Preparation And Properties Of La Or Sm Doped PMN-PT Ceramics

Lead magnesium niobate-lead titanate（PMN-PT）based piezoelectric ceramics are typical relaxor ferroelectrics with high dielectric constant,electrostrictive coefficient,and pyroelectric coefficient,which are perfect materials for manufacturing actuators,micro-displacement drivers,multilayer ceramic capacitors,electrical devices,and optical devices.Rare-earth element doping can effectively improve the electrical properties of PMN-PT-based ceramics.However,the ceramics synthesized by mixing oxides methods tend to induce the pyrochlore phase,which deteriorates the performance of ceramics.In this work,rare-earth elements Sm and La were selected as doping elements.The effects of sintering rhombohedral/tetragonal（R/T）mixture powder method and two-step calcination method on the crystalline phase and electrical properties of Sm and La-doped PMN-PT ceramics were investigated.The pyrochlore crystalline phase was avoided successfully.The research work is divided into four parts.First,Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃（componentA）and Pb_1-1.5xSm_x(Mg_1/3Nb_2/3)_yTi_1-yO₃（component B,x=0.0417～0.0833,y=0.7533～0.8367）was prepared by columbite precursor method,respectively.Then components A and B were mixed according to the design proportion.The（1-w）[Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃]-w[Pb_1-1.5xSm_x(Mg_1/3Nb_2/3)_yTi_1-yO₃]（w=0.3,0.4,0.5,0.6）（（1-w）PMN-w PSMN-PT）piezoelectric ceramics were obtained by granulating,molding and sintering.The X-ray diffraction results show that all the samples presented a pure perovskite structure.The grains were closely packed.The average grain size was～5.18μm from the observations of scanning electron microscopy images.The density of ceramics was found to be 97.8%.It was found that the performance of ceramics was significantly enhanced when compared to the ceramics fabricated using the traditional niobate process.Outstanding piezoelectric coefficient d₃₃ of 1103 p C/N,electromechanical coupling coefficient k_p of0.66,relative dielectric permittivityε_r of 9154,dielectric loss tanδof0.044,and temperature of dielectric peak T_m of 81℃could be achieved for the ceramics with w=0.5.Second,The Pb_0.9625Sm_0.025(Mg_1/3Nb_2/3)_1-xTi_xO₃（x=0.28,0.29,0.30,0.31）piezoelectric ceramics were prepared by two-step calcination process.The effects of the precursor calcination process and Ti content on the crystal structure,microstructure,and electrical properties of piezoelectric ceramics were investigated.The results indicated that the piezoelectric ceramics prepared by the two-step calcination process presented a pure perovskite structure.The piezoelectric,dielectric,and ferroelectric properties of ceramics can be significantly improved.The morphotropic phase boundary appeared near the Ti content of 0.30 mole.When x=0.30,the piezoelectric constant d₃₃,electromechanical coupling coefficient k_p,relative dielectric permittivityε_r,dielectric loss tanδ,the temperature of dielectric peak T_mof the ceramics were found to be 1236p C/N,0.69,11177,0.044 and 85℃respectively.Third,the Pb_1-1.5wLa_w(Mg_1/3Nb_2/3)_1-xTi_xO₃（w=0.015,0.025,0.035,x=0.28,0.29,0.30,0.31）（PLMN-PT）piezoelectric ceramics were synthesized by two-step calcination method.The effects of La₂O₃ and PT content on the phase structures,microstructures,piezoelectric properties,dielectric properties,and ferroelectric properties of the ceramics were investigated.It was found that the PLMN-PT ceramics synthesized by the two-step calcination method presented a pure perovskite structure,with an average grain size of 4.46μm.The phase structure of the samples gradually transformed from the rhombohedral phase to the tetragonal phase along with the PT content increasing.The piezoelectric coefficient,electromechanical coupling coefficient,relative dielectric permittivity,remanent polarization intensity,and field-induced strain of the ceramics gradually increased and then decreased after reaching the peak value.The morphotropic phase boundary appeared near the PT content of 0.30 mole.The piezoelectric coefficient d₃₃ of 1010 p C/N,electromechanical coupling coefficient k_p of 0.69,relative dielectric permittivityε_r of 10766,and field-induced strain S of 0.23%at1 Hz,3.5 k V electric field are achieved in the sample of w=0.025,x=0.30.Fourth,the tetragonal phase Pb(Mg_1/3Nb_2/3)_0.66Ti_0.34O₃and rhombohedral phase Pb_1-1.5xLa_x(Mg_1/3Nb_2/3)_1-yTi_yO₃（x=0.0833～0.0417,y=0.2067～0.2733）powders were prepared in advance.The Pb_0.9625La_0.025(Mg_1/3Nb_2/3)_0.70Ti_0.30O₃ ceramics with different R/T crystalline phase ratios were prepared by sintering rhombohedral/tetragonal mixture powder in accordance with the design mole ratio（rhombohedral content w=0.3,0.4,0.5,0.6 mole）.The effects of the R/T crystalline phase ratio on the piezoelectric,dielectric,and ferroelectric properties of ceramics were investigated.The ceramic samples presented the perovskite structures of the rhombohedral and tetragonal phases.The average grain size and relative density of the ceramics were found to be 5.24μm and 96.76%,separately.When the R/T phase ratio is equal to 1,the electrical properties of the ceramics achieve the maximum.The electro-parameters were found to be:d₃₃～1014 p C/N,k_p～0.67,k_t～0.64,?_r～10955,P_r～24μC/cm²,S～0.21%（1Hz,3.5 k V）.