Alignment Of Defect Dipoles Research Articles

Modification of domain construction in PNN-PIN-PT piezoelectric ceramics for high electromechanical performance using defect engineering

Good piezoelectric properties and low mechanical losses are prerequisites for the application of high-power devices. In this study, an appropriate amount of Mn ions was introduced into a piezoelectric 0.46Pb(Ni1/3Nb2/3)O3-0.23Pb(In1/2Nb1/2)O3-0.31PbTiO3 ceramic to enhance its electromechanical properties. The results revealed that the mechanical quality factor(Qm) value of the ceramic was improved while maintaining high piezoelectric properties, and the dielectric loss was reduced. When Mn doping is 1.0 mol%, the d33 component decreased by only ∼21%, while Qm increased by ∼13 times (d33 = 750 pC/N, Qm = 385, tanδ = 0.66%). The measurement of the internal bias field (Ei) in the P-E loops of aged and nonaged samples showed that aging promoted the directional alignment of defect dipoles and improved their Qm. According to the PFM data, significant increase in Qm was mainly due to the "bulk effect" caused by low domain wall density. In turn, widening and lengthening of domains promoted the accumulation of most defective dipoles in the ceramic body, which greatly amplified the hardening effect. This work provides a good example of enhancing Qm in ceramic systems with high d33 values through the construction of low domain wall densities.

Large imprint in epitaxial 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 thin films for piezoelectric energy harvesting applications

Tuning and stabilizing a large imprint in epitaxial relaxor ferroelectric thin films is one of the key factors for designing micro-electromechanical devices with an enhanced figure of merit (FOM). In this work, epitaxial 500 nm-thick 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (PMN–33PT) films, free from secondary phases and with extremely low rocking curves (FWHM < 0.05°), are grown on ScSmO3 (SSO) and DyScO3 (DSO) substrates buffered with SrRuO3 (SRO). The PMN–33PT is observed to grow coherently on SSO substrates (lattice mismatch of −0.7%), which is c-axis oriented and exhibits large tetragonality compared to bulk PMN–33PT, while on DSO substrates (lattice mismatch of −1.9%), the PMN–33PT film is almost completely relaxed and shows reduced tetragonality. Due to the compressive epitaxial strain, the fully strained PMN–33PT film displays typical ferroelectric P–E hysteresis loops, while the relaxed sample shows relaxor-like P–E loops. Samples present large negative imprints of about −88.50 and −49.25 kV/cm for PMN–33PT/SRO/SSO and PMN–33PT/SRO/DSO, respectively, which is more than threefold higher than the coercive field. The imprint is induced by the alignment of defect dipoles with the polarization and is tuned by the epitaxial strain. It permits the stabilization of a robust positive polarization state (Pr ∼ 20 μC/cm2) and low dielectric permittivity (<700). In addition, the relaxed PMN–33PT film shows improved piezoelectric properties, with a 33% enhancement in d33,eff relative to the fully strained sample. The obtained low dielectric permittivity and the high piezoelectric coefficients at zero electric field in the studied PMN–33PT films hold great promise to maximize the FOM toward applications in piezoelectric devices.

Ferroelectric, dielectric, and impedance properties of Sm-modified Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

To investigate the effect of Sm doping on the electrical properties of Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) (x = 40, 50, 60) ceramics, three Sm-modified ceramics were prepared using the conventional solid-state reaction method. Related electrical measurements, including ferroelectric and dielectric investigations and impedance spectroscopy, were recorded for these ceramics. It was found that a tilted morphotropic phase boundary resulted from the addition of Sm, which induced the best piezoelectric properties and insulating behaviour in the Sm-BZT-60BCT sample. An abnormal P-E loop shrinkage appeared in the Sm-BZT-50BCT sample but not in the other two samples. This could be attributable to the different electronegativities between Ca2+ and Ba2+ and between Zr4+ and Ti4+, whose contents are different in varied samples and have an effect on defect-dipole alignment as well as spontaneous polarization. The activation energies for the bulk conductivity in the three composites were calculated to be 0.28 ± 0.01, 0.08 ± 0.01, and 0.36 ± 0.01 eV, confirming the existence of oxygen vacancies in our samples. The Sm dopant is responsible for the oxygen vacancies. This also leads to an increased Curie temperature in the three composites.

