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Research Papers: Evaporation, Boiling, and Condensation

Characterization and Modeling of the Heat Transfer Performance of Nanostructured Cu Micropost Wicks

[+] Author and Article Information
Youngsuk Nam, Stephen Sharratt, Gilhwan Cha

Mechanical and Aerospace Engineering Department,  University of California, Los Angeles, 90095-1597just@seas.ucla.edu

Y. Sungtaek Ju1

Mechanical and Aerospace Engineering Department,  University of California, Los Angeles, 90095-1597just@seas.ucla.edu

1

Also with California NanoSystems Institute, UCLA.

J. Heat Transfer 133(10), 101502 (Aug 11, 2011) (7 pages) doi:10.1115/1.4004168 History: Received July 08, 2010; Revised May 02, 2011; Published August 11, 2011; Online August 11, 2011

Micro heat pipes incorporating advanced wicks are promising for the thermal management of power electronics. We report the heat transfer performance of superhydrophilic Cu micropost wicks fabricated on thin silicon substrates using electrochemical deposition and controlled chemical oxidation. For a fixed post diameter, the interpost spacing and hence solid fraction is found to be a main design factor affecting the effective heat transfer coefficient and critical heat flux. The effective heat transfer coefficient >10 W/cm2 K and the critical heat flux >500 W/cm2 over 2 mm × 2 mm heating areas are demonstrated. Copper oxide nanostructures formed on the micropost surfaces significantly enhance the critical heat flux without compromising the effective heat transfer coefficient. An approximate numerical model is developed to help interpret the experimental data. A surface energy minimization algorithm is used to predict the static equilibrium shape of a liquid meniscus, which is then imported into a finite element model to predict the effective heat transfer coefficient. The advanced wick structures and experimental and modeling approaches developed in this work will help develop thin and lightweight thermal management solutions for high-power-density semiconductor devices.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

SEM image of nanostructured Cu microposts, Dp (post diameter) = 50 μm, Dcc (post spacing) = 50 μm, P (pitch) = 100 μm, (a) × 350, (b) × 900, (c) × 5 k (side wall), and (d) × 10 k (top wall)

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Figure 2

The electrical resistance of the Au heater (5 × 5 mm2 ) as a function of temperature. (inset) Images of the microfabricated heater chips (2 × 2 mm2 and 5 × 5 mm2 ).

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Figure 4

The schematic (a) and the photos (b) of the vacuum system we developed for the heat transfer tests

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Figure 5

The wick sample with an installed heater chip during a typical heat transfer characterization experiment. The sample is 3 cm × 3 cm in total size.

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Figure 6

The finite element model used to estimate the wall superheat

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Figure 7

Temperature distributions across the heater chip and the substrate obtained from either the 3D numerical simulations or the 1D model (Dp  = 50 μm, Dcc  = 30 μm, tested with a 5 × 5 mm2 heater)

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Figure 8

Calculated and measured temperature distributions on the backside of a wick sample (Dp  = 50 μm, Dcc  = 50 μm, 5 × 5 mm2 heater)

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Figure 9

Heat flux as a function of the corrected wick superheat (TwTv ) for (a) 2 × 2 mm2 and (b) 5 × 5 mm2 heaters

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Figure 10

3D unit cell with a volumetric mesh and boundary conditions for finite element modeling

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Figure 11

The predicted heff for apparent contact angles of 7 deg and 30 deg compared with the values extracted from the experimental data

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Figure 12

(a) Heat flux as a function of the wick superheat (TwTv ) before and after the surfaces of the microposts are nanostructured. In the label, Dp (post diameter, micrometer), Dcc (post spacing, micrometer), and fs (solid fraction). The surface morphology of (b) bare Cu post and (c) nanostructured Cu.

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Figure 3

Measured transient temperature rise in the heater as a function of time after current pulse initiation. Such temporal temperature profiles are analyzed using finite element simulations to determine the thermal resistance of the solder interface between the heater chip and the wick sample.

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