Nanoparticle packed beds are of interest because of their extremely low thermal conductivities. However, thermal nanoparticle packed beds exposed to ambient air show unusual increases in conductivity. The causes for that increase in conductivity are examined with experimental measurements. Sorption isotherms and thermal conductivity measurements for 20 nm and 200 nm silica nanoparticle packed beds exposed to a range of humidity are presented. Both nanoparticle beds demonstrate hygroscopic behavior, absorbing significant amounts of moisture upon exposure to humid air. The thermal conductivity of the nanoparticle beds are shown to increase monotonically with relative humidity. More than an order of magnitude increase in conductivity is seen, with a large increase at very low humidity. The measurements illuminate the effects of moisture on the thermal properties of nanoparticle-based materials.

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