Super-suppression of long phonon mean-free-paths in nano-engineered Si due to heat current anticorrelations

• Published in: Materials today physics Vol. 27; p. 100719
• Main Authors: Hosseini, S. Aria, Davies, Alathea, Dickey, Ian, Neophytou, Neophytos, Greaney, P. Alex, de Sousa Oliveira, Laura
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2022
• Subjects: Equilibrium molecular dynamics; Heat current anticorrelation effect; Monte Carlo ray tracing model; Nanoporous si; Nanostructured thermoelectrics; Phonon mean-free-path suppression; Phonon scattering; Phonon transport; Equilibrium molecular dynamics; Phonon transport; Heat current anticorrelation effect; Nanoporous si; Nanostructured thermoelectrics; Phonon mean-free-path suppression; Phonon scattering; Monte Carlo ray tracing model
• ISSN: 2542-5293; 2542-5293
• DOI: 10.1016/j.mtphys.2022.100719

The ability to minimize the thermal conductivity of dielectrics with minimal structural intervention that could affect electrical properties is an important capability for engineering thermoelectric efficiency in low-cost materials such as Si. We recently reported the discovery of special arrangements for nanoscale pores in Si that produce a particularly large reduction in thermal conductivity accompanied by strongly anticorrelated heat current fluctuations [1] – a phenomenon that is missed by the diffuse adiabatic boundary conditions conventionally used in Boltzmann transport models. This manuscript presents the results of molecular dynamics simulations and a Monte Carlo ray tracing model that teases apart this phenomenon to reveal that special pore layouts elastically backscatter long-wavelength heat-carrying phonons. This means that heat carriage by a phonon before scattering is undone by the scattered phonon, resulting in an effective mean-free-path that is significantly shorter than the geometric line-of-sight due to the pores. This effect is particularly noticeable for the long-wavelength, long mean-free-path phonons whose transport is impeded drastically more than is expected purely from the usual considerations of scattering defined by the distance between defects. This “super-suppression” of the mean-free-path below the characteristic length scale of the nanostructuring offers a route for minimizing thermal conductivity with minimal structural impact, while the stronger impact on long wavelengths offers possibilities for the design of band-pass phonon filtering. Moreover, the ray tracing model developed in this paper shows that different forms of correlated scattering imprint a unique signature in the heat current autocorrelation function that could be used as a diagnostic in other nanostructured systems. [Display omitted] •Large-scale MD calculations show that special arrangements of nanopores result in anticorrelated heat flux fluctuations.•Thermal conductivity is reduced by 80% beyond that expected by porosity alone, due to super-suppression of long MFP phonons.•Different forms of correlated scattering imprint a unique signature in the heat current autocorrelation function.•Monte Carlo simulations show that these phenomena can occur in experimentally realizable systems.•The super-suppression of long MFP phonons provides opportunities for these porous systems to act as phonon band-pass filters.