Correlation Between Microstructure and Electric Behavior of (1−x)Ba_0.96Ca_0.04TiO₃-xBa(Mg_1/3Nb_2/3)O₃ Ceramics Prepared via Chemical-Furnace-Assisted Combustion Synthesis

Abstract

The (1−x)Ba_0.96Ca_0.04TiO₃-xBa(Mg_1/3Nb_2/3)O₃ (x = 0–0.20) lead-free ceramics were prepared through the chemical-furnace-assisted combustion synthesis (abbreviated as CFACS). The phase structure, microstructure, dielectric, and piezoelectric properties were systematically investigated. Phase analysis revealed the coexistence of orthorhombic and tetragonal phases in the vicinity of x = 0.07. More importantly, the composition with x = 0.07 exhibited optimal overall electrical properties, including a high piezoelectric coefficient (d₃₃) of 495 pC/N, the planar electromechanical coupling factor (K_p) of 41.9%, and the Curie temperature (T_c) of 123.7 °C. In addition, the average grain size was observed to progressively decrease with increasing x.

1. Introduction

Piezoelectric devices have become indispensable core components in modern sensors, drives, and energy harvesting systems derived from their excellent electromechanical conversion capabilities, high sensitivity and rapid response characteristics [1,2,3]. Driven by growing environmental concerns and legislation against lead-based products, BaTiO₃ (BT)-based piezoelectric ceramics are considered a prime alternative to the dominant lead zirconate titanate (PZT) systems [4]. Moreover, the various phase structures of BaTiO₃ ceramics can cause lattice mismatch and internal stress. This internal stress produces a pinning effect that hinders domain steering in the field, which is not favorable for obtaining excellent piezoelectric properties. Chen et al. found that introducing Ca²⁺ ions into BaTiO₃-based ceramics enhances the piezoelectric properties to 578 pC/N at a Ca content of 0.04. This is attributed so that the suppression of the hexagonal phase can promote the transformation from orthorhombic to tetragonal phase transition, and reduce long-range ferroelectric domain ordering [5]. However, poor temperature-stability of Ba_0.96Ca_0.04TiO₃ (BCT) ceramics restricts its practical application. The introduction of other elements is an effective approach to enhance electric performance [6,7,8,9]. Ba(Mg_1/3Nb_2/3)O₃ (BMN) was reported to be effective to improve dielectric properties in BaTiO₃ ceramics [10], as well as Curie temperature (T_c) to 380 °C in (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1) ceramics and to 280 °C in Bi_1/2Na_1/2TiO₃ ceramics, respectively. In addition, the Mg²⁺ and Nb⁵⁺ ions introduced in BMN can induce field-induced strain effects, which is conducive to further improving the piezoelectric performance [11,12]. Noteworthy, the study of microstructural evolution and electrical properties of BT-based composite ceramics doped with BMN is still rare. This is mainly attributed that the introduction of BMN ceramics fails to achieve a small grain size. A large grain size makes it difficult for the electric domain to turn, which is not conducive to further improving the piezoelectric performance.

Conventionally, the preparation of BT-based ceramics relies on the solid-state reaction method, which suffers from disadvantages including extended processing durations and substantial energy requirements [13]. The long-time sintering in high temperature inevitably accompanies grain growth and degradation of the microstructures, which leads to the deterioration of the electrical properties. The CFACS is a furnace-free and energy saving approach to prepare inorganic ceramics [14,15]. Rapid grain growth can be limited by the CFACS process as reported in superconductor material, cermets, and their composites [16,17,18]. The highly exothermic combustion reaction is a self-sustained process that can be completed rapidly in a short period of a few seconds [19,20].

Therefore, in this work, the CFACS method is introduced to synthesize (1−x)Ba_0.96Ca_0.04TiO₃-xBa(Mg_1/3Nb_2/3)O₃ (x = 0–0.20) ceramics. The phase evolution, microstructural characteristics along with electrical performance of composite ceramics were systematically investigated.

