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Date of Award

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering–Engineering Mechanics (PhD)

College, School or Department Name

Department of Mechanical Engineering-Engineering Mechanics

First Advisor

Desheng Meng

Abstract

The thermal management systems for electronic devices and their power sources are facing increasing challenge to accommodate the ever-changing environmental and operational conditions. The conventional thermal management systems, with a predominant focus on cooling, are often not sufficient in those cases. In addition, to support miniaturization, complex systems and broader applications (e.g., space and military), the thermal management system often needs to be compatible with smaller device and their fabrication processes, dissipate heat efficiently for localized heat spot, and meet the requirement of light weight and low power consumption. In order to address such issues, a self-adaptive thermal switch array (TSA) is proposed based on microelectromechanical systems (MEMS) technology which has the capability automatically change its thermal conductance according to the environmental and operational conditions. This TSA was actuated by low melting alloy (LMA) with neither control unit nor parasitic energy consumption.

The idea has been first demonstrated by a prototype device with the stabilization temperatures under various power inputs investigated both experimentally and theoretically. When the power input was changed from 3.8W to 5.8W, the stabilization temperature of the device was increased only by 2.5°C due to the stabilization effect of TSA. The experimental data were found in good agreement with their theoretical value. Based on the theoretical model, two types of TSA, namely high-on and low-off, were further developed to increase on-state thermal conductance and decrease off-state thermal conductance, respectively. Compared with the low-off TSA, the high-on TSA can more efficiently cool the devices and stabilize their temperature at a value closer to the melting point of LMA even under higher power inputs. On the other hand, the startup time and energy consumption were significantly reduced with the low-off TSA design due to the enhanced off-state thermal insulation, making them more suitable for cold start applications. A few key design factors have been identified to increase the on-state thermal conductance, reduce the off-state thermal conductance and enhance their ratio (switching ratio).

The TSAs were then applied to a Li-polymer battery stack to demonstrate the self-adaptive thermal management capability. When cold-started from -10°C, the TSA-regulated battery stack reached 20°C in ~10min. The operational temperature was sustained with a moderate discharge current until depletion, while maintaining a fairly uniform temperate distribution within the battery stack. Compared with the open-cooling case, the performance of the Li-polymer battery was significantly improved by TSA-regulated thermal management. The capacity and output energy were increased by 16% and 23%, respectively. With the low-off TSA, the cold-start time has been shortened to ~ 7min, while the capacity and output energy of battery stack were increased by 18% and 27%, respectively, as compared to open-cooling case. The promising results have also paved the way for improving the performance of self-adaptive thermal management through the key design parameters.

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