偵測水穿透質子交換膜的聲波感測器

Abstract

質子交換膜燃料電池運轉時，質子交換膜必須維持一定濕度，以維持良好的離子導電率。本研究設計一組聲導波感測器，黏貼於流道板背面兩端，監測流道板的反應生成水及質子交換膜的濕度。感測器是由反對稱指叉電極壓電纖維複材(簡稱AE-PFC)所構成，具有窄頻、質量輕、高度指向性等優點。 實驗以碳紙做為氣體擴散層，覆蓋於流道區，根據聲導波時域訊號包絡線與頻譜峰值變化，探討反應生成水濕潤碳紙對於導波的影響。實驗結果顯示碳紙的含水量會使行經流道區工作頻率內的第二模態導波振幅下降，減緩第一模態導波在流道區的反射，使工作頻率內的第一模態導波振幅上升，由振幅下降幅度則可判斷水在肋板或流道的分布狀態。感測器於肋板與流道的靈敏度分別為0.03與0.3 mV/μl，可量測水量範圍為20至220 μl，解析度可達到5 μl。以有限元素法分析壓電纖維的電場及聲波傳遞，數值結果顯示驅動及極化電場大多平行AE-PFC之纖維方向，暫態分析得知AE-PFC與主結構之間的黏著劑厚度會改變訊號的頻譜特性。

Water transport rate in the proton exchange membrane fuel cell (PEMFC) should be in a specific range to keep a good electric conductivity in the membrane. This thesis develops an in-situ inspection method to detect water transport through the membrane into flow channels by use of a novel acoustic guided wave transducer. Two anti-symmetric interdigitated piezoelectric fiber composite (AE-PFC) transducers used as the guided wave transmitter and receiver were adhered to the back surface of flow field plate. Guided waves propagate over the flow channel area and carry water information from the transmitter to the receiver. The transducer comes with advantages of narrowband, small mass and high acoustic directivity. In experiment, a carbon paper used as the gas diffusion layer overlays the area of flow channels. The influence of wet carbon paper on the guided acoustic wave was investigated in accordance with changes of the waveform envelop and the peak value of the signal spectrum. The result indicates the water in the carbon paper reduces reflected echoes of the first guided mode and increases the peak amplitude. On the contrary, the amplitude of the second guided mode decreases as it propagates over the wet area of flow channels. Based on the decreasing of amplitude, the water quantity distributed throughout either the ribs or the flow channels can be determined. The sensitivity of detection is 0.03 and 0.3 mV/μl for water on the ribs and in the flow channels. The range of measuring is 20 to 220 μl. The detecting resolution is up to 5 μl. Furthermore, finite element analysis was used to explore the electric field of AE-PFC in the process of polarization and guided wave propagation in the flow field plate. The numerical results indicate the electric field is nearly parallel to the piezoelectric fibers. The transient analysis shows the forking phenomenon in the guided mode signal spectra might be caused by the varying adhesive thickness between AE-PFC and the host structure.