Defect Control of Donor Doping on Dielectric Ceramics to Improve the Colossal Permittivity and Temperature Stability

Abstract

As the demand for miniaturization of electronic devices increases, ceramics with an ABO₃ structure require further improvement of the dielectric constant with high permittivity. In the present work, Ba_1−1.5xBi_xTiO₃ (BB100xT, x = 0.0025, 0.005, 0.0075, 0.01) ceramics were prepared via a solid-state reaction process. The effect of Bi doping on dielectric properties of lead-free relaxor ferroelectric BaTiO₃-based ceramics was studied. The results showed that both colossal permittivity (37,174) and a temperature stability of TCC ≤ ±15% (−27–141 °C) were achieved in BB100xT ceramics at x = 0.5%. The A-site donor doping produces A-site vacancies, a larger space for Ti⁴⁺, and fluctuation of the component, which is partially responsible for the high permittivity and responsible for the temperature stability. Meanwhile, the contribution of defect dipoles, and IBLC and SBLC effects to polarization leads to the colossal permittivity. The formation of a liquid phase during sintering promotes mass transfer when the doping content is higher than 0.5%. This work benefits the exploration of novel multilayer ceramic capacitors with colossal permittivity and temperature stability via defect engineering.

1. Introduction

Oxide materials play an important role in many fields, such as thermoelectrics [1], memory [2], nanogenerator [3], etc. As a member of the oxide family, relaxor ferroelectric materials with a perovskite structure have been widely used in manufacturing multilayer ceramic capacitors (MLCCs), electrostrictive devices, and energy storage capacitors [4,5,6,7], which have typical characteristics of diffuse phase transition and frequency dispersion. Most relaxor ferroelectrics have a lead-based composite perovskite solid solution structure, for example, PZT [8,9] and PMN [10,11], which has been widely studied because of its excellent dielectric properties. However, this type of solid solution has obvious disadvantages, namely, volatility and toxicity. Therefore, a lot of effort has been put into developing environmentally friendly “lead-free” relaxation ferroelectrics [12]. Barium titanate, which has a perovskite structure and can easily be modified, has therefore become an important substitute for lead-based ceramics.

Lead-free barium titanate ceramics have high dielectric constants and low dielectric loss, especially low cost and processing reliability, which have become the foundation of the electronics industry. However, the Curie temperature (Tc) of BaTiO₃ is around 120 °C, and there is a sharp peak in the dielectric constant at the Curie point, which greatly limits the application of BaTiO₃ in MLCCs. Moreover, in recent years, the fields of 5G communication, electric vehicles, and aerospace and aviation have put forward higher requirements for MLCCs, which must be able to withstand high working temperatures (>150 °C) and maintain high and stable dielectric properties. A large number of studies have shown [13,14] that phase transition temperature can be effectively controlled at higher temperatures by appropriately donor doping into the barium site, such as Bi³⁺ ion, which has 6s² lone pair electrons, similar to Pb. In addition, Bi³⁺ doping can flatten the dielectric response of the ceramic material in a wider temperature range and transform it into a relaxor ferroelectric [15].

One of the main challenges in the production of BaTiO₃ ceramics is achieving a high sintering density that is close to its theoretical value. The density of the ceramic is determined by its microstructure. The unevenly distributed pores, as well as the inevitable trace water that is trapped in the pores, will result in a lower density and deterioration of its dielectric properties. Bismuth oxide (Bi₂O₃) is a low-melting flux that helps improve material density and reduce the sintering temperature [16,17]. As reported previously [18], the addition of Bi₂O₃ to BaTiO₃ matrix can effectively promote ceramic densification during sintering because a liquid phase is formed. Therefore, the introduction of Bi₂O₃ into BaTiO₃ matrix not only dopes Bi³⁺ ions into the A site to stabilize the dielectric performance in a wide temperature range but also improves the sintering properties. However, Bi₂O₃ usually volatilizes above 820 °C, so it is necessary to explore suitable doping concentrations.

