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The plate-fin heat exchanger (PFHE) is the core equipment used to achieve the high efficiency of solid oxide fuel cell (SOFC), however, the issues of high-performance brazed joints in the manufacturing of the PFHE have been a challenge due to the poor mechanical properties. This study proposes a brazing manufacturing technology of isothermal solidification with the optimized post bonding heat treatment strategy, to synergistically improve the strength-ductility property and homogenize microstructure of brazed joints, aiming at guaranteeing the high energy efficiency of SOFC. The results show that brazing at 1065 °C for 25 min achieves the complete isothermal solidification and an intermetallic-free joint centerline with numerous borides generating in the diffusion-affected zone. Solution treatment then dissolves large quantities of acicular and blocky borides. The uniformity of grain size and kernel average misorientation distribution is also improved due to recrystallization, which becomes more pronounced after solution aging treatment. In addition, solution aging treatment results in an improvement in the ultimate tensile strength of the brazed joint, which is more prominent than that after solution treatment. However, the increase in elongation after solution aging treatment is smaller than after solution treatment, while still much higher than the as-brazed joint due to the dissolution of boride precipitates and growth of twin boundaries. The results demonstrate that the proposed brazing manufacturing technology not only homogenizes microstructure, but also significantly improves strength and ductility, further promoting the long-life operation of SOFC. • A proper brazing technology to fabricate plate-fin heat exchanger of SOFC is proposed. • An appropriate heat treatment strategy to enhance strength and ductility is proposed. • Solution aging gets superior strength but inferior ductility to solution treatment. • Borides dissolution, solid solution and precipitation strengthening enhance strength. • More uniform microstructure, increased recrystallization and twins raise ductility. [ABSTRACT FROM AUTHOR]

In order to improve product design efficiency and guarantee the high temperature structural integrity during the long-term creep of solid oxide fuel cell (SOFC), the creep strength design method is studied by using the finite element method (FEM) and the response surface method (RSM) with considering the interaction between the geometric parameters. A multi-regression model representing the correlation between the sealant failure probability and the geometric parameters is established for rapid estimation of creep strength and optimization design of geometric dimensions. The sealant failure probability is decreased from 0.994 to 0.015 by the optimization of SOFC geometrical size. And the error between results predicted by the FEM and results predicted by the multi-regression model is less than 10%. Therefore, the multi-regression model is proven to be an excellent tool for creep failure prediction and structural design optimization, reducing research costs and time, and improving design efficiency. • Correlation between creep failure probability and geometric parameter is established. • Interaction between anode thickness H a , sealant thickness H s and frame thickness H f is considered. • The optimal geometric parameters is obtained through regression equation analysis. • 50,000 h creep life is met by optimizing the thicknesses of the components. [ABSTRACT FROM AUTHOR]

During long term operation at high temperature, creep is inevitable and can cause damages and cracks, which should be decreased to ensure the stack integrity. A strain based creep damage model is used to predict the creep damage behavior of a planar solid oxide fuel cell (SOFC). It demonstrates the maximum creep damage locates at the corner of glass-ceramic (GC) facing the frame after 50 000 h creep. The effect of frame material on the creep is studied. By increasing the creep parameter B of frame, the creep and damage in the cell and GC are decreased. This indicates the frame with a larger creep parameter can alleviate the interaction between components and decrease the deformation of the system. It recommends to use frame materials which have creep parameters larger than 1.3752 × 10−15 MPa-nh−1 besides CTEs closed to the cell to compensate and mitigate the creep and damage of SOFC system. • The creep and damage of cell and GC can be mitigated and compensated by the frame. • This is a new discovered advantage for frame material with a larger creep parameter. • The interaction between components is alleviated and the deformation is decreased. [ABSTRACT FROM AUTHOR]

