Micro-displacement actuators are widely used in micromachining, precision measurement, aerospace and biomedical. The performance of micro-displacement actuators can be evaluated in terms of displacement, hysteresis, linearity and temperature stability. For a long time, researchers have been hoping to obtain a ferroelectric material with high field-induced strain, low hysteresis and good temperature stability for a variety of different applications. Pb(Mg_1/3Nb_2/3)O₃ (PMN) is a typical perovskite type relaxor ferroelectric, and PMN-PbTiO₃ (PT) solid solution possesses a morphotropic phase boundary (MPB) region from PT mole fractions of 0.27 to 0.35. The field-induced strain and hysteresis of PMN-PT could meet the requirements of high-performance micro-displacement actuators. However, the temperature stability of field-induced strain still needs to be improved before practical application. In this work, effect of Sm³⁺ doping on the temperature stability of field-induced strain in PMN-0.32PT ceramics was investigated.

Experimental samples of nominal composition Pb_1-1.5xSm_x[(Mg_1/3Nb_2/3)_0.68Ti_0.32]O₃ (xSm-PMN-0.32PT, x=0, 0.020, 0.030, 0.035, 0.040, 0.045, 0.050) were prepared via a two-step columbite precursor method. The sintered disks were sintered at 750 ℃ for 15 min, poled in silicone oil bath under 1 kV/mm for 10 min at room temperature. The electrochemical properties of sintered samples were measured 24 h late after poling. The crystal structure of sintered samples was determined by PANalytical X'Pert3 X-ray diffractometer (XRD) at room temperature. The dielectric, piezoelectric, ferroelectric and resonant properties of sintered samples were measured by TH2816A LCR meter, ZJ-3 quasi-static d₃₃ meter, Radiant Precision Multiferroic Materials Analyzer and ZX80A impedance analyzer, respectively. The field-induced strain of sintered samples was measured using LVDT linear variable differential transformer probe controlled by MDS micro-displacement meter and Keithley 2 410 source meter. The temperature environment was provided by GZ-ESPEC MC710P environmental test chamber.

All samples exhibited typical perovskite phase, and the chemical compositions of them were similar as that of MPB, a mixture of rhombohedral R, monoclinic M_C, monoclinic M_B and tetragonal T phases. The MPB shifted towards to the PMN-rich direction with the increase of x value. The dielectric temperature spectra showed that the dispersion coefficient (γ) increased slightly with the increase of x, 1.374 ≤γ ≤1.67. As the frequency increased, values of ε_r-m shifted to a higher temperature and the frequency dispersion increased significantly with the increased x value. d₃₃ values decreased from 929 pC/N at x=0.020 to 546 pC/N at x = 0.050. These results indicate that all samples are relaxor ferroelectrics. The substitution of Sm³⁺ for Pb²⁺ broke the long-range ordering of lattice, resulted in the existence of numerous nanodomains below depolarization temperature (T_d), and the spontaneous polarization vector was more easily switchable with external electric field. The ferroelectric properties showed that both the remanent polarization (P_r) and coercive field (E_C) decreased monotonically with increased x value. In addition, the maximum value of field-induced strain (S_m) was very small at low temperature, but increasesd rapidly nears -10 ℃. The temperature stability of S_m for x=0.040 sample was much better than other samples when temperature ranged between -10 ℃ to +60 ℃. It is known that the ferroelectric domains in relaxor ferroelectric would be gradually into the small domains with increased temperature, which were easily oriented for external electric field. The field-induced strain was the largest near T_d, but decreased sharply when the temperature was higher than T_d. For xSm-PMN-0.32PT ceramics, value of T_d shifted to lower temperature due to substitution of Sm³⁺ for Pb²⁺, and T_d also showed a broad distribution due to the increased inhomogeneity of chemical composition in the microregion. As the temperature approached the T_d of certain microregions, the ferroelectric domains will be gradually cleaved into the small domains, and thus the field-induced strain will show good stability over a wide temperature range. As the field-induced strain decayed remarkedly below -10 ℃, it may be related to the diffuse phase transition of PMN.

PMN-PT was the preferred material for micro-displacement actuators due to its high field-induced strain and low hysteresis. The substitution of Sm³⁺ for Pb²⁺ broke the long-range ordering of lattice, increased the inhomogeneity of chemical composition in the microregion, resulted in a strong dielectric frequency dispersion, and the appearance of numerous nanodomains and polar nanoregions made samples exhibit large field-induced strains and high temperature stability. The field-induced strain was much smaller below -15 ℃ and increased rapidly near -10 ℃ under the influence of diffuse phase transition of PMN. A field-induced strain of 4.2×10^-4 and hysteresis of less than 13.1% in x=0.040 sample were achieved with excellent temperature stability that the strain fluctuation was smaller than 3.8% in the temperature between -10 ℃ and +60 ℃.