Herein, an A-site nonstoichiometric doping strategy was proposed to increase the polarization type and attempt to stabilize the dielectric constant, as well as increase the dielectric constant. Trace Bi³⁺ donor ion-doped BaTiO₃ ceramics (Ba_1−1.5xBi_xTiO₃, abbreviated as BB100xT) were synthesized by the traditional solid-phase method. Pure BaTiO₃ was prepared as a control using the same method. The dielectric properties of all samples were studied at temperatures ranging from −100 to 200 °C. The mechanism of colossal dielectric properties and temperature stability was explained systematically through impedance spectra, dielectric frequency spectrum, and XPS energy spectra.

2. Materials and Methods

2.1. Preparation of Materials

Non-stoichiometric Ba_1−1.5xBi_xTiO₃ (BB100xT, x = 0.0025, 0.005, 0.0075, 0.01) powders were prepared by a conventional solid-state method, with BaCO₃ (99%), TiO₂ (≥99%), and Bi₂O₃ (≥99%) as raw materials. The powders of raw materials were accurately weighed and ball-milled in ethanol for 12 h, followed by pre-sintering at 800 °C for 2 h. Subsequently, the pre-sintered powders were mixed with a 5% mass fraction of polyvinyl alcohol and pressed into pellets with a diameter of 8 mm. A series of ceramic samples were obtained by sintering at an optimal temperature in the range of 1350–1450 °C for 2 h. The ceramic pellets were well polished, with silver plating on both sides, and heat-treated at 550 °C for 10 min to ensure good electrical contact. Au electrodes were also deposited on the ceramic pellets for comparison.

2.2. Characterization

X-ray powder diffraction (XRD) using a Bruker D8 advance device (Ettlingen, Germany) was carried out to study the phase structure, with Cu Kα (λ~1.54056 A) as a radioactive source at a speed of 10 °/min in a scanning range of 20–70° at room temperature. The surface microstructures were observed by JSM-7500 F scanning electron microscopy (SEM, JEOL, Tokyo, Japan). The average grain size was estimated using size measurements from SEM images using the image processing program Nano Measurer 1.2. The density of the ceramic samples was measured by the Archimedean method. The dielectric temperature spectra of the ceramics were measured by an LCR automatic tester (TH2827A, Tonghui, Changzhou, China) and a dielectric test system (DPTS20005P1, Yanhe Technology Co., Ltd., Wuhan, China). The dielectric frequency spectra were investigated using a combination of a physical properties measurement system (PPMS) and precision LCR instrumentation (Agilent E4980A, Santa Clara, CA, USA) in the frequency range of 10²–10⁶ Hz. A precision impedance analyzer was adopted to measure the changes in impedance and modulus at different temperatures and frequencies. The test results were analyzed by ZView 3.4 software. X-ray photoelectron spectroscopy (XPS) was collected through an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), with the amorphous carbon’s C 1s orbital of 284.8 eV as the standard for all binding energies.

3. Results and Discussion

Figure 1 shows the XRD patterns of all BB100xT samples, where all peaks match well with the standard card (JCSD#05-0626). The XRD peaks indicate that all samples have a single tetragonal phase structure with a space group of P4mm. No second phase is formed, indicating that Bi³⁺ has been fully incorporated into the BaTiO₃ to form a solid solution structure. The ion radius of the twelve-coordinate Bi³⁺ is 0.145 nm [19], which is smaller than the ion radius of the twelve-coordinate Ba²⁺ (0.161 nm), so the A-site doping of Bi³⁺ is theoretically expected to cause cell contraction and the diffraction peaks to shift towards higher angles. However, there is no obvious movement of the peaks, which may be due to the trace doping of Bi³⁺.

The crystallographic study assesses crystalline features such as crystallite size, microstrain, and dislocation density by utilizing Equations (1)–(3) based on XRD data [20,21,22], which are plotted in Figure 1f–i.

$$D=0.9\lambda /\beta cos\theta$$

(1)

$$\delta =1/D^2$$

(2)

$$\epsilon =\beta /4 tan\theta$$

(3)

where D, λ, θ, β, δ, and ε represent crystallite size, wavelength of the incident X-rays (1.5406 Å), diffraction angle, full width at half maximum (FWHM), dislocation density, and microstrain, respectively. Crystallite size decreases with increasing bismuth content due to smaller ionic radii of Bi as compared to Ba and the generation of A-site vacancies at x = 0.5%. While x > 0.5%, the mass transfer effect promoted by Bi³⁺ volatilization increases the crystallite size. With the crystallite size decreasing first and then increasing, the microstrain shows the opposite trend, and the change is slightly delayed, which illustrates the dependence relationship of crystallite size and stress. Additionally, the dislocation density increases with the increment of Bi content, manifesting that introduction of Bi results in more defects and compositional fluctuation. The correlating parameters are shown in

Table 1. Refined structural parameters and crystalline features of BB100xT ceramic samples.

