Carbon Nanotube Coatings for Enhanced Capillary-Fed Boiling from Porous Microstructures

Abstract

Owing to their high intrinsic thermal conductivity, carbon nanotubes (CNTs) have previously been incorporated into a variety of thermal management applications to improve cooling performance. Implementation of controlled CNT growth techniques and functionalization methods are applied herein to enhance boiling heat transfer from the porous capillary wicking surfaces widely used in high heat flux thermal management devices. A microwave plasma chemical vapor deposition (MPCVD) synthesis process resulted in growth of a permeable CNT coating, and physical vapor deposition of copper over these nanotubes yielded the requisite hydrophilic wickingsurface. An array of test samples was fabricated and then evaluated using an experimental test facility to determine the reduction in surface temperature resulting from CNT coating and micropatterning of the porous surfaces under two-phase heat transfer conditions with water as the working fluid. Both CNT coating and micropatterning techniques were able to provide significant performance enhancements, reducing the surface superheat up to 72% compared to baseline tests and eliminating disadvantageous temperature overshoot corresponding to boiling incipience. Such performance gains are attributable to the formation of nanoporous cavities which increase nucleation site density and high permeability vents through which vapor can readily depart the surface under vigorous boiling conditions. The synthesis procedures developed which result in the observed enhancement can be readily incorporated into currently employed devices.

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

“…For the 1 mm-thick sintered powder sample, an array of 1 mm × 1 mm square recesses improved performance in the boiling regime due to the reduced resistance to vapor exiting the wick structure, as described in Section 2.2; however, addition of the CNT array did not alter performance in the boiling regime because the square regions remained largely flooded during operation [227]. Conversely, for the 200 μm-thick sintered powder wicks, the CNT regions were observed to form a thin-liquid film that receded at high heat fluxes during intense evaporation.…”

“…In a pair of studies conducted by Weibel et al [50,227] Experimental evaluation of capillary-fed evaporation/boiling was performed in the experimental facility described in Section 2.1.1. For the 1 mm-thick sintered powder sample, an array of 1 mm × 1 mm square recesses improved performance in the boiling regime due to the reduced resistance to vapor exiting the wick structure, as described in Section 2.2; however, addition of the CNT array did not alter performance in the boiling regime because the square regions remained largely flooded during operation [227].…”

Metal functionalization of carbon nanotubes for enhanced sintered powder wicks

“…Nevertheless, this methodology is not expected to be feasible for continuous operation in thin vapor chambers due to likely degradation of surface wettability of UV-exposed CNT-coated wicks over time. The hydrophilic behavior of CNTs conformally coated with copper was previously demonstrated by our group [16,21]. This method is advantageous for use in vapor chambers due to long-term compatibility of the copper coating with water.…”

“…Several modifications to traditional monoporous wicks have been proposed in prior work [12][13][14][15][16] to improve the dryout heat flux and to decrease the thermal resistance of the wick structure to meet the cooling requirements for ultra-high heat flux applications. Semenic and Catton [12] compared the critical heat flux (CHF) of monoporous and biporous wicks (wicks with two different pore sizes) using water as the working fluid, under sub-atmospheric saturation pressure (< 13 kPa) conditions and reported high CHF values of 520 W/cm 2 and 990 W/cm 2 at superheats of 50 °C and 147 °C for 800 μm and 3000 μm thick biporous wicks, respectively.…”

“…In recent work [16], the thermal characteristics of 1 mm thick monolithic and patterned sintered copper powder wicks were evaluated with and without a functionalized CNT coating. Patterning of the wick structure with a grid of square recesses reaching down to the substrate provided a low resistance vapor exit path to improve performance during vigorous boiling.…”