Goniopolar Thermoelectrics

Goniopolar materials display p-type behavior along certain crystallographic directions, and n-type behavior along others. Examples include NaSn2As2 and Re4Si7. They can be metals or semiconductors. A review will be given of the properties of these two classes of material, and of the physical origin of their goniopolar behavior. In NaSn2As2, the behavior is due to a single band of carriers, which form a Fermi surface of characteristic topological properties, which in turn arise from a very specific change in the shape of the wavefunction of specific Sn-As bonds with wavevector. In Re4Si7, a semiconductor, goniopolarity can have two origins: at low doping levels, it is a two-band effect, and at high doping levels, it is also due to the shape of the Fermi surface of a single band. One particulr application for goniopolar materials in transverse thermoelectric generators (TTEGs) will be reviewed in detail. The transvewrse geometry enables the construction of thermoelectric generators that do not have electrical contacts on the hot end of the generator, where they degrade, and avoid the efficiency losses due to contact resistances.<br/>Funding: DOE grant DE-SC0020923, AFOSR # FA9550-18-1-0335<br/>References: <i>Energy Env. Sci.</i> <b>14</b>, 4009-4017 (2021) DOI: 10.1039/D1EE00923K<br/><i>Nature Materials </i><b>18 </b>568-572 (2019) https://doi.org/10.1038/s41563-019-0309-4