Parameter-free model to estimate thermal conductivity in nanostructured materials

Abstract

Achieving low thermal conductivity and good electrical properties is a crucial condition for thermal energy harvesting materials. Nanostructuring offers a very powerful tool to address both requirements: in nanostructured materials, boundaries preferentially scatter phonons compared to electrons. The computational screening for low-thermal-conductivity nanostructures is typically limited to materials with simple crystal structures, such as silicon, because of the complexity arising from modeling branch-and wave vector-dependent nanoscale heat transport. The phonon meanfree-path (MFP) dependent Boltzmann transport equation (MFP-BTE) approach is a model that overcomes this limitation. To illustrate this, we analyze thermal transport in 75 nanoporous half-Heusler compounds for different pore sizes. Our calculations demonstrate that, in most cases, the optimization of thermal transport in nanostructures should take into account both bulk thermal properties and geometry-dependent size effects, two aspects that are typically engineered separately. To enable efficient calculations within this paradigm we derive a model, based on the "gray" formulation of the BTE, that can decouple the influence of the geometry and the material on the effective thermal conductivity with relatively little loss in accuracy compared to the MFP-BTE. Our study motivates the need for a holistic approach to engineering thermal transport and provides a method for high-throughput low-thermal conductivity materials discovery.

Phonon hydrodynamics in frequency-domain thermoreflectance experiments

“…This is indirect evidence that Fourier's law is not valid at these scales. In previous works, an effective form of this equation with a reduced thermal conductivity [3,[7][8][9][10] or a size-dependent thermal boundary resistance [4,11,12] has been used to interpret experimental observations of size effects. A paradigmatic example in which this approach has been applied are time-domain and frequency-domain thermoreflectance (TDTR/FDTR) experiments in which an oscillating laser heats a metal thin film (the transducer) on top of a semiconductor substrate and the temperature evolution of the transducer is obtained by thermoreflectance techniques [2,[13][14][15][16][17][18].…”

Strong anharmonicity and high thermoelectric performance of cubic thallium-based fluoride perovskites TlXF3 (X = Hg, Sn, Pb)

“…More importantly, the strong anharmonic phonon scattering and the low group velocity of acoustic phonons lead to an ultralow κ L , where for TlPbF 3 this is only 0.42 W m −1 K −1 at 300 K. Meanwhile, the strong bonding of X (X = Hg, Sn, Pb) and F atoms guarantees good electrical conductivity. We captured higher PF and ZT values in p-type TlHgF 3 , and in n-type TlSnF 3 and TlPbF 3 , respectively, where the ZT of TlHgF 3 reached 1.58 at 900 K. Furthermore, a smaller grain boundary size can significantly reduce κ L but has little effect on the electron relaxation time, 26 so we point out ways to further improve the thermoelectric performance by reducing the grain boundary size. This paper reports the excellent thermoelectric properties of three thallium-based compounds and elucidates the microscopic origins of ultralow κ L .…”