Doping And Flexoelectric Enhancement Of Lead-free Piezoelectric Ceramics Based On Bismuth Sodium Titanate

As a kind of force-electric conversion functional ceramic material,piezoelectric ceramics are often used in the manufacture of electronic components such as transducers,filters and energy harvesters.In order to protect the environment,people hope to replace lead ceramics with lead-free ceramics gradually.The performance of BNT lead-free piezoelectric ceramics has excellent plasticity,so it has a good development prospect.The main contents of this paper include three parts.Firstly,the performance of adding the BT to form a binary solid solution system was studied.Then on the basis of 0.94BNT-0.06BT,the effects of component modification on the structure and properties have been studied,including non-stoichiometric modification,addition of BKT to form a ternary solid solution system and Nb₂O₅ doping.Finally,structural modification has been performed on the flexoelectric effect of the reduction heat treatment of 0.94BNT-0.06BT ceramics.The components and structural modification methods have been combined to modify the components.Reduction heat treatment is performed on a group with better performance in component modification.A composite piezoelectric ceramic with high flexoelectric properties has been obtained.The BNT-BT ceramics have been fabricated by the solid-state reaction method in this paper.The calcination temperature is 850?and the sintering temperature is1150¹200?.It is found that when the BT content is 6%,the crystal structure of ceramics belongs to the morphotropic-phase-boundary with good properties:the piezoelectric constant d₃₃ is 120 pC/N,the dielectric constant?_r is 787,the dielectric loss tan?is 0.0709 and field-induced strain S_max is 0.0977%.The Bi_0.5Na_0.5?BN?non-stoichiometric modification of 0.94BNT-0.06BT ceramics reveal that the perovskite structure is unchanged,but the grain size become smaller.The properties are relatively best when A-site is missing by 1%:the piezoelectric constant d₃₃ is 136 pC/N,the dielectric constant?_r is 960,the dielectric loss tan?is 0.0730 and field-induced strain S_max is 0.15%.BKT has been added to 0.94BNT-0.06BT ceramics to form a ternary solid solution system.It is found that the the diffraction peak shifts to the low angle.When BKT amount is 8%,the properties are relatively best:the piezoelectric constant d₃₃ is 148 pC/N,the dielectric constant?_r is 1088 and field-induced strain S_max is 0.32%.After doping Nb₂O₅ in 0.94BNT-0.06BT ceramics,its main crystal phase is a perovskite structure,and the diffraction peak shifts to the low angle.When the doping amount is more than 4mol%,the second phase Bi₂Ti₂O₇ can be generated in XRD patterns.It is found that Nb₂O₅ doping can enhance the dielectric properties of ceramics,but it is bad for piezoelectric properties.The maximum dielectric constant?_r is 1212.When the doping amount is 0.8mol%,the positive field-induced strain S_max is 0.35%.After Nb₂O₅doping,the instantaneous breakdown voltage of the ceramic becomes larger,and the withstand voltage is enhanced.0.94BNT-0.06BT ceramic has been subjected to single-sided high-temperature reduction heat treatment to prepare a lead-free ceramic with high flexoelectric and piezoelectric response.New phases of Bi and Bi₂O₃ appear on the ceramic surface after reduction.The reduced surface seems loose with fine voids and grooves,and the composition is not uniformly distributed with the metal Bi precipitates.The reduction that results in flexoelectric polarization and internal stress in the 0.94BNT-0.06BT ceramic promotes electrical domain steering.Due to the stress gradient,chemical composition gradient and material semiconductorization caused by the reduction,the piezoelectric constant d₃₃ and the flexoelectric coefficient is significantly increased.On the basis of the component-modified ceramics,single-sided high-temperature reduction is performed.It has been studied that the reduction heat treatment temperature of the component-modified ceramics decreases.Because the dielectric constant increases,the strain gradient and the component gradient generated after reduction increase of the component-modified ceramics,the flexoelectric properties of the ceramics further increase.Therefore,ceramics with flexoelectric effect and piezoelectric effect composite structure prepared by single-sided high-temperature reduction have good application prospects.