Structure,Electric And Depolarization Properties Of BNT-BZT Based Lead Free Relaxor Ceramics

The massive utilization of lead based ceramics has caused serious environmental pollution in the past decades,hence,the development of lead free substitution has become a significant work.Bi_0.5Na_0.5TiO₃?BNT?based ceramics are one of the most concerned lead free systems.After years of continuous exploration,modified BNT-based ceramics have made great strides,and some polynary systems has been able to meet specific applications.Therefore,BNT based ceramics have been deemed as a substitute for lead based counterparts.Although many exciting results of BNT based ceramics in performance optimization have been achieved,there is still a great amount of improvement room for diverse requirements,and some mechanism issues remain to be researched.This thesis chooses three problems which need to be improved emphatically,i.e.,the enhancement of pyroelectric and piezoelectric properties,the elevation of relative strain under low driving electric field,and the improvement of depolarization behavior as research directions.Binary system(Bi_0.5Na_0.5)TiO₃-Ba?Ti,Zr?O₃?BNT-BZT?has been chosen as matrix because of outstanding pyroelectric and piezoelectric performance.We have adopted different modification strategies to promote electric properties and explore corresponding mechanisms.In order to enhance pyroelectric and piezoelectric properties of BNT based ceramics,this thesis has synthesized Mn doping BNT-BZT-x mol%Mn?x=0,0.125,0.25,0.375,0.5,0.625,0.75,1?ceramics.All the components locate in the rhombohedral-tetragonal morphotropic phase boundary?R-T MPB?.The result of XRD and XPS reveals low valence Mn ions enter into B site of perovskite structure for substitution after sintering.The researches on P-E loops at RT,temperature-dependent P-E loops,and S-E curves demonstrate there are two impacts induced by Mn doping: the ?hard? doping effect originated from the substitution of lower valence Mn at B site and the deepening of ergodic degree.The variation of electric properties is determined by the co-work of these impacts.The PFM results show that the stability of ceramic domains decreases at the same temperature with higher ergodic degree.The piezoelectric constant d33 of the component x=0.25 reaches up to 187 p C/N for enhanced ergodicity.Meanwhile,corresponding pyroelectric coefficient p at room temperature?RT?significantly increases to 0.061 ?C/?cm² K?.Since the stability of oxygen vacancy decreases at high temperature,the pyroelectric coefficient p of the component x=0.5 at depolarization temperature reaches up to 3.97 ?C/?cm² K?,which is higher than that in other lead based or free ceramics.To elevate large strain of BNT based ceramics under low driving electric field at room temperature,this thesis has prepared?1-x??BNT-BZT?-x ST?x=0,0.05,0.1,0.15,0.18,0.2,0.22,0.25?ceramics by solid state reaction method.With ST content increasing,the phase structure transits from the R-T MPB to pseudocubic phase.The solid solution of ST is able to decrease the ferroelectric-to-relaxor transition temperature TF-R and depolarization temperature Td obviously,which gives rise to the shift of the transition area between ferroelectric phase and ergodic relaxor phase in a large temperature range.The investigation of P-E curves shows that the increasement of ST content decreases the ferroelectric properties of ceramics at room temperature.According to the PFM results of polarized samples,the domains of the component x=0.2 will decompose while unloading to bring the system back to initial nonpolar state.It should be the critical component located at the boundary of ferroelectric and ergodic relaxor phase.Bipolar and unipolar S-E loops demonstrate the large strain of BNT based ceramics only occurs at the boundary of relaxor and ferroelectric phase.Since there are many intrinsic strain contributions in the critical component,under driving electric field of 5 k V/mm,the relative strain of the component x=0.2 reaches up to 0.4%,and corresponding d33* to 800 pm/V.In view of the low depolarization temperature of BNT based ceramics,this thesis utilized Zn O incorporation to prepare typical 0-3 composite ceramics BNT-BZT:x Zn O?x=0?0.1?0.2?0.3?0.4?.EDS and XRD results demonstrate there are two distributions of Zn ions in the composites: some entered into the lattice to form doping substitution and the others grew up to form Zn O enrichment regions.Similar distribution of Na is observed as well.Since the B site substitution of lower valence Zn ions and the occurrence of Zn O enrichment regions are conductive to the formation of oxygen vacancies,the promotion of Qm becomes the main variation of electric properties after Zn O incorporation,and the maximum attained in the component x=0.3 is up to 182,which increases about 80%.Temperature dependent dielectric permittivity and thermally stimulated depolarization current results indicate Td and TF-R increase 17 ? and 19 ?,respectively,after Zn O incorporation.Combined with Na distribution,this thesis offers a new model that modified matrix associated with the formation of Bi(Zn_0.5Ti_0.5)O₃ is the key factor to bring about the enhancement of depolarization temperature.