Advanced Thermoelectrics: From Materials to Devices

Abstract

The development of advanced thermoelectric materials for efficient thermoelectric devices is a major challenge in energy conversion technologies. To overcome limitations of toxic and expensive state-of-the-art bulk semiconductors, an improved fundamental understanding of the materials properties and of the thermoelectric conversion processes is needed. At the Symposium "Advanced Thermoelectrics: From Materials to Devices" held at the E-MRS Spring 2013 conference, May 27–31, 2013, Strasbourg, France the topic of thermoelectricity was discussed from a very broad perspective. Not only was the subtitle – from materials to devices – taken very literally, but also were other axes explored, from simple model materials to materials of extreme complexity, from plastics to borides, from chemical to solid state synthesis, from electrochemical thin film deposition to molecular beam epitaxy, from nanoparticles to nanocomposites, from lab-scale to large-scale materials fabrication, from steady state to time resolved measurements, from conventional thermoelectric concepts to entirely new approaches, from phonon to electron engineering, from molecular dynamics simulations to strongly correlated band theory, from degradation to encapsulation, from micro- to macro-devices. This breadth of topics is also reflected in this special issue. Many different materials classes are covered. Graphene, bismuth, silicon, bismuth tellurides and antimonides, lead tellurides, half-Heusler compounds, oxides, chalcogenides, oxy-chalcogenides, borides, skutterudites, clathrates, complex Zintl compounds, Kondo insulators, topological insulators, organic conductors, glasses, phase change materials, CrN. Most of these materials are investigated as bulk single or polycrystals or as single or polycrystalline thick or thin films. Some of the materials are also investigated as nano- or microwires, nanosheets, or membranes. While some articles focus on the bare intrinsic materials properties, most of them present optimization studies. Many different concepts (electron and phonon engineering) and techniques are explored to improve the thermoelectric efficiency of a given material, frequently leading to interesting ZT values of the order of 1 to 1.5 in varying temperature ranges: doping/substitutions (up to pentenary compounds!), nanograin formation, phase separation, strain/shear, introduction of precipitates (including for instance C60), fabrication of composites, fabrication of ordered and even aperiodic heterostructures, superlattices and quantum dots, sometimes based on different isotopes, etc. It becomes more and more common to study also the stability of the materials and in particular of their artificial nano/microstructuring upon thermal cycling. However, real long term studies have not been presented. Instead, many different strategies are explored to stabilize the structures. On the characterization side, the spectrum has broadened considerably. While the standard steady state techniques to measure the thermoelectric quantities of bulk samples are still broadly used, alternative techniques are getting more widespread, e.g., scanning thermal probe micro-imaging, the 3-omega technique, laser flash diffusivity measurements, time domain thermal reflectance, pump-probe techniques, Raman thermometry, and others. Some articles also deal with the fabrication of devices. The progress on microgenerators, frequently based on nanostructures, is impressive. Also a number of traditional thermoelectric generators, fabricated typically from compacted and sintered powders, are discussed. Some presentations also address upscaling. The integration of thermoelectric modules into automobiles and roadmaps to the targeted efficiencies and cost levels are another important issue. Furthermore also entirely new approaches to explore thermoelectric effects for energy generation are discussed, e.g., pn junction thermoelectric generators, thermionic converters, spin Seebeck plus spin Hall effect generators, and liquid chemical thermocouples. And finally, just a few highlights: A new world record in thermoelectric efficiency of clathrates over the relevant temperature range was shown. An entirely new thermoelectric device concept based on spin current plus spin Hall effect was presented and realized in a microgenerator. The simultaneous drastic enhancement of the electrical conductivity and the thermopower in half-Heusler compounds by certain additives was demonstrated. The symposium as well as this special issue is a testimony of great worldwide activity in the field of advanced thermoelectrics. A waste amount of different subtopics are addressed by a broad community. The results are getting more reliable and reproducible, application relevant issues are considered by an increasing number of scientists. Overall, there is hope in the community that thermoelectric applications in waste heat generation will not remain a dream. The symposium was financially supported by INTELBIOMAT (European Science Foundation) and the Swiss Federal Office of Energy (SFOE). The symposium organizers Silke Buehler-Paschen, Antonio Pereira Gonçalves, Sascha Populoh, and Jan D. König