The Effect of Doping Modification on the Piezoelectric Properties of Ba_1−xCa_xZr_0.1Ti_0.9−ySn_y Lead-Free Piezoelectric Ceramics

Abstract

Lead-free piezoelectric ceramics have attracted substantial attention in environmental protection and energy storage applications due to their excellent performance. In this study, the Ba_1−xCa_xZr_0.1Ti_0.9−ySn_yO₃(BCZTS) lead-free piezoelectric ceramic system was synthesized. The effects of doping ratios of Ca and Sn, as well as sintering temperature, were systematically investigated on the phase structure, microstructure, and piezoelectric properties of BCZTS ceramics. The results showed that the Ba_0.88Ca_0.12Zr_0.1Ti_0.81Sn_0.09 ceramics synthesized with a Ca doping content of x = 12 mol% and a Sn doping content of y = 9 mol % had a homogeneous phase structure with an Orthorhombic–Tetragonal (O-T) morphotropic phase boundary (MPB) and uniform grain size. At a sintering temperature of 1300 °C, the ceramics achieved optimal piezoelectric performance, with a piezoelectric coefficient d₃₃ = 319 pC/N. These lead-free piezoelectric ceramics have superior properties compared to conventional lead-based piezoelectric ceramics in the local market, providing a novel and feasible way to replace lead-based ones in civilian applications.

1. Introduction

At present, piezoelectric ceramic materials are extensively studied as information-functional ceramic materials, and exhibit characteristics such as piezoelectricity, dielectricity, ferroelectricity, and elastic strain [1,2,3,4,5]. Currently, the annual market size of the piezoelectric ceramics industry reaches several hundred billion US dollars. Among these materials, lead-based ceramics, especially lead zirconate titanate (PZT) and its derivatives, are still the most widely used piezoelectric materials [6,7]. Due to their superior piezoelectric properties, they hold over 90% of the piezoelectric materials’ market share [8,9,10]. However, the presence of volatile lead components in PZT ceramics during their preparation, utilization, and disposal presents substantial risks to both human health and environmental safety [11,12,13,14]. Consequently, major countries and regions globally, including the European Union, the United States, Japan, and China, have introduced relevant laws and regulations that prohibit or restrict the future use of lead-based piezoelectric materials [15,16]. In response to the stricter domestic and international environmental protection requirements, the exploration of alternative materials for lead-based piezoelectric ceramics has become a popular research topic [17,18,19,20].

Barium calcium zirconate titanate (BCZT)-based lead-free piezoelectric ceramics show outstanding piezoelectric properties comparable to those of lead-based PZT piezoelectric ceramics [21,22,23,24]. They are widely applied in various electronic components, such as sensors, actuators, ultrasonic transducers, and filters [25]. Although both domestic and international scholars have extensively research on lead-free BCZT-based piezoelectric ceramics in the early stages [26,27,28], numerous pressing issues still need urgent attention and solutions. As the simplest and most direct method for enhancing the piezoelectric properties of BCZT-based lead-free ceramics, doping modification has garnered considerable attention from researchers and has experienced rapid advancement [29,30,31,32,33].

Building on the previous work of this research group on fabricating BCZT-based lead-free piezoelectric ceramics [34,35,36], this project designed the Ba_1−xCa_xZr_0.1Ti_0.9−ySn_yO₃ system. We investigated the impacts of modulating the contents of Ca and doped Sn in the BCZT-based system on the piezoelectric properties of BCZT-based lead-free piezoelectric ceramics. This approach aims to broaden the application fields of BCZT-based lead-free piezoelectric ceramics, implement environmental protection, and promote the development of the lead-free piezoelectric ceramics industry based on practical considerations [37].

2. Experimental Procedures

Lead-free piezoelectric ceramics with compositions of Ba_1−xCa_xZr_0.1Ti_0.9−ySn_y (x = 0, 4, 8, 12, 16, 20 mol%; y = 0, 3, 6, 9, 12, 15 mol%) were prepared by the conventional solid-state synthesis method. The raw materials in this study were barium carbonate (BaCO₃, 99.8%, Aladdin(Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China)), calcium carbonate (CaCO₃, 99.5%, Aladdin), titanium dioxide (TiO₂, 99%, Aladdin), zirconium dioxide (ZrO₂, 99.99%, Aladdin), tin (III) oxide (Sn₂O₃, 99.99%, Aladdin), paraffin wax (binder), and anhydrous ethanol.

