Microstructure And Electrical Properties Of BNT-Based Lead-free Piezoelectric Ceramics Modified By Perovskite Comounds

Piezoelectric ceramics have attracted much attention because of their excellent electromechanical response.With the increasing attention of society to the ecological environment,lead-based ceramics,once the main force of piezoelectric materials,are also limited by laws and regulations because of the toxicity of raw materials such as Pb O.Therefore,the research and development of lead-free piezoelectric ceramics with excellent performance has become an important research direction in the international functional ceramics industry.Among many lead-free piezoelectric ceramic systems,Bi_0.5Na_0.5TiO₃（BNT）-based piezoelectric ceramics have attracted much attention because of their excellent ferroelectric properties,high Curie temperature and relaxor properties.However,its practical application value is limited due to its large coercivity field and low depolarization temperature.Therefore,it needs to be further improved by A/B doping and solid solution modification in order to obtain more excellent properties.In this paper,the experimental design and research are carried out by doping modification method.Through the analysis of phase structure,micro morphology and electromechanical properties,BNT-based ceramics with excellent properties are obtained.Along the above ideas,this paper mainly studies the following three groups of systems:First of all,（0.94-x）Bi_0.5Na_0.5TiO₃-0.06Ba TiO₃-xSr_0.7La_0.2TiO₃（x=0,0.15,0.2,0.25,0.3）ceramics are prepared by traditional solid-state sintering method to explore the properties of low hysteresis and large strain.The comprehensive analysis of XRD,Raman and TEM shows that after doping SLT,the ceramic system changes from the coexistence of rhombohedral and tetragonal phase to pseudo cubic phase,and SL doping will lead to the transformation from large-size ferroelectric domain to small-size PNRs.The resulting high dynamic PNRs have a significant effect on large strain and low hysteresis.SEM shows that the maximum grain size of x=0.2 is 1.19μm.The EDS results show that all elements are evenly distributed.The dielectric temperature spectrum results show that the incorporation of SLT will adjust the T_f peak whose frequency dependence disappears to near room temperature（27°C）,which can reduce the dielectric relaxor behavior of ceramics and help to maintain significant polarity characteristics at the same time.The sample with x=0.2achieved the excellent performance of high strain value（0.325%）and ultra-low hysteresis（8.02%）at the same time,maintained excellent strain temperature and frequency stability,obtained the maximum strain of 0.394%at 150°C,and maintained only 7%strain fluctuation in a wide frequency range.In addition,the sample with x=0.2 also has an energy storage density of 1.72 J/cm³.Secondly,on the basis of the research in the previous chapter,Nb is doped in to B-site of BNT-BT-SLT ceramics to prepare（0.94-x）Bi_0.5Na_0.5TiO₃-0.06Ba TiO₃-xSr_0.2La_0.2Nb O₃（x=0,0.01,0.02,0.03,0.04）ceramics to explore the large strain response under low electric field:XRD shows that the ceramics have a pseudo-cubic phase.The Raman results indicate that the composition induces a local structure transition from an ordered to a disordered state.The maximum grain size is 1.85μm for the x=0.02 sample,and the incorporation of SLN has little effect on the grain size.The results of dielectric thermography show that the incorporation of SLN makes the system more relaxed.The positive and inverse piezoelectric properties show that the maximum inverse piezoelectric coefficient d₃₃*is 677 pm/V for the x=0.02 sample,and the incorporation of SLN will cause the disappearance of the negative strain S_neg.The x=0.02 sample obtained a maximum strain of 0.54%.The energy storage density of 1.28 J/cm³ is obtained for the sample with x=0.04.Finally,ternary（0.3-x）Bi_0.5Na_0.5TiO₃-0.7Bi_0.1Sr_0.85TiO₃-xNa Ta O₃（x=0.02,0.04,0.06,0.08,0.1,0.12）ceramics were prepared by traditional solid-phase sintering method:XRD results show that the PNRs of the samples can remain disordered during both electric field loading and unloading.The Raman results indicate that the ferroelectric long-range order can be reversibly transformed into disordered PNRs with the incorporation of NT.The largest grain size is 2.36μm for the x=0.06 sample.The sample with x=0.06 obtains a maximum effective energy storage density of 2.276 J/cm³ and has good frequency stability（5-30 Hz）.The leakage current of the x=0.06 sample is 6.1×10^-7 A/cm²,indicating that the ceramic has good insulating properties.The dielectric constant-bias electric field strength results show that the（0.3-x）BNT-0.7BST-xNT ceramics have dual dielectric tunability,and the negative dielectric tunability of ceramics disappears at the x=0.06 sample.