Unipolar Fatigue Research Articles

Enhanced unipolar electrical fatigue resistance and related mechanism in grain-oriented Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3 piezoceramics

Piezoceramics with high and fatigue-resisted piezoelectric properties are strongly desired for actuator applications. In this work, textured Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3 ceramics with Lotgering factor F001∼98% were fabricated by templated grain growth technique. Strong [001]c-grain orientation (f∼90% and r∼0.22) of the textured ceramics effectively produced about 230% enhanced piezoelectric coefficient d33* (i.e., Smax/Emax) and substantially improved unipolar electrical fatigue resistance. Unipolar polarization Pmax and d33* of the textured ceramics were nearly maintained up to 106 unipolar cycles, while 19% and 14% degradations were respectively observed from randomly oriented counterparts. Especially, normalized d33* of the textured ceramics shows better unipolar fatigue resistance than those of piezoceramics reported previously. Much lower bipolar strain asymmetry γs (∼4%) was observed from the textured samples fatigued after 106 unipolar cycles as compared to γs∼23% for randomly oriented counterparts. While charged defect accumulation model described the serious fatigue deteriorations in randomly oriented ceramics, the current work revealed that substantially enhanced unipolar fatigue resistance of the textured ceramics is mainly associated with the inherent fatigue anisotropy, weakened local bias fields owing to both enhanced domain mobility and lower defect density near grain boundaries/interfaces, and increased intrinsic contribution due to more tetragonal content. These superior characteristics suggest the great potential of textured ceramics for high-performance and robust actuator applications.

Improving the piezoelectric strain and anti-reduction properties of K0.5Na0.5NbO3-based ceramics sintered in a reducing atmosphere.

Lead-free 0.945K0.48Na0.52Nb0.96Ta0.04O3-0.055BaZrO3 + 6%MnO + xZrO2 piezoelectric ceramics sintered in a reducing atmosphere were prepared by conventional solid-state reaction methods. The use of the ZrO2 dopant resulted in an increase in the rhombohedral (R) phase in orthorhombic (O)/R coexisting phases. Nonstoichiometric ZrO2 dopant addition could effectively improve the anti-reduction properties of KNN-based ceramics via controlling the oxygen vacancy concentration. In particular, 2% mol nonstoichometric ZrO2 dopant addition could improve the activation energy of the grain boundary (Egb) via increasing the grain boundary thickness. The addition of the ZrO2 dopant could improve the fatigue resistance of the unipolar piezoelectric strain of 0.945K0.48Na0.52Nb0.96Ta0.04O3-0.055BaZrO3 + 6%MnO ceramics. The optimum inverse piezoelectric coefficient of ceramics at x = 0.01 reached up to ∼465 pm V-1 at a low driving electric field E of 20 kV cm-1 at room temperature, and the temperature stability of reached 155 °C. After 106 unipolar fatigue cycles, the β value of 0.945KNNT-0.055BZ + 6Mn + xZr ceramics could be preserved to more than 86%. The 0.945K0.48Na0.52Nb0.96Ta0.04O3-0.055BaZrO3 + 6%MnO + xZrO2 ceramic is a lead-free material with great potential to be applied in the fabrication of multilayer ceramic actuators with Ni inner electrodes in the future.

Effect of polarization fatigue on the electrocaloric effect of relaxor Pb0.92La0.08Zr0.65Ti0.35O3 thin film

The electrocaloric effect represents an adiabatic temperature change or isothermal entropy change upon the application or removal of an external electric field. However, the effect of the repetitive bipolar/unipolar electric field on the electrocaloric effect in ferroelectrics is not well understood. In this work, the electrocaloric effect after both bipolar and unipolar electrical cycling in Pb0.92La0.08Zr0.65Ti0.35O3 relaxor thin films has been indirectly evaluated using the Maxwell relationship. It was found that at room temperature, the electrocaloric temperature change ΔT under higher electric fields decreases gradually from a positive value to a negative one with the increase in the bipolar/unipolar fatigue cycle number. Intriguingly, around the Curie temperature, the positive ΔT of the film after bipolar/unipolar cycling was found to decrease under low or intermediate electric fields, while it remains almost unchanged under high electric fields. This behavior is attributed to the pinning effect of defects caused by phase decomposition during both bipolar and unipolar fatigue. Note that the increase in polarization by temperature induced domain depinning close to room temperature after the fatigue process is manifested by using the Maxwell relation indirectly. Our work further reveals the origin of both the electrocaloric effect and ferroelectric electrical fatigue and is important for the applications of practical electrocaloric refrigerators.

