Doping Modification And Electrical Properties Of Bismuth-based Perovskite Relaxor Ferroelectric Ceramics

Bi-based perovskite relaxor ferroelectric ceramics have excellent dielectric,ferroelectric and piezoelectric properties and can further enhance their electrical properties by doping.However,there are still many problems in the current Bismuth-based perovskite ceramics system,such as:the strain and energy storage properties have not been significantly improved,the electrical performance parameters are more sensitive to temperature,the relaxor process and strain mechanism of ferroelectric ceramics are not clear,and so on.In view of the above problems,bismuth-based perovskite ferroelectric ceramics represented by Bi_0.5Na_0.5TiO₃(BNT)and BiFeO₃(BFO)were selected as the substrate materials.BNT has good ferroelectric properties and high strain coefficient,while BFO has high Curie temperature and large dielectric constant,which can be used in high temperature dielectric and piezoelectric fields.The relaxor properties,temperature stability,field strain and energy storage properties are improved by doping and optimizing design of components,such as substitution at A site,substitution at A site and co-substitution at A/B site.The main research contents are as follows:Firstly,0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃(BNT-BT)binary ceramic was substituted by complex ions(K_0.5Nd_0.5)²⁺at A site by conventional solid solution technology,and studied related dielectric properties and ferroelectric properties.It is seen from the XRD pattern that the(K_0.5Nd_0.5)²⁺complex ions have completely dissolved into the crystal lattice and formed a single perovskite structure,and the crystal structure of the ceramic system is at the Morphotropic Phase Boundary(MPB)position.By adjusting the concentration of doped(K_0.5Nd_0.5)²⁺complex ions,the depolarization temperature T_f–r can be adjusted to room temperature,and it is found that at x=0.04,the strongest relaxor behavior can be obtained,and the high field-induced strain value of?0.52%with the normalized strain value of?612pm/V is achieved,which is related to the drop of T_f–r to room temperature.More importantly,the strain varies by less than 10%over a wide temperature range and maintains a high strain value(>0.4%).Secondly,doped Sr_0.7La_0.2TiO₃(SL)containing A-site vacancies into BNT-BT binary substrate materials.The results show that the incorporation of SL has little effect on the crystal phase and grain morphology of ceramic systems,but the incorporation of SL can significantly improve the relaxor properties of ceramic samples over a wide temperature range,and produces good dielectric properties.In addition,with the increase of SL content,the ferroelectric to relaxor temperature T_f–r is adjusted to be lower than the ambient temperature,and finally the best relaxor characteristics and optimal temperature stability are obtained.At an electric field of 80 kV/cm,the x=0.04 sample obtained a large strain of 0.5%and a normalized strain of 625 pm/V.Importantly,the field-induced strain of the component is insensitive to temperature over a wide range,and the strain value remains above 0.4%with a variation of less than 5%.Thirdly,a novel binary ceramic(0.7Bi_0.5Na_0.5TiO₃-0.3Sr_0.7Bi_0.2TiO₃,BNT-SBT)was co-substituted at A and B sites by LaTi_0.5Mg_0.5O₃.The phase transition,dielectric and energy storage characteristics were analyzed.It is found that the increase in LaTi_0.5Mg_0.5O₃(LTM)content not only increases the degree of relaxor diffusion,but also achieves high temperature stable dielectric properties.With proper doping,the ceramic material achieves high temperature stability over a wide temperature range while maintaining a large dielectric constant.When x=0.05,the dielectric constant is stable at2170±15%in the temperature range of 35-363?,and the dielectric loss is less than 0.05in this range.In addition,at an excitation field of 100 KV/cm,the energy storage density of the composition is greatly increased to 1.32 J/cm³,and the energy storage efficiency is as high as 75%.This is because the relaxor phase and the ferroelectric phase are more likely to change each other when they coexist.More importantly,the storage density has good temperature stability within the measurement range,and is maintained within 5%from 30? to 110?.In addition,the x=0.05 sample exhibits excellent fatigue resistance in 10⁶fatigue cycle tests.Then,the Bi-based perovskite ferroelectric ceramics BiFeO₃ was taken as the research object to study its application in high temperature dielectric and strain fields.The conventional bismuth ferrite binary ceramics(0.67BiFeO₃-0.33BaTiO₃,BF-BT)were doped by SrZrO₃ at A and B sites position..The surface structure,dielectric,impedance,electrical conductivity and piezoelectric properties were studied in detail.It is found that these three perovskite compounds have been completely dissolved to form a single structure with pseudocubic phase.In addition,scanning electron microscopy tests have shown that proper doping can make the lattice arrangement of ceramic systems more compact,while excessive SrZrO₃ content leads to abnormal grain growth.In addition,the analysis of the dielectric temperature spectrum shows that as x increases,the dielectric peak gradually broadens,especially the x=0.04 sample has the strongest relaxor characteristics.More importantly,doping SrZrO₃ can greatly improve impedance and electrical insulation properties.In addition,at x=0.04,a high field-induced strain value of 0.24%is obtained,corresponding to a normalized strain value of 400 pm/V,which is related to the relaxor response of the polar nano-region(PNRs)under the electric field.Compared with pure BF-BT binary ceramics,impedance,electrical insulation,ferroelectricity and piezoelectricity are significantly improved after doping with SZ content.High impedance(40 k?·cm at 420?),low conductivity(4×10^-5?·cm^-1 at 460?),and low leakage current density(5.1×10^-6A/cm²)at 30 kV/cm,in addition,good ferroelectric properties(38?C/cm²)and good piezoelectric properties(400 pm/V)were observed.Finally,the BF-BT binary ceramics were co-substituted by the rare earth contained compound(K_0.5Nd_0.5)TiO₃ at the A and B sites.The analysis found that all samples were pseudocubic crystal structures,and the incorporation of the(K_0.5Nd_0.5)TiO₃(KNT)compound had little effect on the grain size.Dielectric analysis showed that the degree of dispersion of the dielectric spectrum increased with KNT,indicating that better relaxor characteristics were obtained.In addition,a high dielectric constant of 23,000 and a dielectric loss of less than 0.5 were obtained in the x=0.02 sample in the temperature range of 30-400?,indicating excellent electrical insulation properties.Besides,the Curie temperature is higher than 390?.In particular,the doping of KNT had a significant effect on the strain of the piezoelectric ceramic,and a large strain of 0.26%was obtained in the x=0.02 sample,which was attributed to an improvement in electrical insulation properties.More importantly,the ceramics have good fatigue resistance,and the strain of the ceramic sample increased significantly with increasing temperature.The maximum strain value is0.75%at 110? with a small hysteresis factor of 30%.This is attributed to the coexistence of non-ergodic and ergodic relaxor domain.