Effect of moisture on nanoparticle packed beds

Abstract

Nanoparticle packed beds are of interest because of their extremely low thermal conductivities. However, measurements for nanoparticle packed beds exposed to humid air have shown anomalously high thermal conductivity. This work examines the cause for that increase in conductivity. Both experimental measurements and an analytical model for the hygrothermal properties of nanoparticle packed beds are presented. Sets of silica and copper nanoparticles are first characterized using electron microscopy (SEM, TEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Sorption isotherms for the nanoparticle beds were then found by exposing them to controlled humidity environments. The thermal conductivities of the nanoparticle beds were measured both with the beds under vacuum and then at ambient pressure over a range of controlled humidity. The thermal conductivities of the nanoparticle beds are found to be monotonic functions of humidity, increasing more than order of magnitude. An analytical model of nanoparticle packed bed heat transfer controlled by the nanoscale constriction resistances between nanoparticles is extended to account for liquid condensation between nanoparticles for packed beds exposed to humid air. Good agreement is found between the analytical model and experimental conductivity measurements for 200nm silica nanoparticle packed beds under vacuum and over the range of humidity tested. Good agreement is also found between the analytical model and the experimental measurements for 20nm silica nanoparticle packed beds under vacuum and for humidity up to 20%. However, the model underpredicts the experimental measurements of bed conductivity for humidity above 20%. Together the model and the measurements demonstrate the impact of moisture on the thermal properties of these materials and identify important limitations on the use of nanoparticles for very low thermal conductivity insulating materials.