Thermal conductivity in nanostructured materials and analysis of local angle between heat fluxes

Abstract

The phonon Boltzmann transport equation with the frequency-dependent model is solved numerically to study the thermal conductivity in nanoporous thin film and nanocomposite. Local angle between heat fluxes, defined as the angle between the directions of heat flux component qx and the local heat flux q, is introduced. At a fixed porosity or interface area, the thermal conductivity, local angle distribution, and the average angle of the two-dimensional nanoporous thin films with circular, hexagonal, square, and triangular pores are reported, and the thermal conductivity decreases with the increase in the interface area or porosity. Furthermore, the relationship between the thermal conductivity and average angle is also discussed for the three-dimensional nanoporous thin films with aligned or staggered pores, and silicon-germanium embedded and compacted nanocomposites. All the results show that the nanostructured material with a larger average angle between heat fluxes has a lower thermal conductivity.

Heat guiding and focusing using ballistic phonon transport in phononic nanostructures

“…Assuming purely specular surface reflections ( p =1), Tang et al 42. predicted a 15% difference between the thermal conductivities of aligned and staggered structures, whereas assuming purely diffuse scattering ( p =0) other theoretical works4145 captured no significant difference. Recently, Hao et al 46.…”

“…Assuming, for instance, a dominant wavelength of λ =2 nm and η =2 nm (Methods), we can roughly estimate that even at room temperature the scattering on hole surfaces can be partly specular ( p >0) if phonons approach the surfaces tangentially ( α >80°). Such tangential incidences often occur 41 42 in the passages between the holes of the aligned lattice as phonons develop directionality passing through the structure ( Supplementary Fig. 6 ).…”

Thermal phonon transport in silicon nanowires and two-dimensional phononic crystal nanostructures

“…It was predicted that the local angle between the heat fluxes, defined as the angle between the directions of the global and local heat fluxes, alters the reduction in thermal conductivity, even when the porosities are identical. 11 This numerical investigation produces some strategies for more efficient reduction of thermal conductivity using porous thin film, but experimental results relating to these predictions have not yet been achieved.…”

“…Fu et al reported that a nanostructured material with a larger average local angles between heat fluxes has a lower thermal conductivity. 11 The local angles are much larger in the PnC structure than in the NWs, where the surface is parallel to the direction of the global heat flux.…”

“…Phonon transport nanoengineering, where phonon transport is controlled by nanostructuring, has attracted significant attention for its contribution to our understanding of fundamental physics 1 and possible application in the development of thermoelectric materials. [2][3][4] Recent experimental and theoretical investigations have demonstrated that thermal conductivity can be dramatically reduced by shortening the thermal phonon mean free path (MFP) using nanopatterning in semiconductors, such as nanowires (NWs), 5,6 thin films, 7,8 porous structures, [9][10][11][12] and phononic crystal (PnC) nanostructures. [13][14][15] In these structures, surface scattering of phonons is the main mechanism for reducing thermal conductivity at room temperature.…”