Phase Composition Tuning of Lanthanum-Doped PMN-PT Ceramics at Morphotropic Phase Boundary

Abstract: The Pb_{0.962 5}La_0.025(Mg_1/3Nb_2/3)_1-zTi_zO₃ (z=0.28, 0.29, 0.30, 0.31) piezoelectric ceramics were synthesized by two-step pre-sintering process. The morphotropic phase boundary (MPB) of ceramics was found at 0.29 mol and 0.30 mol of PbTiO₃.Selecting the chemical composition on both sides of morphotropic phase boundary, and the tetragonal phase Pb(Mg_1/3Nb_2/3)_0.66Ti_0.34O₃ powder and rhombohedral phase Pb_1-1.5xLa_x(Mg_1/3Nb_2/3)_1-yTi_yO₃ (x=0.083 3~0.041 7, y=0.206 7~0.273 3) powder rhombohedral phase were prepared, respectively. Two crystal phase powders (rhombohedral phase mole fraction w=0.3, 0.4, 0.5, 0.6) were mixed and pressed into disks. The ceramics with the same chemical compositionand different crystal phase proportion of Pb_{0.962 5}La_0.025(Mg_1/3Nb_2/3)_0.70Ti_0.30O₃ were sintered. The effect of crystal phase ratio on the piezoelectric, dielectric, and ferroelectric properties of ceramics were investigated. The results show that the rhombohedral and tetragonal phase ratios of ceramics are pasically the same as the proportion of ingredients. For the sample with w=0.5, the rhombohedral and tetragonal phase ratios of ceramics sintered at 1 250 ℃ is about 0.47, 0.53, respectly. The average grain size is (5.24±0.23) μm. The relative density of ceramics is 96.76%. The piezoelectric constant d₃₃, planner electromechanical coefficient k_p, thickness electromechanical coefficient k_t, relative permittivity ε_r, remnant polarization P_r, and field-induced strain S (1 Hz, 3.5 kV/mm) of ceramics are 1 014 pC/N, 0.67, 0.64, 10 955, 24 μC/cm², and 0.21%, respectively. For the chemical composition near MPB, the rhombohedral and tetragonal phase ratios of ceramics could be tuned artificially via this method.

Key words: PLMN-PT ceramics, piezoelectric ceramics, lanthanum doping, morphotropic phase boundary, piezoelectric property, dielectric property, ferroelectric property