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) [1-3] 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 standing on one or both free surfaces. Heat is transported along the membrane portion of this nanostructured material as a succession of propagating vibrational waves, phonons. The atoms making up the minuscule pillars on their part generate resonant vibrational waves, which we describe as vibrons. The vibrons represent new modes added to the system. These two types of waves 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) reduces the base membrane phonon group velocities around the coupling regions, and (2) enables mode localization in the nanopillar portions. These two effects bring rise to a unique form of conductive 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, this effect intensifies as the size of the nanopillar(s) increases, and in principle may be tuned to influence 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 its phonon properties using lattice-dynamics calculations and its thermal conductivity using molecular dynamics simulations. Preliminary electrical properties predictions using density functional theory will also be presented, as well as preliminary experimental results and plans for integration into practical device architectures. Finally, projections of record-breaking values of the thermoelectric energy conversion figure of merit ZT will be provided. Given the relatively large size of our unit-cell modes, I will also overview–if time permits–a Bloch-mode substructuring technique we developed to speed up phonon and electron band structure calculations [4].
[1] Davis, B.L. and Hussein, M.I., Phys. Rev. Lett. 112, 055505, 2014.
[2] Honarvar, H. and Hussein, M.I., Phys. Rev. B. 93, 081412(R), 2016.
[3] Honarvar, H. and Hussein, M.I., Phys. Rev. B. 97, 195413, 2018.
[4] Krattiger, D. and Hussein, M.I., J. Comput. Phys. 357, 183-205, 2018.