Phase Structure And Electrical Properties Of Perovskite Lead-free Piezoelectric Ceramics

Piezoelectric materials have been widely used intransducers,sensors,actuators and so on.Although PZT-based materials have excellent physical properties,high levels of lead oxides（⁷0 wt%）inevitably cause environmental problems during preparation and processing.Therefore,lead-free piezoelectric materials have attracted more and more attention.Since 2000,research and development of lead-free piezoelectric materials have made significant progress,but its physical properties are not good enough to completely replace PZT.Therefore,it is necessraty to develop new high-performance perovskite lead-free piezoelectric ceramics.In this thesis,a series of lead-free ceramics have been developed and prepared and their crystal structure,microstructure and electrical properties have been systematically investigated.The main contents and results were summarized as below:1.（1-x）BaTiO₃-xCaHfO₃ ceramics were prepared by a conventional ceramic technique and the phase transition,microstructure and electical properties of the materails were investigated.All samples have a perovskite crystal structure.With the increase of CaHfO₃（x）,there are different phases coexistence.Rhombohedral（R）,orthogonal（O）and tetragonal（T）phases coexist in the composition with x=0.08,resulting in the largest values of piezoelectric coefficient（d33⁴00 pC/N）,inverse piezoelectric coefficient(d^*₃₃⁵47 pm/V)and plane electromechanical coupling coefficient（k_p⁵8.2%）.This is because the composition located at a region where the R-O-T three ferroelectric phases coexist would have the lowest energy barrier and thus greatly promote the polarization rotation,resulting in a strong piezoelectric response.2.（1-x）BaTiO₃-xCaSnO₃ lead-free ceramics were prepared by an ordinary solid-state sintering method,and the relationship between the phase structure and the piezoelectricity of the materials was studied to ascertain potential factors for strong piezoelectric response.When 0.00≤x≤0.06,the ceramics exhibit the coexistence of orthogonal（O）and tetragonal（T）phases.At x=0.08,the ceramic has a ferroelectric coexistence of rhombohedral,orthogonal and tetragonal（R-O-T）phases.When x is increased to 0.10,the phase structure of the ceramics is transformed into the R-O phases coexistence.And the R phase and cubic（C）phase coexist in 0.12≤x≤0.14.For x=0.08,the ceramic exhibits the lowest energy barrier and thus promotes polarization rotation and extension,resulting in the optimal piezoelectricity of d₃₃ and kp values of 550 pC/N and 60%,respectively.3.A new（1-x）(K_0.5Na_0.5)NbO₃-x[NaSbO₃+Bi_0.5(Na_0.8K_0.2)_0.5(Zr_0.5Hf_0.5)O₃]lead-free ceramics were designed and prepared by a conventional solid state method and the relationship between phase structure and piezoelectricity in ceramics was studied.As x increases,the rhombohedral-orthorhombic phase transition temperature(T_R-O)and the orthorhombic-tetragonal phase transition temperature(T_O-T)are shifted toward room temperature simultaneously.When x=0-0.01,the ceramics exhibit a typical O phase structure.When x=0.02-0.03,the ceramics possess the coexistence of O and T phases.At x=0.04-0.05,three ferroelectric phases of R,O and T coexist.When x is increased to 0.06,the phase structure is transformed to R phase.When R-O-T three ferroelectric phases coexist in the materials,the free energy of the ceramics is unstable,resulting in a large increase in piezoelectricity.At x=0.04,the ceramic has excellent piezoelectric properties of d₃₃=452 pC/N,k_p=63%andε_r=4414.4.[(Bi_1-xLa_x)_0.5Na_0.5]_0.94Ba_0.06(Ti_1-5y/4Nb_y)O₃ ceramics were prepared by a conventional solid state method.All ceramics can be sintered well at 1200 ^oC and have small grain sizes（¹.5μm）,which increases the dielectric breakdown strength（DBS）of the ceramics.Furthermore,as the content of La³⁺and Nb⁵⁺increases,the P-E loop appears to be significantly refined,the residual polarization is significantly reduced,and the maximum polarization remains the same large value as the ceramic without La³⁺and Nb⁵⁺doping.Therefore,high storage density and discharge efficiency are achieved at x/y=0.07/0.02,resulting in the large storage density of 1.83J/cm³ and high discharge efficiency of 70%.