The reduction and re-oxidation of anode have significant effects on the integrity of the solid oxide fuel cell (SOFC) sealed by the glass-ceramic (GC). The mechanical failure is mainly controlled by the stress distribution. Therefore, a three dimensional model of SOFC is established to investigate the stress evolution during the reduction and re-oxidation by finite element method (FEM) in this paper, and the failure probability is calculated using the Weibull method. The results demonstrate that the reduction of anode can decrease the thermal stresses and reduce the failure probability due to the volumetric contraction and porosity increasing. The re-oxidation can result in a remarkable increase of the thermal stresses, and the failure probabilities of anode, cathode, electrolyte and GC all increase to 1, which is mainly due to the large linear strain rather than the porosity decreasing. The cathode and electrolyte fail as soon as the linear strains are about 0.03% and 0.07%. Therefore, the re-oxidation should be controlled to ensure the integrity, and a lower re-oxidation temperature can decrease the stress and failure probability. [ABSTRACT FROM AUTHOR]

The effects of anode porosity on thermal stress and failure probability of a planar solid oxide fuel cell (SOFC) are investigated by finite element method (FEM). The thermal stress and failure probability in planar SOFC with and without considering porosity (CP) were compared. The results show that the compressive stresses are generated in the electrolyte layer, while tensile stresses are generated in Ag–CuO and BNi2 layer because of the compatibility of deformation. In the case of considering anode porosity, the compressive stresses in anode and electrolyte layer and the tensile stresses in cathode layer are all decreased. Comparing to without CP, the thermal stress with CP in electrolyte and cathode layer decreases by 33.2% and 46.9%, respectively. The anode layer has a large risk of failure (5.6489 × 10 −4 ) than that of cathode and electrolyte layer at as-fabricated state. And the failure probability at cathode layer at start-up reaches 0.998. The failure probability decreases gradually in the period of creep. The failure probability in anode, cathode and electrolyte layer with CP all decrease comparing to without CP. The porosity of anode could lead to the decrease of tensile thermal stress, resulting in the decrease of failure probability in cathode layer. The reduction speed of failure probability at creep stage with CP is bigger than that without CP. As the increase of porosity, the failure probability decreases due to the decrease of the tensile stress. [ABSTRACT FROM AUTHOR]

Planar solid oxide fuel cell (SOFC) operates at high temperature and requires a good creep strength to ensure the structure integrity. This paper presents a creep and damage analysis of a bonded compliant seal (BCS) structure of a planar SOFC considering the effect of as-bonded residual stress and thermal stress, as well as the effect of filler metal and foil thickness. A modified continuum creep-damage model is used in the finite element simulation. It demonstrates that the BCS structure meets the requirement of the long-term operation at the high temperature of 600 °C with an appropriate braze bonding process. The results show that the failure location is not in the region of maximum creep deformation due to the effect of high level multi-axial stress which drastically decreases the multi-axial ductility. Reasonably reducing the thickness of filler metal and foil can decrease the damage of the BCS structure. Based on the consideration of creep and damage, it is proposed that the thickness of filler metal and foil should not exceed 0.1 and 0.05 mm, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Bonded compliant seal is a new sealing design for planar solid oxide fuel cell. During brazing, large residual stresses are generated which have a great effect on failure of the brazing connection. Therefore, how to decrease the residual stresses is a critical issue for structure integrity. This paper presents a study on decreasing residual stresses by using short-time creep relaxation effect. A sequential-coupled calculation model is developed based on finite element method. The brazing temperature field is firstly obtained by simulating the convection and radiation heating, and then the residual stress is calculated by a thermal-elasto-plastic-creep model. The calculated results are verified by neutron diffraction measurements. During cooling, a short holding time at 600 °C is designed to relax the residual stress by creep effect. The results show that this effect has a remarkable impact on decreasing the residual stress. The stresses in cell, Ag–CuO and foil have been reduced by about 26.9%, 13.6% and 22.1%, respectively, as the holding time increases up to 40 h. When the holding time exceeds 40 h, the residual stresses remain almost unchanged. It is thus suggested that the holding time should be reasonably determined to allow sufficient stress relaxation. [Copyright &y& Elsevier]

