Photonically enhanced flow boiling in a channel coated with carbon nanotubes

High heat dissipation rates are enabled by multi-phase cooling schemes owing to latent heat uptake. We demonstrate enhanced flow boiling from a carbon nanotube (CNT)-coated copper surface exposed to low-intensity ultraviolet (UV)-visible excitation. Compared to non-illuminated results, the average boiling incipience temperature decreased by 4.6 degrees C and heat transfer coefficients improved by 41.5% with light exposure. These improved results are attributed to augmented hydrophilicity upon exposure to UV light and possible nanoscale opto-thermal effects, and suggest opportunities for active temperature control of temperature-sensitive devices. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681594

Characterization and Nanostructured Enhancement of Boiling Incipience in Capillary-Fed, Ultra-Thin Sintered Powder Wicks

Next-generation thermal management applications will require passive heat spreading at a lower thermal resistance, higher dryout tolerance, and with thinner profile devices than current vapor chambers. Such performance improvements may be achieved by augmenting evaporation and boiling heat transfer via patterning the internal wick or nanostructuring the wick surface in the region of heat input. Test samples composed of 200 μm thick sintered copper powder layers are investigated because they can be integrated into vapor chambers with an overall thickness of 1 mm. Carbon nanotubes (CNTs) are grown onto patterned and monolithic samples by a microwave plasma chemical vapor deposition synthesis technique, and are functionalized to ensure high wettability with the test fluid, water. Performance of the test samples is evaluated in an experimental facility which replicates the heat input and capillary fluid-feeding mechanisms at the evaporator section of a vapor chamber. High-speed visualizations are performed to identify the vapor formation regimes. Monolithic samples are shown to dissipate heat fluxes greater than 400 W/cm2 over 0.25 cm2 prior to dryout. A noteworthy heat transfer enhancement mechanism observed is reduction of the required superheat for boiling incipience by addition of a CNT coating. Predictable transition from the evaporation to boiling regimes at a lowered superheat is critical due to the lower thermal resistance associated with boiling. Multiple repeated tests on identically prepared samples reveal that the CNT coating reduces the average incipience substrate superheat by 5.6 °C compared to uncoated samples

Graphene: An effective oxidation barrier coating for liquid and two-phase cooling systems

Graphene is studied as an oxidation barrier coating for liquid and liquid-vapor phase-change cooling systems. Forced convection heat transfer experiments on bare and graphene-coated copper surfaces reveal identical liquid-phase and two-phase thermal performance for the two surfaces. Surface analysis after thermal testing indicates significant oxide formation on the entire surface of the bare copper substrate; however, oxidation is observed only along the grain boundaries of the graphene-coated substrate. Results show that few-layer graphene can act as a protective layer even under vigorous flow boiling conditions, indicating a broad application space of few-layer graphene as an ultra-thin oxidation barrier coating. (C) 2012 Elsevier Ltd. All rights reserved

Metal Functionalization of Carbon Nanotubes for Enhanced Sintered Powder Wicks

Phase change cooling schemes involving passive heat spreading devices, such as heat pipes and vapor chambers, are widely adopted for thermal management of high heat-flux technologies. In this study, carbon nanotubes (CNTs) are fabricated on a 200 micrometer thick sintered copper powder wick layer using microwave plasma enhanced chemical vapor deposition technique. A physical vapor deposition process is used to coat the CNTs with a varying thickness of copper to promote surface wetting with the working fluid, water. Thermal performance of the bare sintered copper powder sample (without CNTs) and the copperfunctionalized CNT-coated sintered copper powder wick samples is compared using an experimental facility that simulates the capillary fluid feeding conditions of a vapor chamber. A notable reduction in the boiling incipience superheat is observed for the nanostructured samples. Additionally, nanostructured samples having a thicker copper coating provided a considerable increase in dryout heat flux, supporting heat fluxes up to 457 W/cm^2 from a 5 mm x 5 mm heat input area, while maintaining lower surface superheat temperatures compared to a bare sintered powder sample; this enhancement is attributed primarily to the improved surface wettability. Dynamic contact angle measurements are conducted to quantitatively compare the surface wetting trends for varying copper coating thicknesses and confirm the increase in hydrophilicity with increasing coating thickness