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

Abstract

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. Nano-scale 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 § Author to whom correspondence should be addressed. Phone: +1 (765) 494-5621 2 their use as passive flow-conveying media, and identify geometrical parameters which 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 cm 2 for heat flux inputs as low as 1 W/cm 2 due to the large flow resistance in the nanowick. To overcome nanowick flow resistance, we propose a nanostructureenhanced sintered particle wick microstructure which leads to a decrease in the wick thermal resistance by 14% relative to the corresponding wick with the same flow resistance and without nanostructures.

Recent Advances in Vapor Chamber Transport Characterization for High-Heat-Flux Applications

“…Analysis as a function of nanowire density found that the maximum wicking length occurred at a non-dimensional pitch of 5 due to the tradeoff between capillary pressure and permeability; however, the maximum wicking lengths were only on the order of 1 cm even for modest heat loads due to the low permeability of the structure. This suggested that use of nanowick arrays over a large area on a heated smooth substrate would perform poorly [218].…”

“…Ranjan et al [218] considered evaporation from a uniform nanowire array fed by capillary action. The evaporative resistance of the liquid meniscus formed in the array was computed numerically using the previously developed model discussed in Section 3.2.3 [155].…”

“…Ranjan et al [218] developed theoretical and numerical models to approximate these quantities for representative aligned vertical cylinders in hexagonal and square packing arrangements.…”

“…A wicking length was used to assess the feasibility of nanowick structures in this regard, and was defined as the maximum length over which a given mass flow rate (i.e., heat load) can be transported through the wick structure via capillary action [218], as shown in Figure 30c. Analysis as a function of nanowire density found that the maximum wicking length occurred at a non-dimensional pitch of 5 due to the tradeoff between capillary pressure and permeability; however, the maximum wicking lengths were only on the order of 1 cm even for modest heat loads due to the low permeability of the structure.…”

Patterning the condenser-side wick in ultra-thin vapor chamber heat spreaders to improve skin temperature uniformity of mobile devices

“…This has been achieved through patterning the wick structure [3] or using biporous wicks [4] to enable continuous feeding of liquid to the evaporator under boiling conditions. Alternate strategies aim to reduce the evaporator wick thermal resistance and preserve operation in the evaporative regime (avoiding boiling) using thin nanostructure arrays [5,6] or thin monoporous copper particles with arterial liquid return paths [7,8].…”