After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

Piezoelectric and piezomagnetic materials and their function in transducers

Review of State of Art of Smart Structures and Integrated Systems

3 – Piezoelectric and Piezomagnetic Materials and Their Function in Transducers

Ferroelectric and ferroelastic switching cause ferroelectric ceramics to depolarize and deform when subjected to excessive electric field or stress. Switching is the source of the classic butterfly shaped strain vs electric field curves and the corresponding electric displacement vs electric field loops [1]. It is also the source of a stress—strain curve with linear elastic behavior at low stress, non-linear switching strain at intermediate stress, and linear elastic behavior at high stress [2, 3]. In this work, ceramic lead lanthanum zirconate titanate (PLZT) is polarized by loading with a strong electric field. The resulting strain and polarization hysteresis loops are recorded. The polarized sample is then loaded with compressive stress parallel to the polarization and the stress vs strain curve is recorded. The experimental results are modeled with a computer simulation of the ceramic microstructure. The polarization and strain for an individual grain are predicted from the imposed electric field and stress through a Preisach hysteresis model. The response of the bulk ceramic to applied loads is predicted by averaging the response of individual grains that are considered to be statistically random in orientation. The observed strain and electric displacement hysteresis loops and the nonlinear stress—strain curve for the polycrystalline ceramic are reproduced by the simulation.

Ferroelectric/ferroelastic interactions and a polarization switching model

The amount of 90° reorientation during poling was determined from mechanical strains measured during the poling process. With tetragonal lead titanate zirconate 53% of the possible 90° reorientation occurred during poling, but this figure dropped to 44% upon removal of the poling field. With barium titanate the figures are only 17% and 12%, respectively. Comparison of the polarization of poled polycrystalline barium titanate with that for single crystals indicates that 180° reorientation is virtually perfect. Application of very high compressive stress parallel to the polar axis causes 90° switching of nearly all aligned domains, and, therefore, removes virtually all polarization. Curves of released charge as function of mechanical strain are nearly linear, but curves of released charge as function of stress are strongly nonlinear. Application of high compressive stress perpendicular to the polar axis also causes 90° domain reorientation and a reduction in the total polarization of the ceramic. This domai...

Domain Processes in Lead Titanate Zirconate and Barium Titanate Ceramics

Piezoelectric ceramic elements in high‐power acoustic transducers are subjected to high static as well as dynamic stress. This is particularly true of well‐matched transducers operating in deep water, since the static stress in the piezoelectric element may be several times the water pressure. The present study was undertaken in an effort to determine the effects of static compressive stress on the piezoelectric properties of two commercial lead titanate zirconate compositions, PZT‐4 and PZT‐5, and of two barium‐titanate compositions, commercial Ceramic B (a barium calcium titanate), and the composition 88 wt% barium titanate, 12 wt% lead titanate (BaPb12Ti). The permanent effects of stress exposure, determined at zero stress after exposure to a given stress, were found to be more severe with stress parallel to the polar axis than with perpendicular stress, as expected. Under maintained stress, however, the effects of perpendicular stress are more severe. PZT‐4 and BaPb12Ti, generally better suited for use as radiating transducers, show effects dependent upon exposure time but independent of the number of stress cycles. Ceramic B and PZT‐5 show effects dependent upon the number of stress cycles and less dependent upon the total period of stress exposure. Of the compositions tested, PZT‐4 and BaPb12Ti were least affected by high static stress, suffering relatively little from exposure to stress as high as 15 000 psi. Of these two compositions. PZT‐4 has markedly higher coupling (k33∼0.64 compared to 0.365) and therefore offers higher transducer bandwidth.

Effects of High Static Stress on the Piezoelectric Properties of Transducer Materials

The ratio of piezoelectric coefficients $\ensuremath{-}\frac{{d}_{33}}{{d}_{31}}$ was found to decrease from about 2.5 for polarized barium titanate ceramic of high porosity to 2.2 for porosity near zero. The ratio increases up to 2.9 for ceramic with 2% porosity containing 12 weight% calcium titanate. For single crystal barium titanate the ratio $\ensuremath{-}\frac{{d}_{33}}{{d}_{31}}$ is 2.5, which checks well with x-ray data for the ratio of thermal strain in the $c$ and $a$ directions. The value for ceramics of lowest porosity does not, therefore, approach that for the single crystal.

Dependence of the Ratio of Piezoelectric Coefficients on Density and Composition of Barium Titanate Ceramics

: Design of transducers for lower frequency, high power density, and deeper submersion puts increasing demands on piezoelectric transducer materials It has been demonstrated that appropriate lead titanate zirconate ceramics can operate under high bias stress and at high power levels without failure However, changes in physical properties, especially in permittivity as a result of a change in static compressive stress and changes in permittivity and tan delta with drive amplitude, have become increasingly of concern The mechanisms whereby the nonlinear characteristics of ferroelectric ceramics bring about dependence of physical properties on electric and mechanical stress are discussed The basic differences in behavior under high electric and mechanical stress of alkaline earth-substituted lead titanate zirconate ('hard') and donor- doped lead titanate zirconate ('soft') are outlined Data are given also for a new 'very hard' lead titanate zirconate, PZT-8 Stabilizing treatments which tend to minimize pressure effects are described Emphasis on behavior under environmental and operating conditions is placed on the following: (1) Variations in the piezoelectric constant d sub 33 and permittivity with static compressive stress parallel to the polar axis (2) Changes of permittivity with time under maintained stress (3) Changes of permittivity and tan delta with electric field at various levels of compressive stress parallel to the polar axis (4) Variations in permittivity and d sub 33 with stress cycling (stress parallel to the polar axis)

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Behavior of piezoelectric ceramics under various environmental and operation conditions of radiating sonar transducers

Rochelle salt 45° X‐cut square plates were exposed to Co60 radiation of 8.6×103 to 2.6×107 R and reactor radiation up to 2.4×106 R gamma plus 3.5×1015 neutrons/cm2. Measurements were made over a temperature range including both Curie points (−30° to +30°C) of dielectric constant and loss, elastic stiffness coefficients c44D, and mechanical Q. In addition, double‐crystal x‐ray rocking curves were made above the upper Curie point to check line broadening. All the data indicate an initial improvement in crystal perfection (Curie points are sharper, linewidths narrower) up to 8.6×104 R, with subsequent degradation (Curie point and linewidth broadening: finally coalescence of Curie points) including elimination of the ferroelectric phase after 2.6×106 R Co60 radiation or 2.4×106 R gamma plus 3.5×1015 neutrons/cm2 reactor radiation. The Q rose with radiation dosage as the ferroelectric phase disappeared, dropping only at the highest dosage. Destruction of the ferroelectric phase by radiation is compared to simi...

Helmut H. A. Krueger

Papers

Domain Processes in Lead Titanate Zirconate and Barium Titanate Ceramics

Effects of High Static Stress on the Piezoelectric Properties of Transducer Materials

Dependence of the Ratio of Piezoelectric Coefficients on Density and Composition of Barium Titanate Ceramics

Behavior of piezoelectric ceramics under various environmental and operation conditions of radiating sonar transducers

Radiation Damage and the Ferroelectric Effect in Rochelle Salt