Effect of electrical fatigue on the electromechanical behavior and microstructure of strontium modified lead zirconate titanate ceramics

The fatigue behavior of strontium doped lead zirconate titanate (PSZT) ceramics is investigated. Different compositions near the antiferroelectric–ferroelectric (AF–F) phase boundary are synthesized by tape casting and sintering route. The influence of electric field-induced AF to F phase transition on piezoelectric and strain behaviors is studied. Very high maximum polarization (～41 μC/cm²) and ultrahigh strain (0.8%) are seen for some of the PSZT compositions near the morphotropic phase boundary. The samples are subjected to low frequency (30 Hz) electric field up to 10⁷ cycles. Although the maximum polarizations of most of the PSZT ceramics showed fatigue-free behavior (less than 10% degradation), the strains in most of them showed degradation as high as over 50%. Electron microscopy of the fractured surface of the electrically cycled samples showed some intergranular fracture below the electroded surface. Results indicated that diffusion of silver electrode into the PSZT ceramics is responsible for the electro-coloration and degradation in strain response with fatigue cycles. Thermal annealing and removal of the damaged layer under the electrode showed the complete recovery of the strain to its original as-sintered value. X-ray diffraction technique is used before and after the fatigue cycles to investigate the influence of electrical cycles on the changes at the crystal structure level, which are also related to the fatigue-induced changes to electromechanical properties of PSZT ceramics.     

The Prager Medal Lecture: micromechanics and some aspects of phase fields in ferroelectric crystals

Ferroelectric crystals represent a very unique class of multifunctional materials. In addition to strong electromechanical coupling, there exist ferroelectric domains, which can be switched through the application of an electric field or mechanical stress. In addition, these crystals possess several distinct crystal structures over a wide temperature range. As such, phase transition can take place as the crystals are cooled down or heated up, without or with the additional effect of stress or electric field. Domain switch and phase transition represent the two fundamental processes that can affect their microstructures and electromechanical characteristics. In this lecture, we highlight the applications of micromechanics to bulk ferroelectrics and phase fields to nano-structures. The starting points of micromechanics are crystal structures and the Eshelby mechanics, whereas those of the phase fields are the time-dependent Ginzburg–Landau kinetic equation and the Landau–Ginzburg–Devonshire energy density function. We explain how micromechanics can have wide applicability in the study of domain switch and phase transition, and change of dielectric constants, of bulk BaTiO₃ crystals, and how phase fields can provide the nano-scale domain patterns, influence of surface tension on free-standing BaTiO₃ nano-thin films, and grain-size dependence of ferroelectric characteristics in nano-grained BaTiO₃ polycrystals.     

Optical Study of Domains in Antiferroelectric Ceramics

Antiferroelectric ceramics are now highly focused as giant strain actuator materials. In this study, domain formation in antiferroelectric lead zirconate based ceramics (Pb_0.99Nb_0.02[(Zr_0.6Sn_0.4)_1-yTi_y]_0.98O₃) was observed dynamically under an electric field at various temperatures using a high-resolution charge-coupled-device (CCD) microscope system. Field induced polarization and field induced strain were also measured. No domain was observed without an electric field, but clear domains appeared with an electric field due to the phase transition from an antiferroelectric to a ferroelectric state. Results of the optical study can explain well the electrical properties. The behavior of the field induced domains showed a shape memory effect and the domains were well oriented compared with normal ferroelectric ceramics.     

Influence of Zr-substitution on phase transitions character in polycrystalline Ba(Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript>

The results of X-ray diffraction, scanning electron microscopy, and dielectric measurements performed for polycrystalline Ba(Ti_1−x Zr _x )O₃ are presented. Data from these measurements show a change of the type of paraelectric–ferroelectric (PE–FE) phase transition (PT) from sharp PT (x < 0.10), through diffuse phase transition (0.10 ≤ x ≤ 0.20), to relaxor type one (x = 0.30). The lack of structural PT at the temperature T _m (the temperature of the maximum of real part of the electric permittivity ε′) for 0.10 ≤ x ≤ 0.30, suggest that this PE–FE transition is not connected with the change of crystal structure. A phase angle of about Φ ≈ −90°, between electric current and applied voltage, suggests the occurrence of polar regions (clusters) below 400 K. The change of these clusters polarizability and their lability at the T _m is postulated as possible origin of the observed maximum of ε′.     

