Study On Impedance And Electrical Properties Of Non-Stoichiometric Sodium Bismuth Titanate Solid Solution

Sodium bismuth titanate(Na0.5Bi0.5TiO3,NBT)is the most promising lead-free piezoelectric material for piezoelectric and dielectric applications,but it is also a good oxygen ion conductor,which has potential application value in electrolyte as solid oxide fuel cell,so it has been attracted wide attention.Compared with other oxygen ion conductors,NBT based oxygen ion conductors have the following advantages:1.High grain conductivity can be obtained at medium and low temperature operating temperatures;2.For most materials,they have only simple oxygen ion conduction in oxidation or reduction atmosphere,and do not produce electronic conductivity;3.No large degradation can be produced in reduction atmosphere,etc.These advantages make it have great research value.However,the relationship between the structure and properties of the material is not clear,and the properties of conductive,dielectric and other aspects need to be improved.By doping,the conductive mechanism and electrical properties of bismuth titanate sodium-based ceramics are discussed according to the results obtained from the characterization of XRD,SEM,Raman,AC impedance,dielectric temperature spectrum,ferroelectricity,etc.Ferroelectric Na0.5Bi0.5TiO3 is inclined to exhibit oxide ion conductive characteristics at higher temperature due to the large leakage current.K and Ga co-doped Na0.5Bi0.5TiO3 ceramics were prepared by the conventional solid-state reaction method and the conduction mechanism and electrical performance are investigated.The compounds present dominant oxygen ion conduction and their stability under nitrogen are disclosed.The bulk conductivity is improved with the decreased concentration of K+associated with an enhancement of the Na/K-O vibration.Excess K+dopants hinder oxygen vacancy migration attributing to the attraction effect on oxygen vacancies VO and A-site acceptor KBi" and narrow the oxygen vacancy migration channel.Oxygen vacancy concentration,tolerate factor,specific free volume,relaxation parameter and critical radius are used to discuss the conductive property,and conclude that suitable slight quantity of K+and Ga3+doping is beneficial to promote the oxide ionic conductivity,which is helpful for tailoring the electrical properties of Na0.5Bi0.5TiO3 based oxygen ion conductors.A site double doping of Sr and Ba/Ca substituted(Na0.5Bi0.49-x(SrBa)xTiO3-?(NBSB),Na0.5Bi0.49-y(SrCa)yTiO3-?(NBSC),x,y=0.01,0.02,0.04,0.06)ceramics were prepared by conventional solid-state reaction.XRD results showed that NBSB and NBSC are single perovskite structure without secondary phase.With the increasing of the Sr,Ba content,the oxygen vacancy concentration of the NBSB system increases and the grain conductivity is not obvious.The conductivity of NBSB1 is the largest,reaching 1.48 mS/cm(450?).As a result,the NBSB structure distortion makes the oxygen vacancy migration path longer,thus limiting the increase of conductivity.For NBSC,with the increase of doping amount,the conductivity decreases first and then increases.The maximum conductivity of NBSC6 is 1.26 mS/cm,indicating that the high Sr and Ca content are beneficial to the enhancement of conductivity.With increasing temperature,peaks of Tm for NBSB and NBSC shift to low temperature,showing relaxation characteristics.Defect dipoles(Vo··-2M'Bi)(M=Sr2+,Ba2+,Ca2+)induced by the A-site substitution would promote ferroelectricty.NBSB and NBSC possess saturated ferroelectric hysteresis loops.NBSB6 has large S of 0.15%(70 kV/cm)for bipoalr.Na0.5Bi(0.49-x)KxTiO3 was prepared by solid-state reaction and to study its phase structure and electrical properties.The results show that under the same K+doping concentration,grain size increases with temperature.Appropriate K+content(x?0.01)helps to refine the crystal grains and the large free volume and oxygen vacancy concentration are beneficial to improve the conductivity,and the activation energy of the NKBT system is small,the minimum is ENKBT05=0.505 eV.On the basis of x=0.005 and x=0.01 components,the MPB of the NBT were constructed by soluting Bi0.7Sro.2O3 and to study the electrical properties of the solid solution materials.The results show that:after solid solution,the grains are refined and the conductivity of the grains is reduced by nearly two orders of magnitude.The activation energy is increased(For 0.6NKBT05-0.4BST,the activation energy is 1.856 eV).In addition,the dielectric constant reduced.