Bismuth layer‐structured ferroelectric ceramics with high mechanical quality factor

“Comments on: [High temperature stability and mechanical quality factor of donor-acceptor co-doped BaTiO3 piezoelectrics

Piezoelectric materials play a key role in various kinds of modern electronic devices. High Curie temperature piezoelectric materials are required for piezoelectric sensors under critical conditions for nuclear power plants, automotive, aeronautic, and aerospace applications. These piezoelectric sensors usually withstand high temperature of 500°C or higher. To meet the requirements of these applications, piezoelectric materials must possess high piezoelectric performance and high electrical resistivity as well as high Curie temperature. The Aurivillius-type bismuth layer-structured ferroelectrics (BLSFs) are the most promising high-temperature piezoelectric materials because of their very high Curie temperature ( T c). Structurally, the BLSF compounds are composed of fluorite-like (Bi2O2)2+ layers and perovskite-like (A m −1B m O3 m +1)2− blocks, which arrange along the c axis alternately and regularly. The general formula is (Bi2O2)2+(A m −1B m O3 m +1)2−, where A is a mono-, di-, or trivalent metallic ion (or their combination) suited to dodecahedral coordination; B is a transition metallic ion suited to octahedral coordination, and m is the number of BO6 octahedral in the perovskite-like blocks and varies from 1 to 4. In this paper, the crystal structure, piezoelectric properties (herein piezoelectric constant d 33 value), and Curie temperature ( T c) of prototype BLSF ceramics have been summarized, and the summarized results indicate that the piezoelectric properties of high T c BLSF compounds are very low. The prototype BLSF ceramics with T c of >700°C exhibit low piezoelectric d 33 value of no more than 10 pC/N, and the prototype BLSF ceramics with T c of >900°C exhibit low piezoelectric d 33 value of no more than 5 pC/N. Calcium bismuth niobate, CaBi2Nb2O9 (CBN), was firstly found and synthesized by Sweden scientist Aurivillius B, in 1949. But until 2005, the investigations on the piezoelectric properties of CBN were firstly reported by Yan H X and Reece M J. After that, the piezoelectric properties of CBN have attracted much attention. The compound CBN has a very high Curie temperature T c of ~940°C, and it is considered to be potential material as piezoelectric sensors and other electronic components for high-temperature applications. However, its particular bismuth layer-structured crystal structure restricts the rotation of the spontaneous polarization ( P s) in the two-dimensional orientation ( a - b plane), and large coercive field ( E c) makes the material difficult to be poled sufficiently. The prototype CBN ceramic exhibits very low piezoelectric performance, with low piezoelectric d 33 value of 5 pC/N. In this paper, the piezoelectric properties of bismuth layer-structure ferroelectric calcium bismuth niobate (CaBi2Nb2O9) piezoelectric ceramics have been summarized. The results indicate that A-site ions substitution is more effective than B-site ions substitution to enhance the piezoelectric properties of CaBi2Nb2O9 ceramics, and A-site and B-site ions co-substitution is more desirable to optimize the piezoelectric performance. The piezoelectric d 33 can be promoted to >16 pC/N by compositions adjusting, while the Curie temperature remains higher than 900°C. Texture technologies, such as templated grain growth (TGG) and spark plasma sintering, have been introduced, and they are good methods to further enhance the piezoelectric performance of CaBi2Nb2O9 ceramics. The piezoelectric d 33 can be promoted to higher than 20 pC/N by texture technologies. Overall, the CaBi2Nb2O9 ceramics are promising materials for high-temperature piezoelectric sensor applications. Finally, perspectives on how to further enhance the piezoelectric performance of CaBi2Nb2O9 ceramics are presented.

For applications in high power ultrasonic transducers, both high piezoelectric coefficients d 33 and high mechanical quality factor Q m are essential. In this work, acceptor-doped Pb(Zr0.26Sn0.26Ti0.48)1−x Fe x O3−x/2 ferroelectric ceramics with compositions near the morphotropic phase boundary were prepared by routine solid-state reaction. By introducing Fe3+ ion into the perovskite crystal lattices, defect dipoles resulting from Fe3+ ions and oxygen vacancies are formed, pinning extrinsic domain wall motion, and hence increasing Q m but reducing d 33. It was found that substitution with 0.5 mol% Fe3+ resulted in high d 33 (390 pC/N) and high Q m (540). The nonlinear dielectric responses of these ceramics were evaluated on the basis of the Rayleigh law. Both Rayleigh coefficients, $$\alpha$$ and $$\kappa_{\rm{init}},$$ are reduced by doping with the acceptor ion Fe3+, indicating that the intrinsic contribution and reversible domain wall mobility are both reduced.