2. Experimental Section

The (1−x)Ba_0.96Ca_0.04TiO₃-xBa(Mg_1/3Nb_2/3)O₃ [BCT-xBMN x = 0–0.20] ceramics were fabricated through the CFACS. The BaCO₃ (99.0%), CaCO₃ (99.0%), MgO (99.7%), TiO₂ (99%), and Nb₂O₅ (99.99%) materials were weighted according to the stoichiometric formula. The first step is to mix the powders in ethanol through a Planetary ball mill for 12 h, followed by drying at 120 °C for 2 h. Subsequently, the dried powders were compacted into pellets with size of Ø20 mm × 3 mm for further measurements.

Specifically, the CFACS occurred in a closed environment, as shown in Figure 1. A total of 100 g of thermite (Ti + C + 2Al + Al₂O₃ + Fe₂O₃) powers were placed in a graphite crucible with a diameter of 80 mm, where the disk wrapped with carbon paper was buried. Firstly, the reaction chamber was evacuated and inflated with argon to a pressure of 0.5 MPa. Then, at an electric current of 10 A, the top of the thermite was ignited using a tungsten coil. The thermite reaction kept burning until the powder products generated in a self-sustained process. The powders were naturally cooled down in the furnace and then pressed into particles with a diameter of 12 mm. Finally, the samples were burnt off at 1280 °C for 3 h in air.

The phase structure of the samples was investigated using an X-ray diffraction (Japan Rigaku, D/max-2500/PC, Cu Kα1, λ = 1.5406 Å, Tokyo, Japan). The Raman spectrum was recorded through Raman spectrometer with a wavelength of 514 nm (InVia Refex, Renishaw, London, UK). The surface morphology was observed using scanning electron microscopy (SEM; S-4800, Hitachi, Tokyo, Japan). The bulk density of the sample was determined via the Archimedes method. Processing of the sample for the electrical performance test: the sample was polished, then both sides were coated with silver pastes, and finally fired at 650 °C for 30 min to obtain electrodes. The TH2818 Automatic Component Analyzer (Changzhou Tonghui Electronic Co., Ltd., Changzhou, China) was used to measure dielectric properties. A Radiant Precision Premier LC ferroelectric material test system (Radiant Technologies Inc., Albuquerque, NM, USA) was employed to record the polarization-electric field (P-E) hysteresis loops. The piezoelectric constants of the samples were characterized by quasi-static d₃₃ m (ZJ-3AN, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China).

3. Results and Discussions

Figure 2a illustrates the reaction temperature profile of the (1−x)Ba_0.96Ca_0.04TiO₃-xBa(Mg_1/3Nb_2/3)O₃ (x = 0.07) powders fabricated by the CFACS. The temperature of the reacting curve rises quickly to above 1600 °C for about 10 s corresponding to the reaction initiating, then the temperature drops quickly, and the reaction lasts for about 200 s above 1200 °C. The XRD pattern of the (1−x)BCT-xBMN (x = 0.07) powders by the CFACS at around 1200 °C is shown in Figure 2b. The sample exhibits mainly perovskite structure. Some additional peaks arose from inadequately reacted powders during the rapid reaction process. The SEM diagram inserted in Figure 2b shows that the microstructure of (1−x)BCT-xBMN (x = 0.07) powder is uniformly distributed with an average particle size of approximately 300 nm. Because of the faster heating and cooling process during the chemical synthesis, the prepared powder exhibited smaller particle size than that fabricated by traditional preparation method [21]. Such a process is too fast for the particles to grow, and fine powers are naturally associated with high sintering ability.

Figure 3a displays the XRD patterns of the BCT-xBMN ceramics. All compositions crystallize in a pure perovskite structure, with the absence of secondary phases confirming the formation of a solid solution as Mg²⁺ and Nb⁵⁺ ions dissolve into the BaTiO₃ lattice. As illustrated in Figure 3b, the samples exhibit a single peak with x = 0 to 0.05 at 2θ around 45°, then split into two peaks with further x increasing. This indicates that the samples exhibit single orthorhombic phase at 0 ≤ x< 0.07, and the coexistence of orthorhombic and tetragonal phase (PDF#79-2264) at 0.07 ≤ x < 0.20. Moreover, Figure 3c shows that all diffraction peaks consistently shift to lower 2θ values with increasing x. This peak shift indicates an expansion of the unit cell volume, which is ascribed to the replacement of smaller Ti⁴⁺ (0.605 Å) ions by the larger (Mg₁/₃Nb₂/₃)⁴⁺ (0.693 Å) complex at the B-site of the B-O₆ octahedra [22,23,24].

