| Bismuth-sodium-titanate(Bi0.5Na0.5TiO3,abbreviated as BNT)based ceramics are important lead-free piezoelectric materials.Compared with other lead-free ceramics,such as barium-titanate and potassium-sodium niobate,the BNT-based ceramics have high Curie temperature,high spontaneous polarization,giant electric-field induced strain,etc.And making it a broad application prospects in actuators,energy storage devices,and so forth.They show potential to replace the traditional lead-containing Pb(Zr,Ti)O3(PZT)based piezoelectric ceramics in the future.However,BNT-based ceramics have some shortcomings that limit their practical applications in actuators and energy storage fields,and shortcomings including low piezoelectric coefficient,large coercive electric field and high electric field required to obtain large spontaneous polarization or large strain.In this thesis,we have designed and fabricated BNT-based ceramics with acceptor,equivalent,donor doping and multi-phased doping,and then systematically studied the relationship between the structure/defects and properties,and the mechanism of the BNT-based materials with the aim of improving the piezoelectric and ferroelectric properties of BNT-based ceramics.Consequently,large electric-field induced strain and high energy storage density have been achieved in this work.Firstly,Mg2+-acceptor doped 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3 ceramics were fabricated by using solid-state reaction technique.The doped Mg3+ did not change the perovskite structure of the ceramics,but mostly work on the B-sites and their related chemical bonds.The doped Mg2+ will replace Ti4+ via(?).and increase the total number of oxygen vacancies,and further affect the conduction mechanism and relaxor behavior.The Fe3+-doping is expected to show similar effects as Mg2+doping.In the Fe3+doped 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics,the piezoelectric factor is up to 168 pC/N,and its strain is 0.21%with the normalized strain d33*(Smax/Emax)of 305 pm/V.With increasing the concentration of the Fe3+,the conductivity characteristics of the ceramics also change.When the doping content is high,the conduction mechanism experienced a shift from being bulk effect dominant to grain boundary effect dominant,and finally the electrode dominant.Secondly,we carried out investigations on the effect of equivalent Li+doping on the structural and electrical properties of 0.97Bi0.5Na0.4K0.1TiO3-0.03Ba0.7Sr0.3TiO3 ceramics.The Li+doping decreases the lattice constant of the ceramics and enhances their relaxation properties.When the doping level is low,the grain conductance dominates the mechanism.When the doping amount is high,both grain and grain boundary conductance play an important role.Then,in order to fabricate BNT-based ceramics with bigger strain,Nb5+-donor doped 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3 ceramics were prepared.In the Nb5+doped ceramics,the highest strain is 0.34%,accompanied with d33*of 625 pm/V.To further enhance the strain property,we prepared a group of excessive Li+-doped ceramics,in which the Li+cations act as sintering aid and promote the grain growth.As a result,the strain was enhanced further up to 0.4%,with d33*of 683 pm/V.Besides,it also exhibits excellent fatigue-resistance behavior,making it a promising candidate in high-precision displacement drivers.In the study of donor-doped ceramics,we found the energy-storage properties were improved remarkably in relaxors that in ferroelectrics.As a result,investigations on the energy-storage properties of BNT based ceramics were carried out.A series of Hf4+-doped 0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3 ceramics were prepared.Hf4+doping promotes the transition of the material from the ferroelectric-to-relaxor phase transition.As a result,the coercive field Ec decreases,reflecting the enhanced relaxation characteristics.The maximum energy storage density reaches 0.51 J/cm3.In addition,when 2mol%of Hf4+is substituted into the ceramics,the strain of the sample reaches 0.41%.Our work provides a way to prepare ceramics with both energy storage and strain characteristics.Finally,to further enhance the energy storage density and efficiency of the BNT-based ceramics,we prepared 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics with different Sr0.8Bi0.1?0.1Ti0.8Zr0.2O2.95 doping level.The ceramics exhibit remarkable relaxor behavior.The 0.91(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-0.09Sr0.8Bi0.1?0.1Ti0.8Zr0.2O2.95 ceramic has high breakdown electric field of 230 kV/cm,meanwhile,the highest recoverable energy storage density is 2.68 J/cm3 with an efficiency of 74.4%.Further study show that the ceramic has good temperature stability and anti-fatigue properties.When the temperature increases from 25? to 140?,the energy storage density only decreases by 11.1%.Also,after 105 charge-discharge electrical cycling at 165 kV/cm,the energy storage density of the ceramics only decreases by 5.2%.Our work enriches the application of lead-free ceramics in the field of energy storage. |
PDF Full Text Request