Preparation And Modification Of KNN-based Lead-free Piezoelectric Ceramics

Lead-based piezoelectric ceramics,especially represented by PbZr_1-x-x Ti_xO₃?PZT?and?1-x?Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃?PMN-PT?,have been widely utilized owing to their excellent electrical properties.However,the toxic Pb poses great threat and harm to humanbeings and environment.As a consequence,it is high time to develop environmentally friendly lead-free ceramics to replace the lead-based ones.K_xNa_1–xNbO₃?KNN?based lead-free piezoelectrics were considered as a potential and promising one because of its superior piezoelectric properties and high Curie temperature.In this dissertation,K_x Na_1–xNbO₃?KNN?lead-free piezoceramics were respectively modified by A-site,B-site and the second ABO₃-type perovskites.The effect of modification on the crystal structure and electrical properties of the ceramics were investigated.First,?1-x?(K_0.49Na_0.51)(Nb_0.97Ta_0.03)O₃–xBi_0.5Na_0.5ZrO₃?abbreviated as KNNT–BNZ,x=0.00⁰.05?lead-free piezoelectric ceramics were prepared by a solid state reaction method.The effect of Bi_0.5Na_0.5ZrO₃?BNZ?doping amount on the crystal structure,microstructure and electrical properties of the KNNT–BNZ ceramics was investigated.X-ray Diffraction?XRD?results indicated that all the samples possessed a pure perovskite structure.With the increase of BNZ doping amount,the phase structure of the samples changed from orthorhombic to tetragonal,and a polymorphic phase transition?PPT?region with the coexistence of orthorhombic and tetragonal phases can be found in0.03?x?0.04.The ceramics with the composition of x=0.04 in the tetragonal side of the PPT region exhibited optimal electrical properties as follows:piezoelectric coefficient d₃₃=317 pC/N,planar electromechanical coupling k_p=36.4%,mechanical quality factor Q_m=68,dielectric permittivity?^T₃₃/?₀=1225,dielectric loss tan?=3.1%,remnant polarization P_r=20.5?C/cm²,coercive field E_c=1.16 kV/mm and Curie temperature T_c=310?.Second,basing on the composition of 0.96KNNT–0.04BNZ,the Li⁺ion was introduced to A-site of the crystal lattice of the ceramics.0.96(K_0.49Na_0.51–xLi_x)(Nb_0.97Ta_0.03)O₃–0.04Bi_0.5Na_0.5ZrO₃?abbreviated as 0.96KNNTL_x–0.04BNZ,x=0.00⁰.04?lead-free piezoelectric ceramics were prepared via a solid state reaction method.The effect of Li doping amount x on the crystal structure,microstructure and electrical properties of the ceramics was investigated.The results indicated that all the samples possessed a pure perovskite structure.The phase structure gradually changed from the coexisting orthorhombic and tetragonal phases to tetragonal phase as Li doping amount x increased.A polymorphic phase transition?PPT?structure with orthorhombic and tetragonal phases coexisting appeared when x?0.01,while the structure was transformed to tetragonal phase when x?0.02.The ceramics when x=0.02 on the compositional side from PPT to tetragonal exhibited the optimal electrical properties as follows:piezoelectric coefficient d₃₃=335 pC/N,planar electromechanical coupling k_p=38.40%,mechanical quality factor Q_m=43,dielectric permittivity?^T₃₃/?₀=1350,dielectric loss tan?=2.70%,remnant polarization P_r=23.50?C/cm²,coercive field E_c=1.52 kV/mm,and the Curie temperature T_C=325?as well.The complex impedance spectra of the ceramics when x=0.02 were measured at different temperatures and frequencies,and it is indicated that the conduction mechanism is referred to both the grains and the grain boundaries.The activation energy for dielectric relaxation calculated is 1.15 eV,which well coincides with the activation energy associated with the movement of the oxygen vacancy at high temperatures.Last,basing on the composition of 0.96KNNT–0.04BNZ,the(Co_1/3Sb_2/3)complex ion was introducing to the B-site of the crystal lattice of the ceramics.0.96(K_0.49Na_0.51)(Nb_0.97-x(Co_1/3Sb_2/3)_xTa_0.03)O₃–0.04Bi_0.5Na_0.5ZrO₃?abbreviated as0.96KNNT–xCS–0.04BNZ,x=0.000⁰.010?lead–free piezoelectric ceramics were prepared via a conventional solid state reaction method.The effects of Co_1/3Sb_2/3 doping amount x on the crystal structure,microstructure and electrical properties of the ceramics were explored.The results indicated that all the samples possessed a pure perovskite structure with the coexistence of orthorhombic and tetragonal phases over the whole x value range,and the vibrations shifted towards low frequencies slightly in the Raman scattering spectra.The ceramics with x=0.008 exhibited optimal electrical properties as follows:piezoelectric coefficient d₃₃=347 pC/N,planar electromechanical coupling k_p=42.90%,mechanical quality factor Q_m=47,dielectric permittivity?^T₃₃/?₀=1435,dielectric loss tan?=2.70%,remnant polarization P_r=18.55?C/cm²,and the Curie temperature T_c=303?.