Simultaneous enhancement of piezoelectric constant and thermal stability in lead-free Fe-doped 0.94(Na1/2Bi1/2)TiO3-0.06BaTiO3 ceramics

https://doi.org/10.1016/j.jallcom.2021.161880Get rights and content

Highlights

  • Enhanced d33 and Td were simultaneously achieved in 1.0 mol% Fe-doped NBT-6BT ceramics.

  • The enhancement is attributed to the stabilization of ferroelectric order, induced by the enhanced lattice distortion.

  • Fe-doping is an effective way to optimize the thermal stability and piezoelectric properties of NBT-based ceramics.

Abstract

The low depolarization temperature has hindered implementation of (Na1/2Bi1/2)TiO3 (NBT)-based lead-free piezoceramics in practical application, in particular, in high temperature environment. In this work, the impact of Fe modification on the microstructure, depolarization temperature, ferroelectric and piezoelectric properties of 0.94(Na1/2Bi1/2)TiO3-0.06BaTiO3 (NBT-6BT) lead-free ceramics were studied. The results illustrates that the piezoelectric constant and depolarization temperature can be significantly tuned by Fe doping, and more importantly, both can be simultaneously enhanced. Compared to the undoped material, the room-temperature piezoelectric constant of NBT-6BT with 1.0 mol% Fe doping increases from 133 pC/N to 164 pC/N, and the depolarization temperature increases from 106 °C to 125 °C. These observations are mainly attributed to the stabilization of ferroelectric order, induced by the enhanced lattice distortion.

Introduction

Lead zirconate titanate (PZT)-based ceramics have been widely used in actuation and sensing applications due to their excellent electromechanical coupling properties and peculiar thermal stability [1], [2]. However, the application of PZT-based materials is facing rigorous restrictions in recent times, as it contains Pb element, which is toxic, and particularly harmful to children. These restrictions have triggered tremendous efforts in developing lead-free alternatives to replace PZT-based compositions in the past twenty years [3], [4], [5]. Among the various reported lead-free candidates, (Na1/2Bi1/2)TiO3 (NBT)-based lead-free piezoelectric ceramics are considered to be one of the most promising material systems to replace PZT in the area of ultrasonic power applications [5]. Unfortunately, the depolarization temperature Td (at which the material loses its piezoelectricity) of NBT-based piezoelectric ceramics is much lower than that of PZT, e.g., the Td of 0.94(Na1/2Bi1/2)TiO3-0.06BaTiO3 (NBT-6BT) is only ~ 100 °C [6], [7], about 200 °C lower than PZT. This shortcoming, to some extent, has limited its implementation in practical application, in particular, in high temperature environment.

Therefore, many strategies such as doping [8], [9], [10], composite design [11], [12], [13], quenching [14], [15], [16], [17], [18] and grain-size refinement [19] have been explored to improve the thermal stability of NBT-based ceramics. Most of the reports show that an increase in Td is often accompanied by a decrease in the piezoelectric constant d33. For instance, in the case of NBT-6BT:ZnO composites, Td increases from 100 °C to 140 °C, but d33 decreases from 140 pC/N to 125 pC/N [12]. This is because, the increase of Td normally originates from the stabilization of the domain structure, and consequently, reorientation of non-180° domains becomes hard, which in turn, lead to a lower d33 [20].

Selected literature reveals that simultaneously enhancing Td and d33 is also possible. By doping acceptor-Fe in Bi1/2(Na0.8K0.2)1/2TiO3 ceramics, Zhang et al. reported the d33 and Td can be simultaneously improved, and the results were rationalized on the basis of the stabilization of long-range ferroelectric order [21]. Zhou et al. achieved enhanced d33 and higher Td by introducing the (001)-textured structure and the in-plane tensile strain in NBT-6BT thin films [22]. By doping BaTiO3 nanowires in 0.88(Na0.5Bi0.5)TiO3-0.06(K0.5Bi0.5)TiO3-0.06BaTiO3 ceramics, Sheng et al. observed that the Td was increased from 95 to 133 °C and d33 enhanced from 105 to 192 pC/N [23]. By optimizing the material composition, combined with rapid quenching technique, a large d33 (232 ± 5 pC/N) and high Td (188 °C) were simultaneously realized in Bi0.5Na0.5TiO3-BaTiO3-Bi0.5Li0.5TiO3 ceramics by Yin et al. [24].

In this study, we systematically investigated the influence of Fe doping on the performance of NBT-6BT. Here NBT-6BT was chosen as it is at the morphotropic phase boundary (MPB), possessing excellent electromechanical properties. We found that both Td and d33 can be simultaneously enhanced at 1 mol% Fe doping. Potential mechanisms for simultaneous improvement of piezoelectric property and thermal stability are discussed based on the microstructural evolution, temperature-dependent polarization-electric field (P-E) hysteresis loops, and the temperature-dependent relative permittivity (εr) of NBT-6BT with different Fe-dopant concentration.

Section snippets

Experimental procedure

x mol% (x = 0, 0.2, 0.4, 1.0, 2.0) Fe-doped NBT-6BT lead-free ceramics were fabricated using the solid-state reaction technique. Bi2O3 (99.9%), Na2CO3 (99.5%), BaCO3 (99.8%), TiO2 (99.0%), and Fe2O3 (99.9%) were selected as raw materials (purchased from Aladdin, China). They were first dried at 120 °C for 24 h in order to remove any moisture and then weighed immediately according to the stoichiometric ratio. The powders of each composition were ball-milled with ethanol for 24 h and subsequently

Results and discussion

Fig. 1a depicts the X-ray diffraction patterns of undoped and Fe-doped NBT-6BT ceramic pellets. All the ceramics possess pure perovskite structure without secondary phases or impurities. The subscript of “pc” in Fig. 1 denotes pseudo cubic indexing. Typical splitting in (111)pc around 40° and (200)pc around 46.5° were observed (Fig. 1a and b), demonstrating rhombohedral and tetragonal distortions respectively, indicating that all the materials are a phase mixture [25], [26]. As an example, we

Conclusions

In summary, Fe-doped NBT-6BT lead-free piezoceramics were fabricated and the grain size, lattice distortion, dielectric behavior, polarization hysteresis and depolarization behavior were characterized. Results show that Fe-doping can significantly influence the thermal depolarization and lattice strain. At a critical doping concentration, simultaneous increase in Td and d33 is noted. These observations are mainly attributed to the stabilization of ferroelectric order, induced by the enhanced

CRediT authorship contribution statement

Xiangyang Cheng: Methodology, Validation, Investigation, Data curation, Visualization, Writing – original draft, Writing – review & editing. Lalitha Kodumudi Venkataraman: Writing – original draft, Writing – review & editing. Yingwei Li: Conceptualization, Methodology, Writing – original draft, Writing – review & editing, Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 11972262).

References (35)

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