Preparation And Electrical Properties Of Bismuth-based Perovskite Structure Materials

Bismuth based electronic materials of perovskite structure have attracted much attention due to their unique dielectric,ferroelectric,piezoelectric properties.Among them,bismuth ferrite（BiFeO₃,BFO）has good electromagnetic properties,which is the only multiferroic material with both ferroelectric order and antiferromagnetic order at room temperature.At the same time,BFO also exhibits unique gas sensitive properties as a semiconductor material.In addition,sodium bismuth titanate(Bi_0.5Na_0.5TiO₃,BNT)usually has excellent piezoelectric properties.By constructing morphotropic phase boundary（MPB）with potassium bismuth titanate（Bi0.5K0.5TiO3,BKT）,BNT can exhibit high electrostrictive properties and excellent dielectric properties.Therefore,Bi FeO3 and Bi_0.5Na_0.5TiO₃ based materials are selected as the research object.It is studied that the effects of substitution at A-site or compositing,on ferroelectric,piezoelectricity,dielectric tunability,electric induced strain and gas sensing properties.A-site La substituted BiFeO₃（Bi1-xLaxFeO3,BLF）ceramics were synthesized by solid-state reaction method,and the effect of substitution concentrations on the nonlinear electrical properties was studied.BLF was annealed in different atmosphere and its electrical properties was measured.It was found that BLF is dominated by p-type hole conduction mechanism.The higher the oxygen pressure of the atmosphere is,the better the conductivity is.And BLF has the characteristic of nonlinear I-V characteristic.When the applied electric field exceeds a certain threshold,the current increases significantly,while the resistance decreases obviously.The nonlinear coefficient decreases with the increase of La substitution,yet increases with the temperature oxygen pressures increasing.BLF has the highest nonlinear coefficient after annealed in oxygen.This resistive change effect is derived from the microelectronic heterogeneous structure of ferroelectric domain and domain wall,which have different conductivities and result in rectification effect in the domain wall.A-site Ba substituted BiFeO₃（Bi0.9Ba0.1FeO2.95,BBFO10）was prepared by sol-gel method,and the gas sensitive properties of several typical volatile gases were studied.Compared with pure BFO,gas detection sensitivity of BBFO10 has greatly improved,and BBFO10 sensor has a fast response and recovery time,good selectivity and long-term stability.The reason is ascribed to the large specific surface area of BBFO10,which leads to the increase of the gas molecule contact area in gas sensitive reaction process,which can improve the reaction sensitivity.In addition,due to the divalent Ba2+ partial substitution of trivalent Bi3+,the concentrations of oxygen vacancy increase in BBFO10,which also improves the gas sensing properties of BBFO10.A-site Ce substituted BiFeO₃（Bi0.9Ce0.1FeO3,BCFO）powders were prepared by hydrothermal and microwave hydrothermal method,and their magnetic properties were studied.The BFO prepared by microwave-hydrothermal method shows a paramagnetic behavior,and the BFO and BCFO-H prepared by conventional hydrothermal method shows a weak ferromagnetic behavior.Substitution of Ce significantly improved the magnetic properties of the BCFO,which is due to the transfer of internal and external orbital hybrid electron,and the BFO spiral modulation structure change caused by ionic radius difference.（1-x）（0.8BNT-0.2BKT）-xNaNbO3（BNKT-xNN）ceramics were prepared,and the electric-induced strain,ferroelectric and dielectric performance with the variation of composition and temperature were studied.With the content of NN increasing,BNKT-xNN gradually changes from nonergodic relaxor ferroelectric to ergodic relaxor ferroelectric,which is characterized by the fact that is ferroelectric-to-relaxor temperature temperature（TF-R）drops gradually from above the room temperature to below room temperature,the piezoelectric coefficient d33 drops significantly,the hysteresis loops changes from the saturated loop to pinched shape,the electric-field-induced strain S-E curve changes into sprout-shape from butterfly-shape.Under the electric field,the nonergodic relaxor composition exhibits an electric-field-induced irreversible structural transition from pseudocubic to rhombohedral phase,and the strain increases significantly when the temperature rises up above the TF-R temperature.For BNKT-0.04 NN,as a ergodic relaxation component,its electrostrictive strain under room temperature is 0.445%,Smax/Emax value can reach 810 pm/V,which is ascribed to the reversible phase transitions between ergodicity relaxation phase and the ferroelectric phase arouse by electric field.The tunable dielectric properties,pyroelectric properties and conduction mechanism of BNKT-x NN ceramic were studied.It has significant nonlinear dielectric properties and its dielectric tunable behavior is largely dependent on NN content.The tunable dielectric behavior of nonergodic relaxation component changes from non-linear to linear relationship after the electric field induced phase transition,while the ergodic relaxation component remain non-linear relationship.Besides,BNKT-x NN is dominated by n-type electronic conduction mechanism,and its conductivity increases with the decrease of the oxygen pressure.The electrical conductivity activation energy decreases with the decrease of the oxygen pressure.The electric-induced strain,ferroelectric and dielectric properties of（1-x）（0.8BNT-0.2BKT）-xBiMg2/3Nb1/3O3（BNKT-xBMN）ceramics were studied.The BNKT-x BMN phase is pseudo cubic phase.The BNKT-0BMN as non-ergodic relaxation ferroelectric shows typical ferroelectric feature at room temperature,and the irreversible phase transition occurs under the applied electric field.For BNKT-0.02 BMN,as a ergodic relaxor ferroelectric,its electric-induced strain values can be up to 0.431%,and its Smax/Emax can reach 862 pm/V under the low electric field of 40 kV/cm.The high field induced strain efficiency is beneficial for practical applications.