To further characterize the structure evolution, Raman spectra were acquired for all the samples. Figure 4 shows room-temperature Raman spectra of the BCT-xBMN (x = 0–0.2) ceramics in the frequency range of 100–1000 cm⁻¹. The Raman spectra of the ceramics are similar to that of BaTiO₃, suggesting that these compositions share the basic unit structure [25]. The Raman peaks located near 717, 518, 306, and 270 cm⁻¹ are the characteristic vibrational modes of the ferroelectric orthorhombic phase of BaTiO₃ at room temperature [26]. It is noteworthy that the peak located at around 188 cm⁻¹ undergoes significant broadening with increasing doping. This phenomenon is attributed to the intensified tilting or rotation of BO₆ octahedra, which induces local structural disorder [27]. The enhancement of this structural disorder weakens the original long-range ferroelectric order in the crystal, manifesting macroscopically as a reduction in the orthorhombic phase content. The evolution is consistent with the XRD results shown in Figure 3.

Figure 5 exhibits SEM micrographs of the BCT-xBMN ceramics. The microstructures were observed to be more homogeneous with x increasing. For the undoped composition, the sample exhibits a relatively small grain size of approximately 1.31 μm, which can be attributed to the ultra-fast crystallization process of the powder products synthesized during the CFACS. As shown in the inset of Figure 5, the average grain size of each composition is 1.31 μm, 1.21 μm, 1.14 μm, 1.10 μm, 0.96 μm, 0.92 μm, 0.86 μm, respectively. The reduction in grain size is likely governed by the solute drag mechanism. The disparity in ionic radii between the larger dopant ions (Mg²⁺ 0.72 Å, Nb⁵⁺ 0.64 Å) and the smaller host ion (Ti⁴⁺, 0.605 Å) generates significant lattice strain energy (ΔGstrain). As these dopants occupy the Ti⁴⁺ sites, the accumulation of this strain energy impedes grain boundary migration, thereby suppressing grain growth. As can be seen from Figure 5h, the corresponding relative density increases from 88.7% (x = 0) to a maximum value of 93.7% (x = 0.07). The high densification of the ceramic in this range reduces surface defects, which in turn mitigates charge concentration and contributes to an enhanced polarization. However, as x increases further, the density subsequently decreases to 84.6% at x = 0.2. This phenomenon is closely correlated with the complexities of domain switching and domain wall mobility at higher dopant concentrations [12].

Figure 6 shows the temperature dependence of the dielectric permittivity (ε_r) for the BCT-xBMN ceramics at 100 kHz. With the increase in x, the dielectric peak widens gradually, suggesting polarization fluctuations in the ceramic volume or fluctuations in spatial components. To quantitatively evaluate this peak broadening, the Full Width at Half Maximum (FWHM) from the dielectric-temperature spectrum is employed, which can be expressed based on the following equation: FWHM = T₂ − T₁, where T₁ and T₂ represent the temperatures on either side of the dielectric peak corresponding to half its maximum value. The calculated FWHM values of each component are 62, 67, 77, 92, 101, 107, and 109, respectively, indicating excellent relaxation properties in the ceramics during the transition from ferroelectric phase to paraelectric phase. The dielectric constant increases first and then decreases with the increase in x, and reaches a peak value at x = 0.07. This is related to the transformation of phase structure. Compared with the tetragonal structure, the orthorhombic structure has more polarization vectors, so it can obtain higher dielectric constant. With the further increase in x, the dielectric constant decreases slightly, which is related to the small grain size and the change in density [26,28]. Moreover, the ε_r decreasing with further addition of x content is derived from the increased volume fraction of the grain boundaries. A similar decrease in grain size and the ε_r on BMN doping was reported in the BT and (K,Na)NbO₃ (KNN) system [29]. At the same time, it can be seen from the inset that the T_c of the BCT-xBMN ceramics increases gradually from 107 °C (x = 0) to 124.7 °C (x = 0.10), then decreases to 110 °C (x = 0.20). Under rapid reaction conditions, the limited diffusion time prevents the complete penetration of BMN into the BaTiO₃ grains, leading to the formation of a core–shell structure with a BaTiO₃-rich core and a BMN-rich shell [10]. The results show that the ferroelectric-paraelectric transition temperature caused by dipole polarization is above 110 °C [30].

