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) [1-4] 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 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, these effects intensify as the size of the nanopillars 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.
About the Speaker
Mahmoud I. Hussein is the Alvah and Harriet Hovlid Professor at the Smead Department of Aerospace Engineering Sciences at the University of Colorado Boulder. He holds a courtesy faculty appointment in the Department of Physics and an affiliate faculty appointment in the Department of Applied Mathematics, and he serves as the Engineering Faculty Director for the Program of Exploratory Studies. He received a BS degree from the American University in Cairo (1994) and MS degrees from Imperial College, London (1995) and the University of Michigan‒Ann Arbor (1999, 2002). In 2004, he received a PhD degree from the University of Michigan, after which he spent two years at the University of Cambridge as a postdoctoral research associate.
Dr. Hussein’s research focuses on the dynamics of materials and structures, especially phononic crystals and metamaterials, at both the continuum and atomistic scales. His research considers areas that range from vibrations and acoustics of engineering structures and passive flow control to lattice dynamics and thermal transport in semiconductor-based nanostructured materials. His studies are concerned with physical phenomena governing these systems, associated theoretical and computational treatments, and analysis of relevant phenomena such as dispersion, resonance, dissipation, and nonlinearity. His team also conducts experiments to support some aspects of the theoretical work.
Dr. Hussein received a DARPA Young Faculty Award in 2011, an NSF CAREER award in 2013, and in 2017 was honored with a Provost’s Faculty Achievement Award for Tenured Faculty at CU Boulder. He has co-edited a book titled Dynamics of Lattice Materials published by Wiley. He is a Fellow of ASME and has served as an associate editor for the ASME Journal of Vibration and Acoustics. In addition, he is the founding vice president of the International Phononics Society and has co-established the Phononics 20xx conference series which is widely viewed as the world’s premier event in the emerging field of phononics.
Host: Professor Kathryn Matlack
