Date of Award

Spring 1-1-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Ronggui Yang

Second Advisor

Yung-Cheng (Y. C.) Lee

Third Advisor

Elizabeth Julie Steinbrenner

Fourth Advisor

Dragan Maksimovic

Fifth Advisor

Xiaobo Yin

Abstract

Power electronics have become important components in low to high voltage electrical devices including portable electronics, inverters for concentration photovoltaic cells, light-emitting-diode (LED), and electric vehicles. Emerging technologies implementing wide bandgap materials such as SiC and GaN enable smaller and more compact devices with higher power density, this calls for more efficient thermal management technologies with matched ultra-thin form factors. Thermal ground planes (TGPs) utilizing phase-change heat transfer to achieve high thermal conductance offer a promising solution. However, both performance limitations and fabrication challenges are encountered when the thickness of TGPs becomes less than 0.50 mm. Capillary evaporation is the key determinant of the thermal performance of TGPs. The onset of nucleate boiling has been shown to significantly reduce the evaporator thermal resistance. However, it is challenging to maintain stable nucleate boiling in the wicking structures of ultra-thin TGPs. The study of capillary evaporation on the wicking structures can lead to the design optimization of ultra-thin TGPs with improved heat transfer performance.

In the first part of this thesis, ultra-thin TGPs have been developed by using a hybrid wicking structure fabricated by bonding single-layer #500 stainless steel micro-mesh onto copper micropillars via electroplating. Such fabrication approach can encapsulate stainless steel wires with copper to prevent corrosion and create microscale structures on the surface to enhance the capillary pressure by increasing the surface areas. The assembled TGP prototypes were around 0.30-mm thick, among the thinnest TGPs in the world so far, and the footprint of TGPs was 10 cm × 5 cm. The maximum effective thermal conductivity of the best-performed TGP of the five tested prototypes was around 2600 W/m-K, more than six times that of a copper reference. This TGP device could operate with a heat load up to 10.5 W without dryout over a heating area of 8 mm × 8 mm.

In the second part of this thesis, the fundamental fluid and heat transport mechanisms in the multilayer micro mesh wicking structures have been studied. A custom-made experimental setup has been developed to characterize the capillary evaporation heat transfer performance of copper micro mesh wicking structures with different sizes. In addition, liquid-wicking (capillary rate-of-rise) experiments have been conducted to determine the effective permeability and capillary force of the micro mesh wicking structures. As the thickness of micro mesh wicking structures increased from 2 layers to 4 layers, the maximum heat flux has been improved from 67.5 W/cm2 to 102.2 W/cm2 due to the increased cross-sectional area for liquid flow. The onset of nucleate boiling was observed to enhance the heat transfer coefficient, with the superheat varied less than 2 °C as heat flux became greater than 50 W/cm2. However, further increasing the thickness to six layers couldn’t increase the dryout heat flux further owing to the suppressed vapor bubbles removal in thicker wicking structures. To improve the vapor bubbles removal, micro mesh wicking structures with different spacings, but similar wire diameter and the meshes studied included #100, #145, and # 200 have been employed. Finer micro mesh demonstrated higher heat transfer coefficients due to the enlarged area of thin film evaporation. However, the test samples with different pore sizes all dried out around 100 W/cm2 because of the boiling limit. Visual inspection revealed that the effective pores for vapor venting were much smaller than the intrinsic openings of micro mesh due to stackings of micro wires, and the difference caused by mesh sizes was negligible.

Two different methods have been developed to promote the heat transfer performance of micro mesh wicking structures. The first method implemented inline-aligned micro mesh wicking structure, of which the micro-cavities formed between micro wires in the direction perpendicular to the substrate provided numerous nucleation sites and the large openings between microwires enabled fast vapor removal. The experimental results have demonstrated that the dryout heat flux was increased by 45%, and the heat transfer coefficient was increased from 9.0 W/cm2·K to 23.5 W/cm2·K for 4-layer # 145 micro mesh. The second method applied a nanostructured surface to increase the dryout heat flux by enhancing the liquid wicking performance. The dryout heat flux of a single-layer micro mesh wicking structure has been improved by three times, from 13.5 W/cm2 to 44.2 W/cm2, Furthermore, the dryout heat flux of a four-layer micro mesh wicking structure has been improved from 102.5 W/ cm2 to 188.6 W/cm2.

Plans for future work are outlined based on the current findings. The next steps will be to develop a methodology to precisely control the alignment of micro mesh, explore other geometric designs for enhancing heat transfer performance of capillary evaporation on thick wicking structures, and develop generalized and analytical models to predict the heat transfer performance and critical transitions in copper micro mesh wicking structures.

Comments

Sixth advisor: Yifu Ding.

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