The powders were precisely weighed according to the stoichiometric ratio and ball-milled in an alcohol medium for 12 h. Subsequently, the mixture was calcined in air at 1100 °C for 2 h. The calcined powders were ball-milled again for 12 h; subsequently, the obtained powder was mixed with paraffin wax (PW) as a binder. After thorough mixing, granulation and sieving, granulated powder with good flowability and homogeneity was obtained, which was then pressed into samples under pressure of 5 MPa. The samples were placed in a tube sintering furnace, heated to 600 °C at a rate of 4 °C/min, and held for 1 h to complete binder removal, followed by high-temperature sintering at 1300–1380 °C for 5 h. Both sides of the sintered samples were coated with silver paste to facilitate poling. Finally, the piezoelectric properties were characterized. To satisfy the requirements for electrical property characterization, the ceramic samples were mechanically ground and polished. Silver electrodes were applied to both surfaces, and the samples were poled in silicone oil for 30 min prior to the electrical performance measurements.

The phase structure of the prepared samples was characterized by X-ray diffraction (XRD) with Cu-Ka radiation (XPERTPRO, PANAlytical Co., Almelo, The Netherlands) in the 2 h range of 10–80°. The microstructure of ceramic samples was observed by scanning electron microscopy (SEM) (JSM-5900, Japan Electronics Corporation, Tokyo, Japan). The quasistatic d₃₃ m (SINOCERA, Yangzhou, China) was used to measure the piezoelectric coefficient (d₃₃). Ferroelectric hysteresis loops (P-E) were measured using a ferroelectric test system (RT66A, Radiant Technologies Inc., Albuquerque, NM, USA). The temperature-dependent dielectric spectra were tested by a dielectric temperature spectrum analyzer (1200HTDE-LTC, Changsha Sanqi Electronic Technology Co., Ltd., Changsha, China).

3. Results and Discussion

Figure 1 shows the X-ray diffraction (XRD) patterns of Ba_1−xCa_xZrTiSn lead-free piezoelectric ceramics sintered at 1300 °C with different CaCO₃ doping concentrations (0, 4, 8, 12, 16, 20 mol%). The results demonstrate that Ca²⁺ ions are fully incorporated into the BCZTS lattice structure, forming a single ABO₃-type perovskite solid solution. Figure 1 presents that the diffraction peaks of the ceramic samples shift slightly to higher angles as Ca²⁺ doping content increases. According to Bragg’s law (2d sinθ = nλ), the increase in the diffraction angle 2θ indicates a reduction in interplanar spacing (d). Based on the atomic tolerance factor principle, ions with a radius r(Rn⁺) < 0.087 nm tend to dope at the B-site, while those with r(Rn⁺) > 0.094 nm occupy the A-site. In the Ba_1−xCa_xZrTiSn ABO₃-type perovskite structure, the ionic radius of Ca²⁺ (0.106 nm) exceeds 0.094 nm, which confirms that Ca²⁺ acts as an A-site dopant. Considering the similar ionic radii (Ca²⁺: 0.106 nm; Ba²⁺: 0.138 nm) and identical valence states, Ca²⁺ substitutes for Ba²⁺ at the A-site, leading to a slight shrinkage of the unit cell.

Figure 2a,b presents the XRD patterns of BaCaZr_0.1Ti_0.9−ySn_y lead-free piezoelectric ceramics doped with different Sn contents (0, 3, 6, 9, 12, 15 mol%) and prepared by the conventional solid-state reaction method at 1350 °C. To further analyze the evolution of the crystalline phase, Figure 2b shows a magnified view of the diffraction patterns within the 2θ range of 45–46°.