Cyclic properties and impedance analysis of (Li,Na,K)NbO3-based multilayered piezoceramics

(Li,Na,K)NbO3-based multilayered piezoceramics were characterized by performing unipolar polarization electric field measurements and impedance spectroscopy. Subjecting an as-poled sample to up to 107 cycles in an applied 4 kV mm−1 unipolar cyclic field at 100 °C reduced its polarization by 8.9%. The impedance spectra of as-sintered, as-poled, and fatigued samples were acquired and analyzed. Two relaxation processes reported to occur in single-layered (Na,K)NbO3-based ceramics were identified. They occurred at inner domains and interfaces, including domain walls (DWs) and grain boundaries, below the Curie temperature (TC, 450 °C). The electrical properties of the samples were evaluated by two resistance-constant phase element (2 R-CPE) equivalent circuit analysis. The results indicated that DW pinning was the primary cause of unipolar fatigue.

Bipolar and unipolar fatigue property in Bi0.5Na0.5TiO3–Bi0.5K0·5TiO3–SrTiO3 lead-free piezoelectric ceramics

The bipolar and unipolar fatigue property of lead-free piezoelectric solid solution Bi0.5Na0.5TiO3–Bi0.5K0·5TiO3–SrTiO3(BNKST) was studied. Under a bipolar electric field of 4 kV/mm at 10 Hz, the remnant and maximum polarization, maximum strain value of all the studied compositions changed less than 2% after 106 bipolar cycles, while the coercive field exhibited different behavior for BNKST composition with different ferroelectric order states. The unipolar fatigue results also indicated that the BNKST with SrTiO3 concentration of 0.15 had excellent fatigue property after 107 unipolar cycles under 4 kV/mm. The underlying mechanism was discussed. The excellent fatigue property and large strain response make this system suitable for piezoelectric actuator applications.

Domain disruption and defect accumulation during unipolar electric fatigue in a BZT-BCT ceramic

0.5Ba(Zr0.2Ti0.8)O30.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) is a promising lead-free piezoelectric ceramic with excellent piezoelectric properties (e.g., d33 > 600 pC/N). As potential device applications are considered, the electric fatigue resistance of the ceramic must be evaluated. In this Letter, electric-field in situ transmission electron microscopy is employed to study the microstructural evolution in the BZT-BCT polycrystalline ceramic during unipolar cycling. It is shown that the large ferroelectric domains are disrupted and replaced with accumulated defect clusters and fragmented domains after 5 × 104 unipolar cycles. In this fatigued state, the grain becomes nonresponsive to applied voltages.

High-power impedance spectroscopy for lead-free alkali niobate piezoceramics

Impedance spectroscopy for piezoceramics under high-power application was investigated. Impedance spectra under bipolar high electric field were completely different below and above TC. Below TC, high-power impedance spectra could be successfully fitted to a three resistance-constant phase element (3 R-CPE) equivalent circuit. Above TC, however, a 2 R-CPE equivalent circuit was sufficient to fit with high-power impedance spectra. For this reason, there was another relaxation factor caused by high-power application in the ferroelectric phase. From the high-power impedance spectra below TC, the nonlinear response of capacitance under a high electric field was calculated. Hence, domain wall motion would appear as a relaxation factor. Subsequently, high-power impedance spectra before and after cyclic unipolar fatigue treatment were compared. Considering the assumption that domain wall motion appeared as a relaxation factor, the capacitance caused by domain wall motion was decreased after cyclic fatigue. In conclusion, we believe that domain wall pinning could be evaluated by separating other electrical properties. In this work, the measurement possibility by impedance spectroscopy for high-power application for piezoceramics was investigated.

