不同封裝設計對固態氧化物燃料電池堆熱應力之影響; Effect of Sealing Design on the Thermal Stresses in SOFC Stack

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题名:	不同封裝設計對固態氧化物燃料電池堆熱應力之影響;Effect of Sealing Design on the Thermal Stresses in SOFC Stack
作者:	黃令豪;Ling-Hao Huang
贡献者:	機械工程研究所
关键词:	燃料電池;熱應力;雲母封裝;thermal stress;mica seal;SOFC
日期:	2008-06-26
上传时间:	2009-09-21 12:01:11 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	固態氧化物燃料電池相對其他燃料電池，擁有固態的電解質以及較高的操作溫度，使得其擁有高能源轉換率並可使用多種燃料。然而，在高溫運作之下，由於電池堆中各元件之間熱膨脹係數的不匹配，將產生相當大的熱應力值，造成元件毀損或結構破壞。為了評估電池堆在運作及停機階段下的結構可靠性與耐久性，本研究使用商用有限元素分析軟體ABAQUS建立一個三單元平板式固態氧化物燃料電池堆模型，並帶入相對應的元件材料性質及邊界條件進行應力分析，此模型乃是基於核能研究所開發中的平板式固態氧化物燃料電池堆設計所建構。模型中雙極連接板和金屬框架之間是使用雲母墊圈封裝，雲母封裝擁有方便拆解裝配與允許相鄰元件間有相互滑移的優點，可減少相鄰元件因為受到封裝拘束所產生的應力，然而，此種封裝方式需要在電池堆上方施加一個裝配負載，此裝配負載有可能會對電池堆應力分佈造成影響，所以有必要對其進行分析。本研究的第一個目標為使用有限元素法計算使用雲母封裝的三單元平板式固態氧化物燃料電池堆中各元件在不同運作階段下的應力值，並探討裝配負載對結構應力的影響。分析結果顯示，當裝配負載小於0.6 MPa時，電池板以及金屬框架會產生部分彎曲，使得各元件接觸不良；當裝配負載達到6 MPa時，玻璃封裝陶瓷以及雲母墊圈有著破壞的可能性。而0.6 MPa的裝配負載可使各元件之間有著良好接觸，並使玻璃封裝陶瓷以及雲母墊圈的應力值維持在一個可接受的範圍內，由以上兩點可以推論0.6 MPa為最佳裝配負載。當電池堆承受循環溫度負載時，金屬框架的內角落有著應力累積的現象，金屬框架的最大應力值隨著溫度循環次數增加而增加，然而，此應力累積現象並沒有在其他元件中觀察到。第二個目標為比較完全使用玻璃陶瓷封裝以及部分使用雲母封裝對電池堆應力分佈的影響。兩種不同的封裝方式會對電池板的應力值造成顯著的影響，由於此二種不同封裝方式具有不同電池堆元件間的接觸條件，使得二者的電池板在高低溫的應力值呈現相反的趨勢。第三個目標為對核能研究所所提供的Slip-41及Slip-48陽極試片進行可靠度評估，兩種試片分別在高溫及低溫下進行雙軸向抗折測試。兩種試片的抗折強度以及楊氏模數與溫度之間並沒有明顯的相關性。由於兩種試片之間孔隙率的差異，Slip-48試片的抗折強度明顯高於Slip-41試片，SEM破斷面觀察也得到同樣的佐證；Slip-48試片擁有較緻密的微結構以及較少的缺陷，所以抗折強度較高。將Slip-48陽極材料機械性質帶入應力分析中並比較應力分析結果與其材料強度後，可判斷Slip-48材料在電池堆室溫時可承受其最大應力值，但在穩態工作時有著破壞的可能性。 Solid oxide fuel cell (SOFC) usually operates at a high temperature with a good fuel flexibility and a high efficiency. However, the coefficient of thermal expansion mismatch between ceramic electrodes and metallic components under this high temperature gives rise to a large amount of stresses in the cell stack. In order to predict the durability and reliability of a SOFC stack, a comprehensive three-dimensional finite element analysis (FEA) model based on a stack design being developed at the Institute of Nuclear Energy Research (INER) was constructed. The model was designed for using mica sealant in a compressive sealing design. The first objective of this study is using FEA to calculate the thermal stress distribution in a three-cell planar SOFC stack with a compressive mica sealing design under cyclic thermal loading and to investigate the effects of the applied assembly load on the stress distribution. Simulation results indicate that an applied compressive load of 0.6 MPa could eliminate the bending deformation of the PEN and frame leading to a well joined structure. For a greater applied load, the critical stresses in the glass-ceramic and mica sealants were increased. The glass-ceramic and mica sealants might fail for an applied compressive load of 6 MPa. In this regard, a 0.6 MPa compressive load might be an optimal assembly load. When the SOFC stack was subjected to cyclic thermal loading, the stress accumulation behavior was observed at the inner corners of the frames. The critical stress in the metallic interconnect/frame was increased with increasing number of operating cycles. However, the critical stresses in the PEN, nickel mesh, mica gasket, and glass-ceramic sealant barely changed with cycle number. The second objective of the current study is to make a comparison of the stress distributions between two sealing designs, namely the rigid and compressive seals. Changing a rigid type of glass-ceramic sealant to a compressive type of mica gasket would influence the stress distribution, especially in the PENs. The critical stress in the PEN was decreased at room temperature but significantly increased at operation temperature. Such difference in the stress distribution could be ascribed by the difference in the constrained conditions at the interface of connecting components under various sealing designs. The third objective of this work is to assess the structural reliability of the PEN subjected to such a high thermal stress by measuring the flexural strength of two anode materials, namely Slip-41 and Slip-48. The Slip-48 anode material has a higher flexural strength than does the Slip-41 anode material. The difference in the strength might be caused by their different porosity. Fractography analysis results indicate the Slip-48 anode has a smoother fracture surface and a less amount of defects, compared to the Slip-41. Both anode materials have no obvious temperature dependence on the flexural strength. By importing the mechanical properties of the Slip-48 anode material into stress analysis, the critical stress in the PENs at steady-operation stage would exceed its strength. Therefore, the strength of the tested anode materials needs to be improved for future use in the given SOFC stack design.
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