Piezoelectric Effect And Temperature Stability Of High-Performance Potassium Sodium Niobate-based Lead-Free Ceramics

As a kind of typical advanced function materials,piezoelectric materials can be used in many crucial regions,including electronic information,aerospace,communications,defense,and medical field.Herein,lead-based piezoelectric materials which is represented by lead zirconate titanate [Pb(Zr,Ti)O3,PZT],have been widely used because of the outstanding piezoelectric properties and good temperature stability.However,Due to the high content of Pb,lead-based piezoelectric materials would cause serious damage to human body and environment during the process of preparation,recovery,and so on.With the proposal of sustainable development strategy and the improvement of environmental awareness,many countries or regions in the world,represented by the European Union,have issued the relevant laws to reduce or even prohibit the use of lead and other toxic elements in electronic devices.Therefore,it is of great significance for the strategic development of China to research and develop high-performance lead-free piezoelectric materials.Potassium sodium niobate [(K,Na)Nb O3,KNN]-based ceramics become one of the most potential lead-free piezoelectric materials due to the preferable electrical properties and high Curie temperature(Tc).After decades of research and development,the piezoelectric properties of KNN ceramics have got obvious promotions by ion doping,phase boundary construction,process optimization,and so on.Among these,new phase boundary construction is one of the most efficient strategies to enhance piezoelectricity.However,is not yet clear whether new strategies can be presented to further improve the piezoelectric property and the physical mechanism of high piezoelectricity in KNN ceramics is still unclear.In addition,poor temperature stability is often exhibited in KNN ceramics due to the temperature dependence of polycrystalline phase boundary(PPB).Therefore,simultaneously gaining KNN ceramics with high piezoelectricity as well as good temperature stability is still a research difficulty and hotspot.In this paper,KNN-based ceramics were studied via forming the new phase boundary with relaxation characteristics by inducing relaxation behavior,so as to further improve the piezoelectric properties of KNN-based ceramics.And the physical mechanism of high piezoelectricity is revealed from the polarized structures at the atomic scale and domain structure characteristics.In addition,the temperature stability of piezoelectric effect in high-performance KNN-based ceramics was explored,and the physical mechanism of improved temperature stability was described with domain structure and atomic characteristics.The results obtained in this paper are summarized as follows:(1)KNN-based ceramics with new phase boundary [rhombohedralorthorhombic-tetragonal(R-O-T)] and relaxor behavior have been designed and prepared along with excellent electrical performance in a wide composition region,realizing a further improvement of piezoelectric properties.In this work,a ceramic system(0.96-x)KyNa1-y Nb1-z Sbz O3-0.04Bi0.5(Na0.82K0.18)0.5Zr O3-0.4%Fe2O3-x Ag Sb O3 was designed by inducing Ag Sb O3 to 0.96K0.48Na0.52Nb0.95Sb0.05O3-0.04Bi0.5(Na0.82K0.18)0.5Zr O3 ceramics possessing R-T phase boundary and adding a sintering aid of Fe2O3.Combining with XRD patterns,εr-T curves,Rietveld refinement,and temperature-dependent Raman spectra,it can be gained that R&O&T multiphase coexistence with relaxor behavior is formed when1.2%≤x≤3.0%,0.45≤y≤0.60 and 0.04≤z≤0.055,a kind of new relaxor behavior is exhibited where frequency dispersion occurs around the R-O-T phase boundary,and then,enhanced electrical properties are obtained.For example,by modifying the content of Ag Sb O3,K+ and Sb5+,large dielectric constant(εr≥4000)is gained for1.6%≤x≤3.0%,0.45≤y≤0.55 and z≥0.048.Especially,giant piezoelectric constant(d33≥500 p C/N)is achieved in a wide composition region(1.5%≤x≤2.5%,0.45≤y≤0.53,0.04≤z≤0.055),as well as the largest d33 value(650±20 p C/N)for x=1.6%,y=0.48,z=0.05,which becomes the highest value of reported non-texture KNN-based ceramics.(2)The structure mechanism of high piezoelectricity promoted by the slush polar state with nanoscale multiphase coexistence with relaxor behavior is explored with experiments and theory.The coexistence of nanoscale R,O,T phase and the refined nanodomain of highperformance 0.944K0.48Na0.52Nb0.95Sb0.05O3-0.04Bi0.5(Na0.82K0.18)0.5Zr O3-0.4%Fe2O3-1.6%Ag Sb O3 ceramics are firstly observed by STEM,and then the contribution of polar structure to the high piezoelectric properties is analyzed by simulating the local polar structure combining with phase field simulation calculation.According to the displacement vector of Nb-O in the nanodomains,the distribution of the corresponding polarization vectors of R,O and T phases is presented.The research results show that the distribution of spontaneous polarization in local structure is heterogeneous in the high-performance ceramics.And the domain size is very small(~ 2 nm)with highdensity domain wall.Then,the slush polar state is formed with many polar nanoregions.However,there is a sharper boundary(2~5 unit cells)among the coexisting nanoscale R,O and T phases,implying the easily rotated polarization vectors.The polarization states of R,O and T phases are simulated theoretically utilizing Landau free energy and phase field simulation.The Landau free energy simulation verifies the lowered free energy barrier and polarization anisotropy energy of the slush polar state.The phase field simulation shows that nanoscale polarization rotation exists between R,O and T phases and is widely distributed.Finally,lowered polarization rotation energy barrier and domain