Poling above the Curie temperature driven large enhancement in piezoelectric performance of Mn doped PZT-based piezoceramics

Sm and Mn co-doped PMN-PT piezoelectric ceramics: Defect engineering strategy to achieve large d33 and high Qm

The wide application of KNN-based lead-free piezoelectric ceramics is constrained by the contradictory relationship between its mechanical quality factor and piezoelectric constant. From an application point of view, searching for chemical composition with enhanced piezoelectric constant (d33) and mechanical quality factor (Qm) is one of the key points of KNN-based ceramics. In this work, KNN-based ceramics with enhanced d33 and high Qm values were obtained by the solid solution method via a donor-acceptor codoping strategy. The donor dopant Ho3+ enhanced d33 values by refining the domain size, while the acceptor dopant (Cu1/3Nb2/3)4+ improved Qm by the formation of defect dipoles. The composition (KNN-5Ho-4CN) exhibits optimal integrated performances, of which d33, Qm, and TC values are 120 pC/N, 850, and 392 °C, respectively. Moreover, the temperature coefficient of resonant frequency (TCF = -429 ppm/K) indicates that KNN-5Ho-4CN ceramic has good temperature stability. This work provides a new insight for developing KNN-based ceramics with enhanced d33 and high Qm.

Achieving a high piezoelectric coefficient (d33) and a high mechanical quality factor (Qm) simultaneously in ferroelectric ceramics has been a persistent challenge, as conventional doping strategies, i.e., donor doping and acceptor doping, inevitably sacrifice one for the other. Here, the inherent trade-off is breaking through a new gradient-acceptor-doping strategy. By judiciously introducing gradient Cu2+-acceptor ions in "soft" (K, Na)NbO3 (KNN) ceramics, a record combination of properties is achieved - a d33 of 350 pC/N and a Qm of 340,-the first co-elevation of both parameters exceeding 300 in KNN-based systems. This represents a fourfold improvement in Qm with only a 10% reduction in d33. Multi-scale structural analysis reveals that the gradient doping retains a "soft" interior for high d33, and creates a "hardened" surface layer that effectively restrains domain switching, accounting for the enhanced Qm. This work offers a new paradigm for designing high-power ferroelectric ceramics by spatially tailoring functional properties.

Temperature-dependent dielectric and electromechanical properties of co-modified calcium bismuth titanate (CaBi4Ti4O15) ceramics

High-density ceramics with compositions of (Bi0.5-xNa0.5TiO3)0.94(BaTiO3)0.06 (BNBT-6), where x=0−0.03, were fabricated to develop potentially useful lead-free piezoelectric materials with high mechanical quality factors, Qm. Effects of Bi-deficiency on the physical properties of the fabricated ceramics were investigated. Microstructures of the samples with a Bi-deficiency of x≥0.015 showed no appreciable grain boundaries. This is due to the liquid-phase sintering that occurs for a Bi-deficiency concentration of x≥0.015. The mechanical quality factors for the radial vibration mode and the thickness vibration mode exhibited a maxima at x=0.022 and 0.024 with values of 674 and 128, respectively. The temperature coefficients of resonant and antiresonant frequencies (TCF) of the BNBT-6 samples with x=0.019−0.030 were almost constant with TCFfr1=−400 ppm/°C for the fundamental resonant frequency and TCFfa1=−320 ppm/°C for the fundamental antiresonant frequency, respectively. This method of creating a deficiency of a very small quantity of Bi in the BNBT-6 composition produces substantial improvement in Qm characteristics without deterioration of other piezoelectric parameters.

