Thermoelectric (TE) devices are ‘fuel-free’ solid-state devices which convert a difference in temperature into a comparable difference in voltage or vice-versa, so they can be designed as a cooling device or an electric power generator. They are attractive because they require no moving parts and therefore are extremely reliable. TEs can harvest residual low-grade energy which otherwise is wasted. To date, their use is limited by low conversion efficiency. Until recently, research in semiconductor materials was primarily focused on their thermoelectric effects (Seebeck effect, Peltier effect and Thomson effect) with the aim to replace mechanical refrigerators with thermoelectric (TE) modules. However, low cooling efficiency of such TE devices restricts the commercial applications to niche areas like ‘ice-less’ picnic baskets, ‘climate controlled’ car seats, and as power source for wrist watches. The efficiency of any TE material is determined by the dimension-less parameter, ZT and is defined as ZT = S2σT/k, where S is Seebeck coefficient, σ is the electrical conductivity, k is the thermal conductivity, and T is the absolute temperature. Since S, σ, and k are interdependent, optimizing ZT is a major challenge.
In semiconductors, charge carriers are electrons and holes, and heat transport is done by phonons (lattice vibrations). Both electrons (or holes) and phonons have their corresponding wavelengths (λ) and mean free path. With recent advances in Nanotechnology and techniques for making nanostructured materials, it has become possible to design semiconductor materials with unique properties. For example it is now possible that the size of a semiconductor is smaller than the mean free path of a phonon, but larger than that of its electrons or holes, thereby reducing the thermal conductivity by boundary scattering without affecting its electrical transport. Another concept is to fabricate a composite material with nano-sized inclusions that will scatter or absorb only the phonons.
NexTec project will design and develop thermoelectric materials mainly in the form of bulk nanostructures. This essentially involves synthesis of thermoelectric semiconductor materials in the form of nanoparticles with tailored composition by chemical methods. Specialized compaction techniques will be developed to preserve the nanostructures while achieving a dense bulk-like form that is easy to integrate into modules and eventually working devices. Close collaboration in research between leading universities and industries will enable rapid implementation of research ideas into next generation thermoelectric devices.