Fabrication technique and thermal insulation properties of micro- and nano-channeled polymer composites

Abstract

Sustaining the extra-terrestrial presence of humans in the solar system will require the ability to provide structural, thermal insulation composites (STICs) that have thermal insulation properties in the range of 10–50mW/mK and mechanical properties at least capable of supporting an anticipated lunar habitat internal pressure of 100kPa. A technique has been developed which permits the introduction of micro- and nano-scale channels in polymers, in controlled configurations that will allow for optimization of both thermal insulation and mechanical properties. In this approach, conductive heat transfer mechanisms are being limited by the introduction of crisscrossed channel obstacles into an epoxy matrix, which creates a tortuous pathway and inhibits thermal conduction. By varying the volume fraction of micro-channels, control over final density and effective thermal conductivity can be achieved. The micro-channeled materials were created via sacrificial removal of poly(lactic acid) (PLA) fiber networks embedded in a high-temperature epoxy matrix, using either selective thermal degradation or solvation of the thermoplastic fiber. These processes allowed for acceptable rigidity and strength to be retained by the epoxy matrix. Furthermore, it is expected that as channel diameter decreases from micro-scale to nano-scale, gas diffusion will be constrained by the Knudsen effect, since the gas molecules will increasingly collide with the channel wall, and less frequently with each other, as channel diameter becomes comparable to, or smaller than, the molecules׳ mean free path. This effect, which may limit convective heat transfer through the channels, may be augmented by an even higher degree of tortuosity (or pathlength) imposed by the tightly packed nano-channels. In this report, the thermal conductivities of micro-channeled specimens were compared over a range of channel fractions to provide insight into the mechanisms controlling and limiting heat transfer in these systems.