Prof. Mahmoud Hussein -Large-scale vibron-phonon couplings: A new strategy in thermoelectricity

Prof. Mahmoud I. Hussein
Alvah and Harriet Hovlid Professor
Ann & H.J. Smead Department of Aerospace Engineering Sciences
Department of Physics
University of Colorado Boulder

Abstract: Thermoelectric materials convert heat into electricity or vice versa through a solid-state process. For the conversion efficiency to be competitive with fluid-based technologies, a thermoelectric material must be a good insulator of heat while, simultaneously, exhibit good electrical properties‒a combination that is hard to find in common materials. Here we present the concept of a locally resonant nanophononic metamaterial (NPM) to overcome this natural trade-off in properties. One realization of an NPM is a freestanding silicon membrane (thin film) with a periodic array of nanoscale pillars erected on one or both free surfaces. Heat is transported along the membrane portion of this nanostructured material as a succession of wavenumber-dependent propagating vibrational waves, phonons. The atoms making up the minuscule pillars on their part generate wavenumber-independent resonant vibrational waves, which we describe as vibrons. These two types of waves linearly interact causing a mode coupling for each pair which appears as an avoided crossing in the pillared membrane’s phonon band structure. This in turn (1) enables the generation of new modes localized in the nanopillar portion(s) and (2) reduces the base membrane phonon group velocities around the coupling regions. In addition, the phonon lifetimes drop due to changes in the scattering environment, including both phonon-phonon scattering and boundary scattering. These effects bring rise to a unique form of transport through the base membrane, namely, resonant thermal transport. The in-plane thermal conductivity decreases as a result. Given that the number of vibrons scales with the number of degrees of freedom of a nanopillar, these effects intensify as the size of the nanopillar(s) increases–possibly reaching millions of vibrons–up to the limits of the phononic mean-free-path distribution. In principle, the vibron density of states may be tuned to conform with that of the phonons across the entire phonon spectrum (which for silicon extends up to over 17 THz). This novel phenomenon thus provides an opportunity for achieving exceptionally strong reductions in the thermal conductivity. Furthermore, since the mechanisms concerned with the generation and carrying of electrical charge are practically independent of the phonon-vibron couplings, the Seebeck coefficient and the electrical conductivity are at most only mildly affected, if not at all. In this talk, I will introduce the concept of an NMP and present thermal conductivity predictions using lattice-dynamics-based calculations and molecular dynamics simulations, as well as preliminary electrical properties predictions using density functional theory. Some early experimental results will be presented as well. In conclusion, projections of record-breaking values of the thermoelectric energy conversion figure of merit ZT will be provided.