Abstract: Solid oxide fuel cell (SOFC) requires good sealant because it works in harsh conditions (high temperature, thermal cycle, oxidative and reducing gas environments). Bonded compliant seal (BCS) is a new sealing method for planar SOFC. It uses a thin foil metal to bond the window frame and cell, achieving the seal between window frame and cell. At high temperature, a comprehensive evaluation of its creep strength is essential for the adoption of BCS design. In order to characterize the creep behavior, the creep induced by thermal stresses in SOFC with BCS design is simulated by finite element method. The results show that the foil is compressed and large thermal stresses are generated. The initial peak thermal stress is located in the thin foil because the foil acts as a spring stores the thermal stresses by elastic and plastic deformation in itself. Serving at high temperature, initial thermal displacement is partially recovered because of the creep relaxation, which becomes a new discovered advantage for BCS design. It predicts that the failures are likely to happen in the middle of the cell edge and BNi-2 filler metal, because the maximum residual displacement and creep strain are located. [Copyright &y& Elsevier]

Abstract: This paper uses finite element method to study the effect of Al2O3 film on thermal stresses in the bonded compliant seal (BCS) design of a planar solid oxide fuel cell. The effects of Al2O3 thickness, operating temperature, window frame thickness, foil thickness and cell length on thermal stresses have been discussed. The results show that compressive stresses are generated in Al2O3 film. A bowing deformation is generated due to the BCS design, which can trap and relax some thermal stresses. With Al2O3 thickness increase, compressive stresses in Al2O3 film and foil are decreased slightly, while tensile stresses in BNi2 and frame are increased. With operating temperature increase, compressive stress in Al2O3 is increased greatly, while the stresses in foil, BNi2 and frame are increased slightly. The bowing deformation is increased with operating temperature increase. The window frame thickness has little effect on thermal stresses and bowing deformation. With sealing foil thickness increase, thermal stresses and bowing deformation are increased. The cell length has little effect on thermal stresses, but reducing the cell length can decrease the bowing deformation. [Copyright &y& Elsevier]

Abstract: This paper uses finite element method (FEM) to predict the residual stress and plastic strain in the brazed joint of sealing foil-to-window frame in bonded compliant seal (BCS) design in a planar solid oxide fuel cell (PSOFC). The effects of window frame material type, sealing foil thickness, filler metal thickness and window frame thickness on residual stress and plastic strain are discussed. Large residual stress is generated in the joint, and the stress and strain are concentrated around the fillet. It is proved that the BCS design can mitigate and trap some residual stress by plastic deformation within the sealing foil. The residual stress and the ability of trapping stress of sealing foil are affected by window frame material and structure thickness. Based on the comprehensive considerations of the impact of residual stress and plastic strain, Alloy 625 as a window frame material is found to be better than Haynes 214, Hastelloy X and SUS 316L. The optimum thickness of sealing foil and filler metal BNi2 are found to be 150μm and 75μm, respectively. The residual stress and plastic strain are increased with the increase of window frame thickness. [Copyright &y& Elsevier]

A typical failure mode of the anode-supported solid oxide fuel cell (SOFC) is the cracking of electrolyte due to the excessive tensile stress caused by the expansion of anode during re-oxidation. This paper builds a three-dimensional model based on the finite element method (FEM) to investigate the effect of inhomogeneous oxidation on mechanical degradation of SOFC. The stress distributions and critical oxidation strains with considering inhomogeneous oxidation are compared to those of supposed homogeneous oxidation. The results indicate that the gradient of oxidation strain induces a large stress gradient and bending moment in anode, which increase the stresses and curvature of the cell, leading to the deterioration of SOFC degradation. The critical oxidation strains are increased in the SOFC with considering the inhomogeneous oxidation except for the anode. By calculating the stress under homogeneous re-oxidation, the resistance to failure is underestimated for cathode, electrolyte and glass-ceramic (GC), but the resistance to failure of the anode is overestimated. The effects of creep and thickness of the graded-oxidized zone on the redox stability are also investigated. This study helps to understand the effect of inhomogeneous oxidation and enhance the performance and lifetime of SOFC. • The effect of inhomogeneous oxidation on mechanical degradation of SOFC is studied. • The first paper to study the effect by using a three dimensional model. • The inhomogeneous oxidation induces a large stress gradient in anode. • The curvature of the anode aggravates the degradation of SOFC. • The critical oxidation strains are underestimated for the cell except for anode. [ABSTRACT FROM AUTHOR]

