Study On Preparation And Physical Properties Of Element-modulated NBT-based Oxygen Ion Conductors

Na0.5Bi0.5Ti O3（NBT）-based oxygen ionic conductor has good ionic conductivity,which has attracted great attention in the field of electrolyte materials for solid oxide fuel cells.At present,NBT-based oxygen ion conductor materials have been extensively studied in depth,such as crystal structure,doping modification and conductivity mechanism.However,the research on the preparation process of NBT-based materials is not well developed.In addition,the preparation process plays a crucial role in the performance of the new NBT-based oxygen ion conductor materials.Therefore,this thesis focuses on the preparation process of new NBT-based materials and the effect of elemental doping on the electrical properties.The main findings of this paper are as follows.（1）The electrical properties of Na/Ga co-doped NBT-based ceramics were studied.The bulk conductivity of Na0.52Bi0.47Ti1-x Gax O3-δsamples decreases with the increase of Ga³⁺doping.When the test temperature is 673 K,the bulk conductivity of Na0.52Bi0.47Ti1-x Gax O3-δis 7.19×10^-4 S/cm,which is lower than that of Na0.52Bi0.47Ti O3-δat the same temperature.With the introduction of Ga³⁺into the Na0.52Bi0.47Ti1-x Gax O3-δsamples,the total conductivity first shows an increasing pattern and then decreases,indicating that the trace amount of Ga³⁺doping can improve the total conductivity of the materials.In the internal friction temperature spectrum of the Na0.52Bi0.47Ti1-x Gax O3-δsample,a relaxation internal friction peak is observed,and the study of this relaxation peak reveals that the peak height of this relaxation peak decreases with the increase of Ga³⁺doping.At the same time,the relaxation activation energy also increases,indicating that the introduction of Ga³⁺decreases the concentration and motility of movable oxygen vacancies in the Na0.52Bi0.47Ti1-x Gax O3-δmaterial,which leads to decrease in the bulk conductivity of Na0.52Bi0.47Ti1-x Gax O3-δsamples.（2）The synthesis of Na0.54Bi0.46Ti O2.96 electrolyte material by sol-gel method and its electrical properties were studied.The phase structure,microscopic morphology,electrical properties and oxygen ion diffusion behavior of the Na0.54Bi0.46Ti O2.96 material were investigated by X-ray diffraction analysis,scanning electron microscopy analysis,AC impedance technique and dielectric relaxation technique.Compared with the sample produced through the conventional solid-phase reaction means,the materials prepared by the sol-gel method have less heterogeneous phases and higher bulk conductivity.The dielectric loss spectrum reveals that the Na0.54Bi0.46Ti O2.96 specimen prepared by the sol-gel method has lower relaxation activation energy and higher concentration of movable oxygen vacancies,which led to a higher bulk conductivity of the specimens fabricated through the sol-gel method.Compared with the sample produced through the conventional solid-phase reaction means,the sol-gel method is able to achieve mixing at the atomic and molecular levels during the preparation of materials where the metal ions in the raw material can react in a liquid-phase environment.Meanwhile,the lower calcination temperature of the sol-gel method can reduce the loss of bismuth elements during the material preparation,which results in a more accurate stoichiometric ratio of the material and reduces the generation of heterogeneous phases.（3）The effect of sintering temperature on electrical properties of Na0.54Bi0.46Ti O2.96 materials was investigated.The results of AC impedance test show that the bulk conductivity of Na0.54Bi0.46Ti O2.96 sample first increases and then decreases when the sintering temperature increases.The highest bulk conductivity of Na0.54Bi0.46Ti O2.96 samples sintered at 1273 K can be derived,and it can reach3.35×10^-4 S/cm when the test temperature is 573 K.The total conductivity of the samples shows a monotonic increasing trend as the sintering temperature increases.The results of internal friction temperature spectrum show that when the sintering temperature increase,the concentration of mobile oxygen vacancy in the Na0.54Bi0.46Ti O2.96 sample increase accompanied by the increase of relaxation activation energy and the decrease of mobility.The higher bulk conductivity of the Na0.54Bi0.46Ti O2.96 sample sintered at 1273 K can be explained by the higher concentration and the better motility of movable oxygen vacancies.（4）The electrical properties of Na element doped and Na/K/Mg co-doped NBT-based compounds sintered by spark plasma sintering method were studied.Compared with the conventional sintering method,the shorter sintering time and lower temperature during sintering process are the advantages of the spark plasma sintering method.Spark plasma sintering method is beneficial to increase the activity of ions in the material,reduce the activation energy,and improve the concentration and mobility of movable oxygen vacancies.