Thermal Conductivity at Solid/Fluid interfaces: From Adsorption to Phonon Scattering through the Rattle Effect
Résumé
Of relevance to adsorption, separation and catalysis, fluids in nanoporous materials display intriguing phenomena arising from their underlying structural, thermodynamical and dynamic behavior. While many effects at the solid/fluid interface in nanoporous materials are well-documented, the thermal behavior of such solids subjected to fluid adsorption remains to be deciphered. Among exotic mechanisms, the so-called rattle effect -which corresponds to the decrease in the solid thermal conductivity induced by phonon scattering at the fluid/solid interface -has received only little attention. This phenomenon, which challenges existing mixing rules and effective medium approaches, has now been identified in nanoporous materials (e.g. zeolite, MOF). However, considering that this nanoscale effect is necessarily restricted to a very small region near the fluid/solid interface, its impact for less finely divided materials (i.e. with thicker solid/fluid domains) remains to be established. Here, we address this question by employing a molecular simulation strategy to investigate thermal transport in a prototypical nanoporous silica whose porosity is filled with a simple fluid. While the conventional behavior predicting an overall thermal conductivity increase upon fluid addition is qualitatively recovered, a reminiscent rattle effect at the solid/fluid interface is observed. By tuning the fluid/solid interaction strength, we establish simple scaling relations between the adsorbed amount per unit of surface area, as measured through the typical density profile, and the contributions of the fluid, solid and solid/fluid interface to the thermal conductivity. Understanding such intrinsic relations and the rattle effect that govern the thermal conductivity in fluid/solid systems paves the way for the design of novel materials to harness thermal processes in practical applications.
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