Phonon Transport within Periodic Porous Structures — From Classical Phonon Size Effects to Wave Effects (original) (raw)
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Journal of Applied Physics, 2016
In the past two decades, phonon transport within nanoporous thin films has attracted enormous attention for their potential applications in thermoelectrics and thermal insulation. Various computational studies have been carried out to explain the thermal conductivity reduction within these thin films. Considering classical phonon size effects, the lattice thermal conductivity can be predicted assuming diffusive pore-edge scattering of phonons and bulk phonon mean free paths. Following this, detailed phonon transport can be simulated for a given porous structure to find the lattice thermal conductivity [Hao et al., J. Appl. Phys. 106, 114321 (2009)]. However, such simulations are intrinsically complicated and cannot be used for the data analysis of general samples. In this work, the characteristic length K Pore of periodic nanoporous thin films is extracted by comparing the predictions of phonon Monte Carlo simulations and the kinetic relationship using bulk phonon mean free paths modified by K Pore. Under strong ballistic phonon transport, K Pore is also extracted by the Monte Carlo ray-tracing method for graphene with periodic nanopores. The presented model can be widely used to analyze the measured thermal conductivities of such nanoporous structures.
Analytical model for phonon transport analysis of periodic bulk nanoporous structures
Applied Thermal Engineering, 2017
Phonon transport analysis in nano-and micro-porous materials is critical to their energy-related applications. Assuming diffusive phonon scattering by pore edges, the lattice thermal conductivity can be predicted by modifying the bulk phonon mean free paths with the characteristic length of the nanoporous structure, i.e., the phonon mean free path () for the pore-edge scattering of phonons. In previous studies (Jean et al., 2014), a Monte Carlo (MC) technique have been employed to extract geometry-determined for nanoporous bulk materials with selected periods and porosities. In other studies (Minnich and Chen, 2007; Machrafi and Lebon, 2015), simple expressions have been proposed to compute. However, some divergence can often be found between lattice thermal conductivities predicted by phonon MC simulations and by analytical models using. In this work, the effective values are extracted by matching the frequency-dependent phonon MC simulations with the analytical model for nanoporous
Thermal transport in 2-and 3-dimensional periodic “holey” nanostructures
Understanding thermal transport in two-and three-dimensional periodic "holey" nanostructures is important for realizing applications of these structures in thermoelectrics, photonics and batteries. In terms of continuum heat diffusion physics, the effective medium theory provides the framework for obtaining the effective thermal conductivity of such structures. However, recently measured nanostructures possess thermal conductivities well below these continuum predictions. In some cases, their thermal conductivities are even lower than predictions that account for sub-continuum phonon transport. We analyze current understanding of thermal transport in such structures, discussing the various theories, the measurements and the insights gained from comparing the two.
Nature Communications, 2015
Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures. However, the role of coherent boundary scattering in such structures has become a matter of some debate. Here we report on the first experimental observation of coherent phonon boundary scattering at room temperature in 2D phononic crystals formed by the introduction of air holes in a silicon matrix with minimum feature sizes 4100 nm. To delaminate incoherent from coherent boundary scattering, phononic crystals with a fixed minimum feature size, differing only in unit cell geometry, were fabricated. A suspended island technique was used to measure the thermal conductivity. We introduce a hybrid thermal conductivity model that accounts for partially coherent and partially incoherent phonon boundary scattering. We observe excellent agreement between this model and experimental data, and the results suggest that significant room temperature coherent phonon boundary scattering occurs.
High-temperature phonon thermal conductivity of nanostructures
Physical Review B, 2002
Phonon propagation in the disordered nanostructures at a high ͑about the Debye temperature or higher͒ temperature is considered. Scattering at the grain boundaries is assumed to be the main mechanism restricting the thermal conductivity. Influence of the structure ͑the grain size and its dispersion, the pore diameter and their volume concentration, and the intergrain interface structure͒ as well as temperature on the thermal conductivity is discussed.
Lattice Thermal Conductivity in Nano- to Micro-scale Porous Materials
Metallurgical and Materials Transactions E, 2014
We study the effect of thermal phonon scattering on the reduction of lattice thermal conductivity (LTC) in porous materials with spherical pores and inclusions of varying diameters from nano-to microscale sizes. Using a model based on the Gamma distribution of the pore sizes, we calculate effective phonon mean free paths at scattering on randomly distributed pore boundaries and obtain a general relationship in ''gray medium'' approximation for the LTC of the material. Then, we determine the LTC of multiple phases in the presence of inclusions of various size scales embedded within a single host material and obtain a simple analytic expression for the effective LTC of a three-phase composite with nano-and microscale randomly distributed inclusions. We show that the presence of hollow pores and (or) inclusions with the all-scale hierarchical disorder leads to a considerable reduction in the LTC of the composite. For example, the thermal conductivity of such a composite on the basis of PbTe in some specific cases may possibly be reduced by more than one order of magnitude, which is very useful to achieve a large enhancement in the thermoelectric figure of merit ZT. Results of our model are compared with the existing experimental data.
Thermal transport in nanostructures
AIP Advances, 2012
This review summarizes recent studies of thermal transport in nanoscaled semiconductors. Different from bulk materials, new physics and novel thermal properties arise in low dimensional nanostructures, such as the abnormal heat conduction, the size dependence of thermal conductivity, phonon boundary/edge scatterings. It is also demonstrated that phonons transport super-diffusively in low dimensional structures, in other words, Fourier's law is not applicable. Based on manipulating phonons, we also discuss envisioned applications of nanostructures in a broad area, ranging from thermoelectrics, heat dissipation to phononic devices.