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

Abstract

Due 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-enhanced 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 wicking surface. 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 that increase nucleation site density and high permeability vents through which vapor can readily depart the surface under vigorous boiling conditions. The synthesis procedures developed that result in the observed enhancement can be readily incorporated into currently employed devices.

Keywords:

• carbon nanotubes
• microporous materials
• microwave plasma-enhanced CVD synthesis
• heat transfer
• wetting

Acknowledgments

This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA) and Space and Naval Warfare Systems Center (SPAWAR/SYSCEN) San Diego, California, under Contract No. N66001-08-C-2011.