| Piezoelectric ceramics are a class of functional materials capable of converting mechanical energy and electrical energy,widely employed in electronic industry.Lead zirconate titanate(abbreviated as PZT)-based piezoelectric ceramics have dominated the market at present.However,the considerable lead content in PZT-based ceramics poses health and environmental risks due to volatilization during production,usage,and disposal.Consequently,developing environmentally-friendly,lead-free piezoelectric ceramics has become an urgent matter.Among several researched lead-free piezoelectric ceramic systems,potassium sodium niobate(abbreviated as KNN)-based ceramics,with their high Curie temperature and low production costs,hold promise as potential alternatives to lead-based piezoelectric ceramics.Presently,the piezoelectric performance of KNN-based ceramics has reached or even surpassed some PZT-based ceramics.However,several issues warrant further exploration:For highperformance KNN-based piezoelectric ceramics,related research of dielectric properties has been scarce in recent years,and there is limited research on the impact of polarization and phase transitions on domain wall structures in recent years.In addition,reported high-performance KNN-based piezoelectric ceramics often have poor piezoelectric temperature stability,and are often doped with Sb elements with certain toxicity.Based on the research background,this paper investigates the dielectric properties,domain structure,and temperature stability of potassium sodium niobate-based piezoelectric ceramics.The specific research contents and main results are as follows:1.A series of(1-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3-x(Bi0.5Na0.5)ZrO3 ceramics were prepared,and their crystal structure,piezoelectric properties,dielectric spectra,and temperaturedependent dielectric properties at room temperature were examined.As the doping content of(Bi0.5Na0.5)ZrO3 increased,the rhombohedral-orthorhombic phase transition point significantly increased,and the orthorhombic-tetragonal phase transition point substantially decreased,causing the ceramics to gradually transition from an orthorhombic phase to a coexistence of orthorhombic and tetragonal phases.At x=0.04,the optimal room temperature piezoelectric properties were achieved,with d33=480 pC/N,kp=0.58,k2=0.43.Through dielectric property testing,several interesting phenomena were observed:(1)the dielectric spectrum at room temperature exhibited noticeable dielectric relaxation at frequencies above 105 Hz,accompanied by high tanδ values;(2)In piezoelectric ceramic materials,it has been observed that under a polarized state,parameter displays multiple resonance peaks as the frequency increases.This is accompanied by a notable step-like decrease;(3)At x≥0.04,the lowfrequency value of ε’ in the polarized state at room temperature surpasses the low-frequency value of ε’ in the non-polarized state.This phenomenon can be attributed to the coexistence of multiple phases within the ceramic material.Furthermore,possible physical mechanisms causing these phenomena were discussed.2.The(1-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3-x(Bi0.5Na0.5)ZrO3 ceramics with x=0.03 and x=0.04 were selected to the crystal phase composition and domain structure changes at different temperatures and before and after polarization at room temperature.X-ray diffraction(XRD)analysis was performed on ceramic samples under non-polarized and polarized conditions at room temperature.The {200} diffraction peaks in the obtained XRD spectra were fitted using a Lorentzian function to investigate the changes in crystal phase composition with varying compositions and polarization states.The domain patterns on the acid-etched surfaces of nonpolarized and polarized ceramic samples were observed at different temperatures(25℃,50℃,and 80℃).The domain structures of ceramic samples under different compositions,temperatures,and polarization states were examined and compared,and the relationship between the abnormal changes in low-frequency value after polarization and the domain structure of the ceramics was further discussed.3.The preparation of non-Sb doped KNN-based piezoelectric ceramics with excellent comprehensive properties was attempted.By leveraging the distinct regulation effects of doping various ABO3 compounds on phase transition points in KNN-based ceramics,compositions such as 0.93(K0.48Na0.52)NbO3-0.05BaZrO3-0.02(Bi0.5Na0.5)HfO3-1wt%MnO2,0.94(K0.48Na0.52)NbO3-0.02BaSnO3-0.03BaZrO3-0.01(Bi0.5Na0.5)ZrO3 ceramics have been designed and prepared.The room-temperature crystal structure,piezoelectric performance,dielectric temperature and frequency spectra,temperature stability of piezoelectric properties,and thermal aging characteristics have been investigated.The composition 0.94(K0.48Na0.52)NbO3-0.02BaSnO3-0.03BaZrO3-0.01(Bi0.5Na0.5)ZrO3 ceramics exhibit optimal electromechanical coupling performance at room temperature,with kp=0.53,kt=0.47,and k33=0.66 and the kp,variation rate is only 1%within the temperature range of 25℃ to 100℃.For the piezoelectric coefficient d33,the composition 0.94(K0.48Na0.52)NbO3－0.01BaSnO3-0.03BaZrO3-0.02(Bi0.5Na0.5)ZrO3 ceramics demonstrate the best comprehensive performance,with d33=270 pC/N at room temperature,and the d33 variation rate is only 6%within the temperature range of 25℃ to 100℃.Through testing dielectric properties,it was found that Sb-undoped ceramics generally have lower tanδ values at high frequencies compared to Sb-doped ceramics.Additionally,at the orthorhombic-tetragonal phase transition point,a polarization-induced increase in the low-frequency value is also observed. |
