具有高散熱性能之可撓式微型聚對苯二甲酸乙二酯熱管

Abstract

本論文提出一種低成本且快速的創新製程技術來製作可撓式微型熱管(Flexible Micro Heat Pipe: FMHP)，有別於一般傳統的金屬熱管，本研究以聚對苯二甲酸乙二酯(polyethylene terephthalate : PET)作為熱管的管殼材料(Container)。聚對苯二甲酸乙二酯為一熱塑性高分子，經由低溫熱壓(Low temperature hot lamination)的方式可以與護背膜做黏接並具有高強度的韌性，而在聚對苯二甲酸乙二酯管殼內置入銅網與甲醇以作為熱管的毛細結構(Wick structure)與工作流體(Working fluid)，由於管殼材料與毛細結構皆具有可撓性，因此本熱管可以彎曲且在撓曲力矩的作用下毛細結構並不會被破壞。此外，為了找出可撓式微型聚對苯二甲酸乙二酯熱管(PFMHP)的最佳操作條件與熱傳極限(Heat transfer limitations)，本實驗以不同的輸入功率(Input power)、工作流體的填充率(Filling ratio)與彎曲角度(Bending angle)來探討這些實驗參數對熱管性能的影響。 實驗結果顯示當熱管在26瓦的輸入功率與體積百分比36.9%的填充率下具有最佳的性能，其最低熱阻(Minimum thermal resistance)與最大等效熱傳導係數(Maximum effective thermal conductivity)為0.146 (oC/W)與18310 (W*m-1K-1)，然而，當輸入功率大於30瓦時，由於甲醇具有很高的蒸氣壓(Vapor pressure)所以會導致聚對苯二甲酸乙二酯與護背膜作分離而使得熱管失效(Failure)，因此本研究的熱管在真空封裝(Vacuum packaging)下所能承受最的大蒸氣壓為115.39 kPa，而最大熱傳量(Maximum heat transfer)則為30瓦。 本研究的另外一個主題為探討當熱管在不同的彎曲角度下其對性能的影響，當彎曲角度在30至45度時，由於熱管截面積(Cross-sectional area)縮小的關係，經冷凝後的工作流體會被阻塞(Blocked)在彎曲的地方而無法經由毛細結構傳送到蒸發端(Evaporator)，同時甲醇蒸氣會被冷凝液擋住而無法傳送至冷凝端(Condenser)， 因此熱管的整體熱阻會快速的提高。 雖然聚對苯二甲酸乙二酯其熱傳導係數(0.15-0.24 (W*m-1K-1))遠低於銅(401 (W*m-1K-1))，但本研究以聚對苯二甲酸乙二酯作為熱管的管殼材料卻仍然具有與銅質熱管相當的散熱性能(Heat thermal dissipation performance)，另外，聚對苯二甲酸乙二酯材料具有成本較低、重量較輕且易於封裝的優點，因此可以大幅降低熱管的製作成本與時間，未來此具有高散熱性能之可撓式微型聚對苯二甲酸乙二酯熱管可以應用在消耗功率小於30瓦的電子產品上。

A high thermal dissipation performance polyethylene terephthalate flexible micro heat pipe (PFMHP) has been fabricated and tested in this research. The polyethylene terephthalate (PET) is applied as the container material, which is different from conventional copper heat pipe. PET is a thermoplastic polymer resin which can be easily bonded together with another PET laminating film by low temperature hot lamination. The copper mesh and methanol are sealed inside the middle of two PET films as the wick structure and working fluid, respectively. Because these materials are all flexible, the wick structure of the heat pipe will not collapse under bending moment. The effect of filling ratio, input power and bending angle on thermal resistance are also investigated to find out the optimal conditions and operating limits of the heat pipe.

The experimental results reveal that the minimum thermal resistance and maximum effective thermal conductivity in this PFMHP is 0.146 (oC/W) and 18310 (W*m-1K-1) with 36.9 vol% filling ratio at 26W input power. However, when the input power is larger than 30W, the laminated PET will debond owing to high vapor pressure of methanol. The highest endurable pressure of PFMHP is 115.39 kPa. The comparison of the thermal resistance difference under different bending angle is another object in this research. As the PFMHP is subjected to a pure bending with 45o bend, the condensing liquid will block at bending section because of the cross-sectional area reduction. Therefore, the methanol vapor cannot deliver from evaporator to condenser and the total thermal resistance of the heat pipe will increase suddenly. Although the thermal conductivity of PET (0.15-0.24 (W*m-1K-1)) is much smaller than copper (401 (W*m-1K-1)), the PET heat pipe still has a very low thermal resistance. Besides, compared with the copper pipe, PET is an inexpensive, light and simple sealing material. In the future, with the high thermal dissipation performance, PFMHP can be applied for the cooling of electronic devices with power consumption smaller than 30W.