Electric-field-induced volume change and room temperature phase stability of (Bi1/2Na1/2)TiO3-x mol. % BaTiO3 piezoceramics

Phase stability of (1 - x) (Bi1/2Na1/2)TiO 3-x BaTiO3 (0 ??? x ??? 0.15) under electric field was investigated by measuring volume changes during a bipolar poling cycle. The unique nature of field-dependent phase stability with three distinctive regions is revealed by comparative studies using commercial soft PZT and relaxor PLZT. For x ??? 0.06 and x 0.13, similarly with PZT and PLZT, the axial strain expands with the contracting radial strain, but the former results in a remanent volume demonstrating an electric-field-induced phase transition. For 0.08 ??? x ??? 0.12, this field-induced phase transition is distinguished by negligible radial contractions implying polarization rotation. A "poling-induced" morphotropic phase boundary forms at x = 0.07.open433

Quenching-induced circumvention of integrated aging effect of relaxor lead lanthanum zirconate titanate and (Bi1/2Na1/2)TiO3-BaTiO3

The effect of quenching on the dielectric properties of ceramic Pb0.92La0.08(Zr0.65Ti0.35)(0.98)O-3 (PLZT 8/65/35) and (Bi1/2Na1/2)TiO3-6 mol. %BaTiO3 (BNT-6BT) was investigated after annealing at a wide range of temperatures. The dielectric permittivity showed that the magnitude and shape profile of the permittivity were significantly affected by the quenching process in comparison to furnace-cooled specimens. We propose that the changes originate from a circumvention of integrated aging that takes place during cooling process. A comparison between PLZT 8/65/35 and BNT-6BT allowed us to conclude that two different types of polar nanoregions exist in BNT-6BT and the transition between them peaks at around 300 degrees C.open3

Frequency and temperature dependence of actuating performance of Bi1/2Na1/2TiO3-BaTiO3 based relaxor/ferroelectric composites

Recently, composites of relaxors (matrix) and either ferroelectric or nonergodic relaxor (seed) were proposed as a solution to resolving one of the main drawbacks of incipient piezoceramics, namely the requirement for high driving electric fields. In this study, we investigate the temperature and frequency dependence of the actuating performance of Bi1/2Na1/2TiO3-BaTiO3-based composites. Apart from the reduction of driving field, the composite architecture offers an extra degree of freedom for tailoring the temperature stability for different operational conditions for actuators. High strain values appear to be sensitive especially to driving frequency. This is originated by the time-dependent process of the coalescence of polar nanoregions. In effect, proximity of driving field and poling field leads to high strain sensitivity. Hence, the driving electric field needs to be adjusted in order to meet the desired frequency specifications for given applications.open0

Cyclic Fatigue Crack Growth In PZT Piezoelectric Ceramics Induced By Mechanical Load

In this work the crack propagation behaviour of a commercially obtained lead zirconate titanate (PZT) piezoceramic is studied under monotonic and cyclic loading. Piezoelectric ceramics contain an electrical dipole in their crystal structure such that when a mechanical load is applied an electrical potential is formed, and vice versa. Permanent electrical domain orientation may be induced by the application of a large electrical potential in a process known as poling. Piezoelectric ceramics are used for applications such as actuators, sonar transducers and microphones. These and most other application involve high levels of cyclic loading and fatigue degradation is will known. However, cyclic fatigue to date has only been considered under electrical loading despite the fact that most components are subjected to high levels of mechanical load. Crack growth studies are rare. In the vicinity of a crack tip, local stress causes switching of the crystal domains leading to dilation perpendicular to the crack propagation direction. This explains the observed crack growth resistance (R-curve) toughening which is characterised in this work. Furthermore, it is shown that the extent of toughening is dependent upon the crack propagation direction relative to the poling direction. When a cyclic mechanical load is applied, subcritical crack growth is shown to occur below the plateau value of the R-curve and at rates which are independent of poling direction. A measurement of static fatigue rates show that a true cyclic fatigue degradation process exists. Another experiment which measured the R-curve behaviour as a function of intermittent loading time showed that the observed fatigue behaviour could not be explained by reverse switching of the crystal domains. The question arises therefore as to the mechanism of cyclic fatigue degradation in PZT and whether it is the same under both cyclic electrical and mechanical loading. In both cases it is currently unknown however a parallel study has shown that plastic strain accumulates in bulk PZT under mechanical loading in a strain-softening type process. The link between this and crack propagation behaviour will be discussed

Origin of the large strain response in (K0.5Na0.5)NbO3-modified (Bi0.5Na0.5)TiO3-BaTiO3 lead-free piezoceramics

The mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92(Bi0.5Na0.5)TiO3-0.06BaTiO(3)-0.02(K0.5Na0.5)NbO3 [S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rodel, Appl. Phys. Lett. 91, 112906 (2007) was investigated. The validity of the previously proposed mechanism that the high strain comes both from a significant volume change during the field-induced phase transition, from an antiferroelectric to a ferroelectric phase and the domain contribution from the induced ferroelectric phase was examined. Monitoring the volume changes from the simultaneously measured longitudinal and transverse strains on disk-shaped samples showed that the phase transition in this specific material does not involve any notable volume change, which indicates that there is little contribution from a volume change due to the phase transition to the total strain response. Temperature dependent hysteresis measurements on unpoled samples of a nearby ferroelectric composition, 0.93(Bi0.5Na0.5)TiO3-0.06BaTiO(3) -0.01(K0.5Na0.5)NbO3 demonstrated that the origin of the large strain is due to the presence of a nonpolar phase that brings the system back to its unpoled state once the applied electric field is removed, which leads to a large unipolar strain.open1539

Influence of electric fields on the depolarization temperature of Mn-doped (1-x)Bi1/2Na1/2TiO3-xBaTiO(3)

The transition between induced long-range order and relaxor-like behavior upon heating is investigated in lead-free (1-x)Bi1/2Na1/2(Ti0.995Mn0.005)O-3-xBa(Ti0.995Mn0.005)O-3 piezoceramics with x = 0.03, 0.06, and 0.09 (BNT-100xBT:Mn). Temperature-dependent permittivity epsilon'(T) and thermally stimulated depolarization currents (TSDC) of poled samples were measured under identical heating conditions to clarify the depolarization mechanism. In both methods, the influence of electric bias fields on the transition temperature was investigated. Fields applied in the poling direction shift the transition to higher temperatures, with corresponding results in epsilon'(T) and TSDC measurements. While the response of transition temperature to external fields displays a similar trend in all investigated compositions, the shape of TSDC is clearly connected with the composition and, hence, the crystal symmetry of the sample. Furthermore, the comparison of epsilon'(T) and TSDC data reveals a systematic shift between transition temperatures obtained with the two different methods.open322

Two-stage processes of electrically induced-ferroelectric to relaxor transition in 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO(3)

The stability of electrically induced long-range ferroelectric order in a relaxor 0.94(Bi1/2Na1/2) TiO3-0.06BaTiO(3) ceramic material has been investigated by temperature-dependent X-ray diffraction and electrical property measurements. The depolarization and ferroelectric-to-relaxor transition are identified as separate and discrete processes. It is observed that the induced ferroelectric domains first lose their ferroelectric/ferroelastic texture coincident with a peak signal in the thermally induced depolarization current. With further increase in temperature, the detextured ferroelectric domains are dissociated into nanoscale entities. This fragmentation marks the ferroelectric-to-relaxor transition. It is suggested that the ferroelectric-to-relaxor transition has features of a second order phase transition.open302

Morphotropic phase boundary in (1-x)Bi0.5Na0.5TiO3-xK(0.5)Na(0.5)NbO(3) lead-free piezoceramics

The electromechanical behavior of (1-x)Bi0.5Na0.5TiO3-xK(0.5)Na(0.5)NbO(3) (BNT-KNN) lead free piezoelectric ceramics is investigated for 0 <= x <= 0.12 to gain insight into the antiferroelectric-ferroelectric (AFE-FE) phase transition on the basis of the giant strain recently observed in BNT-based systems. At x approximate to 0.07, a morphotropic phase boundary (MPB) between a rhombohedral FE phase and a tetragonal AFE phase is found. While the piezoelectric coefficient is largest at this MPB, the total strain further increases with increasing KNN content, indicating the field-induced AFE-FE transition as the main reason for the large strain.open756

Fatigue-free unipolar strain behavior in CaZrO3 and MnO 2 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.open4

Enhanced bipolar fatigue resistance in CaZrO3-modified (K,Na) NbO3 lead-free piezoceramics

The bipolar fatigue behavior of (K,Na) NbO3 (KNN)-based lead-free piezoceramics was investigated. A comparative analysis demonstrated that CaZrO3-modified KNN ceramics exhibited highly enhanced bipolar fatigue resistance due to the reduced lattice distortion (c/a ratio) and coexistence of orthorhombic and tetragonal phases. The hypothesis was verified by systematical studies of cycle-dependent large and small signal parameters and micro-morphologies. It was identified that mechanical stress induced microstructure damage beneath the electrodes renders the KNN ceramics to be vulnerable to bipolar cycling; while the mild fatigue behavior for CaZrO3-modified ones mainly originates from a thermally reversible domain wall pinning.open8