessary temperature difference effectively.
Different strategies have been developed to optimize these properties. Doping has commonly been used to increase the concentration of mobile charge carriers and holes, or to manipulate the electronic structure of semiconductors. This strategy has worked well in the case of lead telluride, leading to the development of heavily doped substances such as PbTe1−xSex (ref. 2; Se is selenium), and to a class of thermoelectric3 materials known as lead antimony silver tellurides. The Seebeck coefficients of thermoelectric materials can also be improved by tailoring their electronic structures4.
Most efforts to improve the thermoelectric properties of materials, however, have involved the reduction of thermal conductivity. This requires sophisticated methods, such as altering the nanometre-scale structure of a bulk material (in some cases generating well-defined low-dimensional substructures, such as quantum dots and quantum wells), or forming precipitates of another substance within a thermoelectric material. These approaches prevent heat transport5 by scattering phonons — the heat carriers in thermoelectric materials. But at least some of these widely used scattering procedures will be hard to scale up for the manufacture of commercial products in the near future.
The brilliance of Biswas and co-workers'study of lead telluride is that they have canalized almost every known strategy for optimizing thermoelectric materials into one system. They used a fast, highly effective technique known as spark plasma sintering (SPS) to synthesize bulk lead telluride, identified strontium telluride as the most suitable candidate to form nanoscale precipitates during the synthesis, and determined the optimal amount of sodium to use as a dopant. This combined approach improves the thermoelectric performance of lead telluride to previously unattainable levels.
The success of the authors' strategy depends on the interplay and occurrence of units at several different length scales: from mesoscopic grains of lead telluride at the micrometre scale, to nanoscale precipitates of strontium telluride, and all the way down to dopants that act at the atomic scale (Fig. 1). The authors call this interplay a panoscopic approach, but the embedding of progressively smaller subunits within the material reminds me of matryoshka (Russian) dolls.
Materials scientists have long dreamt of a fast, reliable method for producing bulk thermoelectrics that does not require complicated optimization procedures and intensive material structuring. Biswas and colleagues' work certainly provides a practical method for making bulk lead telluride, but it also shows us that we really do need to screen for the best additives and dopants, and to tailor structural units, to realize the panoscopic approach for every thermoelectric system. It should also be noted that lead telluride is toxic — for commercial applications, other thermoelectric materials must be found that are non-toxic and inexpensive.
Nevertheless, I am sure that the authors' findings will trigger exponential progress in the performance of thermoelectric materials in general. Indeed, I believe that many surprising and encouraging aspects of thermoelectric behaviour will be discovered as a result of their work. If so, then thermoelectric devices might eventually be improved until their efficiency becomes at least comparable to that of other state-of-the-art energy-conversion devices, such as those that convert solar or geothermal energy.
Biswas, K. et al. Nature 489, 414–418 (2012).
Pei, Y. et al. Nature 473, 66–69 (2011).
Hsu, K. F. et al. Science 303, 818–821 (2004).
Nilges, T. et al. Nature Mater. 8, 101–108 (2009).
He, J. et al. J. Am. Chem. Soc. 132, 8669–8675 (2010).
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