Super-suppression of long phonon mean-free-paths in nano-engineered Si due to heat current anticorrelations. (October 2022)

• Record Type: Journal Article
• Authors: Hosseini, S. Aria; Davies, Alathea; Dickey, Ian; Neophytou, Neophytos; Greaney, P. Alex; de Sousa Oliveira, Laura
• Is Part Of: Materials today physics. Volume 27(2022)
• Publication Date: 2022-10
• Subjects: Nanoporous si -- Phonon transport -- Nanostructured thermoelectrics -- Phonon mean-free-path suppression -- Phonon scattering -- Heat current anticorrelation effect -- Equilibrium molecular dynamics -- Monte Carlo ray tracing model; Materials science -- Periodicals; Physics -- Periodicals; Electronic journals; 530.41
• DOI: 10.1016/j.mtphys.2022.100719
• Languages: English
• ISSNs: 2542-5293

Abstract

Abstract: 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. Graphical abstract: Image 1 Highlights: 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.