Microstructure Evolution And Pizoelectric Properties Of Bi_0.5Na_0.5TiO₃-BiFeO₃-based Ceramics

The pizoelectric material could transform the applied stress to electrical signal or generate strain while under the electrical field.This intrinsic electrical coupling effect of piezoelectric ceramic has been widely applied in engineering realm.But with the emerge of environmental pollution,the high performance lead-free piezoelectric ceramic hopefully replaced the lead based piezoelectric ceramic of the applications in daily life or emerging technologies.Sodium bismuth titanate is a common lead-free piezoelectric material with rhombohedral perovskite structure,it’s high remanent polarization（P_r=38 μC/cm2）attracted widely research.However,pure Bi_0.5Na_0.5TiO₃ is difficult to saturate polarization due to the high conductivity and coercive field（E_C=73 kV/cm）,witch leading to the poor piezoelectricity.This paper researched Bi_0.5Na_0.5TiO₃-BiFeO₃ based ceramic material,further characterized the effect of phase structure,dielectric property and piezoelectric property with different BFO doping concentration.Furthermore,we use doping elements（Sm,La,BiGaO₃ and Sc）and quenching methods to improve the depolarization temperature（Td）and the d₃₃ under high temperature of x=0.60 sample.The main work ware as follows:（1）We use the traditional solid state reaction sintering method to prepare the（1-x）Bi_0.5Na_0.5TiO₃-xBiFeO₃ ceramic with different BFO concentration（x=0,0.05,0.10,0.20,0.30,0.40,0.50,0.60）.With the increasing of BFO content,the d₃₃ at room temperature of this material first drop（0<x<0.30）and then increase（0.30<x<0.60）,furthermore,the grain size decreased with the increasing of BFO contents.With the increase of BFO,the material transform from relaxation state to ferroelectric state,the depolarization temperature increased because there is no ferroelectric-relaxation phase transition at high temperature.（2）The previous prepared（1-x）Bi_0.5Na_0.5TiO₃-xBiFeO₃ samples were quenched,all sample dwelling at 1000℃ for 20 min,then take out from furnace and cooling with fan or different quenching medium.The measuring result of quenched samples verified while the concentration of BiFeO₃ lower than 0.3,the quenching process cloud obviously increase the piezoelectric constant of BiFeO₃ doped samples.Furthermore,the quenching process could significantly improve the piezoelectric parameter at low BiFeO₃ concentration（x<0.30）,furthermore,the depolarization temperature has a great improvement while the BiFeO₃ concentration exceed 0.30 after quenching.For 0.40Bi_0.5Na_0.5TiO₃-0.60BiFeO₃ quenched sample,witch obtained ultra high depolarization temperature of 640℃ without deteriorated piezoelctric constant of 59 pC/N.However,the 450℃ annealing temperature would lead to phase decomposition of air quenched samples,obviously deteriorated the piezoelectric property.（3）The piezoelectric property of doping and quenching process with different elements such as Sm,La,BiGaO₃ and Sc of 0.40Bi_0.5Na_0.5TiO₃-0.60BiFeO₃ was studied to reach both high depolarization temperature and giant piezoelectric constant.The experimental results suggests that quenched samples of Sm or La doped possessed higher piezoelectric constant,but the stability of d₃₃ at high temperature was deteriorated.Fortunately,the doping of BiGaO₃ and Sc could reach high Td（640℃）.In addition,the doping of BiGaO₃ and Sc could increase the piezoelectric constant stable temperature of 20℃.Witch could be a promise piezoelectric material for high temperature application.