Dielectric,ferroelectric, and field-induced strain properties of Ta-doped 0.99Bi0.5(Na0.82K0.18)0.5TiO3–0.01LiSbO3 ceramics

Ta-doped 0.99Bi_0.5(Na_0.82K_0.18)_0.5TiO₃–0.01LiSbO₃ (BNKTT–LS) ceramics were prepared through a conventional mixed oxide solid-state sintering route. Partial substitution of Ta for Ti decreased the dielectric constant and depolarization temperature. The dielectric curves, polarization and strain hysteresis loops demonstrated that the incorporation of Ta stabilized the canonical relaxor phase of BNKT–LS ceramics leading to the degradation of piezoelectric and ferroelectric responses. The destabilization of field-induced ferroelectric order at x = 0.013 was accompanied by substantial enhancement in strain level. A unipolar field-induced strain of 0.39 % with a normalized strain (S _max/E _max =  $ d_{33}^{*} $ ) of 650 pm/V was achieved at a driving field of 6 kV/mm. The observed large strain can be attributed to the non-ergodic relaxor phase at zero electric field that transformed into an ergodic relaxor phase under the influence of the applied electric field.     

Defect-dipole-induced two important macroscopic effects: The dielectric relaxation and the ferroelectric aging in hybrid-doped (Ba,Ca)TiO3 ceramics

To observe the dielectric relaxor behavior and the ferroelectric aging effect, the structural and electrical properties of (Ba_0.90Ca_0.10)(Ti_1−xCa_x)O_3−x (BCT-C) ceramics prepared using a conventional dry route were investigated. With increased concentration of Ca on the B-site in BCT-C ceramics, the tetragonal phase is decreased while the multiphases with the cubic pervoskite structure as a major constituent is increased and remained predominant in the BCT-C ceramics with x > 0.03. The increased amount of Ca on the B-site causes the high temperature phase transition to shift to the low temperature and results in an increase in the degree of diffusion and relaxation of the BCT-C ceramics. The observed dielectric relaxation behavior may be understood by a defect-dipole formed random electric field induced domain state. The aging effect controlled by the migration of mobile oxygen vacancies is rarely observed in aged BCT-C ceramics while it can be markedly observed in Bi-doped BCT ceramics. Based on the microscopic mechanism of the aging effect, the possible reasons for the aging effect are given.     

Effect of Sn content on the dielectric and piezoelectric properties of the ternary system (0.975-<Emphasis Type="Italic">y</Emphasis>)BaTiO<Subscript>3</Subscript>–0.025SrTiO<Subscript>3</Subscript>–<Emphasis Type="Italic">y</Emphasis>BaSnO<Subscript>3</Subscript>

Design of the polymorphic phase composition in the (0.975-y)BaTiO₃–0.025SrTiO₃–yBaSnO₃; BT-ST-yBSn ternary system was based on the ferroelectric phase diagram. The dense ceramic of BT-ST-yBSn, with y = 0.00, 0.02, 0.04, 0.06, 0.08 and 0.10 compositions, was fabricated successfully via the solid-state reaction method. The effect of Sn substitution on the ferroelectric phase transition and piezoelectric properties was explored in order to achieve high-performance piezoelectric properties. All of the ceramics exhibited pure perovskite structures. Orthorhombic to tetragonal phase transition was evidenced clearly as a function of Sn content. The orthorhombic to tetragonal phase transition shifted close to ambient temperature by increasing the Sn content. The coexistent tetragonal and orthorhombic phases were exhibited at the composition, y = 0.04, and showed outstanding dielectric and piezoelectric properties, maximum relative permittivity (ε _{r max}) of 11500 and piezoelectric coefficient (d ₃₃) of 450 pC/N. An outstanding reversible strain of about 0.12%, with a normalized piezoelectric coefficient (S _max /E _max) of 1280 pm/V at a low electric field (10 kV/cm), was observed clearly at the composition of the coexistent phase. The BT-ST-BSn ceramics are the most promising candidate for lead-free piezoelectric materials.     