Influence of composition on the unipolar electric fatigue of Ba(Zr0.2Ti0.8)O3‐(Ba0.7Ca0.3)TiO3 lead‐free piezoceramics

AbstractThe lead‐free (1−x)Ba(Zr0.2Ti0.8)O3‐x(Ba0.7Ca0.3)TiO3 system is considered as promising candidate for the replacement of lead‐based piezoceramics in actuation applications, during which electric fatigue is a major concern. This issue was addressed in this work, where the unipolar fatigue resistance of three (1−x)Ba(Zr0.2Ti0.8)O3‐x(Ba0.7Ca0.3)TiO3 compositions with different crystallographic structures (rhombohedral, orthorhombic, and tetragonal) was evaluated. Strain asymmetry and development of an internal bias field were observed in all compositions. The decrease in the remanent polarization and the large signal piezoelectric coefficient after 107 unipolar cycles was found to lie between 6%‐12% and 2%‐13%, respectively. The most pronounced fatigue was observed for the orthorhombic composition, which has the largest extrinsic contribution to strain. On the other hand, the best fatigue resistance was observed for the tetragonal composition, which has a predominantly intrinsic strain response. The correlation of fatigue resistance with strain mechanism was corroborated with determination of the Rayleigh parameters and changes in the domain morphology after cycling as confirmed by piezoresponse force microscopy.

Temperature dependence of mechanical degradation in lead-free alkali niobate ceramics under unipolar loading

Piezoelectric actuators are used under high electric fields, which can cause crack formation as a consequence, and therefore require sufficient mechanical properties to guarantee long lifetime and reliability. We measured the flexural strength before and after unipolar fatigue under different temperatures of undoped Li0.06Na0.52K0.42NbO3 (LNKN6) and the same compound with additives (LNKN6-A). Before performing fatigue tests, the flexural strength σ0 was 111MPa and 177MPa for LNKN6 and LNKN6-A, respectively. After electric cycling over a certain amount of time, only slight changes in the piezoelectric properties were observed. However, mechanical degradation appeared in LNKN6 whereas LNKN6-A remained the as-poled flexural strength throughout each test. The results suggest that a smaller grain size is an advantage in case of crack formation during unipolar fatigue. In addition, LNKN6 showed after cycling a bimodal Weibull distribution. Under room temperature, the highest mechanical degradation of about 35% was observed. Noteworthy is the fact that after 50°C, less mechanical degradation was observed. It is suggested, that phase transition from tetragonal to orthorhombic lead to improved mechanical properties.

Unipolar Fatigue Behavior of BCTZ Lead‐Free Piezoelectric Ceramics

(Ba,Ca)(Ti,Zr)O3 lead‐free piezoelectric ceramics have been considered to be one of the most potential lead‐free alternatives for PZT in the room‐temperature range. The stability of the piezoelectric performance during unipolar cycling is investigated in this study. It is found that the unipolar fatigue behavior is similar to soft PZT. Developments of bias field, offset polarization, asymmetry in strain, and dielectric hysteresis loops are observed during bipolar measurements. The changes are mainly contributed to the migration of charge carriers to the grain boundaries driven by the unscreened depolarization field. Redistribution of the accumulated charge carriers by bipolar electric cycling or thermal annealing can significantly recover the unipolar fatigued state. The unipolar strain response stabilized after 1000 cycles at 0.053% for an electric field of 0.6 kV/mm (d33*= 883 pm/V), which is a good characteristic for actuator applications.

Temperature dependence of unipolar fatigue behavior of lead-free alkali niobate ceramics

Temperature dependence of unipolar fatigue behavior of lead-free alkali niobate ceramics

Robust CaZrO3-modified (K, Na)NbO3-based lead-free piezoceramics: High fatigue resistance insensitive to temperature and electric field

Robust resistivity against electrical cycling at not only ambient conditions but also enhanced temperatures is an essential requirement for high-end actuator applications. In this study, the temperature and electric field dependence of unipolar fatigue behaviors of CaZrO3-modfied (K, Na)NbO3 lead-free piezoceramics were investigated. The space charge accumulation during unipolar cycling is responsible for the build-up of internal bias field, the dynamics of which with respect to temperature and driving field can be described using a model based on Maxwell-Wagner relaxation process. Besides, clamping of domain walls can be inferred by comparing the large and small signal permittivity. Most intriguingly, the unipolar strain exhibits a fatigue-free behavior even at elevated temperatures, rendering the material exceptionally suitable for actuator applications.

