Manisha, (2008-08). Evaluation of thermal stresses in planar solid oxide fuel cells as a function of thermo-mechanical properties of component materials. Master's Thesis. Thesis uri icon

abstract

  • Fuel cells are the direct energy conversion devices which convert the chemical energy of a fuel to electrical energy with much greater efficiency than conventional devices. Solid Oxide Fuel Cell (SOFC) is one of the various types of available fuel cells; wherein the major components are made of inherently brittle ceramics. Planar SOFC have the advantages of high power density and design flexibility over its counterpart tubular configuration. However, structural integrity, mechanical reliability, and durability are of great concern for commercial applications of these cells. The stress distribution in a cell is a function of geometry of fuel cell, temperature distribution, external mechanical loading and a mismatch of thermo-mechanical properties of the materials in contact. The mismatch of coefficient of thermal expansion and elastic moduli of the materials in direct contact results in the evolution of thermal stresses in the positive electrode/electrolyte/negative electrode (PEN) assembly during manufacturing and operating conditions (repeated start up and shut down steps) as well. It has long been realized and demonstrated that the durability and reliability of SOFCs is not only determined by the degradation in electrochemical performance but also by the ability of its component materials to withstand the thermal stresses. In the present work, an attempt has been made to evaluate the thermal stresses as a function of thermal and mechanical properties of the component materials assuming contribution from other factors such as thermal gradient, mechanical loading and in-service loading conditions is insignificant. Materials used in the present study include the state of art anode (Ni-YSZ), electrolyte(YSZ) and cathode materials(LM and LSM) of high temperature SOFC and also the ones being suggested for intermediate temperature SOFC Ni-SCZ as an anode, GDC and SCZ as electrolyte and LSCF as the cathode. Variation of thermo-mechanical properties namely coefficient of thermal expansion, and elastic and shear moduli were studied using thermo-mechanical analyzer and resonant ultrasound spectroscope respectively in 25-900?C temperature range. A non-linear variation in elastic and shear moduli- indicative of the structural changes in the studied temperature range was observed for most of the above mentioned materials. Coefficient of thermal expansion (CTE) was also found to increase non-linearly with temperature and sensitive to the phase transformations occurring in the materials. Above a certain temperature (high temperature region- above 600?C), a significant contribution from chemical expansion of the materials was also observed. In order to determine thermal stress distribution in the positive electrode, electrolyte, negative electrode (PEN) assembly, CTE and elastic and shear moduli of the component materials were incorporated in finite element analysis at temperature of concern. For the finite element analysis, anode supported configuration of PEN assembly (of 100mm x 100mm) was considered with 1mm thick anode, 10?m electrolyte and 30?m cathode. The results have indicated that cathode and anode layer adjacent to cathode/electrolyte and electrolyte/anode interface respectively are subjected to tensile stresses at the operating temperature of HT-SOFC (900?C) and IT-SOFC (600?C). However, the magnitude of stresses is much higher in the former case (500MPa tensile stress in cathode layer) when compared with the stress level in IT-SOFC (178MPa tensile stress in cathode layer). These high stresses might have been resulted from the higher CTE of cathode when compared with the adjacent electrolyte. However, it is worth mentioning here that in the present work, we have not considered any contribution from the residual stresses arising from fabrication and the stress relaxation from softening of the glass sealant.
  • Fuel cells are the direct energy conversion devices which convert the chemical energy of a

    fuel to electrical energy with much greater efficiency than conventional devices. Solid Oxide

    Fuel Cell (SOFC) is one of the various types of available fuel cells; wherein the major

    components are made of inherently brittle ceramics. Planar SOFC have the advantages of

    high power density and design flexibility over its counterpart tubular configuration.

    However, structural integrity, mechanical reliability, and durability are of great concern for

    commercial applications of these cells. The stress distribution in a cell is a function of

    geometry of fuel cell, temperature distribution, external mechanical loading and a mismatch

    of thermo-mechanical properties of the materials in contact. The mismatch of coefficient of

    thermal expansion and elastic moduli of the materials in direct contact results in the

    evolution of thermal stresses in the positive electrode/electrolyte/negative electrode (PEN)

    assembly during manufacturing and operating conditions (repeated start up and shut down

    steps) as well. It has long been realized and demonstrated that the durability and reliability of

    SOFCs is not only determined by the degradation in electrochemical performance but also

    by the ability of its component materials to withstand the thermal stresses.

    In the present work, an attempt has been made to evaluate the thermal stresses as a function

    of thermal and mechanical properties of the component materials assuming contribution

    from other factors such as thermal gradient, mechanical loading and in-service loading

    conditions is insignificant. Materials used in the present study include the state of art anode (Ni-YSZ), electrolyte(YSZ) and cathode materials(LM and LSM) of high temperature SOFC

    and also the ones being suggested for intermediate temperature SOFC Ni-SCZ as an anode,

    GDC and SCZ as electrolyte and LSCF as the cathode. Variation of thermo-mechanical

    properties namely coefficient of thermal expansion, and elastic and shear moduli were

    studied using thermo-mechanical analyzer and resonant ultrasound spectroscope respectively

    in 25-900?C temperature range. A non-linear variation in elastic and shear moduli- indicative

    of the structural changes in the studied temperature range was observed for most of the

    above mentioned materials. Coefficient of thermal expansion (CTE) was also found to

    increase non-linearly with temperature and sensitive to the phase transformations occurring

    in the materials. Above a certain temperature (high temperature region- above 600?C), a

    significant contribution from chemical expansion of the materials was also observed.

    In order to determine thermal stress distribution in the positive electrode, electrolyte,

    negative electrode (PEN) assembly, CTE and elastic and shear moduli of the component

    materials were incorporated in finite element analysis at temperature of concern. For the

    finite element analysis, anode supported configuration of PEN assembly (of 100mm x

    100mm) was considered with 1mm thick anode, 10?m electrolyte and 30?m cathode. The

    results have indicated that cathode and anode layer adjacent to cathode/electrolyte and

    electrolyte/anode interface respectively are subjected to tensile stresses at the operating

    temperature of HT-SOFC (900?C) and IT-SOFC (600?C). However, the magnitude of

    stresses is much higher in the former case (500MPa tensile stress in cathode layer) when

    compared with the stress level in IT-SOFC (178MPa tensile stress in cathode layer). These

    high stresses might have been resulted from the higher CTE of cathode when compared with

    the adjacent electrolyte. However, it is worth mentioning here that in the present work, we

    have not considered any contribution from the residual stresses arising from fabrication and

    the stress relaxation from softening of the glass sealant.

publication date

  • August 2008