Investigation Of Element-doped BaBiNb₅O₁₅-based Oxygen Ion Conductors

Oxygen ion conductors are promising for applications in solid oxide fuel cells,oxygen sensors,oxygen pumps,etc.,and have now become one of the research hot spots in the field of solid electrolytes.At present,Ba Bi Nb₅O₁₅（BBN）-based oxygen ion conductor materials with tungsten bronze structure have large free space and good ionic conductivity,which have attracted much attention upon report.However,there has not been much research on the crystal structure,doping modification,and conductivity mechanism of BBN-based oxygen ion conductor materials.In this thesis,the physical properties of element-doped Ba Bi Nb₅O₁₅-based oxygen ion conductors are investigated,and the ion diffusion properties and conductivity mechanism of the new BBN-based oxygen ion conductors are studied.The main findings of this paper are as follows:（1）Physical properties study of Na-doped Ba Bi Nb₅O₁₅ electrolyte materials at A-site.The grain conductivity of Ba Bi_1-xNa _XNb₅O_15-δspecimens tended to decreasing and then inhanced with the add of Na⁺doping,but they were all lower than that of the parent phase BBN specimens at the same temperature.The total conductivity of Ba Bi_1-xNa _XNb₅O_15-δsamples showed a trend of decreasing,then increasing and finally decreasing with the increase of Na⁺doping concentration,which indicates that the doping of 4 mol%Na⁺contributes to the total conductivity of the specimens.At the test temperature of 643 K,the total conductivity of Ba Bi_0.96Na_0.04Nb₅O_15-δsample reached 1.12×10^-4s/cm,which is 2.8 times higher than that of BBN sample at the same temperature.In the dielectric-modulus spectrum,a relaxation peak associated with thermally activated relaxation properties is observed,and the magnitude of the relaxation activation energy indicates that Na⁺doping reduces the energetics of the movable oxygen vacancies of the specimens.The poorer energetics of the oxygen vacancies are responsible for the lower grain conductivity of the Ba Bi_1-xNa _XNb₅O_15-δspecimens.For Ba Bi_0.96Na_0.04Nb₅O_15-δspecimens annealed at 773 K for different times,the grain conductivity of Ba Bi_0.96Na_0.04Nb₅O_15-δspecimen remained essentially constant with increasing aging time.When the aging time runs up to 60 h,the grain conductivity of the Ba Bi_0.96Na_0.04Nb₅O_15-δsample is 5.16×10^-5 S/cm at 673 K,which is very close to that of the fresh Ba Bi_0.96Na_0.04Nb₅O_15-δsample.This result indicates that BBN-based ionic conductors have significant thermal stability.（2）Effect of A-site Sr doping and sintering process on the electrical performance and oxygen relaxation behavior of Ba Bi Nb₅O₁₅-based oxide ceramics.The AC impedance test indicates that the grain conductivity of Ba Bi_1-xSr_xNb₅O_15-δcompound increased and then decreased when the Sr²⁺doping amount increased.The grain conductivity of Ba Bi_0.98Sr_0.02Nb₅O_15-δsample was the highest when the doping concentration was 2 mol%.By the dielectric relaxation test,the relaxation activation energy showed a pattern of decreasing and then increasing with increasing doping concentration,and the activation energy of Ba Bi_0.98Sr_0.02Nb₅O_15-δspecimen was the smallest.The higher grain conductivity of the Ba Bi_0.98Sr_0.02Nb₅O_15-δspecimen is due to the higher oxygen vacancy mobility.When the low-temperature sintering temperature was 1433 K and the holding time was 20 h,the grain conductivity and total conductivity of Ba Bi_0.98Sr_0.02Nb₅O_15-δceramics obtained by the two-step sintering method were nearly one and two orders of magnitude higher than those obtained by the conventional sintering method,respectively,accompanied by a reduction in the grain size of the specimens.The main reason is that the Ba Bi_0.98Sr_0.02Nb₅O_15-δspecimens obtained by the two-step sintering method have a higher concentration of movable oxygen vacancies,better motility,larger grain boundary volume and smaller concentration of grain boundary impurities,which are factors that favor the diffusion of oxygen ions and thus improve the electrical properties of BBN-based oxygen ion conductors.（3）B-site Ti doping and the physical properties of Ba Bi Nb₅O₁₅ ceramic materials synthesized by two-step sintering.The crystal structure,micromorphology,electrical performances and oxygen ion diffusion behavior of Bi Bi Nb_5-xTi_xO_15-δspecimens were obtained by X-ray diffraction analysis（XRD）,scanning electron microscope analysis（SEM）,electrochemical impedance and dielectric relaxation experimental techniques.Compared with the traditional sintering process,the embryos of Ti⁴⁺-doped Bi Bi Nb_5-xTi_xO_15-δoxygen ion conductor materials were prepared by the two-step sintering method with good denseness,higher conductivity,less heterogeneous phase content and greatly reduced grain size of the specimens.The two-step sintering method has the following advantages in the preparation of oxide ceramic materials:short sintering time at high temperatures,longer sintering time at lower sintering temperatures,control of calcination temperature and holding time to suppress grain growth to complete the purpose of sintering.