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%and?_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?¹.5?m?,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%.