Grain size effect on the poling of soft Pb(Zr,Ti)O 3 ferroelectric ceramics

As a representative lead-free perovskite compound, Bi0.5Na0.5TiO3 (BNT) is widely recognized as a promising candidate for advanced piezoelectric and pyroelectric ceramics, which is primarily attributed to its relatively high Curie temperature, substantial spontaneous polarization and the intrinsic environmental compatibility. However, further improvement in piezoelectric and pyroelectric properties is critically hindered by its low depolarization temperature (Td), which limits the reliable operation under high-temperature conditions. In this study, the high-entropy engineering was adopted to construct a (1-x)Bi0.5Na0.5TiO3-x(Bi1/6Na1/6Ba1/6Sr1/6Mg1/6Zn1/6TiO3) ceramic system. By introducing A-site multicomponent configurations, lattice distortion and local disorder were effectively modulated, accompanied by a synergistic enhancement of polarization thermal stability and pyroelectric response. Particularly, the sample of x = 0.15 presented an enhanced polarization stability, with maintaining the piezoelectric coefficient d33 of ∼80 pC/N, increasing Td to 182 °C and obtaining the high mechanical quality factor Qm of 1036. Meanwhile, the sample of x = 0.05 exhibited the optimal pyroelectric performance, with Td = 176 °C, Fv = 3.24 × 10-2 m2/C and the piezoelectric activity quality factor of 1658 × 10-15 m2/N. These results demonstrate that high-entropy engineering can effectively overcome the trade-offs in conventional doping, to synergistically improve the piezoelectric and pyroelectric responses. These results also provide an effective strategy for the development of lead-free multifunctional ceramics applied under high-temperature condition.

Emerging high-power piezoelectric applications demand the development of piezoelectric materials featuring both a high mechanical quality factor (Qm) and a large piezoelectric coefficient (d33). However, it is widely accepted that an increase in d33 is usually accompanied with a decrease in Qm, and vice versa. Herein, a multiscale regulation strategy is proposed to improve Qm and d33 simultaneously from the perspectives of phase structure, ferroelectric domains, and lattice defects. A well-balanced combination of electromechanical performances with Qm = 726, d33 = 502 pC N-1, kp = 0.69, tan δ = 0.0024, and TC = 267 °C was obtained. Through structural characterization, it was observed that the morphotropic phase boundary and enhanced dispersion behavior lead to a lowered energy barrier, which contributes to polarization rotation and enhances piezoelectric performance. At the same time, the excellent piezoelectric performances also benefit from the highly oriented domain structure and small domain size after high-temperature poling. Furthermore, the segregation of Ba2+ causes A-site defects in the crystal lattice, accompanied with an increase in oxygen vacancies, which maintains the hardening effect of the ceramics. This study proposes a multiscale regulation strategy, providing insights for the design of high-power piezoelectric ceramics with high d33 and Qm.

In this work, the process of two-stage modifications for (Ba0.97Ca0.03)(Ti0.96Sn0.04-xHfx)O3 (BCTS4-100xH100x) ceramics was studied. The trade-off composition was obtained by Hf substitution for Sn and MnO2 doping (two-stage modification) which improves the temperature stability and piezoelectric properties. The phase structure ratio, microstructure, and dielectric, piezoelectric, ferroelectric, and temperature stability properties were systematically investigated. Results showed that BCTS4-100xH100x piezoelectric ceramics with x=0.035 had a relatively high Curie temperature (TC) of about 112 °C, a piezoelectric charge constant (d33) of 313 pC/N, an electromechanical coupling factor (kp) of 0.49, a mechanical quality factor (Qm) of 122, and a remnant polarization (Pr) of 19μC/cm2. In addition, the temperature stability of the resonant frequency (fr), kp, and aging d33 could be tuned via Hf content. Good piezoelectric temperature stability (up to 110 °C) was found with x =0.035. BCTS0.5H3.5 + a mol% Mn (BCTSH + a Mn) piezoelectric ceramics with a = 2 had a high TC of about 123 °C, kp ∼ 0.39, d33 ∼ 230 pC/N, Qm ∼ 341, and high temperature stability due to the produced oxygen vacancies. This mechanism can be depicted using the complex impedance analysis associated with a valence compensation model on electric properties. Two-stage modification for lead-free (Ba0.97Ca0.03)(Ti0.96Sn0.04)O3 ceramics suitably adjusts the compositions for applications in piezoelectric motors and actuators.

Study of piezoelectric ceramic materials for high-temperature and high-frequency applications

Characterization of acceptor-doped (Ba, Ca)TiO3 “hard” piezoelectric ceramics for high-power applications