Samples	Cell Volume (Å3)	Cell Parameter (Å)	Particle Size (nm)	Microstrain (×10−3)	Dislocation Density (nm−2)
BB0.25T	64.304	a = b = 3.99253	67	2.22995	0.60960
		c = 4.03405
		α = β = γ = 90°
BB0.5T	64.301	a = b = 3.99243	64	2.38064	0.82159
		c = 4.03405
		α = β = γ = 90°
BB0.75T	64.299	a = b = 3.99224	69	2.49030	0.93061
		c = 4.03431
		α = β = γ = 90°
BB1T	64.290	a = b = 3.99176	73	2.40596	1.04808
		c = 4.03472
		α = β = γ = 90°

. Rietveld refinements of XRD patterns for BB100xT are shown in Figure 1b–e. The refined structural parameters and calculated cell volume are shown in Table 1 to compare the changing trends between different compositions. With the increment of the Bi³⁺ doping amount, parameters a and c decrease slightly, as well as cell volume, further verifying the replacement of Bi for Ba and the vacancies produced.

The surface and fracture SEM images of BB100xT ceramics with different Bi³⁺ contents are shown in Figure 2. These images show that all the components have clear and distinct grain boundaries and are tightly packed grains. The addition of different amounts of Bi₂O₃ does not result in significant differences in the morphology of the BBT ceramic. It was found that the grain size changes with the variation of Bi³⁺ doping concentration. For lower concentrations, with the increase in the Bi amount, the related defects may segregate to the grain boundaries, preventing grain growth and causing grain refinement, resulting in a reduction in grain size from x = 0.25% to 0.75%. The maximum grain size appears at BB0.25T, which shows significant pores at the grain boundaries, with a relative density of 93.05%. BB0.5T and BB0.75T exhibit compact microstructures without visible pores. The BB0.5T ceramic has the highest relative density of 95.17%, and then decreases monotonically. As the doping concentration further increases, due to the volatilization of excess Bi³⁺ during sintering, a liquid phase is formed, which promotes mass transfer and causes a further increase in grain size and voids between boundaries at x = 0.01 [23]. The BB1T ceramic is found to be the most unsound, but all components maintain a relative density of above 90%, as shown in

Table 2. Dielectric properties and relative density of BB100xT ceramic samples.

Samples	εr (Max)	Tanδ (25 °C)	Tc/°C	ρ/%	TCC (≤±15%)
BB0.25T	10,388	0.085	143	93.05
BB0.5T	37,174	0.134	120	95.17	−27–141 °C
BB0.75T	24,753	0.191	143	94.17
BB1T	23,198	0.129	151	91.98

.

Figure 3 shows the dielectric properties of BB100xT ceramic samples coated with silver electrodes. As can be seen from Figure 3a, BB0.25T ceramics have a single dielectric peak, similar to that of pure BaTiO₃ due to the low doping content of Bi³⁺. But the addition of Bi³⁺ still increases its dielectric constant and makes the Curie peak move towards higher temperature, which is a sign of Bi³⁺ doping in the A site [24]. When x = 0.5%, the volatility of bismuth oxide can promote the diffusion of Bi element, enhancing the chemical heterogeneity of the material, so as to disperse the Curie peak and enhance the temperature stability of the material by the diffusion broadening effect. Its temperature coefficient of capacitance (TCC) < ±15% is in the temperature range of −27–141 °C, as shown in Figure 3c. With a further increase of Bi doping concentration, BB0.75T and BB1T ceramics still retain a higher Curie temperature, but the temperature stability deteriorates. This is because the excessive content of Bi³⁺ makes the internal components of the ceramic evenly distributed, and the multi-peak superposition effect is lost. The detailed dielectric properties of BB100xT ceramics are listed in Table 2.

