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

In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(Zr
 x
 Ti1-x
 )O3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO3 and (Bi1/2Na1/2)TiO3-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi1/2Na1/2)TiO3-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

This article presents in a unified way, the various optimization criteria used by researchers for optimal placement of piezoelectric sensors and actuators on a smart structure. The article discusses optimal placement of piezoelectric sensors and actuators based upon six criteria: (i) maximizing modal forces/moments applied by piezoelectric actuators, (ii) maximizing deflection of the host structure, (iii) minimizing control effort/maximizing energy dissipated, (iv) maximizing degree of controllability, (v) maximizing degree of observability, and (vi) minimizing spill-over effects. Optimal piezoelectric sensor and actuator locations on beam and plate structures for each criterion and modes of interest are presented in a tabular form. This technical review has two objectives: (i) practicing engineers can pick the most suitable philosophy for their end application and (ii) researchers can come to know about potential gaps in this area.

Optimization Criteria for Optimal Placement of Piezoelectric Sensors and Actuators on a Smart Structure: A Technical Review

(Bi 1/2 Na 1/2 )TiO 3 (BNT) based lead-free incipient piezoceramics are promising candidates for actuator applications due to their giant electromechanical strains originating from the reversible relaxor-ferroelectric phase transition. To decrease the electric field required for this large strain, the templated grain growth method was employed to prepare 〈1 0 0〉 oriented 0.91Bi 1/2 Na 1/2 TiO 3 –0.06BaTiO 3 –0.03AgNbO 3 piezoceramics using plate-like BNT templates. Textured samples provided a high unipolar strain of 0.38% and a corresponding large signal piezoelectric coefficient, d 33 ∗ of 766 pm/V at 5 kV/mm, which are 78% higher than the values of the randomly oriented ones. The enhanced electric-field-induced strain at relatively lower field was attributed primarily to the facilitated phase-transition to form a long range ferroelectric order along the 〈1 0 0〉 direction. It was also found that the textured piezoceramics exhibited significantly reduced frequency dependence in the unipolar strain behavior at room temperature, resulting from the decreased electric field required for the relaxor-ferroelectric phase transition.

Preparation and enhanced electrical properties of grain-oriented (Bi1/2Na1/2)TiO3-based lead-free incipient piezoceramics

The temperature sensitivity of enhanced piezoelectricity in (K,Na)NbO3 perovskite-based lead-free piezoelectrics is one of the technical bottlenecks impeding practical applications. This work found a recipe that possesses a piezoelectric strain coefficient d33* as high as 600 pm V−1 over a wide temperature range up to 100 °C, which is additionally insensitive against the frequency of an applied electrical field. Electrical property measurement and nanostructure observation revealed that the high performance combining high piezoelectricity and excellent stability benefits from a diffused phase transition (DPT), which is closely associated with the transition from ferroelectric to relaxor behavior induced by the local structural disorder due to the complicated compositional modification. The high-resolution transmission electron microscopy images of Moire fringes imply the existence of polar nanoregions (PNRs) that are responsible for the relaxor-like electrical behavior in the present materials. The findings and understanding obtained in this work should be valuable for pushing lead-free piezoelectric ceramics towards practical applications.

High-performance lead-free piezoelectrics with local structural heterogeneity

Multilayer piezoelectric actuators are used in fuel injectors due to their quick response, high efficiency, accuracy, low power consumption, and excellent repeatability. Experimental results for soft lead zirconate titanate (PZT) stack actuators have shown that a significant amount of heat is generated when they are driven under high frequency and/or high electric-field magnitudes, both of which occur in fuel injectors. Self-heat generation in these actuators, mainly caused by losses, can significantly affect their reliability and piezoelectric properties, and may also limit their application. Other studies have demonstrated that at large unipolar electric-field magnitudes, displacement–electric-field loss (displacement hysteresis) shows a direct relation with polarization–electric-field loss (dielectric hysteresis). In this paper, a simplified analytical self-heating model is presented. The model directly relates self-heating in multilayer piezoelectric actuators to displacement–electric-field loss (displacement hysteresis). The model developed is based on the first law of thermodynamics, and accounts for different parameters such as geometry, magnitude and frequency of applied electric field, duty cycle percentage, fuel type, and environmental properties. The model shows reasonable agreement with experimental results at low and high electric-field magnitudes.

