Prof. Joseph P Heremans
October 18 @ 4:00 PM - 5:00 PM - Birck 1001
Solid-State Thermal Energy Conversion
Solid state thermal energy converters are primarily based on thermoelectric materials. The major goal in thermoelectrics research is to simultaneously enhance the thermopower while maintaining a high electrical conductivity, and minimize the thermal conductivity without affecting the electronic properties. We review here three modern physical principles to achieve these goals.
Starting with the thermal conductivity of thermoelectric materials, we know that heat is conducted mostly by phonons. Historically, impeding their transport has been achieved by alloying TE materials, and in the last decade by nanostructuring them or by adding atoms that locally "rattle" and scatter phonons. To these techniques, we add a new one: engineer solids in which the phonons that carry heat have highly anharmonic properties. This promotes phonon-phonon interactions: indeed, when the bond is anharmonic, the atom displacements due to the passage of one phonon change the local bond strength and perturb the field for a second phonon. We will show how specific types of chemical bonds can be selected and designed to maximize these effects, thereby reducing the lattice thermal conductivity of the material to its minimum possible value, the amorphous limit. We will describe one class of thermoelectric solids as examples, the I-V-VI2 compounds (I=group I element, alkali or noble metals; V=group V element, Sb of Bi; VI=group VI element, S, Se or Te) in a CaF2 crystal structure. Here, the anharmonicity arises from the lone pair electrons on the group V element: the atomic displacements that accompany the passage of a phonon distort the orbitals of these electrons (polarizes them) in strongly non-linear ways.
Several years ago, we showed how resonant impurities can lead to an enhancement of the local density of electronic states, which following the theory of Mahan and Sofo leads to an enhancement of the power factor. Our work in this field will be briefly reviewed, with emphasis on the delicate balance that has to be found between the enhancement of the DOS and the loss of mobility.
Finally, the talk will give an introduction to a completely new class of solid-state thermal energy converters based on spin transport. One configuration for such energy converters is based on the recently discovered spin-Seebeck effect. This quantity is expressed in the same units as the conventional thermopower, and we have recently shown that it can be of the same order of magnitude. The main advantage of spin-Seebeck converters is that the problem of optimization is now distributed over two different materials, a ferromagnet in which a flux of magnetization is generated by a thermal gradient, and a normal metal where the flux of magnetization is converted into electrical power. The talk will focus on the basic physics behind the spin-Seebeck effect.
Heremans is an Ohio Eminent Scholar and professor in the Mechanical and Aerospace Engineering Department and in the Physics Department of the Ohio State University since 2005. He is a member of the National Academy of Engineering, and a fellow of the AAAS and the American Physical Society. He joined the Ohio State University after a 21 year career at the General Motors Research labs, and the Delphi Research Labs. He is an experimentalist who specializes in the physics of narrow-gap semiconductors.
- Nancy Black