Assessment of Nanostructured Capillary Wicks for Passive, Two-Phase Heat Transport

The major factors limiting the thermal performance of passive two-phase heat-spreading devices are the ability of the wick structures to transport liquid by means of capillary forces and the thermal resistance posed by the wicks. Nanoscale geometric enhancements to the wick structure, through the use of carbon nanotubes and metallic nanowires, promise to enhance the capillary transport while at the same time decreasing the thermal resistance due to their high intrinsic thermal conductivity. We analyze the performance of nanostructured wicks in heat-spreading applications. We report that the flow resistance of nanostructures constitutes a major barrier to their use as passive flow-conveying media and identify geo- metrical parameters that yield high rates of thin-film evaporation while minimizing the flow resistance. The analysis shows that the use of nanostructures as the sole wicking element in a two-phase thermal spreader restricts its footprint area to a size of 4 cm2 for heat flux inputs as low as 1 W/cm2 due to the large flow resistance in the nanowick. To overcome nanowick flow resistance, we propose a nanostructure-enhanced sintered particle wick microstructure that leads to a decrease in the wick thermal resistance by 14% relative to the corresponding wick with the same flow resistance and without nanostructures