微型無閥門熱氣泡幫浦之研製

Abstract

能源問題日益嚴重，替代能源的研究日益興盛。在這之中，燃料電池又可以說是非常深具潛力的明日之星。在眾多種燃料電池之中，直接甲醇燃料電池(DMFC) 是有機會可以結合微機電製程進行微型化，以符合眾多攜帶裝置的需求。目前DMFC所使用的甲醇燃料其體積濃度約為3% ~ 10%，然而端看化學反應式可以知道總反應是生成水的。若是能將這些水回收並與醇甲醇混合成操作濃度再進入反應區，則就可以提升能量密度。而要能驅動甲醇，水這兩個工作流體且又能搭配DMFC的低操作電壓等特性，及未來要能有微小化的潛力，在此我們選用熱氣泡做為微幫浦的驅動源，製作熱氣泡致動無閥門幫浦。 本研究試著以微機電製程和陽極接合技術製作出微型熱氣泡致動式無閥門幫浦。 此幫浦主要由下列三個部份所組成： 1.致動器(Actuator)：幫浦的運作是由氣泡來推動，而氣泡的產生是由加熱器對工作流體加熱所生產的氣泡。加熱器的設計必須要考慮到電阻值，希冀能夠在1到5伏特的低電壓操作下能降低電阻，提升電功率。 2.腔體(Chamber)：致動器推動工作流體的地方。熱氣泡內是氣相的工作流體，與液相的密度差將近千倍之多，因此在熱氣泡產生後勢必會造成其他還在液態的工作流體產生推擠效應而將流體推出至流道當中。 3.漸縮漸擴管(Nozzle-diffuser channel)：利用漸縮漸擴管兩端在不同的流場方向下會產生不一樣的壓降(Pressure drop)效應，達到整流的目的，可以得到一個特定方向的淨流量(Net flow)。 而本論文著重在下列四個方向： 1.嘗試著在玻璃晶圓(Glass Wafer)鍍上一層金薄膜(Au Thin Film)來當作加熱器電橋，利用不同長度和寬度的電橋，並且考慮製程的誤差來設計不一樣的電阻，俾使能有低電阻值。 2.將腔體和漸縮漸擴管一同製作於矽晶圓(Silicon Wafer)上，並進行特殊設計以利水管路和電路的連接。 3.利用陽極接合(Anodic Bonding)將上述兩晶圓密封接合，並且使用毛細管連接到預留的流道口以及使用導線接到加熱器的接線端。 4.在輸入的功率固定下，利用方波(Square Wave)的高和低準位來當做功電源輸出的ON/OFF，藉由改變頻率和Duty Cycle來量測幫浦所產生的流量差異，並分析輸入功率和流功所計算效能與上述條件的關係。 依照目前所能實驗出的資料顯示，在10赫茲操作頻率及4.375ms的加熱時間下，可以產生流量為5.027μl/min，但所消耗的平均功率約67.2mW。同時也發現提高操作頻率或是加長加熱時間並不一定會增加效能，這也是未來需要更多實驗數據來去分析這個驅勢。 利用調整產生微氣泡的頻率推動工作流體的幫浦致動器，同時搭配上無閥門(valve-less)設計，其優點是不具備可動元件(moving parts)，機械可靠度高，保養容易，且製造程序也比起有可動元件製造簡單。也因為此，在微型化的過程中比其它型微幫浦更具有優勢。對於可精確控制微小流量具有一定貢獻。 加熱器採用金來製造，金是活性最低的金屬元素，在加熱過程中不易與工作流體產生反應，可以不需要再鍍上氧化矽(Silicon Oxide)來當保護層，對於熱量傳輸效率可以提升，另外一點則是金的電阻率也是最低，可以降低電阻值，這樣在同功率操作下，電壓可以降低，在應用層面上是非常重要的因素。

Because the problem about energy need is more and more serious, all countries in Earth are researching the alternative energy. Fuel cell is very popular and important in many energy researches. In many kinds of them, DMFC has potential to miniaturize for the portable equipment above them. It has some advantage: no fuel reformer, low operation temperature, safe fuel storage and transport, and etc. It can be easy to carry on one’s person after miniaturization. In real experiment, it use only 3% ~ 10% solution of Methanol in water, so the energy density is very low. However, the total chemical reaction results in producing power, water and carbon dioxide. So if the water can be recycled to mix with methanol, the fuel concentration in cartridges may raise. The energy density is raised. How does the water and methanol be driven in miniaturized DMFC? A Micropump is wished to be used! The Voltage that DMFC outputs is very low. Few micropumps can be driven. So we choose micropump by thermal bubble actuated according to the properties of DMFC. In the meanwhile, it is looked forward to have high operation efficiency, so the valve-less design is also considered. Except the studies of the operation principles, this thesis focuses on the design issues and manufactures of the micropump. This micropump has three parts: (1) Thermal bubble actuator that is made by electric igniting device because it raises temperature fast in short time. It has low resistance, so it can be driven by low voltage. (2) Chamber where the phase of working fluid changes when it sucks heat transferred from actuator. (3) Nozzle-Diffuser channel that can rectify net flow in specified direction. This micropump chip is come true by MEMS and Anodic bonding successfully. In experiments, the peak power is fixed and the square wave or pulse function can switch the power supply and set the heating time. That is duty cycle. the flow rate and difference between them are be measured by changing the frequency or heating time. Then the average input power is discussed with the flow rate and frequency. According to the experiment that has been done, the maximum flow rate is 5.027μl/min under 10Hz operating frequency and 4375μs(about 0.044% duty cycle) and it consumes 67.2mW. When the operating frequency or heating time is raised, the flow rate isn’t guaranteed to raised, too. Preliminary result expresses that flow rates achieve the research before but consume lower power than them. In the future, the experiment is continued to gather more information to investigate the results.