Trans. Nonferrous Met. Soc. China 30(2020) 756-764

Dielectric properties and relaxor ferroelectric behavior of (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics

Xin ZHONG, Chen ZHANG, Fang-xu CHEN, Zhu-ming TANG, Gang JIAN

Department of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China

Received 29 May 2019; accepted 24 December 2019

Abstract: The microstructure, dielectric and ferroelectric properties of (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ (y=0-0.05) ceramics prepared by traditional solid state method were investigated by X-ray diffractometer, scanning electron microscope, electric parameter testing system and ferroelectric tester. It is found that the barium zirconate titanate based ceramics are single-phase perovskites as y increases up to 0.05 and their average grain size decreases with the increase of y. The permittivity maximum ε_r,max is suppressed from 8948 to 1611 at 1 kHz with increasing y, and the ferroelectric-paraelectric phase transition temperature T_m decreases from 93 to -89 °C at 1 kHz as y increases. The composition-induced diffuse phase transition is enhanced with increasing y. The relaxor-like ferroelectric behavior with a strong frequency dispersion of T_m and permittivity at Tm accompanied by a strong diffuse phase transition is found for the system with high y value. The remnant polarization decreases with increasing y, while the coercive field decreases remarkably and then increases with the increase of y.

Key words: barium zirconate titanate; dielectric properties; relaxor ferroelectric behavior; perovskite; phase transition

1 Introduction

The perovskite materials such as BaTiO₃, Na_0.5Bi_0.5TiO₃, K_0.5Na_0.5NbO₃ and Nd_1-xBa_xCoO₃ are responsible for the development in modern technology to a great extent since they are a vital part of capacitors, pyroelectric detectors, ultrasound sensors, actuators and so forth [1-6]. Among them, barium zirconate titanate (Ba(Ti_1-xZr_x)O₃, or BZT), the solid solution of BaTiO₃ and BaZrO₃, has received extensive attention for its potential applications in ceramic capacitors owing to its eminent dielectric performance [7-9].

BaTiO₃ goes through three phase transitions from cubic to tetragonal (at 120 °C), tetragonal to orthorhombic (at 5 °C) and orthorhombic to rhombohedral phase (at -90 °C) when cooling [10]. The permittivity (ε_r) normally reaches a maximum value at the ferroelectric to paraelectric phase transition temperature known as Curie temperature, T_C. Above T_C, the temperature dependence of permittivity follows the Curie-Weiss law. As the Zr⁴⁺ ions occupy the central sites of oxygen octahedrons in BaTiO₃, the cubic-tetragonal phase transition temperature decreases and the tetragonal- orthorhombic phase transition temperature shifts towards higher value with increasing Zr⁴⁺ content, leading to a pinched phase transition (x<0.15) [11]. For further substitution (x>0.15), only a rhom- bohedral-cubic phase transition takes place [12]. Also, the diffuse paraelectric-ferroelectric phase transition stimulated by increasing the amount of zirconium is noticed [13]. As x approaches 0.3, the ferroelectric behavior of BZT system changes from a “normal” one to a typical relaxor one which mainly occurs in lead-based compositions with more than one type of ions occupying the equivalent six-coordinated crystallographic sites. Although the strong diffuse paraelectric-ferroelectric phase transition of BZT relaxor ferroelectrics helps to improve temperature stability of permittivity, the low transition temperature, for example 198 K for Ba(Ti_0.7Zr_0.3)O₃, results in a very low permittivity at room temperature, which restricts the real applications of the BZT ceramics with high Zr content in capacitors [14].

Trivalent rare earth cations (La, Dy, Sm, Eu, Y, Nd, etc) help to induce a relaxor-type behavior of BZT ceramics in a manner similar to the effects of an increment of zirconium [9,15-18]. Moreover, the diffusivity degree of phase transition increases due to both the increment of the lanthanide content and the increase of the ionic radius of the dopant element. The ytterbium dopant is found to enormously enhance and broaden the relative permittivity maximum of BZT ceramics with the increase of ytterbium content. A novel Y5V dielectric material with the room temperature dielectric constant over 23000 and the dielectric strength higher than 4.0 kV/mm under alternating current (AC) field is developed in the ytterbium- doped BZT ceramics [19]. The composites of the Ba(Zr_0.07Ti_0.93)O₃ ceramic and the CoO nano- particles (1.0-3.0 vol.%) also show broad relative permittivity versus temperature (ε_r-T) curves with frequency dispersion [20]. However, less effort has been put into finding a balance between the fine temperature stability of permittivity and high permittivity at room temperature by introducing an analogous system with different aliovalent ions in the equivalent six-coordinated crystallographic sites in the BZT matrix. Therefore, the present study involves the modification in structural, dielectric and ferroelectric properties of Ba(Zr_0.1Ti_0.9)O₃ matrix by successively adding Ba(Zn_1/3Nb_2/3)O₃. The study highlights the effects of Ba(Zn_1/3Nb_2/3)O₃ on dielectric parameters, phase transition and ferroelectric behavior of BZT ceramics to explore their potential for practical capacitor applications in bypassing or coupling circuits.