Aging effect in Er3+‐doped 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 ceramics

AbstractUsually, aging in poled ferroelectrics leads to degradation of certain physical properties. In this study, we found a remarkable aging effect in tetragonal Er3+‐doped 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT‐BCT) ceramics after poling. It is observed that the domains can spontaneously rotate to keep their spontaneous polarization direction similar to that of the poling electric field during aging for the poled ceramics. Furthermore, compared with freshly poled ceramics, the thermally stimulated current (TSC) peak of the aged ones shifts toward a higher temperature (10°C). And the temperature of the TSC peak in the aged ceramics is exactly equal to their Curie temperature. Such features indicate that aging for the poled ceramics could stabilize the alignment of ordered ferroelectric domains. Additionally, a downward TSC peak above Curie temperature is obtained in both poled and aged ceramics, demonstrating that poling and aging can lead to ordered alignment of defect dipoles. The aging mechanism of poled Er3+‐doped BZT‐BCT ceramics has proposed and discussed in this article.

Improved depolarization temperature via the ordered alignment of defect dipoles in (Na0.5Bi0.5)TiO3-BaTiO3 ceramics

The thermally induced depolarization along with the disappearance/loss of macroscopic piezoelectricity is an inevitable issue in ferroelectrics. Motivated from the Landau phenomenological theory, we have developed a strategy to create inner bias field via forming ordered defect dipoles in the NBTBT via CoFe2O4 (CFO) doping and electric poling. It is proposed that the deferred depolarization is related to the formed inner bias field in the CFO-doped NBTBT ceramics. Temperature-dependent dielectric, piezoelectric, strain and ferroelectric behaviors have been measured to evidence the improved thermal stability after CFO doping. It is found that the depolarization temperature of NBTBT-0.02 with a little sacrifice of piezoelectric response is improved to 138 °C, which is 41 °C higher than the pure NBTBT samples. Our study not only provides a feasible paradigm to enhance the ferroelectric order in NBT based materials though defect dipoles, but also inspires further works to promote the depolarization temperature in other ferroelectric materials.

Electric-Field-Dependent Surface Potentials and Vibrational Energy-Harvesting Characteristics of Bi(Na0.5Ti0.5)O3-Based Pb-Free Piezoelectric Thin Films.

The successful utilization of Pb-free piezoelectric materials is considered as critical since the piezoelectric material-based thin-film cantilever is still the preferred choice for commercial vibrational energy harvesters. Herein, we introduce a highly efficient piezoelectric energy harvester based on a Pb-free representative compound, Bi0.5Na0.5TiO3, which has not been explored so far. Applying a strong electric field for poling purposes brought unexpectedly huge changes in the dielectric constant and piezoelectric coefficient, which were responsible for the promising power density of 21.2 μW/cm2/g2/Hz with 537.7 mV output voltage and 2.22 μW output power for a 2 μm thick 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 thin-film cantilever. The power density value is the best so far compared with any reported values for thin-film-based harvesters. As the origin of the effects of poling, the surface potentials across the grain structure are discussed in conjunction with the defect-dipole alignment, as evidenced by the increased oxygen vacancies on the film surface under an external bias field.

Theoretical study on local domain pinning effect due to defect dipole alignment

We have theoretically studied the domain pinning effect from the internal bias field generated by aligned dipolar defects. By ordering defect dipoles along the polarization direction, the experimentally observed ‘hard’ properties in ferroelectric materials, including shifted polarization–electric field (P–E) loops, biased strain-electric field (S–E) loops, suppressed piezoelectric and dielectric responses in sufficiently poled and aged ferroelectric system, can be well described by our model. As expected, the P–E loops and S–E loops become more asymmetric with the increase of defect concentration. The local domain evolution around dipolar defects under an electric field shows the physical process for the domain pinning effect, i.e. the difficulty of domain switching due to the presence of defect dipoles. Finally, the domain memory effect during heating/cooling cycle is simulated. Interestingly, domain configuration can be memorized when a ferroelectric system with dipolar defects goes through the ferro-para-ferro phase transition cycle, and the simulated time steps of domain recovery are strongly correlated with the defect dipole concentration. All theoretical results from our model are consistent with the experimental results. We can conclude that the local interaction between dipole defects and domains is the main mechanism of ferroelectric hardening.

Large strain induced by the alignment of defect dipoles in (Bi3 +,Fe3 +) co-doped Pb(Zr,Ti)O3 ceramics

(Bi3+,Fe3+) co-doped Pb(Zr,Ti)O3 (PZT-BF) and single Fe3+ dope Pb(Zr,Ti)O3 (PZT-F) ceramics were prepared by solid state reaction method and their ferroelectric properties were investigated. The results showed that ferroelectric property of (Bi3+,Fe3+) co-doped PZT ceramics were very different from that of single Fe3+ doped PZT ceramics due to the different mechanisms on the interaction of defect dipoles with spontaneous polarization or domain walls. Unpoled PZT-BF showed very low remnant polarization and low electric field induced strain. After poling, the remnant polarization and electric field induced strain increased markedly due to the alignment of defect dipoles.

