| CeO2-based solid electrolytes exhibit higher oxygen ion conductivity than those of ZrO2-based solid electrolytes under range(600-800℃),and they are widely recognized as the promising electrolyte used in intermerdiate temperature solid oxide fuel cells(IT SOFCs).However,due to the intrinsic brittleness of the CeO2-based materials,it is easy to be broken due to thermal shocks associated with intentional or accidental shut-down events occurred in SOFCs,and subsequently decreasing the long-term durability and reliability of SOFCs.Therefore,it is urgent need to improve the toughness and thermal shock resistance of CeO2-based solid electrolytes.On the basis of the combination of excellent mechanical properties,high thermal conductivity,high temperature stability and electric insulation associated with boron nitride nanoplatelets(BNNPs),Gd2O3 doped CeO2(GDC)electrolytes reinforced with BNNPs were fabricated using plasma spray,in which agglomerated composite powders with uniform particle size distribution and good flowability through spray drying were employed as feed stock materials.The main aim is to improve the mechanical properties and thermal shock resistance of the plasma sprayed GDC composite electrolytes,and the effects of the added BNNPs on the microstructure and the improved mechanial proerpties were corroborated.The influence of the characteriastics of spray dried agglomerated powders was analyzed,and plasma spray processing parameters are also optimized to synthesis BNNP/GDC composite electrolyte.Microstructure of the as-sprayed electrolytes is carefully characterized by X-ray diffraction(XRD),Raman spectroscopy and scanning electron microscopy(SEM).Results showed GDC sitll has the cubic fluorite structure without phase transformation,and BNNPs can survive their original structure after high-temperature process.Moreover,SEM observation depicted BNNPs homogeneously distribut in the as-sprayed GDC composite electrolytes.As compared with the GDC electrolyte,the measured microhardness,elastic modulus,fracture toughness and indentation yield strength of the as-sprayed 1.0wt.%BNNP/GDC composite are increased by～15.7%,～41.7%,～45.7%,and～58.4%,respectively.Above resultes strongly illustrated that synergetic strengthening and toughening mechanisms through splat boundaries and individual splats contribute greartly to the improved toughness and strength of the as-sprayed composite electrolytes.Among them,these embedded BNNPs induced stronger adhesion between the adjacent splats to resist splat sliding of the GDC composite electrolyte.More importantly,toughening mechanisms such as BNNP pullout,crack bridging by both anchored BNNPs and nanosized GDC grains,crack deflection and crack propagation arrested by the embedded BNNPs are recognized to improve toughness within splats.Also,Orowan-type strengthening effect associated with thermal mismatch between GDC matrix and BNNPs has been corroboraed to the improved strength.Plasma sprayed GDC and BNNP/GDC composites are conducted thermal shock resistance test at 400℃,500℃ and 600℃.The results indicate that the critical temperature difference increases with BNNP addition.Compared with GDC sample,the 0.5wt.%BNNP/GDC and 1.0 wt.%BNNP/GDC have significant improvements of～4.5%and～10.4%,respectively.Also,the strength retentions of the BNNP/GDC coating after thermal shock get improved.Thermal shock resistance factors RⅠ,RⅡ,and RⅣ are used to characterize the thermal shock resistance properties.Results show that the thermal shock resistance properties increase with the BNNP contents.For example,the hardness,elastic modulus and fracture toughness increase by up to～11.1%,48.1%and 54.2%after 600oC thermal shocking.It is concluded that the addition of BNNP is beneficial to not only improve the thermal shock resistance of the GDC sample,but also improve their mechanical properties.These are mainly attributed to the fact that BNNPs with excellent mechanical properties decrease temperature gradient of the composite leading to the improved thermal shock resistance. |
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