Optical properties of (1−<Emphasis Type="Italic">x</Emphasis>)PbZn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>−<Emphasis Type="Italic">x</Emphasis>PbTiO<Subscript>3</Subscript> single crystals

The temperature dependence of the optical transmission and small-angle light scattering with and without applied constant electric field was studied in relaxor single crystals of 0.91PbZn_1/3Nb_2/3O₃-0.09PbTiO₃ (PZN-PT 91/9) and 0.93PbZn_1/3Nb_2/3O₃-0.07PbTiO₃ (PZN-PT 93/7) solid solutions in the region of two phase transitions: (i) from cubic paraelectric to tetragonal ferroelectric phase at T=T _c and (ii) from tetragonal ferroelectric to rhombohedral ferroelectric phase at T=T _rt. In the absence of external electric field, only the phase transition at T _c proceeds in both PZN-PT 91/9 and PZN-PT 93/7 crystals according to a percolation mechanism and is accompanied by the appearance of a sharp maximum in the small-angle light scattering intensity curve. In PZN-PT 93/7 crystals, the application of a relatively weak electric field induces an additional percolation type phase transition at T _rt.     

Polarization-electric field relations of ferroelectric/antiferroelectric layered ceramics in Pb(Nb, Zr, Sn, Ti)O3 system

Ferroelectric/antiferroelectric bi-layer ceramics with different ferroelectric and antiferroelectric phase thickness ratio (FE/AFE thickness ratio) in Pb(Nb, Zr, Sn, Ti)O₃ system were prepared and characterized. With increasing the maximum external electric field from 0 to 40 kV/cm, polarization-electric field relation was always ferroelectric-like or underwent an antiferroelectric-ferroelectric transition, depending on the FE/AFE thickness ratio. All layered ceramics showed ferroelectric-like hysteresis loops with maximum external electric field of 40 kV/cm, and much higher remanent polarizations were attained than those of the ferroelectric/antiferroelectric heterostructures reported previously.     

Electric response of Pb0.99[(Zr0.90Sn0.10)0.968Ti0.032]0.98Nb0.02O3 ceramics to the shock-wave-induced ferroelectric-to-antiferroelectric phase transition

Shock-wave-enforced ferroelectric (FE)-to-antiferroelectric (AFE) phase transition releases a large electrical polarization, having application in pulse power technology. In the present work, the depoling currents under shock wave compression were investigated in Pb_0.99[(Zr_0.90Sn_0.10)_0.968Ti_0.032]_0.98Nb_0.02O₃ (PZST) ceramics with composition close to the FE/AFE phase boundary. Shock wave was generated by gas-gun and propagated in a direction perpendicular to the remanent polarization. It was found that the shock pressure promoted the phase transition under the short-circuit condition. The shock pressure dependence of the released charge was associated with the evolution of FE-to-AFE phase transition. The onset of phase transition was about 0.40 GPa and complete transformation occurred at 1.23 GPa. However, the released charge decreased with increasing load resistance. The reason may be that the electric field suppresses the phase transition in uncompressed zone and/or shock induces conductivity in compressed zone. Results lay the foundation for application of PZST ceramics in shock-activated power supply.     

Direct current electric field adjustable phase transformation behavior in (Pb,La)(Zr,Ti)O3 antiferroelectric thick films

(Pb,La)(Zr,Ti)O₃ antiferroelectric 1.4 μm-thick films have been prepared on Pt (111)/Ti/SiO₂/Si(100) substrates by sol–gel process. The structures and dielectric properties of the antiferroelectric thick films were investigated. The films displayed pure perovskite structure with (100)-preferred orientation. The surface of the films was smooth, compact and uniform. The antiferroelectric (AFE) characterization have been demonstrated by P (polarization)-E (electric field) and C(capacitance)-V (DC bias) curves. The AFE–ferroelectric (FE) and FE-to-paraelectric (PE) phase transition were also investigated as coupling functions of temperature and direct current electric field. With the applied field increased, the temperature of AFE-to-FE phase transition decreased and the FE-to-PE phase shifted to high temperature. The AFE-to-FE phase transition was adjustable by direct current electric field. (Pb,La) (Zr,Ti) O₃ antiferroelectric films have broad application prospects in microelectromechanical systems because of the phase transition.     

