Molecular dynamics simulations of energy accommodation between gases and polymers for ultra-low thermal conductivity insulation

TL Feng and A Rai and D Hun and SS Shrestha, INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, 164, 120459 (2021).

DOI: 10.1016/j.ijheatmasstransfer.2020.120459

Determining the energy accommodation between gases and solids is essential to developing porous thermal insulation materials with ultra- low effective thermal conductivity that reduce energy use, greenhouse gas emissions, and fossil fuel consumption. The energy accommodation coefficients of most gases, however, have been rarely studied, especially with respect to solids that have relatively high thermal resistivity, e.g., polymers. In this work, by using all-atom nonequilibrium molecular dynamics simulations, we reveal the accommodation coefficients of He, Ar, N-2, and O-2 with polymers, mainly polystyrene. We find that their values are around 0.51, 0.72, 0.79, and 0.90, respectively, suggesting a critical reexamination of the commonly used theoretical maximum value of 1. We have also conducted experiments and validated the value for air, which is about 0.81. Such a change in accommodation coefficients can lead to a reduction of about 70%, 50%, 35%, and 20% in the thermal conductivity of He, Ar, N-2, and O-2 gases in nano pores (below 100 nm) or at low pressures (below 1 millibar). With these new accommodation coefficients, we find that in a 10 nm pore with ambient pressure at 300 K, the gas thermal conductivity of He, Ar, N-2, and O-2 in porous polystyrene can be as low as 9.7 x 10(-4) , 3.4 x 10 (-4) , 7.3 x 10 (-4) , and 8.5 x 10 (-4) W m (-1) K (-1), respectively, which are two to three orders of magnitude lower than their bulk values, promising higher thermal resistivity of insulation materials. This work reveals the fundamental energy exchange between gases and polymers, providing important guidance for designing high- performance thermal insulation materials for various applications. (C) 2020 Elsevier Ltd. All rights reserved.

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