Creep damage and creep crack growth (CCG) behavior of Crofer 22 APU as interconnect of solid oxide fuel cell at different temperatures are investigated by the continuum damage mechanics method through CT specimen. It is illustrated that the fracture time of CT specimen reduces with temperature increasing at the same stress level, they are 1297 h, 42 h, 1.8 h and 0.25 h, respectively, at 650, 700, 750 and 800 °C with stress level of 2.73 MPa ⋅ m . The slopes of creep fracture mechanics parameter C * to CCG rate a ˙ (C *- rate of a) are nearly the same at 650 and 700 °C. The slope of C *- a ˙ at 650 and 700 °C is smaller than that at 750 and 800 °C. It is concluded that the temperature effect on CCG behavior of Crofer 22 APU is more evident than that the stress effect. All the data of C *- a ˙ fall within the area of 95% prediction band, and the obtained equation of C * and CCG rate is a ˙ = 3.8 e 2 C ∗ 0.91 . Therefore, the a ˙ of Crofer 22 APU can be predicted under different stress levels and temperatures. • CCG behavior of Crofer 22 APU under different temperatures are investigated. • The Liu-Murakami constitutive equations of Crofer 22 APU are established. • Temperature effect of CCG behavior is more evident than that the stress effect. • All the data of C *- a ˙ fall within the area of 95% prediction band. [ABSTRACT FROM AUTHOR]

The time dependent failure probabilities (TDFP) of solid oxide fuel cell (SOFC) under different geometrical sizes are analyzed by a creep and damage related probability prediction constitutive model. The results demonstrate that sealant is the most possible failure component of the SOFC under different geometrical sizes. Increasing the sealant thickness or width can decrease the TDFP of the sealant. While the cathode thickness and electrolyte thickness have little effect on the TDFP of SOFC components. Decreasing the anode thickness, frame thickness can reduce the TDFP of the sealant. The sealant thickness and frame thickness can greatly affect the life of the SOFC stack. Based on the TDFP analysis of SOFC, it recommends that the sealant thickness should not be smaller than 0.1 mm, the frame thickness should not be less than 0.4 mm considering the stiffness requirement. • Sealant is the potential failure component under different geometrical sizes. • Increasing sealant thickness or width can decrease failure probability of SOFC. • Decreasing anode or frame thickness can decrease failure probability of SOFC. • The sealant thickness should not be smaller than 0.1 mm. • Frame thickness should not be less than 0.4 mm considering the stiffness requirement. [ABSTRACT FROM AUTHOR]

In present paper, a new model is proposed and embedded into the finite element software ABAQUS to estimate the time dependent failure probability of the solid oxide fuel cell stack. The results show that sealant is the potential failure region of the solid oxide fuel cell stack, while the failure probability of the anode, electrolyte and cathode are very small within the operation time of 50,000 h. The creep and damage distribution of the components reflect that the proposed model can reasonably predict the time dependent failure probability of the solid oxide fuel cell stack. Increasing either the characteristic strain, Weibull modulus or decreasing the operating temperature can decrease the failure probability of the SOFC stack. For the sealant, to ensure the high temperature integrity of the SOFC stack, the characteristic strain should be larger than 0.01 or Weibull modulus should be higher than 8.0 under the operating temperature of 600 °C. [ABSTRACT FROM AUTHOR]