Origin of the large strain response in tenary SrTi0.8Zr0.2O3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free piezoceramics

The perovskite oxides (1 ? x)Bi_0.5(Na_0.9K_0.1)_0.5TiO₃–xSrTi_0.8Zr_0.2O₃ (SZT1000x, x = 0, 0.2, 0.4, 0.6, 0.8, and 1 %) were prepared via the conventional solid-state reaction method. The room temperature ferroelectric P–E loops coordinate with polarization current density J–E curves illustrated the changes of ferroelectric domains and polar nanoregions under different driving fields exhaustively. The composition and electric field dependent strain behavior of this system were investigated to develop a lead-free piezoelectric material with a large strain response at a lower electric field. A large strain of 0.44 % (S _max/E _max = 744 pm/V) at an applied field of 50 kV/cm was obtained at the composition of 0.6 mol% SZT. Temperature-dependent hysteresis measurements reveal the primary origin of the large strain is due to the presence of a nonpolar phase at a zero field. Upon the application of an electric field, the nonpolar phase that can easily transform into a long-range ferroelectric phase, and then brings the system back to its unpoled state once the applied electric field is removed. Notably, the electric field required to deliver large strains is reduced to 40 kV/cm while the S _max/E _max reached up to 717 pm/V, indicating that the developed material is highly promising for actuator applications.     

The effect of thermal ageing on carbon fibre-reinforced polyetheretherketone (PEEK)

The static and dynamic mechanical properties of carbon fibre-reinforced PEEK (APC-2) laminates subjected to long-term thermal ageing and cycling treatments have been studied using three-point bend flexure tests. Results are discussed with respect to morphological changes and degradation analysis. S/N curves were modelled using fatigue modulus degradation data. Ageing laminates at high temperatures, for long time periods, between the glass transition temperature, T _g, and the melting temperature, T _m, caused a significant reduction in mechanical properties. However, for short ageing periods, a crystal-perfection process occurs which enhanced the low stress level fatigue resistance of both laminate geometries.     

Giant strain in Nb-doped Bi0.5(Na0.82K0.18)0.5TiO3 lead-free electromechanical ceramics

The effect of Nb substitution on the crystal structure, ferroelectric, and electric field induced strain properties of Bi_0.5(Na₈₂K_0.18)_0.5TiO₃ (BNKT) ceramics has been investigated. The coexistence of rhombohedral and tetragonal phases was found in undoped BNKT ceramics, however, Nb doping induced a phase transition to a pseudocubic phase with high electrostriction coefficients. When 3 mol% Nb was substituted on Ti ions, the electric field induced strain was markedly enhanced up to S_max/E_max = 641 pm/V, which is higher than those previously reported on non-textured lead-free electromechanical ceramics.     

Electric field–temperature phase diagram of sodium bismuth titanate-based relaxor ferroelectrics

The electric field–temperature phase diagrams of three bismuth sodium titanate-based relaxor ferroelectrics are reported, namely 0.94(Na_1/2Bi_1/2TiO₃)–0.06(BaTiO₃), 0.80(Na_1/2Bi_1/2TiO₃)–0.20(K_1/2Bi_1/2TiO₃) and 0.75(Na_1/2Bi_1/2TiO₃)–0.25(SrTiO₃). Relaxor behavior is demonstrated by temperature-dependent dielectric permittivity measurements in the unpoled and poled states, as well as by the field-induced phase transition into a ferroelectric phase from the relaxor phase. From temperature-dependent thermometry measurements, we identified the threshold electric field to induce the ferroelectric phase and obtained the released latent heat of the phase transition. We determined the nonergodic and ergodic relaxor phase temperature range based on the absence or presence of reversibility of the relaxor to ferroelectric transition. For all three compositions, the electric field–temperature phase diagram was constructed and a critical point was identified. The constructed electric field–temperature phase diagrams are useful to find optimum operational ranges of ferroelectrics and relaxors for electromechanical and electrocaloric applications.     