Since the ionic radius and electronegativity of Bi and Ba are almost similar, the doped Bi³⁺ ion (1.45 A) in BaTiO₃ usually replaces the A-site Ba²⁺ (1.61 A). Due to the difference in ion valence, every two Bi³⁺ ions that replace Ba²⁺ ions will create an A vacancy (shown in Figure 4), which will result in compositional fluctuations on a microscopic scale, and then, the sample heterogeneity is enhanced. Therefore, the dielectric peak dispersion of BB0.5T ceramics can be attributed to the compositional fluctuation and substitution disorder of cations in the lattice, resulting in microscopic inhomogeneity in the perovskite structure, namely a superimposed effect of Curie peaks. The relevant defect equation can be expressed according to the Kröger–Vink symbol as follows.

$$B{i}_2{O}_3\stackrel{ Ba }{\to }2B{i}_{Ba}^{•}+V_{Ba}^{″}+3O_O^{\times }$$

(4)

where $B{i}_{Ba}^{•}$ represents the bismuth atom at the barium site with a positive charge, $V_{Ba}^{″}$ represents the barium space with two negative charges, and $O_O^{\times }$ represents the neutral oxygen atom at the oxygen site. Since $B{i}_{Ba}^{•}$ at the A site is positively charged, a barium vacancy is formed through a charge compensation mechanism to maintain charge balance. Through Bi³⁺ doping, the Curie temperature changes from 120 to 150 °C, and the dielectric constant increases from about 6000 of pure BaTiO₃ to 37,174. Meanwhile, the A-site ion volume decreases due to the presence of barium vacancy, which provides a larger space for Ti⁴⁺ ions. In addition, when the valency of the A site increases from +2 to +3, the excess positive charges also enhance the interaction between the A and B sites. Thus, the polarization of Ti⁴⁺ is enhanced, resulting in an increase in dielectric constant. On the other hand, the grain structure is also an important factor of dielectric properties. As the grain structure becomes denser, the number of polarized ions per unit volume increases when an electric field is applied, and the dielectric constant also increases accordingly [6]. Therefore, the BB0.5T ceramic with the most densely packed structure has the largest dielectric constant, while the samples with relatively lower densities, such as BB0.75T and BB1T, have smaller dielectric constants. The dielectric properties of BB100xT ceramics have a strong dependence on their composition and structure.

The dielectric frequency spectra of BB100xT ceramics are shown in Figure 3b. It can be seen that the dielectric constant of BB0.5T ceramic remains above 10⁴ up to 10⁵ Hz. However, the sample’s A-site defects and compositional inhomogeneity lead to an increase in the concentration of free charge carriers, resulting in high dielectric loss at high frequencies. The temperature-dependent dielectric constant curve shows an increase in dielectric loss of BB0.75T and BB1T ceramics before the Curie temperature, because an excessive doping concentration of Bi₂O₃ is prone to volatilize, forming free mobile vacancies. The defect equation is as follows.

$$2B{i}^{3+}+3O^{2-}\stackrel{ }{\to }B{i}_2{O}_3\uparrow +3V_O^{••}+2V_{Bi}^{‴}$$

(5)

Contrary to the above conclusion, Zheng et al. [24] found that a severe deficiency in Bi would lead to the formation of a large number of $V_{Bi}^{‴}$ defects, which would increase the disorder degree of the perovskite structure and severely suppress the εr-T curve. Wang et al. [25] also concluded that the phase transition point of Bi³⁺-doped BaTiO₃ would slightly shift to a higher temperature and reduce ε_max value. Therefore, besides the defect dipole of $B{i}_{Ba}^{•}-V_{Ba}^{″}-B{i}_{Ba}^{•}$ mentioned above, there may be other mechanisms contributing to the appearance of a colossal dielectric constant in the BB0.5T ceramic in this work.

AC impedance spectroscopy is an effective tool for studying the variation of ceramic dielectric properties with temperature. It can better reflect the internal structure of grains and the transport process of charge carriers. As shown in Figure 5a, from 299.15 K to 393.15 K, the ceramic consists of two serially connected parallel RQ circuits, i.e., (RgQg) (RgbQgb). Rg and Rgb represent the grain resistance and grain boundary resistance, respectively. Qg and Qgb are the corresponding constant phase elements. Only a Cole–Cole semicircle is observed at 443.15 K, which is fitted by a single parallel RQ element. These results indicate that the BB0.5T ceramic contains two components below 443.15 K, namely, semiconductor grains and insulating grain boundaries. Therefore, the Maxwell–Wagner interface polarization is one of the important reasons for the enhancement of the dielectric constant, originating from the increased local electric field at the interface between the conductive phase and insulating phase [26]. This led to the BB0.5T ceramic exhibiting colossal dielectric behavior.