Defect-dipole alignment and strain memory effect in poled Li doped (Bi0.5Na0.4K0.1)0.98Ce0.02TiO3 ceramics

Asymmetric ferroelectric hysteresis (P–E) loops and strain hysteresis (S–E) curves are detected in poled-aged Li doped (Bi0.5Na0.4K0.1)0.98Ce0.02TiO3 ceramics at room temperature. Temperature dependence of electrical and structural analysis is used to study the stability of electrically induced long-range ferroelectric order. Interestingly, this asymmetric characteristic can sustain at 80 °C ≤ T ≤ 90 °C, where the ferroelectric and non-ergodic relaxor phase coexist. Our results suggest that the preferentially reoriented defect dipoles have the pinning effect on temperature- and electric field- induced phase transition between long-range ferroelectric and relaxor.

Ferroelectric polarization and defect-dipole switching in an epitaxial (111) BiFeO3 thin film

The time and field control of defect-dipole alignment as well as ferroelectric polarization switching has been investigated in an epitaxial BiFeO3 thin film. Under electric field poling, a double hysteresis loop arising from individual ferroelectric domain switching and defect dipole alignment can be artificially induced after one long enough pre-poling time. Meanwhile, the alignment of defects dipoles can increase the leakage current of the film. The activation field for the defect dipole alignment is extracted to be around 192 MV/m. These results demonstrate the possibility of strengthened polarization contributed by defect dipole poling in ferroelectric thin films.

Self-polarization induced by lattice mismatch and defect dipole alignment in (001) BaTiO3/LaNiO3 polycrystalline film prepared by magnetron sputtering at low temperature

Upward self-polarization induced by lattice mismatch and defect dipole alignment in (001) BaTiO3/LaNiO3 polycrystalline film at low temperature.

Enhancement of ferroelectric Curie temperature in BaTiO3 films via strain-induced defect dipole alignment.

The combination of epitaxial strain and defect engineering facilitates the tuning of the transition temperature of BaTiO3 to >800 °C. Advances in thin-film deposition enable the utilization of both the electric and elastic dipoles of defects to extend the epitaxial strain to new levels, inducing unprecedented functionality and temperature stability in ferroelectrics.

Aging effect evolution during ferroelectric-ferroelectric phase transition: A mechanism study

Aging can significantly modify the dielectric, piezoelectric, and ferroelectric performance of ferroelectrics. However, little attention has been paid to the aging effect during ferroelectric-ferroelectric phase transitions that is essentially correlated with real applications. In this letter, the authors report the aging effect evolution between two ferroelectric phases in an acceptor-doped piezoceramics. The results show that aging-induced double hysteresis loops were exhibited in different ferroelectric phases, but disappeared during ferroelectric-ferroelectric phase transitions, suggesting the mechanism that the intrinsic restoring force for the reversible switching of domains caused by the alignment of defect dipoles was weakened due to ferroelectric dipole reorientation.

Investigating the effect of oxygen vacancy on the dielectric and electromechanical properties in ferroelectric ceramics

The effect of oxygen vacancy on the dielectric and electromechanical properties in lead titanate zirconate based ferroelectric ceramics is discussed in this paper. The presence of oxygen vacancy forms a defect-dipole inside a perovskite cell. This defect-dipole contributes to the unswitchable polarization and influences the alignment of the ferroelectric dipole in the same cell as well. The latter is due to the displacement of the B-site cation, which contributes to the switchable polarization. It is found that a double hysteresis emerges when the alignment of defect-dipole conforms with the crystal orientation, which is perpendicular to the measured polarization. Moreover, the optimal electromechanical property can be obtained by careful adjustment of c-domain volume fraction and the alignment of defect dipoles. Monte Carlo simulation based on the four-state Potts model is presented. Calculation results are compared with experiments.

Domain switching anisotropy in poled lead titanate zirconate ceramics under orthogonal electromechanical loading

Domain switching process in aged lead titanate zirconate ceramics after hyper-TC (TC is the Curie temperature) poling was studied under two types of orthogonal electromechanical loading: (i) axial electric field E3 coupled with lateral compression stresses σ11 and (ii) axial compression stresses σ33 coupled with lateral electric fields E1. It is found that in the lateral direction, both the coercive field and the switchable polarization are significantly smaller than that in the original poling direction and that the lateral coercive field does not decrease but increase with the increasing axial compression, which contradicts with the widely accepted domain switching criterion by Hwang et al. [Acta. Metall. Mater. 43, 2073 (1995)]. The alignment of defect dipoles during aging, together with its effect on domain switching, is employed to explain the observed domain switching anisotropy. Furthermore, the domain stabilizing effect of aligned defect dipoles may lead to a design of large electrostriction actuators.

