The Investigation Of Glass-alumina Composite Seals In Intermediate Temperature Solid Oxide Fuel Cells

The seals are an important component in solid oxide fuel cell(SOFC),whose performance directly affects output characteristics and long-term stability of stack.Therefore,the researching development and performance optimization of sealing materials are significant in SOFC.In thermal cycle of stack,the oxidation of metal and chemical reaction between interconnect and seals in high temperature will cause gas leakage in the interface,so it is important to improve interface performance.When designing glass sealing materials for SOFC,pure glass cannot simultaneously meet comprehensive requirements of glass transition temperature(T_g),softening temperature(T_s),coefficient of thermal expansion(CTE),thermal stability and mechanical properties.Moreover,glass is a thermodynamic metastable phase,which exhibits a tendency to crystallize in high temperature,leading to change in volume and internal stress,and even an excessive amount of crystallization causing sealing failure.The above problems have restricted commercial application of glass sealing materials in SOFC.In this paper,two types of glass(H-2 and H-3)with differences in high-temperature thermal stability are selected.The effects of pre-oxidized alloy surface roughness and thermal stability of glass in the interface wettability and bonding strength are explored.The glass-Al₂O₃ composite seals are prepared by tape casting.The influence of addition amount of Al₂O₃ ceramic powder on the tightness,mechanical properties,thermal cycle stability and interface compatibility are studied.The influence of Al₂O₃ ceramic addition on the crystallization kinetics of glass are investigated by differential scanning calorimetry(DSC).The relationships between viscosity of GA80(80wt%H-2 glass+20wt%Al₂O₃)and HA80(80wt%H-3 glass+20wt%Al₂O₃)composite seals with temperature are analyzed basing on glass deformation characteristics.The following conclusions are drawn:(1)By pre-oxidizing Fe-16Cr alloy to change surface roughness,it can effectively improve high temperature wettability and interface bonding between glass and interconnect.After Fe-16Cr alloy was pre-oxidized at 750°C for 50 h,H-2 and H-3 glass exhibited good high-temperature wettability with them,mainly due to proper surface roughness providing an additional drive for glass infiltrating in high temperature,improving bonding state between glass and interconnect.The inter-diffusion between glass and interconnect oxide forms an intermediate composite oxide layer,whose existence effectively improves the interface bonding strength and interface sealing performance.The interface fracture mechanism model was constructed to clarify the influence of interface oxide layer thickness and glass crystallization on the interface fracture.When thermodynamically stable glass phase is combined with a suitable rough interface,the fracture mainly occurs in the middle composite oxide layer.When the oxide layer of interconnect is too thick,the adhesion between oxide and metal matrix would decrease,and the fracture would be the peeling of oxide.When the glass precipitates a large amount of crystalline phase,the high-viscosity glass would cause imcomplete infiltration between glass and metal oxide,leading to the fracture occurring in the middle composite oxide layer.(2)In glass-Al₂O₃ composite seals,glass fully penetrates and infiltrates Al₂O₃particles,effectively blocking leakage channel,and Al₂O₃ ceramic can enhance mechanical properties in high temperatures.Adding 20wt%Al₂O₃ improves mechanical properties of glass-based composite seals.Compared with H-2 glass,GA80 has a 74%increase in shear strength.This is mainly due to synergistic enhancement effect of Al₂O₃ ceramics and crystalline phase Ba Al₂Si₂O₈.Compared with H-3 glass,HA80 increases its shear strength by 143%,because of Al₂O₃ ceramic reinforcing phase.GA80 and HA80 seals all show better tightness,thermal cycle stability and interface compatibility.The maximum leakage rate of GA80 stabilized at 0.0044 sccm/cm after 10 thermal cycles,and the leakage rate of HA80 stabilized at 0.0022 sccm/cm during 20 thermal cycles.And the interface is tightly connected and there is no element diffusion after 20 thermal cycles.In SOFC stack test,two types of composite seals could meet tightness requirements and maintain their structural integrity.(3)The amount of H-2 glass crystallization can be controlled by adding Al₂O₃ ceramic powder.After adding 20wt%Al₂O₃,the crystallization activation energy of H-2 glass is increased,its crystallization kinetic behavior is changed and final crystallization amount is stabilized at about 45%,improving long-term thermal stability of H-2 glass.The main reason is that Al₂O₃ ceramic phase inhibits uniform nucleation process of glass.The addition of Al₂O₃ ceramic did not change thermal stability of H-3 glass.There is non-reactive liquid phase sintering process between H-3 glass and Al₂O₃.After the temperature is higher than softening temperature of H-3 glass,glass starts to soften and flow into alumina particles,which promotes glass-alumina rearrangement and molten glass completely covers Al₂O₃particles.Viscous flow is generated under glass surface tension to close pores in composite seals,finally completing high-temperature dense sintering process between H-3 glass and Al₂O₃ ceramic.The high temperature viscosity of GA80 and HA80 increased after heating at 750°C for 500 h,but it was still within a controllable range.The former is due to the increase of crystallization phase,while the latter is due to the liquid phase sintering process.A moderate increase in the high-temperature viscosity of seals would improve its high-temperature mechanical properties,which has been verified in SOFC stack.