Study Of Piezoelectric Properties,Temperature Stabilities And Anti-thermal Aging Performance Of KNN-based Ceramics

Piezoelectric materials,which enable the conversion between mechanical and electrical energies,have been widely used in many devices such as piezoelectric actuators,filters and transducers.Lead oxide based ferroelectric,represented by lead zirconate titanate are the most widely used materials due to their excellent piezoelectric properties and temperature stability.However,due to the volatilization of lead element during calcination and sintering process,they can cause large harm to human health and environment.With the improvement of people's environmental protection,developing piezoelectric materials that are environmentally friendly has become a hot research topic.(K,Na)NbO3(abbreviated as KNN)is considered as one of the most promising candidates for lead-free piezoelectric ceramics.It usually exhibit very high Curie temperature and excellent thermal stability.However,the piezoelectric properties of natural sintered KNN ceramics were relatively low;the piezoelectric constant d33 was about 125 pC/N at room temperature.This greatly limit their application.Recently,researchers attempted to improve their piezoelectric performance mainly by doping and changing sintering methods.The doping method usually includes Li substitution for K and Na,Sb or Ta substitution for Nb,and the addition of ABO3 compounds.However,composition and sintering methods may cause changes in crystal structure and microstructure,resulting in a significant impact on piezoelectric properties and temperature stability.With the above background,this paper gives a study on composition,preparation and piezoelectricity of doped KNN ceramics,as follows.Firstly,we studied the influence of sintering methods on the piezoelectric properties,temperature stability and thermal ageing property of(Na0.52K0.4425)(Nb0.885Sb0.08)O3-0.0375LiTaO3(abbreviated as KNLNST)ceramics.The KNLNST ceramics were obtained by either conventional or two-step sintering.Here,all the ceramics show coexistence form of orthorhombic-tetragonal phase at room temperature.It was found that the two-step sintering could effectively improve the density and piezoelectric constant(the density is enhanced from 95.2%to 97.0%,and d33 is enhanced from 363 pC/N to 387 pC/N).The ceramic prepared by conventional sintering shows a comparatively uniform grain-size distribution in microstructure,whereas the one prepared by two-step sintering exhibits a bimodal grain-size distribution in microstructure.Secondly,we studied the piezoelectric properties of ceramics with a compositional of(1-x)K0.48Na0.52Nb0.955Sb0.045O3-xBi0.5Na0.5HfO3(abbreviated as KNNS-xBNH).Here,all the samples were sintered by two-step sintering.When x =0.01,d33 and planar electromechanical coupling coefficient kP are 214 pC/N and 0.46,respectively.This ceramic has a wide orthorhombic phase region,and its kP is very stable in the temperature range from-50 ? to 120 ?.With increasing the doping content of BNH,the R-O phase transition temperature TR-O increases and the O-T phase transition temperature TO-T deceases to room temperature.Due to the reduction of the orthorhombic phase region,piezoelectric temperature stability becomes poor,but room-temperature piezoelectric properties are greatly enhanced.When x = 0.037,crystal structure at room temperature is in a coexistence state of rhombohedral-orthorhombic-tetragonal phases.The d33 and kP are 512 pC/N and 0.53,respectively.Domain structure was investigated by acid etching technique.The unpoled ceramic exhibits a complicated domain pattern consisting of irregularly shaped domains of long parallel stripes separated by 180� domain boundaries from neighboring domains.In addition,the domain pattern contains a hierarchical nanodomain structure.In contrast,upon poling the domain pattern becomes simpler and takes the form of long parallel stripes of diverse widths,with a hierarchical nanodomain structure appearing inside some of the broader stripes.It is suggested that the superior piezoelectric properties are closely associated with the characteristic domain structure.Thermal ageing stability of the ceramics with x = 0.037 is quite good.The d33 remains larger than 450 pC/N after in the thermal treatments down to-50 ? and up to 220 ?.Thirdly,we designed the(1-x)(K0.48Na0.52)1-yLiyNb1-zSbzO3-x(Bi0.5Na0.5)ZrO3(abbreviated as(1-x)KNLyNSz-xBNZ)composition and prepared the ceramics by two-step sintering.By properly adjusting the compositional ratio,a maximum d33 value of 445 pC/N could be obtained in these(1-x)KNLyNSz-xBNZ ceramics.For the ceramics with x = 0.035,y = 0.02 and z = 0.045/0.05/0.06,crystal structure is in a coexistence state of rhombohedral-tetragonal phases at room temperature,and their d33 values are over 410 pC/N.Besides,their kP show the quite similar temperature dependences,increasing slowly with temperature in the temperature range from-50? to the temperature of rhombohedral-tetragonal phase transition but decreasing rapidly above the temperature of rhombohedral-tetragonal phase transition.Thermal ageing stability was further examined.It was found that the ceramic with x = 0.035,y=0.02 and z = 0.045 shows the best thermal ageing stability among the three ceramics.Its d33 and kP show little changes in the temperature range from-50 ? to 140 ?.