Imprint in ferroelectric Pb(Zr,Ti)O3 thin films with thin SrRuO3 layers at the electrodes

The imprint behavior of ferroelectric Pb(Zr,Ti)O3 (PZT) films with thin SrRuO3 (SRO) layers at the platinum electrodes was investigated. Different models (defect dipole alignment, bulk screening and interface screening) are discussed which are usually applied to explain the imprint effect in ferroelectric materials. Based on our experimental results for the material under investigation, we suggest that the interface screening mechanism is the dominant mechanism which is responsible for the imprint scenario in ferroelectric thin films.

Thermally induced voltage offsets in Pb(Zr,Ti)O3 thin films

Voltage offset in the polarization-voltage characteristics of Pb(Zr,Ti)O3 (PZT) capacitors was evaluated by a thermal stress process. A thermally induced voltage shift occurs when heating the sample under either remanence or a saturating bias. It was found that the voltage shifts can, to a large extent, be attributed to the role of charged defects and the defect-dipole alignment throughout the films. PZT film with a high Zr/Ti ratio, i.e., rhombohedral compositions exhibited the best imprint resistance. When these films were doped wit up to 6% La, the imprint resistance was further improved. It was also found that B-site donors were more effective in minimizing the voltage shift than A-site donors. However, dopants with the same charge value as Pb, for example, Ca and Sr, did not affect the thermally induced voltage shifts of the films since they could not reduce the charged defects in the films.

Defect-dipole alignment and tetragonal strain in ferroelectrics

We show the alignment of defect dipoles along the direction of the spontaneous polarization in polycrystalline Pb(Zr,Ti)O3 and BaTiO3 ferroelectric ceramics using electron paramagnetic resonance (EPR). The alignment is demonstrated via orientation dependent paramagnetic centers in the polycrystalline materials and computer modeling of the EPR line shapes. It is shown that defect dipoles can become aligned by oxygen vacancy motion in the octahedron about a negatively charged center for the oxygen vacancy-related dipole complexes or by defect displacement and domain realignment in the lattice for isolated defect centers. We find that the alignment is not observed in nonferroelectric materials such as SrTiO3, and is destroyed in ferroelectric materials by heating above the Curie temperature. These observations suggest an interplay between distortion in the unit cell and the ability to align defect dipoles, as is the case more generally for ferroelectric dipole alignment. We also directly observe aligned intrinsic Ti and Pb ion displacements in the ferroelectric Pb(Zr,Ti)O3 perovskite unit cell.

Voltage Shifts and Defect-Dipoles in Ferroelectric Capacitors

AbstractWe review the processes and mechanisms by which voltage offsets occur in the hysteresis loop of ferroelectric materials. Simply stated, voltage shifts arise from nearinterfacial charge trapping in the ferroelectric. We show that the impetus behind voltage shifts in ferroelectric capacitors is the net polarization, with the net polarization being determined by the perovskite and the aligned defect-dipole components. Some common defect-dipoles in the PZT system are lead vacancy-oxygen vacancy complexes. One way to change the net polarization in the ferroelectric is to subject the PZT capacitor to a dc bias at elevated temperature; this process is spectroscopically shown to align defect-dipoles along the direction of the applied electric field. The alignment of defect-dipoles can strongly impact several material properties. One such impact is that it can lead to enhanced voltage shifts (imprint). It is proposed that the net polarization determines the spatial location of the asymmetrically trapped charge that are the cause for the voltage shifts. An enhanced polarization at one electrode interface can lead to larger voltage shifts since it lowers the electrostatic potential well for electron trapping, i.e., more electron trapping can occur. Defect-dipole alignment is also shown to increase the UV sensitivity of the ferroelectric.

Alignment of defect dipoles in polycrystalline ferroelectrics

Using electron paramagnetic resonance (EPR), we show the alignment of defect dipoles along the direction of the spontaneous polarization in polycrystalline BaTiO3 ceramics by subjecting the capacitors to a dc bias at elevated temperatures. The alignment is demonstrated to occur via orientation dependent EPR signals in the polycrystalline perovskite lattice. The alignment of the defect dipoles is found to strongly enhance the light sensitivity of the defects. This observation may have important implications for photorefractive applications.