Thermoelectrics (TE) are solid state systems which convert heat toelectricity or do active cooling by passing a current through it. They canbe used for power generation or cooling devices. Although TE have been around for more than 100 years they are not widely used because of theirlow efficiency. The efficiency of a TE is determined by a dimensionlessfactor ZT called the figure of merit which depends on both electronic andphononic properties of the solid. Due to competing physical effects it hasbeen very difficult to make ZT much larger than 1. However, in recentyears several theoretical ideas (electron crystal phonon glass, transportin reduced dimensionality, dissipation-less energy transport throughenergy filtering etc ) have been proposed to increase ZT beyond 1. Severalof these ideas have been implemented in designing novel materials and someof these show a ZT value ~2. These concepts and how they operate in thesematerials will be discussed. Conditions for dissipation-less energytransport which occurs in a perfect thermoelectric will be explained.Attempts to design such materials will be described.* Work partly supported by a ONR-MURI grant.