Fatigue-free unipolar strain behavior in CaZrO3 and MnO2 co-modified (K,Na)NbO3-based lead-free piezoceramics

The unipolar fatigue behavior of CaZrO3 and MnO2 co-modified (K,Na,Li)(Nb,Ta)O3 lead-free piezoceramics was investigated systematically. The well-known charge agglomeration model is shown to explain the overall changes observed during unipolar fatigue, such as the development of bias field as well as the anisotropy in bipolar strain hysteresis and field-dependent dielectric permittivity. In addition, it is found that the unipolar strain exhibits only small degradation within 3% at the field amplitude of 2 kV/mm up to 107 cycles. This exceptionally good fatigue resistance is identified due to the presence of additional process, assigned as a “softening” effect that competes against the usual fatigue effect.

Cycling stability of lead-free BNT–8BT and BNT–6BT–3KNN multilayer actuators and bulk ceramics

This study presents the electromechanical properties and cycling stability of lead-free piezoelectric materials 0.92(Bi1/2Na1/2)TiO3–0.08BaTiO3 (BNT–8BT) and 0.91(Bi1/2Na1/2)TiO3–0.06BaTiO3–0.03(K0.5Na0.5)NbO3 (BNT–6BT–3KNN). Both bulk samples as well as multilayer actuators (MLA) with internal Ag/Pd (70/30) electrodes were successfully processed from both materials. Electromechanical characteristics in the non-fatigued state and after different numbers of unipolar fatigue cycles are provided, representing the first direct comparison of the fatigue resistance of lead-free bulk ceramics and the corresponding MLAs. At a maximum field of 6 kV/mm and a frequency of 50 Hz, BNT–8BT MLA delivered a maximum strain of 0.07% and displayed excellent cycling stability. BNT–6BT–3KNN MLA provided a higher strain of 0.15% initially but degraded during cycling and exhibited break down after 107 cycles. Furthermore, the frequency dependence of strain and the self-heating during cycling were investigated. The temperature increase is limited only to 2 °C in BNT–8BT MLA and 13 °C in BNT–6BT–3KNN MLA.

Repetitive impact loading causes local plastic deformation in wood

The relationship between the impactor velocity and the amount of strain localization in a single impact compression of cellular solids is known. However, few studies report on the effects of repeated high frequency compression. We therefore studied the mechanical behavior of Norway spruce, a cellular viscoelastic material, before, during, and after cyclic high frequency, high strain rate, compression. A custom made device applied 5000-20 000 unipolar (constrained compression and free relaxation) fatigue cycles with a 0.75 mm peak-to-peak amplitude at 500 Hz frequency. The consequences of this treatment were quantified by pitch-catch ultrasonic measurements and by dynamic material testing using an encapsulated Split-Hopkinson device that incorporated a high-speed camera. The ultrasonic measurements quantified a stiffness modulus drop and revealed the presence of a fatigued low modulus layer near the impacting surface. Such a localized plastic deformation is not predicted by classical mechanics. We introduce a simple model that explains several changes in the mechanical properties caused by fatiguing. The high speed images indicated pronounced strain localization in the weakest (thinnest walls) parts of the earlywood layers, and revealed strain propagation as a function of time. We present a hypothesis explaining why there is a fatigued layer formed in a piece of wood that has sustained cyclic compression and free relaxation.

Piezoelectric properties and unipolar fatigue behavior of KNN-based Pb-free piezoceramics

Single-phase perovskite ceramics were synthesized using a Pb-free (K(0.44)Na(0.52)Li(0.04))(Nb(0.86)Ta(0.10)Sb(0.04))O(3) (LF4) piezoelectric material both with and without CuO as a dopant additive. Bipolar hysteresis measurements showed a relatively high remanent polarization (20 μC/cm(2), 16 μC/cm(2)) and maximum polarization (25 μC/cm(2), 21 μC/cm(2)) values were found for undoped LF4 and 0.2 wt% CuO-doped LF4, respectively. Unipolar fatigue behavior for this system was found to decrease strongly with increased applied testing fields and increased test durations. For undoped LF4, the maximum polarization values were stable after 10 cycles after testing for 100,000 cycles duration. For the CuO-doped samples, increasing the testing field to 3E(C) resulted in twice the decrease in P(MAX) (¿32%) compared with the 2E(C) tests at 10(6) cycles (-17%). At 2E(C) testing for CuO doping, polarization decreases continually through 10(7) cycles, with the decreases in P(MAX) and d(33) both reaching a maximum (-22% and -30%, respectively) after 10(7) cycles. In the CuO-doped samples, the fatigue is exacerbated because of the influence of space charge on the increased number of defects present.

