Study On Phase Boundary Constructing And Electrical Properties Of BT-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics are functional materials that can realize mutual conversion of mechanical energy and electrical energy.Among them,Pb(Zr1-xTix)O3(PZT)piezoelectric ceramics have attracted widespread attention due to their superior electrical properties,but in view of the hazards of toxic lead to the environment and human health,people have gradually shifted their attention to lead-free piezoelectric ceramics.As a kind of lead-free piezoelectric material,BaTiO3(BT)based piezoelectric ceramic has stable electrical properties,but its relatively low piezoelectric coefficient(d33)limit its application.In order to optimize the electrical properties of BT-based ceramics,researchers have made many attempts.Effective methods include doping modification,microstructure design,and domain engineering,etc.Among these methods,phase boundaries constructed by chemical doping can enable BT-based ceramics to obtain enhanced electrical properties.In the experimental research of the first and second parts of this thesis,the diversity of electric domains in phase coexistence region promotes the polarization rotation between these phases,leading to an enhancement of the piezoelectric properties.In addition,in the third part of the experiment,the phase boundary constructed by introducing Bi(Mg3/4W1/4)O3(BWM)induces the generation of polar nanomicroregions(PNRs),and a phase transition from ferroelectric phase to relaxor ferroelectric phase occurred,thereby optimizing the BT-based ceramics’ energy storage performance.Therefore,not only enhanced piezoelectric coefficient(d33)but also relaxor ferroelectrics with energy storage properties can be obtained in chemically modified BT-based ceramics by constructing phase boundaries.In this thesis,BaTiO3 with tetragonal structure is selected as the research object,and polycrystalline phase boundary(PPB)or morphotropic phase boundary(MPB)are constructed by dissolving BT with other elements or perovskite structure materials;The origin of enhanced electrical properties of the BaTiO3 system and its corresponding internal mechanism were analyzed by systematically investigating the density,microstructure,phase structure,dielectric,ferroelectric and piezoelectric properties of these binary solid solutions.The main research content of this thesis are as follows:(1)A series of of(1-x)BaTiO3-xCa(Sn1/2Zr1/2)O3((1-x)BT-xCSZ,x=0-0.15)ceramics have been prepared by the conventional solid-state reaction method.The structure evolution,microstructure,and piezoelectric properties were investigated.The X-ray diffraction(XRD)results indicated that the phase symmetry strongly depends on the CSZ content.The tetragonal phase(T)is well-maintained in the compositions of 0 ≤x≤0.03,and the MPB with coexistence of tetragonal(T)and cubic(C)phases is obtained in the range of x=0.06-0.09,and the pure cubic phase is obtained when x≥0.09.A significantly enhanced piezoelectric coefficient(d33=388 ± 9 pC/N)is attained in the composition of x=0.06 at the MPB,where a tetragonal ferroelectric phase and an ergodic relaxor phase with average cubic symmetry coexist.Based on the analysis of crystal structure and dielectric properties,a temperature-component phase diagram consisting of four phase regions was established.This study indicates that lead-free BT-CSZ binary solid solution can be applied to electromechanical sensors.(2)Lead-free piezoceramics of Ba0.85Sr0.15Ti1-xZrxO3(BSTZx)were prepared by the conventional solid-state method.In the component interval of 0≤x≤0.15,the microscopic morphology,phase structure and piezoelectric,dielectric and other electrical properties of BSTZx ceramics were systematically investigated.Coexistence of rhombohedral(R),tetragonal(T)and orthorhombic(O)phases was identified in ceramics with 0.06 ≤x ≤0.09.Ceramics with x=0.06 has found enhanced piezoelectric coefficient(d33=325pC/N)and enhanced relative permittivity(er=12200).The electrical performance the ceramic is better than or equivalent to other BT-based piezoelectric ceramics.Therefore,the addition of Zr4+ can effectively improve the electrical properties of BaTiO3-based ceramics,which is beneficial to promote the practical progress of BT-based ceramics.(3)(1-x)BaTiO3-xBi(W1/4Mg3/4)O3((1-x)BT-xBWM)lead-free relaxor ferroelectric ceramics were prepared by solid-state reaction method.The phase structure evolution,dielectric properties,and P-E loops of(1-x)BT-xBWM ceramics were systematically studied and the components with the best energy storage properties were found.XRD shows that the phase structure of the(1-x)BT-xBWM binary solid solution changes from a tetragonal phase at x=0 to morphotropic phase boundary at x=0.02-0.04(the proportion of the tetragonal phasein x=0.04 is much smaller than that of the cubic phase),and completely transformed into pure cubic phase at x=0.06.The x=0.02 sample exhibits an enhanced piezoelectric coefficient(d33=118pC/N)due to being at the morphotropic phase boundary.Grain refinement occurs at x=0.04,which improves the breakdown strength(BDS)of ceramics with this component.When the BWM doping amount increases to 4%,the ferroelectricparaelectric phase transition peak begins to broaden,showing the characteristics of a diffuse phase change;the temperature(Tm)corresponding to the dielectric peak moves to high temperature with the increase of the external field frequency,showing an obvious frequency dispersion behavior.The introduction of BWM induces the generation of polar nanomicroregions(PNRs)and a phase transition from ferroelectric phase to ergodic relaxor phase,resulting in relaxor ferroelectric ceramics(0.96BT-0.04BWM)at room temperature.The 0.96BT-0.04BWM ceramics are located at the phase boundary between the tetragonal phase and the cubic phase,with obvious relaxor characteristics and the best energy storage density(Wrec=0.884 J.cm-3)and energy storage efficiency(η=91.3%).