Soft PZT Material Research Articles

Effect of the electric conductivity on the modeling of the poling process of ferroelectric components

Piezoceramic materials, despite being semiconductors with a high but finite ohmic resistance, are generally modeled as perfect insulators. In this work the influence of the electric conductivity on the poling process of piezoceramic materials is investigated. To solve the electromechanically coupled boundary value problem, a reduced form of the Maxwell equations is implemented inside a hybrid finite element formulation. In this formulation the electric displacement is available as nodal quantity (i.e. degree of freedom) which is used instead of the electric field to determine the evolution of the remanent polarization. The material model is fully ferroelectric/ferroelastic coupled, whereas the material behavior is described by a set of yield functions and the history dependence is stored in internal state variables representing the remanent polarization and the remanent strain. The simulation of poling processes for three different components are presented. First and second, the poling of a radially poled hollow cylinder and of a stack actuator is investigated. Here, a residual electric field appears after poling, leading to significant time-dependent changes of stresses and deformation due to subsequent charge transportation processes. The third example is a bending actuator composed of two layers of hard-PZT and soft-PZT material. It is shown that considering the electric conductivity new strategies for the poling process can be developed in order to improve the bending actuation. In this way, we show the importance of considering charge transportation processes in simulations of the poling of ferroelectrics, which seems not to have been recognized so far.

Non-linear electromechanical behaviour of piezoelectric bimorph actuators: Influence on performance and lifetime

Piezoelectric bimorph bender actuators find application number of areas, ranging from automotive to health care. High voltage operation in harsh environments poses ever more stringent demands on functionality and lifetime. In these high performance benders, the trade-off between functionality and lifetime becomes of increasing importance, and a model-based understanding is imperative for further tailoring actuator performance. An important issue in this is the non-linear electromechanical coupling and mechanical poling/depoling processes. PZT materials are known to exhibit non-linear stiffness when strained. This effect is attributed to domain re-orientation and can result in mechanical depoling. In bimorph benders where only one of the PZT members is actuated, the non-linear stiffness effect results in higher deflections than are predicted by linear elastic models. This effect is larger for soft PZT materials than for hard PZT materials. In this work, the non-linear behaviour of piezoelectric bimorphs was characterized by dynamic mechanical thermal analysis (DMTA). Unimorph benders were characterized in different configurations. Two different PZT materials (one soft, one hard) were used. The non-linear stiffness behaviour is included in a model for piezoelectric multimorphs. The multimorph model predictions are compared to measured deflection and blocking force of bimorph benders made from soft PZT material. The model incorporating non-linear stiffness predicts deflection and blocking force values more accurately than the linear model for the situation studied. Attention is paid to the stresses induced during poling of the bimorphs and their effect on performance and lifetime. © 2008 Springer Science+Business Media, LLC.

Thermal effects on mechanical grinding-induced surface texture in tetragonal piezoelectrics

The effect of temperature on grinding-induced texture in tetragonal lead zirconate titanate (PZT) and lead titanate (PT) has been investigated using in situ x-ray diffraction (XRD) with an area detector. In contrast with previous results on electrical poling, mechanically-ground PT and soft PZT materials retain strong ferroelastic textures during thermal cycling, even after excursions to temperatures slightly above the Curie temperature. The relationship between the residual stresses in the surface region, caused by grinding, and those resulting from domain wall motion is elucidated by in situ texture measurements obtained during thermal cycling.

Domain structures and nonlinear mechanical deformation of soft Pb(Zr xTi 1− x)O 3 (PZT) piezoelectric ceramic fibers

The nonlinear mechanical deformation of two tetragonal variants of soft, morphotropic lead zirconate–titanate Pb(Zr x Ti 1− x )O 3 (PZT) piezoelectric ceramic fiber was correlated with studies of the crystal structure, domain structure, and microstructure. Fibers with dense hierarchical microtwin structures having spatial dimensions of ∼10 nm showed an additional mode of stress accommodation as evidenced by a larger strain accumulation and greater anelastic response relative to fibers with conventional twin structures consisting primarily of 90° and 180° tetragonal ferroelectric–ferroelastic domains. The differing natures of the domain structures in the two variants of soft PZT materials were associated with differences in the defect chemistry. The observed sensitivity of the nonlinear mechanical deformation to the presence of spatially inhomogeneous microtwin structures is expected to be of particular importance in the engineering of flexible fiber composites and other transducers based on soft PZT piezoelectric materials intended for use under conditions of high amplitude dynamic deformation.