Temperature and driving field dependence of fatigue processes in PZT bulk ceramics

The temperature- and field-dependent degradation properties of bulk Pb(Zr,Ti)O 3 material (PZT) under a unipolar electric field were investigated. Unipolar cycling leads to the build-up of an internal bias field based on the agglomeration of charges at grain boundaries. A simple model was developed which describes the general dynamics of unipolar fatigue and its dependence on temperature and driving field. Comparing the large and small signal permittivity before and after fatigue led to the conclusion that domain walls became clamped by the agglomerated charges. This clamping effect could be visualized by transmission electron microscopy (TEM). Additionally, the TEM investigations revealed that unipolar fatigue leads to a weakening of the microstructure and to the development of microcracks.

Effect of Ferroelectric Long‐Range Order on the Unipolar and Bipolar Electric Fatigue in Bi1/2Na1/2TiO3‐Based Lead‐Free Piezoceramics

The fatigue behavior of the lead‐free piezoelectric Bi1/2Na1/2TiO3–BaTiO3−K0.5Na0.5NbO3 (BNT‐BT‐KNN) under bipolar and unipolar electrical loading is investigated. Special attention is paid to the effect of a decreasing ferroelectric order which was affected by an increase in KNN‐content. It is found that, in mainly ferroelectric systems with low KNN‐content, bipolar cycling leads to domain wall pinning and formation of microcracks, while in mainly non‐ferroelectric systems with high KNN‐content fatigue can be rationalized by the repeated field‐induced phase transition occurring during each field cycle. Under unipolar load, fatigue in low‐KNN systems is dominated by domain wall pinning and the formation of a space charge field. Only the latter effect is present in non‐ferroelectric high‐KNN systems, which fatigue to a smaller degree.

Enhanced Unipolar Fatigue Resistance in Ferroelectric Pb(Ni1/3Nb2/3O3)–PbTiO3 Ceramics Prepared via Glycerol‐Assisted Solid‐State Reaction

It is experimentally observed that PNN–PT ceramics processed with glycerol possess superior unipolar fatigue resistance; but the same ceramics processed without glycerol are vulnerable to unipolar fatigue. For PNN–PT–glycerol fatigued by 5 × 107 unipolar cycles, there are no losses in the maximum polarization (Pmax) and remnant polarization (Pr). But for PNN–PT, these losses are ∼14% and 22%, respectively. In addition, strain asymmetryγS and internal bias field Ebias of PNN–PT–glycerol are much less than those of PNN–PT. However, the ferroelectric properties can be recovered by bipolar cycles and thermal annealing; and such recovery is much easier and larger in PNN–PT–glycerol than in PNN–PT ceramics. Scanning electron microscopy micrographs show that the significant loss in properties of the fatigued PNN–PT ceramics is accompanied by a measurable number of severely damaged grains in the samples, especially in the region near the electrode surface, whereas the much less damaged grains in the PNN–PT–glycerol ceramics contribute to their good fatigue performance.

Bipolar and Unipolar Fatigue of Ferroelectric BNT‐Based Lead‐Free Piezoceramics

The bipolar and unipolar fatigue behavior of the lead‐free piezo‐ and ferroelectric 0.94Bi1/2Na1/2TiO3–0.06BaTiO3 (94BNT–6BT) was investigated. To obtain a complete profile of the fatigue behavior, both large‐signal (polarization, strain) and small‐signal (d33, ɛ33) properties were measured at room temperature. The results indicate that the material suffers a significant degradation in both large‐ and small‐signal properties after cycling, with most of the fatigue occurring in the first 104 cycles of the fatigue test.