Progress towards better thermoelectric materials

The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise
with their exceedingly low lattice thermal conductivities. Here we report the successful incorporation of cerium as a guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, through the Kondo interaction, to strong correlation phenomena including the occurrence of giant thermopowers at low temperatures. Indeed, we observe a 50% enhancement of the thermopower compared with a rare earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a rattling enhanced Kondo interaction underlies this effect.

Type-I clathrates are guest host systems with the general composition G8H46. The guest atoms G are situated in polyhedral face-sharing and space-filling cages formed by the tetrahedrally bonded host atoms H. Exceptionally low lattice thermal conductivities are an intrinsic property of these materials as they are observed even in pure single-crystalline specimens. They have been attributed to reduced phonon group velocities resulting from the interaction of acoustic and rattling modes. The charge carriers are essentially unaffected by these lattice anomalies. Thus, type-I clathrates seem to be a realization of the phonon glass electron crystal concept. With this in mind, many groups worldwide have synthesized and investigated a large number of type-I clathrates with various compositions over the past decades. As a result a significant increase has been achieved in the thermoelectric figureof-merit ZT D S 2 = T, where T is the absolute temperature, S is the thermopower, is the electrical conductivity and is the thermal conductivity. The highest ZT values reported are 1.63 at 1,100 K for n-type Ba8Ga16Ge30 and 1.1 at 900 K for p-type Ba8Ga16Ge30.

Record high power factors S^2 have, however, been found in a very different class of materials in strongly correlated rare earth or transition metal compounds. The giant thermopower values observed in these systems can be traced back to an enhanced quasiparticle density of states near the Fermi level that results from the Kondo interaction of the local 4f states with the conduction electrons.

Michigan Thermoelectric with ZT 2.2

Northwestern and Michigan State researchers developed a thermoelectric in 2012 based on the common semiconductor lead telluride. It was the most efficient thermoelectric material known. It exhibits a thermoelectric figure of merit (so-called “ZT”) of 2.2, the highest reported to date.

Nature – High-performance bulk thermoelectrics with all-scale hierarchical architectures

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