Dielectric and piezoelectric performance of soft PZT piezoceramics under simultaneous alternating electromechanical loading

This study is focused on the investigation of the fundamental properties of piezoceramics under loading conditions simulating the in-service environment of high-strain actuators. High electric field induced polarization and strain responses were experimentally evaluated for a commercial soft PZT material subjected to cyclic mechanical load with different mean stresses and amplitudes. When the stress is applied in-phase with electrical loading, the polarization and strain outputs are found to monotonically decrease with an increase in stress amplitude, until mechanical loading completely impedes the piezoelectric response. An inverse effect occurs for the out-of-phase electromechanical loading tests, in which the polarization and strain outputs increase with increasing stress amplitude. In general, the enhanced polarization and strain responses are accompanied by an unfavorable increased hysteresis and nonlinearity. An attempt has been made to explain the experimental findings by simultaneously taking into account the effects of elastic deformation, domain reorientation, and piezoeffects.

Characterization Techniques for Porous Piezoelectric Materials

The applicability of characterization techniques of ferroelectric dense materials to porous ferroelectric materials has been investigated. In fact nonlinearity effects are highly increased in porous materials, due to the discontinuities at the internal boundaries. Resonant and quasi-static techniques, based on direct or converse piezoelectric effect, were employed to obtain the physical properties of various types of ferroelectric porous materials. Soft PZT materials, obtained through the “starch consolidation method,” with porosities ranging from ≈30% to ≈45% and high hydrostatic figure of merit, acoustical matching and mechanical resistance, as requested in many applications, were characterized. The properties of the dense and porous materials were measured and compared and it was evidenced that the same characterization techniques used for dense ferroelectric materials can be safely and reliably employed, with just slight modifications, for porous ferroelectric materials with porosities as high as 45%.

Tip Deflection and Blocking Force of Soft PZT‐Based Cantilever RAINBOW Actuators

A piezoelectric/electrostrictive RAINBOW actuator is a monolithic bending device consisting of an electromechanically active layer and a reduced passive layer formed in a high‐temperature reduction treatment. When the piezoelectric or electrostrictive layer is driven under an electric field or when the environmental temperature changes, bending deflection is produced because of the constraint of the reduced inactive layer or because of the thermal expansion coefficient difference of the two layers. In this study, general analytical expressions relating tip deflection, blocking force, and equivalent moment with an applied electric field and temperature change are derived for a cantilevered RAINBOW actuator. It is shown that optimal actuator performance can be achieved in the RAINBOW actuator by choosing a suitable thickness ratio of the reduced layer to the PZT layer. A series of RAINBOW cantilever actuators have been experimentally prepared from high‐density, soft, lead zirconate titanate (PZT) ceramics. Different reduction layer thickness is obtained by adjusting the processing parameters, such as reduction temperature and time. The measured results on tip deflection and blocking force agree well with theoretical prediction under a weak electric field. However, when a high driving electric field is used, deviation is observed, which can be attributed to a nonlinear piezoelectric response and a nonlinear elastic behavior associated with soft PZT materials under high driving electric fields.

Change of the weak-field properties of Pb(ZrTi)O3 piezoceramics with compressive uniaxial stresses and its links to the effect of dopants on the stability of the polarizations in the materials

The properties of several Pb(ZrTi)O3 (PZT) piezoceramics under compressive uniaxial stresses were characterized. It was observed that uniaxial stresses have a marked effect on the soft PZT materials, including reducing the piezoelectric coefficients and depoling the samples at relatively low stress levels. The effect of the uniaxial stresses on the properties of hard PZT's is more complicated because the domain structure of the materials can be changed substantially without depoling the samples. Therefore, under a compressive stress along the poling direction, the piezoelectric and electromechanical coupling factor can be increased markedly due to both the increased non-180° domain boundary motions and the deaging effect. In addition, the experimental results support the notion that the difference between a hard PZT and a soft PZT lies in the types of defects introduced by dopants. Immobile defects create frustrations in the lattice and result in a soft behavior, and mobile defects stabilize the polarization and produce a hard behavior.