固態氧化物燃料電池元件熱應力與機械性質分析; Analysis of Thermal Stresses and Mechanical Properties for the Components of Solid Oxide Fuel Cell

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题名:	固態氧化物燃料電池元件熱應力與機械性質分析;Analysis of Thermal Stresses and Mechanical Properties for the Components of Solid Oxide Fuel Cell
作者:	田傑文;Jie-Wun Tian
贡献者:	機械工程研究所
关键词:	熱應力;固態氧化物燃料電池;玻璃陶瓷;連接板;interconnect;glass-ceramic;solid oxide fuel cell;SOFC;thermal stress
日期:	2009-07-08
上传时间:	2009-09-21 12:06:15 (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是一種高溫型的燃料電池，較高的操作溫度使它可以得到較高的能量轉換效率，但也因為元件之間在高溫下可能發生交互的化學反應，因此材料可能在高溫下劣化。熱應力影響著固態氧化物燃料電池的結構耐久性，其來源為元件之間的熱膨脹係數差異和溫度梯度。過大的熱應力會造成元件的損壞並危及固態氧化物燃料電池堆的結構耐久性。因此，為了發展可靠的固態氧化物燃料電池堆，對於元件在室溫及操作溫度下的熱應力分佈和材料機械性質有必要作系統化的研究。　　本研究的第一個目標是利用有限元素分析計算三單元平板式固態氧化物燃料電池堆的熱應力分佈，分析的模型是依據核能研究所電池堆設計所建立，分別匯入兩個不同的溫度場，探討溫度梯度對於元件熱應力的影響。分析結果顯示，對於這兩個溫度場，溫度梯度的影響很小，而熱膨脹係數的不匹配仍然是熱應力產生的主要原因。　　第二個目標是測定核能研究所開發的一款陽極材料的機械性質，其代號為Slip-81。藉由雙軸抗折測試分別在室溫及800 oC下測定試片的抗折強度與楊氏模數。將實驗量得的Slip-81之機械性質匯入前述的熱應力分析中，所有元件的最大應力值皆低於其相對應的材料強度。　　本研究的第三個目標是研究玻璃陶瓷與金屬連接板間的接合強度及破壞模式。所使用的玻璃陶瓷為核能研究所開發的一款代號為GC-9的材質，金屬連接板則是使用一款代號為Crofer 22 H的商用肥粒鐵系不銹鋼。藉由設計製作兩款三明治試片，分別量測其在室溫和800 oC下的剪力強度及在室溫下的張力強度。為了評估試片燒結溫度對於接合強度的影響，試片分在850 oC燒結和900 oC燒結。結果顯示，無論在室溫或是800 oC下，900 oC燒結試片的張力強度和剪力強度皆高於850 oC燒結的試片。顯然燒結溫度的增加可以增進試片接合的性質，乃是在較高的溫度下，玻璃陶瓷在金屬連接板上的潤濕性質會較好。　　金屬連接板的預氧化處理對於接合強度的影響也在本研究的討論範圍當中。結果顯示，預氧化處理對於850 oC燒結的剪力試片強度有負面的影響，但是對於900 oC燒結的剪力試片強度卻有提昇的效果。由微結構分析的結果可以發現，此種接合件試片有三種破壞模式。第一，脫層現象發生在玻璃陶瓷基材與其氧化層的界面，此破壞模式所對應的接合強度是最低的。第二，脫層現象發生於金屬連接板基材與其氧化層的界面，並伴隨著中等的接合強度。第三，破壞可能發生在玻璃陶瓷基材之中，此種破壞模式所對應的接合強度是最高的。　　The high operating temperature (OT) enables solid oxide fuel cells (SOFCs) to have a superior efficiency of energy conversion while accompanying concerns such as degradation of materials because of undesirable reactions between components. Structural durability of SOFC is affected by the thermal stresses due to thermal mismatch between components. Excessive thermal stresses may lead to fracture of components endangering the mechanical integrity of an SOFC stack. Therefore, a systematic investigation of thermal stress distribution and mechanical properties of components at room temperature (RT) and OT is essential for development of a reliable SOFC stack. 　　The first objective of this study is using finite element analysis (FEA) to calculate the thermal stress distribution in a 3-cell planar SOFC stack. The simulated model was constructed based on a stack design of compressive sealing being developed at the Institute of Nuclear Energy Research (INER). Two temperature profiles were applied to evaluate the effect of thermal gradient on the critical stresses in the components. Simulation results indicate that the effect of thermal gradient could be neglected for the given two temperature profiles. 　　The second objective is to determine the mechanical properties of a potential anode material, Slip-81, developed at INER. Biaxial flexural strength and Young’s modulus were determined at RT and a high temperature of 800 oC by biaxial flexural ring-on-ring testing. The measured mechanical properties were applied to the aforementioned FEA modeling of thermal analysis. The critical stresses of all components were lower than the corresponding fracture strength by the use of Slip-81 anode. 　　The third purpose in the current study is to investigate the joint strength between the glass-ceramic and metallic interconnect. The utilized materials were the GC-9 glass-ceramic developed at INER and the Crofer 22 H which is a commercial ferritic stainless steel. A methodology of evaluating the joint strength was developed by testing two types of sandwich-like specimens under tensile and shear loading. The shear strength at RT and 800 oC and tensile strength at RT were measured. The effect of sintering temperature on the joint strength was studied. The measured tensile and shear strength of the specimens sintered at 900 oC were greater than those of the ones sintered at 850 oC at both testing temperatures of RT and 800 oC. Apparently, an increase of sintering temperature could improve the joining performance due to a better wetting behavior of glass-ceramic. 　　The effect of pre-oxidization of metallic interconnect on the joint strength was also evaluated. The pre-oxidization treatment degraded the shear strength of the specimens sintered at 850 oC. However, it enhanced the shear strength of the specimens sintered at 900 oC. Through the analysis of interfacial microstructure, fracture modes of the joint were correlated with the measured strength. Three types of fracture modes were identified for the joint specimens. Firstly, the lowest joint strength was accompanied with delamination at the interface between the glass-ceramic substrate and an adjacent oxide layer, chromate (BaCrO4). Secondly, delamination at the interface between the metal substrate and the oxide layer of the metal, chromia (Cr2O3), accompanied with a medium joint strength. Thirdly, the fracture might take place in the glass-ceramic layer for a larger joint strength.
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