Thermoelectric materials provide a means of excess heat to usable electrical energy through simple solid-state devices. When a temperature difference is applied across a thermoelectric material, charge carriers are thermally excited on the hot side and travel to the cold side. This creates a voltage difference across the material. When the ends of the device are connected to a load, a current is generated from the temperature difference. Improvements in thermoelectric materials require maximizing the thermopower (S) and electrical conductivity (s) of the material without subsequently increasing the thermal conductivity (k) of the material. The ratio of these properties provides a measure of the efficiency by which the material converts heat to electrical power and is represented by the thermoelectric figure of merit ZT = S2σT/k.
Epitaxial III-V based nanocomposites
Epitaxial III-V based nanocomposites are comprised of small rare-earth monopnictides (RE-V) nanoparticles embedded in thermoelectric semiconductor alloys during MBE growth. The inclusion of nanoparticles increases the number of interfaces leading to an increase in phonon scattering and a decrease in thermal conductivity. In addition, these particular nanoparticles are believed to provide enhanced electrical conduction through electron filtering mechanisms. RE-V materials are primarily cubic materials with the rock-salt crystal structure and have lattice parameters commensurate with many of the III-V based semiconductors. The RE-V materials are thermodynamically stable with their III-V semiconductor counterparts and the rare-earth elements have low solubilities in the host semiconductors leading to the precipitation of nanoparticles at relatively low concentrations. This supports the growth of single crystal, low dislocation density nanocomposites with coherent interfaces for electron conduction and phonon scattering.
Thermoelectric Half-Heusler Alloys
A not so obvious feature of half-Heusler alloys is that those containing either 8 or 18 valence electrons per primitive unit cell tend to be semiconductors despite being composed of metallic elements. These closed shell electron arrangements (eg. d10 + s2 + p6 = 18) enable a semiconducting gap to open between the bonding and antibonding states. The rule of 18 allows for covalent hybridization of the transition elements; however, the Z element plays a critical role in not only supporting the hybridized bonding with the transition metals but in accommodating the extra d electrons necessary to stabilize the system. The small band gap semiconducting nature of the half-Heuslers and the vast compositional variations that are possible has spawned a great deal of interest in exploiting these material properties for thermoelectrics. The rapidly rising density of states and p-orbital states near the band edges are expected to contribute to the large power factors in half-Heuslers. Additionally, as the valence electron count is slightly increased or decreased through alloying, it ideally provides a means of determining if the material will be p-type or n-type.