https://iopscience.iop.org/article/10.1088/0964-1726/18/4/045008/pdf

Self-heat generation in piezoelectric stack actuators used in fuel injectors

Piezoelectric actuators are increasingly used in fuel injectors due to their quick response, high efficiency, accuracy, and excellent repeatability. Current understanding of their thermo-electro-mechanical performance under dynamic driving conditions appropriate for fuel injection is, however, limited. In this paper, the thermo-electro-mechanical performance of soft Lead Zirconate Titanate (PZT) stack actuators is experimentally investigated over a temperature range of -30°C to 80°C, under driving electric fields of up to 2.0 kV/mm (using an AC drive method and a biased DC offset), different frequencies, and a constant preload of about 5 MPa. Experimental results show that the dynamic stroke of the actuators increases with the magnitude and frequency of the applied electric field, as well as ambient temperature. The dynamic stroke was also found to increase with decreased driving field rise time, which is equivalent to increasing the driving field frequency. At driving frequencies lower than the resonance...

Thermo-electro-mechanical Performance of Piezoelectric Stack Actuators for Fuel Injector Applications

Quasi-static thermo-electro-mechanical performance of annular and solid cylindrical piezoelectric actuators was studied by experimental means under electric fields varying from 0.3 to 1.8 kV mm −1 with a preload of 4.6 MPa over the temperature range of −30 to 125 ◦ C. It was found that, for both annular and solid actuators, the electrically induced stroke increases steadily with temperature. Under electric fields larger than 1.0 kV mm −1 , a nonlinear transition zone exists in the stroke-temperature plot over the temperature range 25-50 ◦ C. The dielectric constant was also found to increase with temperature. Preload dependence of displacement was measured up to 42 MPa at room temperature and found to be negligible below 30 MPa. A mathematical model that includes the linear piezoelectric effect and 90 ◦ domain switching effect was used to model the experimental results. The model shows reasonable agreement with experimental results at low and high driving fields. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0964-1726/17/1/015049/pdf

Quasi-static thermo-electro-mechanical behaviour of piezoelectric stack actuators

Performance of piezoelectric actuators in a hydrogen environment: Experimental study and finite element modelling

Piezoelectric actuators are increasingly used in modern fuel injectors due to their quick response, high efficiency, and
excellent repeatability. Current understanding of the thermo-electro-mechanical performance of piezoelectric actuators
under dynamic driving fields is very limited. In this paper, the dynamic thermo-electro-mechanical performance of Lead
Titanate Zirconate (PZT) stack actuators is experimentally studied over the temperature range of -30 °C to 80 °C, under a
driving field of up to 2.4 kV/mm (with an AC drive method) and a constant preload of about 5MPa. Sinusoidal and
trapezoidal driving fields with rise times varying from 0.1 ms to 2 ms are applied. It is found that dynamic stroke
increases steadily with the temperature. Under driving frequencies lower than the resonance frequency of the testing
system ( ≈ 500Hz), the electric field-strain behavior under different temperatures is very similar to the quasi-static results
obtained previously. In the case of a trapezoidal pulse, decreasing the rise time is found to be equivalent to increasing the
frequency.

D. Mumford

Papers

Self-heat generation in piezoelectric stack actuators used in fuel injectors

Thermo-electro-mechanical Performance of Piezoelectric Stack Actuators for Fuel Injector Applications

Quasi-static thermo-electro-mechanical behaviour of piezoelectric stack actuators

Performance of piezoelectric actuators in a hydrogen environment: Experimental study and finite element modelling

Dynamic thermo-electro-mechanical performance of piezoelectric stack actuators