In order to further analyze the dielectric relaxation performance, the experimental data can be fitted using a modified Curie–Weiss law: 1/ε – 1/ε_m = (T − T_m)^γ/C (T > T_m), where the exponent γ, ε_m, T, T_m, and C represent the dielectric relaxation, maximum value of dielectric permittivity, temperature, temperature for ε_m, and Curie-like constant, respectively [23]. Figure 7 exhibits that the values of γ are 1.84, 1.75, 1.68, 1.69, 1.83, 1.78, 1.81 for the samples of the BCT-xBMN ceramics with x = 0–0.2, respectively. The introduction of BMN into the BCT ceramic matrix serves to break down the long-range ferroelectric order. The mismatch in both valence state and ionic radius from the BMN dopants induces the fragmentation of large domains into a dense arrangement of finer nanodomains. This refined domain configuration is a key factor in the enhancement of the material’s electrical properties. In addition, small grain size can inhibit domain steering and obtain strong relaxation ferroelectric behavior.

Figure 8 shows d₃₃ and K_p of the BCT-xBMN ceramics with x = 0–0.2. The d₃₃ and K_p of samples were greatly improved at 0 < x ≤ 0.07, but then exhibit a decreasing trend with further increases in x. The optimal piezoelectric properties are achieved at the x = 0.07, with a peak d₃₃ value of 495 pC/N and a K_p of 41.9%. This significant enhancement is primarily attributed to the presence of a morphotropic phase boundary (MPB) near this composition, coupled with the microstructural effects arising from the orthorhombic-to-tetragonal phase transition [11]. A comparison of the piezoelectric constant and the plane electromechanical coupling coefficient achieved in this work with those of other lead-free ceramics by other techniques, illustrating excellent d₃₃ and K_p of theBCT-xBMN ceramics [4,9,31,32,33,34]. It is notable that the decrease of d₃₃ and K_p with further x increasing corresponds to the grain sizes decrease, which is related to the domain wall size [35]. The moving of the domain walls becomes difficult with the domain size decreasing, consequently, leading to the reduction in piezoelectric performance [15].

Figure 9 displays the P-E diagrams of BCT-xBMN samples at 30 kV/cm and 1 Hz at room temperature. All the hysteresis loops possess well-saturated characteristics. The inset shows the P_r and E_c under different contents. The P_r and E_c of the BCT-xBMN ceramics decrease gradually with x increasing. This can be explained by the Avrami model for ferroelectrics proposed by Orihara [33]. The decrease of P_r derived from the decrease in grain size, which can deteriorate the polarization reverses. The E_c of the samples decreases gradually from 3.05 kV/cm (x = 0) to 0.82 kV/cm (x = 0.2), indicating that ferroelectric properties of BCT-based samples can be enhanced through BMN-doped. The low value of E_c (<1.77 kV/cm) shows quite “soft” feature in 0.05 < x < 0.2. This trend indicates that the excellent piezoelectric properties of samples can be obtained through tailoring the poling condition.

4. Conclusions

The (1−x)Ba_0.96Ca_0.04TiO₃-xBa(Mg_1/3Nb_2/3)O₃ (x = 0–0.20) ceramics were synthesized via the chemical-furnace-assisted combustion synthesis. The effects of Ba(Mg₁/₃Nb₂/₃)O₃ (BMN) doping content on the phase structure, microstructure, and electrical properties of the ceramics were systematically investigated. Phase analysis revealed the formation of a morphotropic phase boundary (MPB) at around x = 0.07, characterized by the coexistence of orthorhombic and tetragonal phases. Fine-grained and homogeneous microstructures were formed by the CFACS process, and doping of BMN further inhibits the grain growth of grains and form the homogeneous microstructures. Consequently, the ceramic at x = 0.07 demonstrated optimal comprehensive electrical properties with d₃₃ = 495 pC/N, K_p = 41.9%, T_c = 123.7 °C. The CFACS is proved to be a good fabrication method to piezoelectric ceramics.