In this paper, 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-30%PT) single crystal samples were poled using an alternating current (electric field) poling (ACP) method. Compared to the traditional poling method, the piezoelectric coefficient, free and clamped dielectric constants were improved more than 21%. X-ray diffraction result suggests the existence of monoclinic phase (M_A) in ACP samples and piezoresponse force microscopy (PFM) result further depicts the finer engineered domain structures. The ACP sample also showed the unique phase transition sequences during the depoling process. Our work could provide a novel domain engineered method to enhance piezoelectric properties of PMN-PT single crystal. GRAPHICAL ABSTRACT IMPACT STATEMENT Piezoelectric and dielectric properties of relaxor-PT single crystals can be significantly enhanced by employing the new alternating current poling method, attributing to the unique heterogenous domain structure containing unprecedented domain wall density.

In this work, the piezoelectric and dielectric properties of [001]-oriented 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ single crystals under alternating current poling (ACP) at different temperatures were studied. The piezoelectric coefficients (d₃₃ ∼ 1930 pC/N, d₃₁ ∼ −850 pC/N) and free dielectric permittivity (ε^T₃₃/ε₀ ∼ 7570) reached their highest values when the poling temperature (T_poling) was 70 °C. Compared with traditional direct current electric field poling at 20 °C, 70 °C-ACP samples showed an enhancement of 40%, 35%, and 49% for d₃₃, d₃₁, and ε^T₃₃/ε₀, respectively. Meanwhile, d₃₃ and ε^T₃₃/ε₀ were enhanced by about 9% when T_poling increased from 20 °C to 70 °C under ACP, while d₃₁ remained the same value and the dielectric loss was lowered from 0.29% to 0.22%. Moreover, ACP samples with different T_poling have similar electromechanical coupling factors (k₃₁ ∼ 0.44, k_t ∼ 0.60). A discussion of the mechanism for the ACP enhancement was based on the domain observation using piezoresponse force microscopy, and the results showed that the domain densities of ACP samples with different T_poling were positively correlated with their piezoelectric properties. This work demonstrated the enormous potential of ACP optimization for relaxor-PT single crystal applications.

Alternating current (electric field) poling (ACP) was applied on [001]-oriented 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-0.3PT) single crystal samples with dimensions of 5 × 1.25 × 1.25 mm³ (with electrodes on the 1.25 × 1.25 mm² surfaces), and the influence of ACP frequency (f_ACP) was studied. Compared to those from traditional direct (electric field) poling samples, the piezoelectric coefficient (d₃₃) and free dielectric constant (ε^T₃₃/ε₀) of ACP samples could gain up to a 67% increase to 3200 pC/N and 10 500, respectively. The influence of f_ACP was studied on two main aspects: saturated properties and dynamic saturation process. In general, ACP samples with lower f_ACP had higher saturated d₃₃, ε^T₃₃/ε₀, and coupling factor k₃₃, as well as lower dielectric loss and faster saturation speed. The ACP dynamics during the saturation process were studied by measuring the polarization-vs-electric field hysteresis loops (P-E loops). The P-E loops illustrated that the coercive field of ACP samples could be further tuned from 1.84 kV/cm to 3.03 kV/cm by changing f_ACP (0.1–10 Hz). This work demonstrated the enormous potential of ACP optimization in relaxor-PT single crystal-based low-frequency transducer applications.

Alternating current poling (ACP) was performed on Gen III relaxor-PT Mn-doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (Mn: PIN-PMN-PT) single crystals with the poling direction of [001]. Experimental results proved that ACP could bring property enhancement to both k₃₁ and k₃₃ mode crystals. Compared to those from traditional direct current poling (DCP), ACP with the optimized conditions (20 kV/cm, 0.1 Hz, and 20 cycles) enhanced the dielectric and piezoelectric properties of k₃₁-mode mode crystals by more than 30%, where the enhanced free dielectric constant and piezoelectric coefficient d₃₃ reached 5300 and 1750 pC/N, respectively. Furthermore, replacing DCP with ACP could increase the advantages of Gen III relaxor-PT. The coupling factors k₃₁ and k₃₃ were enhanced to 0.472 and 0.915, the mechanical quality factor Q_m was enhanced by 17%, and the depoling temperature was raised by 17 °C to 123 °C. In the following mechanism study, in situ x-ray diffraction (XRD) combined with the temperature-dependent dielectric constant measurement proved the introduction of the monoclinic phases after ACP, while piezoresponse force microscopy (PFM) observation showed “2R”-like “2M” domain morphologies in ACP single crystals. Both these intrinsic and extrinsic factors are believed to be the keys to the mechanisms of property enhancement behind ACP. This study proved that ACP is an effective property enhancement method suitable for Gen III relaxor-PT single crystals and will promote its applications in high-temperature and high-power devices.

The overpoling effect of alternating current poling (ACP) was studied on [001]-orientated rhombohedral Pb(Mg1/3Nb2/3)O3-0.26PbTiO3 (PMN-0.26PT) single crystals. Our experimental results showed that the property enhancement from the ACP was remarkable only when the poling cycle number (NL) was kept low. When ACP was continued after the saturation, dielectric and piezoelectric properties gradually dropped down to traditional direct current poled levels or even lower. Such a decrease in material properties caused by the large NL during ACP was defined as the “overpoling effect of ACP” in this study. The following lattice symmetry and domain structure characterization studies were performed through x-ray diffraction (XRD) and piezoelectric force microscopy (PFM) to find the origin of the overpoling effect. The XRD measurements combined with temperature dependence of dielectric properties demonstrated that the field-induced phase transition continued when the samples became overpoled. Further PFM measurements illustrated that the domain density of the overpoled ACP sample was significantly lower than that of the normal one, while the “2R” domain configuration was maintained through the ACP process. In addition, the hysteresis loop characterization indicated large decreases in the coercive fields. The discovered overpoling effect of ACP could help us understand the mechanisms of ACP and optimize the ACP process.