Figure 5b shows the impedance–modulus plot of the BB0.5T ceramic. The peak height of Z″ is proportional to the R of the component, while M″ is inversely proportional to the C of the component. The modulus curve represents the grain response, while the impedance reflects the resistance caused by the internal barrier layer capacitance effect (IBLC). Therefore, the difference between the M″ and Z″ curves will depend on the electrical uniformity of the ceramic material, which is related to the long-range or short-range transport behavior of the charge carriers [27]. As can be seen, the impedance increases sharply in the low-frequency region, while the modulus fluctuates in the high-frequency part, i.e., the impedance and modulus curves do not overlap. The above results show that the mobile charge carriers within the grains break the long-range order and conduct short-range transport, which provides more evidence for the dispersion of the dielectric peak in the BB0.5T ceramic.

Because the interface polarization includes not only the IBLC effect within grains but also the SBLC effect between grains and electrodes, to further investigate whether the SBLC effect contributes to the colossal dielectric behavior of the BB0.5T ceramic, the ceramic was coated with gold electrodes on both ends for dielectric spectroscopy measurements. The results were compared with those obtained by the sample coating with silver electrodes. As shown in Figure 3d, the dielectric constant and dielectric loss of the sample coated with gold electrodes are significantly increased and become less stable, indicating that the SBLC effect exists in the ceramic. Therefore, the IBLC and SBLC effects cooperate to produce the colossal dielectric properties of the BB0.5T ceramic.

Ion substitution leads to the formation of point defects, which are closely related to the microstructure and dielectric properties of perovskite ceramics. Therefore, the XPS of BB0.5T ceramic samples was used to further prove the above deduction. Figure 6a shows the total spectrum of the ceramic sample, verifying the existence of Ba, Ti, O, and Bi characteristic peaks. The Ba 3d_5/2 and Ba 3d_3/2 peaks are located at 778.8 eV and 793.8 eV, respectively, while the peaks at a higher binding energy of 780.9 eV and 794.8 eV are characteristic of Ba vacancies [28], proving the substitution of Bi³⁺ at the A site, as shown in Figure 6b. The O 1s peak is divided into three small peaks, attributed to Ti-O bonds, oxygen vacancies, and surface water [29], where oxygen vacancies are inevitably produced by the evaporation of Bi₂O₃ in the ceramic. The peaks at 458.8 eV and 464.6 eV correspond to Ti 2p_3/2 and Ti 2p_1/2, respectively, while the peaks at 457.1 eV and 463 eV prove the existence of Ti³⁺ [30]. This is because the substitution of Bi³⁺ at the Ba²⁺ site causes an imbalance in charge, producing not only Ba vacancies but also mobile electrons, by the charge compensation mechanism. Thus, the electrons will reduce Ti⁴⁺ to Ti³⁺ (shown in Figure 6c,d).

4. Conclusions

In this work, a series of Ba_1−1.5xBi_xTiO₃ ceramic samples with different donor contents were synthesized through the traditional solid-state method. The influence of donor ions Bi³⁺ on the phase structure, microstructure, and dielectric properties of the samples were investigated systematically. All the samples are single tetragonal phase structures, with the highest relative density of 95.17% in the BB0.5T ceramic. The grain size of the ceramic samples decreases first and then increases as the Bi³⁺ content increases. The BB0.5T ceramic has a stable colossal dielectric constant (>3 × 10⁴) in the temperature range of −27–141 °C. The donor ion Bi³⁺ is introduced in the A site, increasing the fluctuation and disorder of the cation arrangement in the lattice, thereby enhancing the microscopic inhomogeneity of the grains, resulting in the superposition of materials with different Curie peaks. The colossal dielectric constant is the result of defect dipoles, and IBLC and SBLC effects. This study provides a theoretical basis for exploring new capacitors with colossal dielectric constant and temperature stability.