wall energy are revealed by theory and experimental evidence,signifying the spontaneous polarization is easily re-rotated under external fields,resulting in a large piezoelectric response.In addition,the domain evolution under the external fields is explored in the KNNS-0.05BPNZ-Fe ceramics with relaxor R-T phase boundary.According to the domain morphology change with various frequencies,it can be noted that the polarization rotation of PNRs around domain boundary is easy,inducing the movement of domain walls.Combining with the domain switching under different dwell time and electric fields,one can see that the polarization rotation of ferroelectric domain with PNRs can be promoted(lower switching voltage and faster switching),indicating the reduced polarization energy barrier of KNN-based ceramics with relaxor R-T,which finally benefits piezoelectricity.According to the I-E curves,the promotion of PNRs to domain switching shows little dependence on temperature,leading to the improved temperature stability at low temperature range(30~60 oC),while that is mainly influenced by domain switching at high temperature range(80~120 oC).(3)The temperature stability of piezoelectric response is improved by modifying the relaxor degree of multiphase coexistence or inducing rare earth(RE)element,and the related physical mechanism is analyzed by combining the phase structure,domain structure and local polar structure.Firstly,gradually increased relaxor degree is achieved in 0.944KyNa1-yNb0.95Sb0.05O3-0.04Bi0.5(Na0.82K0.18)0.5Zr O3-0.4%Fe2O3-1.6%Ag Sb O3 ceramics by changing the K+ content,achieving the improvement of strain temperature stability.When y=0.48,a low threshold electric field(Et)is gained,i.e.,improved strain temperature stability is gained with increasing electric fields under low electric fields(E<Et)as well as a weak electric field dependence of good strain temperature stability under high electric fields(E>Et).Both the temperature dependence of piezoelectric response hysteresis and switched area of domain show little change with increasing electric field above a certain voltage,meaning that the optimized strain temperature stability originates from the domain structure evolution of the ceramics.Combining with the proved slush polar state of the ceramics,the refined nanodomains and heterogeneous distribution of polarization vectors are observed,which can promote the domain switching and strengthen the electric field compensation effect for temperature.Thus,the optimized strain temperature stability is achieved.Then,by adjusting the Ho content of rare earth ion(Ho3+),the temperature stability of piezoelectric property and strain is improved in 0.965K0.45Na0.55Nb0.96Sb0.04O3-0.035(Bi1-x Hox)0.5Na0.5Hf O3 ceramics.According to the XRD patterns,εr-T curves,and temperature-dependent Raman spectra,the R-T phase boundary together with high piezoelectricity can be maintained after doping Ho3+.The temperature stability of piezoelectricity and strain is improved for x=0.40.For example,the small signal d33 can remain ~96% at 100 oC,and the strain can remain ~103% at 140 oC.Meanwhile,the promotion of temperature stability is mainly attributed to the increasing phase transition temperature(TR-T)and steady domain structure behavior by introducing Ho content.(4)Based on the optimization of temperature stability of KNN-based ceramics with rare earth(RE)elements,the 0.965K0.45Na0.55Nb0.96Sb0.04O3-0.035(Bi1-xDyx)0.5Na0.5Hf O3 ceramics were designed and synthesized,and the effect of RE on poling temperature(Tp)is explored.The research shows that the ceramics still present the R-O-T phase boundary although the phase transition temperatures increase,and thus the large piezoelectric properties(d33 ~ 440 p C/N)are maintained after the addition of Dy.When x=0.25,large piezoelectricity(d33=400~480 p C/N)can be achieved in a wide poling temperature range(Tp=35~120 oC).And similar phenomenon is found by doping other RE(Eu,Pr,Y,Gd,Yb,Nd,La or Sm),possessing a certain universality.The relevant origin is analyzed by combining with phase structure,domain structure,and ion electronegativity.Compared with Bi3+,RE3+ with a lower electronegativity can promote the lattice softening and release micro-stress produced by the lattice distortion when temperature changes after replacing A site.Then easily rotated domain under external electric fields and steady domain under thermal field are realized.Thus,the sensitivity of high piezoelectricity to poling temperature can be alleviated.In this paper,the relaxor R-O-T phase boundary is constructed by inducing relaxor characteristic into new phase boundary,forming the slush polar state in KNNbased ceramics.And a new kind relaxor behavior that frequency dispersion occurs around the R-O-T phase transition is gained,and then the enhancement of piezoelectricity is realized,becoming highest value of the reported non-textured KNNbased ceramics.Meanwhile,good strain temperature stability with weak electric-field dependence is obtained,improving the deficiency of strong electric field dependence of temperature stability in KNN-based ceramics.And the contribution of relaxor behavior to large piezoelectricity and improved temperature stability is analyzed with the domain evolution under external fields.The mechanisms of high piezoelectric properties and improved temperature stability are analyzed with the local polarization and domain structure.Moreover,by the addition of RE,the temperature stability of piezoelectric response is improved in KNN-based ceramics with little deterioration of piezoelectricity.In conclusion,this paper provides a new idea for the design of highperformance KNN-based lead-free piezoceramic materials,and promotes the research and development of lead-free piezoelectric materials.