As clearly observed in Figure 2a, all BaCaZr_0.1Ti_0.9−ySn_y ceramic samples exhibit a pure perovskite structure, and no diffraction peaks of secondary phases are detected. This suggests that doped Sn ions are well-incorporated into the BCZTS lattice to form solid solutions without changing the primary crystalline structure. In Figure 2b, as the Sn content increases, the diffraction peaks gradually shift to lower angles, which reflects lattice expansion. This peak shift is attributed to the difference in ionic radii of B-site ions. The ionic radius of Sn⁴⁺ (0.069 nm) is larger than that of Ti⁴⁺ (0.060 nm). The increased substitution of Sn⁴⁺ leads to changes in lattice parameters, causing the characteristic diffraction peaks to shift to lower angles. Moreover, Figure 2b reveals a trend of diffraction peaks transitioning from splitting to merging with increasing Sn content. Based on standard crystalline phase data, the crystal structure is tetragonal (T) when the Sn doping is below 6 mol% and changes to orthorhombic (O) when the Sn content exceeds 9 mol%. This indicates that the phase structure of BaCaZr_0.1Ti_0.9−ySn_y ceramics is highly sensitive to Sn doping, leading to the coexistence of multiple phases. Notably, when the Sn doping level reaches 9 mol%, an Orthorhombic–Tetragonal (O–T) morphotropic phase boundary (MPB) is formed in the ceramics. This suggests that the phase boundary in these materials can be effectively tuned by introducing Sn dopants. This is significant because it is well-known that the presence of an MPB can significantly enhance the piezoelectric performance of ceramics.

Figure 3a–f present the SEM micrographs of Ba_1−xCa_xZrTiSn lead-free piezoelectric ceramics with different Ca contents, prepared by conventional solid-state sintering. SEM results indicate that as the Ca content increases, the grain size gradually decreases, indicating grain refinement. When the Ca doping amount is x = 0.12, the grain size is the most uniform, indicating promoted grain growth in the ceramics. SEM images also show that the pore density in the ceramic samples first decreases and then increases as the Ca content rises. Notably, the sample exhibits the highest density at x = 0.12, and density directly affects the piezoelectric properties of ceramic materials. These observations suggest that variations in Ca content directly influence grain growth through Ca doping, which reduces the lattice parameters of Ba_1−xCa_xZrTiSn ceramics and lowers the nucleation energy. However, excessive Ca doping increases lattice distortion in Ba_1−xCa_xZrTiSn, leading to higher nucleation energy and hindered nucleation/grain growth. Therefore, 12 mol% Ca doping is optimal for enhancing the density of these ceramics.

Figure 4a–f display the SEM micrographs of BaCaZr_0.1Ti_0.9−ySn_y lead-free piezoelectric ceramics with different Sn doping contents, prepared by conventional solid-state sintering. The images show that all ceramic samples with varying Sn contents exhibit microstructures with clear grain boundaries and relatively dense packing. Notably, there is a weak correlation between the grain size of the lead-free piezoelectric ceramics and Sn doping levels. At the same sintering temperature, the grain size decreases slightly as the Sn content increases. This indicates that moderate Sn doping promotes densification during conventional solid-state sintering and reduces the formation of coarse grains. Additionally, pore distribution analysis reveals that the ceramic exhibits the highest density at 9 mol% Sn doping. Combining this with the XRD analysis results of BaCaZr_0.1Ti_0.9−ySn_y samples, the optimal Sn doping ratio is determined to be 9 mol%.

Figure 5 shows the piezoelectric property (d₃₃) results of Ba_0.88Ca_0.12Zr_0.1Ti_0.81Sn_0.09 ceramics sintered at different temperatures. The data indicate that the d₃₃ value varies in a non-monotonic trend with increased sintering temperature. Higher temperatures enhance the ceramic densification, thereby improving piezoelectric performance. However, excessively high temperatures lead to uneven grain growth, which deteriorates piezoelectric properties due to the increased microstructural heterogeneity [38]. Separately, at a fixed sintering temperature, as the Ca content increases, the piezoelectric constant d₃₃ of the ceramic samples also exhibits a non-monotonic trend. It reaches a peak of d₃₃ = 319 pC/N when the Ca content is x = 12 mol% and the sintering temperature is T = 1300 °C.

