Effects of nano-void density, size and spatial population on thermal conductivity: a case study of GaN crystal

Abstract

The thermal conductivity of a crystal is sensitive to the presence of surfaces and nanoscale defects. While this opens tremendous opportunities to tailor thermal conductivity, true 'phonon engineering' of nanocrystals for a specific electronic or thermoelectric application can only be achieved when the dependence of thermal conductivity on the defect density, size and spatial population is understood and quantified. Unfortunately, experimental studies of the effects of nanoscale defects are quite challenging. While molecular dynamics simulations are effective in calculating thermal conductivity, the defect density range that can be explored with feasible computing resources is unrealistically high. As a result, previous work has not generated a fully detailed understanding of the dependence of thermal conductivity on nanoscale defects. Using GaN as an example, we have combined a physically motivated analytical model and highly converged large-scale molecular dynamics simulations to study the effects of defects on thermal conductivity. An analytical expression for thermal conductivity as a function of void density, size, and population has been derived and corroborated with the model, simulations, and experiments.

Investigation of size and electronic effects on Kapitza conductance with non-equilibrium molecular dynamics

“…The data points for the longer systems (2:1) fall slightly below the trend; however, the values for the longest systems revert to the linear trend in a manner that is suggestive of a lengthinsensitive bulk value that is approximately equal to the linear extrapolation of the bulk limit, r % 4:73 m 2 K=GW. Similar behavior was observed in bulk resistivity 12,47,48 (see Ref. 48 for a thorough discussion). This linear trend is significant since it appears to be another instance of the Matthiessen scattering rule, analogous to the thermal resistivity of an interface-free material in contact with heat baths being linearly related to the inverse of the system length, which is widely cited and employed.…”

“…This linear trend is significant since it appears to be another instance of the Matthiessen scattering rule, analogous to the thermal resistivity of an interface-free material in contact with heat baths being linearly related to the inverse of the system length, which is widely cited and employed. 12,26,[47][48][49][50] Also, it is clear that size effects are less prominent at higher temperatures, where the phonon mean-free-paths are shorter; and, as Sellan, Landry, McGaughey and co-workers 12,50 suggest, at shorter lengths, when more phonons travel ballistically from bath to interface, the apparent resistances decrease. The lower sensitivity of the Kapitza resistance at higher temperatures and the finding that the length at which the Kapitza resistance is essentially length-independent increases at lower temperatures are consistent with the temperature dependence of the phonon population's mean free paths.…”

“…It can provide guidelines for the design of real devices at applicable length-scales and allow MD results obtained on the order of nm to be extrapolated to the infinite system length limit, as is commonly done with the thermal conductivity. 12,[26][27][28]35,[47][48][49] The fact that both thermal resistivity and Kapitza resistance satisfy the same proportionality to inverse total length suggests similar scattering by the heat source/sink and the interfaces between different materials. As our simulations involve both types of interfaces, the scaling rule also confirms the primary assumption of Matthiessen rule, i.e., the scattering sources are essentially additive.…”

Influence of defects on the thermal conductivity of compressed LiF

“…We did not observe larger reductions in thermal conductivity apparently since the crystal structure in these highly defected system is still largely intact and phonons propagation is relatively unperturbed. One basic type of defect, a volume defect such as a void, was not studied but may have significant effects on thermal conductivity if present in sufficient density 64 . We did consider voids, which are essentially clusters of point vacancies which have a tendency to aggregate 65,66 , but they…”

Tailored morphology and highly enhanced phonon transport in polymer fibers: a multiscale computational framework

“…4 The low TC in bulk polymers have been attributed to various fundamental mechanisms, including the disordered arrangement of polymer chains in the amorphous phase, the presence of grain boundaries between polycrystalline domains, inter-chain entanglements, nanovoids, and other defects and impurities, all of which result in significant phonon scatterings that hinder heat conduction. [5][6][7][8] Nevertheless, fully extended (with all the dihedrals in the trans state) single polyethylene (PE) chain has been proved to possess extremely high TC in the order of 100 W m −1 K −1 through all-atomistic molecular dynamics (AA-MD) simulations. 9 This highlights the potential of highly efficient heat conduction in bulk polymers, if all the chains are extended and aligned along the heat conducting direction.…”