Study On Preparation And Equal-valent Doping Of Lead-free Bismuth Titanate-based Piezoelectric Ceramics

The lead-free binary 0.94Bi_0.5Na_0.5Ti O₃-0.06BaTiO₃（BNBT）piezoelectric ceramics were prepared by traditional solid-state reaction method.First,the influence of sintering temperature on the structure and properties of BNBT ceramics was discussed.The X-ray diffraction suggested the samples that sintering near 1160℃-1200℃displayed single perovskite structure.The microstructures showed that the grain grew up significantly with the increase of sintering temperature.When the sintering temperature reached 1200℃,the largest average grain size was obtained and the grains grew uniformly which also increased the density of the ceramic.In terms of electrical performance,all samples were measured at room temperature obtained Polarization-Electrical Filed loops.It could be seen that the BNT-BT sintered at 1200℃exhibited the most excellent ferroelectric characteristics and energy storage performance:P_r=30.91μC/cm²;P_max=41.10μC/cm²;E_c=2.08 k V/mm;W_rec=21.06 m J/cm³;η=15.70%.Meanwhile,the BNBT sintered at 1200℃under 6 k V/mm also showed the best field-induced strain capability（S₊=～0.17%,S_-=～0.15%）;Finally,in terms of dielectric and piezoelectric properties,with the increase of sintering temperature,both the piezoelectric constant and dielectric constant of BNBT have gradually increased,while the dielectric loss had been reduced.Therefore,the BNT-BT sintered at 1200℃also showed the best piezoelectric and dielectric properties(ε_r=1788,tanδ=0.055,d₃₃=156p C/N).It can be seen from the above results,increasing the sintering temperature could improve various performances of BNBT ceramics,and its optimal sintering temperature is1200℃.On the basis of the previous experiments,the effects of different sintering environments（argon;air）on BNBT ceramics were further studied.All the samples under two sintering environments had a higher density（～98.8%）and appeared as a single perovskite structure.The SEM showed that all grains were well developed and displayed similar sizes.However,the concentration of oxygen vacancies in the ceramic was affected by the partial pressure of oxygen.Sintering under pure argon could effectively promote the formation of oxygen vacancies inside the sample.Due to the inconsistency of oxygen vacancy concentration,the electrical properties of BNBT ceramics sintered in two environments were directly affected.Compared with argon sintered samples,the remanent polarization and coercive field of air sintered samples increased and decreased respectively,showing a better ferroelectricity.At the same time,its electrical strain capacity was also more stable than argon sintered samples,and with a larger inverse piezoelectric constant.In terms of dielectric properties,the sample sintered in air had a higher dielectric constant than sintered in argon sample,which also showed a lower dielectric loss.Therefore,BNBT ceramics is more suitable to sintered in air,because it could obtain more excellent electrical characteristics.Based on the research of the basic preparation process of BNBT,the ions(Gd³⁺,Hf⁴⁺)with the same valence state was droping replaced with 0.94BNT-0.06BT,in order to improve the properties of the ceramics.In the experiment,the samples were prepared by traditional solid-state method with different levels of doping elements,and then analysed the changes of ceramic in phase,microstructure,ferroelectric properties,and electro-induce strain.When the Gd element was added,the BNBT ceramics were still near the morphotropic phase boundary where the trigonal-tetragonal phase coexists without any trace of secondary phase,but the electrical properties were greatly affected.With the incorporation of Gd element,the original long-range ordered domain structure of the sample was destroyed,which caused an increase in the number of nano-polarized micro-domains,led to the ferroelectric properties decreased.When the Gd doping amount reached 4 mol.%,The remanent polarization strength of the ceramic was only 2.74μC/cm².In terms of field-induced strain,the electro-strain capability of BNBT-Gd also became weaker with increasing Gd content.In general,the A-site replacement of BNBT ceramics by Gd³⁺did not achieve excellent electrical properties.In addition,research on Hf-doped BNBT ceramics was continued conducted.When the Hf element was introduced into the BNBT ceramics,the sample still had a single perovskite structure.A small amount of Hf element（Hf content less than 1 mol.%）was conducive to promoting the growth of grains,and the size of the grains would affect the electric-field performance to a certain extent.The polarization-electric field hysteresis loop（P-E）measured at room temperature indicated that the sample would change from a normal ferroelectric phase to a relaxed ferroelectric phase with increasing Hf element.When the amount of Hf added was 2 mol.%,the waist"shrink"was most obvious,BNBT-2.0Hf ceramics exhibited the best energy storage performance that the energy storage efficiency reached 38.23%.In terms of field-induced strain,when the Hf doping amount was 1 mol.%,a large electric field strain of 0.35%and a large normalized strain(S_max/E_max)of 583 pm/V,were obtained at an electric field strength of 6 k V/mm.Therefore,Hf-doped BNT-based ceramics exhibit excellent electrical properties and have the potential to replace lead-based materials.