| Popis: |
The majority of the world’s energy comes from nonrenewable sources, with over 60% rejected as waste-heat. If waste-heat could be recovered, the effect on humanity would be equivalent to that of adding a renewable energy source, majorly increasing society’s energy conversion efficiency. This can be accomplished through the use of thermoelectric materials, which convert a temperature gradient (like that from waste-heat) into a usable voltage output. Conventional thermoelectric materials have not increased in commercial efficiency in recent years, therefore a different approach is taken in this dissertation. Here, metals are explored as thermoelectric materials despite having a low thermoelectric efficiency. Magnon drag, an advective process utilizing magnetization dynamics in a temperature gradient to pull charge carriers through a crystal lattice, is shown to dominate thermoelectric transport in ferromagnetic transition metals. Experimental comparison with theory offers a pathway for increasing the thermoelectric efficiency in metals by increasing their magnon-drag thermopower. Additionally, novel transport is explored in the recently experimentally-realized class of materials called Weyl semimetals, where tuning the electronic band structure gives unique topological transport signatures. Predicted to have large transverse transport coefficients, NbP is experimentally proven to effectively convert a temperature gradient into a perpendicular output voltage via the Nernst effect. Transverse thermoelectric devices have technological advantages over conventional Peltier or Seebeck longitudinal modules (in which the applied temperature gradient is parallel to the output voltage), but they require an externally applied magnetic field. Further control over the band structure in Weyl semimetals offers a solution, where YbMnBi2 is experimentally predicted to effectively utilize a transverse geometry without the need for an external magnetic field. This effect is predicted to arise from the Berry curvature of the electronic band structure, which functions like an internal magnetic field. The novel and unique signatures of Weyl semimetals indicate their strong potential as candidate materials for thermoelectric energy generation and cooling. |
