Lead-free piezoceramics - Where to move on?

Lead-free piezoceramics aiming at replacing the market-dominant lead-based ones have been extensively searched for more than a decade worldwide. Some noteworthy outcomes such as the advent of commercial products for certain applications have been reported, but the goal, i.e., the invention of a lead-free piezocermic, the performance of which is equivalent or even superior to that of PZT-based piezoceramics, does not seem to be fulfilled yet. Nevertheless, the academic effort already seems to be culminated, waiting for a guideline to a future research direction. We believe that a driving force for a restoration of this research field needs to be found elsewhere, for example, intimate collaborations with related industries. For this to be effectively realized, it would be helpful for academic side to understand the interests and demands of the industry side as well as to provide the industry with new scientific insights that would eventually lead to new applications. Therefore, this review covers some of the issues that are to be studied further and deeper, so-to-speak, lessons from the history of piezoceramics, and some technical issues that could be useful in better understanding the industry demands. As well, the efforts made in the industry side will be briefly introduced for the academic people to catch up with the recent trends and to be guided for setting up their future research direction effectively.ope

Correlation between structure and Rayleigh parameters in the lead-free piezoceramic (1-x)Ba(Ti0.88 Sn0.12)O3-x(Ba0.7Ca0.3)TiO3

Composition dependent Rayleigh and structural analysis was carried out on the lead-free piezoceramics (1-x)(BaTi0.88Sn0.12)-x(Ba0.7Ca0.3)TiO3 at room temperature. The system exhibits tetragonal (P4mm) structure for x > 0.21, rhombohedral (R3m) for x < 0.13 and orthorhombic (Amm2) for 0.13<x<0.21. Rayleigh analysis suggests that the irreversible contribution to the dielectric response is enhanced in the single phase orthorhombic compositions in the vicinity of the R3m-Amm2 and Amm2-P4mm phase boundaries, and not in compositions exhibiting phase coexistences (x = 0.12 and 0.22). We also found a correspondence between the irreversible Rayleigh parameter and the coercive field in this system.Comment: 18 pages 5 figure

(Co)-Doping of Lead-Free Piezoceramics

This work encompassed an in depth investigation of the defect chemistry and piezoelectric properties of Bi0.5(Na(1-w)Kw)0.5Ti(1-x-y)CuxVyO3 with w = 0.1, 0.2 and 0.3 and x and y < 0.01, synthesized by a solid state process. A pseudocubic perovskite phase with no detectable impurities was verified by X-ray diffraction, while doping tended to reduce the rhombohedral character of Bi0.5(Na0.9K0.1)0.5TiO3 (BNKT10) ceramics. Doping BNKT10 was shown to result in more homogenous grain size distributions, elucidated by analyzing scanning electron microscopy images. Small and large signal piezoelectric measurements also revealed that the tetragonal phase of Bi0.5(Na0.7K0.3)0.5TiO3 is disproportionally affected by doping. The transition temperature from ferroelectric to relaxor phase (Tf-r), the coercive field, and the remanent polarization drop for compositions with high K-content, while the maximum strain is mostly increased. The general trend of decreasing Tf-r in these high K tetragonal materials can be rationalized by A-site defects and the resulting increased distribution of random fields. Impedance spectroscopy at resonance indicates a high electromechanical coupling factor of planar samples in thickness mode (kt up to 0.56), which is useful for sensing applications in combination with the observed low mechanical quality factor (QM) of 9 in thickness direction. Impedance spectroscopy at elevated temperatures revealed that doping resulted in a decrease in the activation energy (EA) of 110±10 meV from 1.37 eV of undoped Bi0.5(Na0.9K0.1)0.5TiO3 for both single element doping with Cu or V, as well as simultaneous doping with both Cu and V. Furthermore, the resistivities of doped BNKT10 ceramics (3.3×103 Ωm to 1.3×10E7 Ωm) were consistently lower than those of the undoped BNKT10 ceramics (1.6×10E5 Ωm to 2.0×10E8 Ωm). The variation of the oxygen partial pressure from 0.21 bar (synthesis condition) to 1 bar, 2.1×10E-6 bar and < 10-18 bar likewise lead to consistently decreased EA and resistivities with values as low as 0.23 eV and 1.9×10E0 Ωm to 2.7×10E3 Ωm, respectively. Electron paramagnetic resonance spectroscopy (EPR) and X-ray photoelectron spectroscopy (XPS) indicated the presence of Cu2+, V4+ and V5+ in the doped ceramics, while lower oxidation states of Cu and V could be excluded. EPR spectroscopy indicated Cu2+ and V4+ in a rhombic environment with major distortion in one direction and minor distortion in the other directions. While V4+ is introduced into the B-site of the bulk, Cu2+ was shown to dominantly segregate from the bulk, most likely at the grain boundary due to the creation of a liquid phase during sintering. This correlates with the increasing density with increasing Cu content and the observed trend of a lowered QM with increasing Cu content, presumably due to leakage. The solubility limit of Cu in the bulk was found to be < 0.05 at.%. All doped BNKT10 ceramics, containing Cu, were shown to possess Cu in two different rhombic electronic environments, namely CuO6 and CuO4. For V doping, a discernable, albeit, small (<400 meV) increase in Fermi level was determined by XPS, suggesting a donor doping effect corroborated by the lower resistivities and EA of these samples likely due to charge compensation induced electrons. The overall defect chemistry of the Cu and V doped ceramics was revealed to be influenced by (I) bismuth, sodium and potassium acceptor vacancies, resulting from the host material, (II) additional n-type doping induced electrons, (III) few bulk Cu2+ acceptor centers, resulting from Cu doping, (IV) V5+ donor states, introduced through V doping, and (V) in all cases the concentration and mobility of oxygen vacancies

