How Gas–Solid Interaction Matters in Graphene-Doped Silica Aerogels

Abstract

It was interesting to experimentally find that the thermal insulation of silica aerogels was improved by doping graphene sheets with high heat conductivity. The underlying mechanism is investigated in the present work from the perspective of gas−solid interaction using a comprehensive analysis of molecular dynamics (MD) simulations, theoretical modeling, and experimental data. The MD-modeled small pores are demonstrated to effectively represent big pores in silica aerogels because of similar heat conduction physics, because it is found that adsorption does not contribute to gas heat conduction. Meanwhile, based on the experimentally measured density, the porous structures are schematically re-engineered using molecular modeling for the first time. The evaluated pore size distributions numerically present a consistency with available experimental data. Inspired by the visualization of the 3D pore structure, we proposed a graphene/silica/nitrogen model to evaluate the role of graphene in heat conduction: it can not only reduce effective gas collision (impede heat transport) but also enhance the gas−solid coupling effect. The former is dominant because of the high porosity, leading to an improvement in thermal insulation. The competition between them can be the reason for the "trade-off" phenomenon in the graphene doping effect in the available experiment.

Near-Wall Cavitation Effect: A Molecular Dynamics Study

“…Subsequently, since the role of the vibrating wall is to continuously perform reciprocating motion, it is necessary to apply a force f z to each atom on the vibrating wall in the z direction. The sum of forces is the total force applied to the vibrating wall, namely, Σf z = P × A, so that the pressure fluctuates in the system, as introduced by Yang et al 33 To each atom is applied −0.1 kcal/mol•Å, and the negative sign indicates the downward direction of the applied pressure.…”

Silica‐based core materials for thermal superinsulations in various applications

“…Like the sphere model of Swimm et al it is a unit cell model with consideration of local Knudsen numbers. The difference is that the surface of the model particles is not described by the sphere function, but by Equation (15).…”

“…Although this fundamental relationship is clear since Knudsen, 14 the applicability of the corresponding analytical calculation models for real porous media is not yet fully understood. 15 Heat transfer in porous media is mainly composed of three mechanisms: gas thermal conductivity λ g , solid thermal conductivity λ s , and radiation λ r . Moreover, a coupling λ c between the above mechanisms must be considered which strongly depends on the microstructure of the materials.…”

Thermal transport properties of gas-filled silica aerogels

“… 21 Yang et al focused on the impact of graphene doping on the TC of SA, finding that the addition of graphene effectively reduces collisions between gas phases, thereby lowering heat transfer efficiency. 22 Additionally, they investigated the effect of water molecules on the TC of SA, discovering that the adsorption of water molecules can fill molecular-level defects, reducing phonon scattering and thus enhancing TC. 23 However, despite the significant progress made by the MD method in revealing the TC mechanisms of SA, current research still has limitations.…”