Figure 6 displays the polarization-electric field (P-E) hysteresis loops of the Ba_1−xCa_xZr_0.1Ti_0.81Sn_0.09 ceramics measured at room temperature. The remanent polarization (P_r) and coercive field (E_C) exhibit a systematic evolution with varying compositions, with the optimal x = 0.12 and y = 0.09 delivering the maximum P_r and moderate E_C. This significant enhancement in polarization is primarily attributed to the formation of the Orthorhombic–Tetragonal (O-T). The coexistence of multiple phases significantly flattens the polarization anisotropy energy profile, thereby facilitating easier ferroelectric domain switching under an external electric field. Additionally, the octahedral expansion induced by Sn⁴⁺ substitution provides a larger displacement space for the central cations, further contributing to the enhanced spontaneous polarization. Apart from the phase structure, appropriate co-doping promotes grain growth and reduces grain boundary density, which weakens the domain wall pinning effect and enables sufficient polarization reversal, thus achieving a high P_r.

Figure 7 shows the dielectric performances of the Ba_1−xCa_xZr_0.1Ti_0.81Sn_0.09 ceramic sintered at 1300 °C. As shown in Figure 7a, all samples exhibit a characteristic dielectric peak, corresponding to the Curie temperature. As the Ca doping content increases, the maximum dielectric constant of the samples first rises and then falls. The x = 0.08 composition exhibits the highest dielectric peak. The ceramic sample of x = 0.12 composition lies within the morphotropic phase boundary region. The coexistence of multiple phases induces a phase transition diffusion effect, leading to a broadening of the dielectric peak and a reduction in the dielectric constant at the Curie point. Figure 7b shows that the dielectric loss at room temperature is below 0.04 for x ≤ 0.08, indicating excellent loss performance. Dielectric measurements at varying frequencies were conducted using the optimal composition of x = 0.12 in Figure 7c. The results show that an increase in frequency slightly reduces the dielectric peak. The system exhibits a slight dispersion phase transition. As shown in Figure 7d, the Curie temperature exhibits a trend of first decreasing, then increasing, and finally decreasing again with increasing Ca doping. The Curie temperature (Tc) reaches a peak at x = 0.12. This evolution of dielectric properties is consistent with the trends observed in the samples of SEM, ferroelectric and piezoelectric properties.

By analyzing Ba_1−xCa_xZr_0.1Ti_0.9−ySn_yO₃ samples, the optimal doping ratio and sintering conditions for this system were determined. Subsequently, the piezoelectric performance was compared with commercially available lead-based piezoelectric ceramic samples. Figure 8a shows samples of common lighters available in the local market, and Figure 8b their piezoelectric constant. Among them, the sample 7 represents the piezoelectric constant of Ba_0.88Ca_0.12Zr_0.1Ti_0.81Sn_0.09 piezoelectric ceramic prepared in our laboratory. A comparison reveals that samples from a market-available lighter only have the highest piezoelectric constant, d₃₃ = 309 pC/N, and the other samples are below 300 pC/N. The ceramic sample prepared in this project has a piezoelectric constant of d₃₃ = 319 pC/N, outperforming all common lighter samples in the market.

4. Conclusions

In this study, Ca²⁺ and Sn⁴⁺ were employed as doping ions to construct the Ba_1−xCa_xZr_0.1Ti_0.9−ySn_y lead-free piezoelectric ceramic system. The effects of the doping concentrations of Ca/Sn and the sintering temperature on the phase structure, microstructure, and piezoelectric properties of BCZTS ceramics were investigated. The research indicated that when the Ca doping concentration was x = 12 mol% and the Sn doping concentration was y = 9 mol%, the fabricated Ba_0.88Ca_0.12Zr_0.1Ti_0.81Sn_0.09 piezoelectric ceramic had a single-phase perovskite with coexistence of orthorhombic and tetragonal phases, and an O-T morphotropic phase boundary formed in the crystalline phase. Under these conditions, the ceramic grain size was relatively uniform. When sintered at 1300 °C, the ceramics in this system exhibited the optimal piezoelectric performance, achieving d₃₃ = 319 pC/N. And the ceramic system showed excellent dielectric properties and Curie temperature. All samples’ P-E hysteresis loops exhibit a high saturation and symmetry.

The Ba_0.88Ca_0.12Zr_0.1Ti_0.81Sn_0.09 ceramic exhibited superior piezoelectricity (d₃₃ = 319 pC/N) than conventional lead-based ceramics. This demonstrates the potential of BCZTS ceramics as lead-free alternatives for civilian applications (e.g., piezoelectric igniters), which can expand the practical use of these lead-free materials.