Relaxor behavior and superior ferroelectricity of Y2O3-doped (Ba0.98Ca0.02) (Ti0.94Sn0.04Zr0.02)O3 lead-free ceramics

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Abstract

To upgrade the electric properties of lead-free piezoceramics, (1–x)(Ba0.98Ca0.02Ti0.94Sn0.04Zr0.02)O3-xY2O3 (abbreviated as (1–x)BCTSZ-xY, x = 0 mol%, 0.02 mol%, 0.04 mol%, 0.06 mol%, 0.08 mol% and 0.1 mol%) ceramics were successfully synthesized by traditional solid-state sintering method. The phase structure and microstructure of ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and piezoresponse force microscopyeramics (PFM). The electric properties of ceramics were researched through piezoelectric, dielectric and ferroelectric test instruments. The results show that all samples have pure perovskite structure and favorable electric properties. The optimal electric properties which especially include superior ferroelectric properties are gained when Y2O3 content is 0.06 mol% (d33 = 419 pC/N, kp = 52%, Tc = 89.5 °C, εr = 26900, tanδ = 2.86%, Pr = 14.41 μC/cm2, Ec = 1.8 kV/cm). Moreover, the temperature-dependent dielectricity of samples shows apparent relaxor behavior under different frequencies. The Curie–Weiss law further proves that all samples are typical relaxor ferroelectrics, and the relaxor degree of samples decreases with increase of Y2O3 content. In conclusion, Y2O3 plays a significant role in enhancing electric properties of BCTSZ ceramics.

Graphical abstract

Doping rare earth oxide Y2O3 in BCTSZ lead-free piezoceramics can enhance the electric properties (the ferroelectric properties, especially) and reduce the relaxor degree of ceramics.

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Introduction

Piezoceramics that convert electric and mechanical energy into each other are extensively utilized in many electronic devices (e.g., sensors, transducers and ultrasonic motors). Currently, the lead-based piezoelectric ceramics are significant materials due to their high and stabilized piezoelectric constant and Curie temperature.1, 2, 3 Nevertheless, noxious Pb limits the employments of lead-based piezoelectric ceramics and hence intangibly facilitates the progress of lead-free piezoelectric ceramics since Pb will intensely injure environments and human health. Therefore, the lead-free piezoceramics satisfy the requirements of building an environment-friendly society, which is expected to be a candidate for lead-based piezoceramics.

In recent years, extensive researches have been conducted on lead-free ceramics systems such as BaTiO3 (BT), (Bi0.5Na0.5)TiO3 (BNT) and (K0.5Na0.5)NbO3 (KNN). Especially, BaTiO3-based ceramics, the earliest discovered perovskite ferroelectrics, had been widely used in the manufacture of dielectric capacitors since 1950s.4 In addition, the BaTiO3-based ceramics co-doped with Ca2+ and Zr4+ or Ca2+ and Sn4+, gave birth to outstanding electric properties.3,5 Recently, Zhao et al.6 adopted a new phase-boundary engineering strategy using the multiphase convergence, which induced extensive structural flexibility in a wide phase-boundary region with continuous polymorphic phase transitions. Simultaneously, Zhao et al. also achieved an ultrahigh piezoelectric constant (d33) of 700 ± 30 pC/N in BaTiO3-based ceramics.6 Bai et al. studied phase transition behavior of (Ba0.85Ca0.15)(ZrxTi1–x)O3 ceramics, and large piezoelectric constant (d33 = 349 pC/N) was gained at diffuse ferroelectric phase transition field.5 Moreover, Zhu et al. researched effects of different Sn4+ addition on electric properties of (Ba,Ca)(Ti,Sn)O3 ceramics. The results showed that piezoelectric constant of BCTS ceramics reached 670 pC/N when Sn4+ content was 0.11 mol%.7 However, the Curie temperature of this ceramics was too low to be applied widely. In terms of rare earth oxide chemical modification, Chen et al. researched microstructure and electric performance of (Ba0.9Ca0.1) (Ti0.9Sn0.1)O3 ceramics, and found that La2O3 improved Curie temperature of ceramics (Tc = 85 °C).3 Han et al. studied the microstructure and ferroelectricity of (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 ceramics, and found that Pr2O3 improved the ferroelectricity of ceramics (Pr = 11.05 μC/cm2).8

The rare earth elements are widely used in the fields of biomedicine and electronics due to their extremely high melting point and special physical and chemical properties.9 The method of doping rare earth oxides (La2O3, Pr2O3 and Nd2O3) adjusted the phase structure and crystal grain distribution of ceramics, which enhanced the electric properties of ceramics.3,8,10 The result indicates that the rare earth oxide Y2O3 with similar properties to La2O3, Pr2O3 and Nd2O3, may also enhance electric properties of piezoceramics. Moreover, in our previous work, the influence of different sintering methods on electric properties of (Ba0.85Ca0.15)(Ti0.9Sn0.02Zr0.08)O3 ceramics was researched.11 Further the (Ba0.98Ca0.02)(Ti0.94Sn0.04Zr0.02)O3 ceramics possessed O−T phase structure by adjusting the composition contents of (Ba0.85Ca0.15)(Ti0.9Sn0.02Zr0.08)O3 ceramics. Therefore, the upgraded BCTSZ ceramics host was chosen and the electric properties of BCTSZ ceramics were optimized by chemical modification (Y3+ doping). Simultaneously, the relaxor behavior and the relationship between microstructure and electric properties of ceramics with different Y3+ contents were researched in detail.

Section snippets

Experimental

The raw materials, BaCO3, CaCO3, TiO2, SnO2, ZrO2 and Y2O3, were used as starting chemicals. Conventional ceramics sintering technique was used to fabricated (1–x)BCTSZ-xY ceramics. First, the starting materials of stoichiometric amount were ball-milled for 16 h, then, mixed powders were calcined at 1200 °C for 4 h in air. After calcination, these compounds were ball-milled for 12 h again to improve homogeneousness. Then, the dried powders were granulated with paraffin (fully refined paraffin

Phase structure

The normalized XRD patterns of (1–x)BCTSZ-xY ceramics with different Y3+ contents are shown in Fig. 1. One can see that all samples possess single perovskite structure, manifesting Ca2+, Zr4+, Sn4+ and Y3+ can diffuse into the BaTiO3 lattice to constitute homogeneous solid solution. Normalized diffraction peaks are quoted from BaTiO3 with orthorhombic phase (PDF#81–2200) and tetragonal phase (PDF#05–0626), which are manifested by vertical lines (see the bottom of Fig. 1(a) and (b)). It can be

Conclusions

The lead-free piezoceramics (1–x) (Ba0.98Ca0.02Ti0.94Sn0.04Zr0.02)O3-xY2O3 (x = 0, 0.02 mol%, 0.04 mol%, 0.06 mol%, 0.08 mol%, 0.1 mol%) were successfully synthesized by traditional solid-state sintering method. The relationship between phase structure, microstructure, and electric properties was systematically researched. The appropriate addition of Y3+ can enhance Curie temperature, piezoelectricity, dielectricity and ferroelectricity of (Ba0.98Ca0.02Ti0.94Sn0.04Zr0.02)O3 ceramics. The

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.

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Foundation item: Project supported by the Guizhou Province Graduate Research Fund (YJSCXJH[2020]029), Specialized Funds from Industry and Information Technology Department of Guizhou Province (2016056), the National Natural Science Foundation of China (51602066) and High-level Innovative Talents Plan of Guizhou Province ((2015)4009).

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