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

Universal Behavior of Highly Confined Heat Flow in Semiconductor Nanosystems: From Nanomeshes to Metalattices

“…In line with previous work, this contradicts the ballistic assumption wherein Λ is determined by the neck size and challenges the applicability of the standard Matthiessen's rule in estimating scattering times. 49,50 In the fully ballistic limit where porosity approaches 100%, μ declines to zero. However, we observe that, even at high porosities above 80% (ϕ = 0.8), MD simulation geometries with dramatically different values of κ collapse to the same value of μ, indicating that a hydrodynamic-like description of the phonon flow can be extended to extremely constrained systems.…”

Super-Suppression of Long-Wavelength Phonons in Constricted Nanoporous Geometries

“…In this case (green structure), a larger decrease in the HCACF values is achieved, and this change enhances the reduction in the thermal conductivity in Figure 3c. In fact, we find that the AC effect is better controlled by the neck-to-pore-radius ratio than by either of the two quantities alone [16,17], as the combination of both quantities reflects phonons more effectively. We also find that beyond a certain point, corresponding to ~80% thermal conductivity reductions, reducing the neck does not enhance the AC effect.…”

“…For our EMD simulations, we used the LAMMPS software (October 29, 2020 stable version) [20], and the Stillinger-Weber (SW) potential [21], which is commonly used for heat transport in silicon by us and others [12,16,17,22,23], as it provides a good estimation of the phonon dispersions, despite overestimating the thermal conductivity to values of ~250 W/mK [24]. Simulation domain dimensions of up to ~108 nm in length and ~5.4 nm in the width were employed, while the thickness of the domain was ~5 nm in all cases.…”

“…In Refs. [16,17], using equilibrium molecular dynamics (EMD) simulations, we presented a specially designed nanoporous structure of only a small porosity percentage, which allows strong reductions in thermal conductivity. Reductions even up to 80% larger than one would have expected from Matthiessen's rule, taking into account the pore geometry and distances, can be achieved.…”

Effects of vacancies on the thermal conductivity of Si nanowires

“…23,24 The results are similar and feature a non-linear reduction of the thermal conductivity for an increasing concentration of vacancies. Interestingly, Wang et al 23 considered both vacancies and nanovoids – like those of porous Si samples 25,26 – and found that the latter are much more effective in reducing the thermal conductivity. The effect of vacancies on the thermal conductivity of Si NWs was studied by Gholipour Shahraki and Zeinali using reverse nonequilibrium molecular dynamics, 27 but their computational cell only allowed for a separation of 11 nm between the hot and the cold reservoir, and thus their results are likely to be severely affected by boundary scattering.…”