Frequency-dependence of large-signal properties in lead-free piezoceramics

The dependence of large signal properties of (1-x)(0.81Bi 1/2Na 1/2TiO 3-0.19Bi 1/2K 1/2TiO 3)-xBi(Zn 1/2Ti 1/2)O 3 with x 0.02, 0.03, and 0.04 on the measurement frequency was investigated for a wide range of frequencies from 0.1 Hz to 100 Hz. A significant frequency dispersion in the characteristic parameters representatively maximum and coercive values was denoted. On extension with the temperature dependent dielectric permittivity measurement, it was shown that the observed frequency dependence is primarily correlated with the dynamics of field-induced phase transition from a relaxor state to a long-range ferroelectric state. Increasing the substitutional disorder introduced by Bi(Zn 1/2Ti 1/2)O 3 addition was demonstrated to pronounce the frequency dependence. It was proposed that the change be due to the increase in random fields and consequent dominance of ergodicity, based on the frequency-dependent hysteresis measurements at an elevated temperature above so-called induced-ferroelectric-to-relaxor transition temperature.open8

Electrical And Mechanical Properties of BZT − xBCT Lead‐Free Piezoceramics

In this study, lead‐free (1 − x)Ba(Zr0.2Ti0.8)O3 − x(Ba0.7Ca0.3)TiO3 compositions are synthesized via conventional solid oxide route, and the ceramics are fabricated with normal sintering in air. The effects of composition fluctuations on dielectric, piezoelectric, and mechanical properties are investigated. The phase structure and the microstructure are analyzed with X‐ray diffraction and scanning electron microscopy. The best dielectric and piezoelectric properties of εr = 11 207 and d33 = 330 pC/N were obtained for BZT−0.35BCT and BZT−0.5BCT ceramics, respectively. The mechanical behavior—in terms of Vickers hardness and compressive and flexural strengths—was investigated, and the best mechanical behavior was found in the vicinity of the phase transition boundary with x values between 0.5 and 0.6

Ultrahigh electrostrain > 1% in lead-free piezoceramics: A critical review

Recently, a series of reports showing ultra-high electrostrain (> 1 %) have appeared in several Pb-free piezoceramics. The ultrahigh electrostrain has been attributed exclusively to the defect dipoles created in these systems. We examine these claims based on another report arXiv:2208.07134 which demonstrated that the measured electric field driven strain increased dramatically simply by reducing the thickness of the ceramic discs. We prepared some representative Pb-free compositions reported to exhibit ultrahigh strain and performed electrostrain measurements. We found that these compositions do not show ultrahigh electrostrain if the thickness of the discs is above 0.3 mm (the disc diameters were in the range 10- 12 mm diameter). The ultrahigh strain values were obtained when the thickness was below 0.3 mm. We compare the electrostrain obtained from specimens designed to exhibit defect dipoles with specimens that were not designed to have defect dipoles in Na0.5Bi0.5TiO3 (NBT) and K0.5Na0.5NbO3 (KNN) -based lead-free systems and could obtain much higher strain levels (4- 5 %) in the defect dipole free piezoceramics in the small thickness regime. Our results do not favor the defect dipole theory as the exclusive factor for causing ultrahigh strain in piezoceramics. A new approach is called for to understand the phenomenon of ultrahigh electrostrain caused by the thickness reduction of piezoceramic discs.Comment: 9 pages, 5 figures, lab repor

Fabrication of Grain-oriented Lead-free Piezoceramics

富山県立大学平成29年