Study On The Microstructure And Electrical Conductivity Of Na_0.5+xBi_0.5-xTiO_3-?

Sodium bismuth titanate(Na0.5Bi0.5TiO3,NBT)has been widely studied as a lead-free piezoelectric material,but some researchers have found that it also can be regarded as a promising solid oxide fuel cell(SOFC)electrolyte.Up to now,the application of NBT in SOFC electrolyte materials is still in its infancy.Some studies of NBT with A-site full stoichiometry,Na0.5+xBi0.5-xTiO3-?,have been reported,but these materials are limited in some special Na/Bi ratios,and it is necessary to investigate systematically.Besides,the absolute dry condition can't be achieved during experiment,therefore,the existence of H+(proton)conduction in NBT electrolyte,in which the oxygen ion is the predominant contributor to the total electrical conductivity,is still worth to discuss.a series of NBT materials,Na0.5+xBi0.5-xTiO3-?(0?x?0.1)were prepared by conventional solid-state reaction method.The effect of nominal Na/Bi ratios in a wide range on the NBT's microstructure,phase composition and electrical conductivity was investigated systematically in this study.Considering the possibility of proton conduction in NBT materials,a three-carrier equivalent circuit model with oxygen ion,proton and electron conduction is proposed,which tries to improve the traditional measurement method of ion transference number.And the X-ray diffraction,scanning electron microscopy,energy spectrum analysis and electrochemical impedance spectroscopy were used to characterize the crystal structure,micromorphology,phase composition and conductivity on related materials.Relevant results showed that all the prepared as-calcined powders exhibited the rhombohedral perovskite structure.After sintering,secondary phases could be observed in samples,and the amounts of which increased with increasing nominal Na/Bi ratio.Further elemental analysis indicated that the secondary phases were mainly composed of sodium titanates with different Na/Ti ratios.When x ?0.03,the grain bulk and grain boundary conductivities of Na0.5+xBi0.5-xTiO3-? increased significantly with increasing Na/Bi ratio,while these two conductivities almost remained unchanged at higher x.When x = 0.03,the grain bulk conductivity of Na0.53Bi0.47TiO2.97 is about 5.5-7.5 times higher than that of Na0.5Bi0.5TiO3 at the same test temperature.In addition,considering only the oxygen ion and electron conduction,the three-carrier transference model proposed in this study can derive the classical Liu's modification.