Case Western Reserve University

Thermoelectrics

Over the past decade, considerable research efforts have focused on understanding and manipulating thermal transport in micro/nanostructures. Although results have benefited many technologies, promising research has prompted renewed interest in thermoelectric devices. In particular, considerable attention has been paid to fabricating micro/nanostructures that, due to their reduced dimensionality, exhibit desirable thermoelectric properties including low thermal conductivity and enhanced thermopower. For a thermoelectric cooler to be comparable in efficiency to the standard vapor-compression cycle refrigerator, the dimensionless thermoelectric figure of merit, ZT , which is a measure of the performance of the material, must be higher than ~3.0 at room temperature [ [i] ]. The formulation for ZT is given as:

where S is the Seebeck coefficient, T is absolute temperature, and s and k are the electrical and thermal conductivity, respectively. Much of the research in this area has investigated the use of superlattices, two-dimensional structures consisting of alternating layers of thin films, which impede heat flow in the cross-plane direction. Many promising experimental discoveries have reported dimensionless figures of merit of approximately 1.0, with the most recent unmatched claim of 2.4 in Bi 2 Te 3 /Sb 2 Te 3 superlattices [ [ii] ]. Even though superlattices hold promise as good thermoelectric materials, theoretically a device composed of nanowires or nanoparticles may provide a superior alternative. In certain nanoscale systems, the reduced dimensionality of the structures may result in confinement of the charge carriers and phonons, thereby affecting transport characteristics, and consequently leading to an enhanced thermoelectric figure of merit [ [iii] ] – [ [iv] [v] [vi] [vii] ]. More specifically, quantum confinement of electrons in nanowires allows tailoring of the electronic band structure. Boundary scattering (which may dominate phonon interaction) and phonon confinement (which influences the phonon spectra and lifetime [ [viii] ]) can lead to a further reduction in the thermal conductivity. An important application focus of the nanoEngineering laboratory is to investigate nanomaterials for use in thermoelectric devices for superior performance.

Although the principles and theory of thermoelectrics has been rigorously developed for nearly two hundred years, devices based on this technology have not seen widespread use. This is largely because thermoelectric efficiencies are considerably lower than comparable technologies. Nonetheless, thermoelectric coolers and generators contain no moving parts and, in the case of power generation, can be used to capture “free" waste heat to produce electricity. In the recent past, thermoelectric generators have been employed as power sources in satellites, space probes and unmanned remote facilities – the heat released by the decay of radioactive material is converted to electricity. Moreover, the automobile industry is actively pursuing the capture of waste heat from exhaust to provide additional power to a car. The nanoEngineering laboratory is also currently investigating the use of thermoelectric devices in various waste heat capture applications.