Electrochemical Aging Protocol Governed Capacity Losses and Structure Degradations in Layered LiNi0.8Co0.1Mn0.1O2 Oxides

The comparatively poor endurance of Ni-rich cathode materials restricts their application in high-energy lithium-ion batteries. A thorough understanding of the degradation characteristics of such materials under complex electrochemical aging protocols is required to further improve their reliability. In this work, the irreversible capacity losses of LiNi_0.8Mn_0.1Co_0.1O₂ under different electrochemical aging protocols are quantitatively evaluated via a well-designed experiment. In addition, it is discovered that the origin of irreversible capacity losses is highly related to electrochemical cycling parameters and can be divided into two types. Type I is heterogeneous degradation caused by low C-rate or high upper cut-off voltage cycling and features abundant capacity loss during H2-H3 phase transition. Such capacity loss is attributed to the irreversible surface phase transition that limits the accessible state of charge during the H2-H3 phase transition stage via the pinning effect. Type II is fast charging/discharging induced homogeneous capacity loss that occurs consistently throughout the whole phase transition time. This degradation pathway shows a distinctive surface crystal structure, which is dominated by a bending layered structure rather than a typical rock-salt phase structure. This work offers detailed insight into the failure mechanism of Ni-rich cathodes and provides guidance on designing long-cycle life, high-reliability electrode materials.     

The effect of electric field on metal-insulator phase transition in vanadium dioxide

The effect of a strong electric field on the metal-insulator phase transition in vanadium dioxide was studied. It was found that the field application to a silicon-silicon oxide-silicon nitride-vanadium dioxide (Si-SiO₂-Si₃N₄-VO₂) heterostructure shifts the critical temperature of this transition toward lower values under conditions when the thermal effects are minimized. Numerical modeling of the current-voltage characteristics of the vanadium dioxide-based sandwich-type switches measured at various temperatures in the range from 15 to 350 K showed that the applied electric field influences the critical concentration and temperature of the phase transition.     

Carbon composites based on multi-axial multi-ply stitched preforms – Part 6. Fatigue behaviour at low loads: Stiffness degradation and damage development

This article studies the fatigue properties of a carbon-fibre cross-ply non-crimp fabric reinforced epoxy composite. Tensile–tensile fatigue cycling was carried out at load levels corresponding to the onset of damage in a static tensile test, in machine, cross and bias direction. Specimens in machine and cross direction did not fail up to 10⁶ cycles; specimens in bias direction had an average fatigue life N_max of 3 × 10⁵ cycles. Stiffness degradation in bias direction samples was found to be more severe than in machine or cross direction. Damage development in the samples was studied by means of X-ray photography and appears to show remarkable resemblance to the development under a static tensile test and can be qualitatively compared to the behaviour of non-stitched UD laminates. Post-fatigue tensile tests were done at various stages of the fatigue life. Small differences in damage onset strain level can be found. Failure strain of bias direction tested samples shows significant decrease upon cycling.     

Magnetic transitions and magnetocaloric effects in Fe0.75Mn1.35As

In Fe_0.75Mn_1.35As compound, a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase can be induced above its phase transition temperature T _s = 165 K by an external magnetic field, which leads to large magnetocaloric effects around T _s. The sign of the magnetic entropy change ΔS _M in the Fe_0.75Mn_1.35As compound is negative, not as expected as an inverse magnetocaloric effect, and the maximum value of ΔS _M is 4.2 J/kg K at 167.5 K for a magnetic field change of 5 T. Although it induces an irreversible lattice expansion, the cycling of a magnetic field does not induce an irreversible change in the magnetic transitions and magnetocaloric behaviors. The antiferromagnetism-related metamagnetic transitions with a large magnetic entropy change may provide with an opportunity in searching novel materials for magnetic refrigeration.     

High Magnetic Field Dependence of Magnetodielectric Properties in Sr2CoSi2O7 Crystal

We have investigated magnetic and dielectric properties of åkermanite Sr₂CoSi₂O₇ single crystal in which the orbital hybridization is controllable by external magnetic fields. Sr₂CoSi₂O₇ shows the electric polarization that is originated from asymmetric orbital hybridization induced by magnetic field. This induced electric polarization was observed even at temperatures considerably higher than the magnetic transition temperature. The polarization data obtained for the paramagnetic phase are scaled with the square of the induced magnetization. By the discussion of the crystal symmetry, the space group of a polar state of åkermanite materials is probably Cmm2.