2 Experimental

The (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ (y=0, 0.01, 0.02, 0.03, 0.04 and 0.05) (short for BZT-BZN) specimens were synthesized by traditional solid state method. High purity BaCO₃ (>99.0%), ZrO₂ (>99.0%), TiO₂ (>98.0%), ZnO (>99.0%) and Nb₂O₅ (>99.0%) powders, namely the starting raw materials, were weighed according to Table 1. After being ball-milled in deionized water for 24 h, the slurries were dried and then the obtained mixtures were calcined in air at 1080 °C for 2 h. After calcination, the powders were reground and added with 5 wt.% polyvinyl alcohol (PVA) for granulation. Sintering was conducted in air at 1360 °C for 2 h after green pellets were obtained by die pressing under 250 MPa. For dielectric measurement, the specimens with BQ-5311 silver paste coated on the top and bottom surfaces were fired at 800 °C for 10 min.

Table 1 Masses of raw materials for (1-y)Ba- (Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics

The crystalline structures of the ceramics were confirmed by X-ray diffraction analysis (XRD, Rigaku D/max 2500v/pc) with Cu K_α radiation at the step of 2 (°)/min. And the scanning electron microscope (JSM-6480 ESEM) was applied for observing the surface morphologies of the as- sintered specimens. The average grain size of BZT-BZN ceramics was calculated using the Nano Measurer software. The temperature dependence of dielectric parameters was measured at 200 Hz, 1 kHz, 10 kHz, 100 kHz, 250 kHz and 500 kHz from -180 to 230 °C with TZDM-200-300C automatic electric parameter measuring system. The polarization-electric field (P-E) hysteresis characteristics were obtained by utilizing the Radiant Precision Premier II ferroelectric tester.

3 Results and discussion

3.1 Crystal structure

Figure 1 shows the XRD patterns of as-sintered (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ bulk ceramics. All of the samples exhibit typical tetragonal perovskite structure (space group P4mm), without obvious evidence of the secondary phase, indicating that the present system is a single phase solid solution with Ba²⁺ ion in the A site and aliovalent ions (Zr⁴⁺, Ti⁴⁺, Zn²⁺ and Nb⁵⁺) in the B site of perovskite structure even when y reaches up to 0.05.

3.2 Surface morphology

Figure 2 shows the surface morphologies of as-sintered (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics. It is obvious that all the samples exhibit dense microstructures and fine grain size distribution. The average grain size of BZT-BZN ceramics shown in Table 2 decreases obviously with the increase of Ba(Zn_1/3Nb_2/3)O₃ content. In other words, the grain growth for pure Ba(Zr_0.1Ti_0.9)O₃ ceramic is significantly inhibited after the combination with Ba(Zn_1/3Nb_2/3)O₃.

Fig. 1 XRD patterns for (1-y)Ba(Zr_0.1Ti_0.9)O₃- yBa(Zn_1/3Nb_2/3)O₃ ceramics

3.3 Frequency characteristics

Fig. 2 SEM micrographs of (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics

Table 2 Average grain sizes of (1-y)Ba(Zr_0.1Ti_0.9)- O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics

Fig. 3 Temperature dependence of relative dielectric constant and dielectric loss for (1-y)Ba(Zr_0.1Ti_0.9)O₃- yBa(Zn_1/3Nb_2/3)O₃ ceramics at different frequencies

The temperature dependence of the relative dielectric constant and the dielectric loss for (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics at 200 Hz, 1 kHz, 10 kHz, 100 kHz, 250 kHz and 500 kHz is shown in Fig. 3. All BZT-BZN ceramics possess the dielectric constant peaks corresponding to the ferroelectric-paraelectric phase transition in the temperature range from -180 to 230 °C. The maximum value of the dielectric permittivity is symbolized as ε_r,max while using T_m as the temperature corresponding to this permittivity maximum. As shown in Fig. 3(a), the relative dielectric constant of pure Ba(Zr_0.1Ti_0.9)O₃ ceramic remains almost same as the test frequency increases from 200 Hz to 500 kHz. Particularly, there is no frequency dispersion of the dielectric maximum ε_r,max of the pure Ba(Zr_0.1Ti_0.9)O₃ ceramic. Also, as the test frequency increases, no shift in T_m for Ba(Zr_0.1Ti_0.9)O₃ ceramic is detected. The dielectric behavior of (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)- O₃ ceramics can be easily distinguished from that of the pure Ba(Zr_0.1Ti_0.9)O₃ ceramic when Tm. The ε_r (in the range of Tm) and especially the permittivity maximum of BZT-BZN ceramics decreases obviously with the increase of test frequency, as indicated in Figs. 3(b-f). The frequency dispersion of ε_r,max clearly demonstrates a relaxation nature, namely a relaxor-type permittivity, for (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3- Nb_2/3)O₃ ceramics. The dielectric loss of all BZT-BZN samples also shows obvious frequency- dependent characteristics.

The T_m for BZT-BZN samples moves toward higher temperatures with increasing frequency. The empirical relaxation strength describing the frequency dispersion of T_m is defined as [7,14]

ΔT_rs=T_{m(500 kHz)}-T_{m(1 kHz)} (1)

The values of ΔT_rs and T_m at different frequencies for BZT-BZN samples are given in Table 3. The ΔT_rs increases from 2 °C for 0.99Ba(Zr_0.1Ti_0.9)O₃-0.01Ba(Zn_1/3Nb_2/3)O₃ ceramic to 10 °C for 0.95Ba(Zr_0.1Ti_0.9)O₃-0.05Ba(Zn_1/3- Nb_2/3)O₃ ceramic, indicating an enhanced frequency dispersion of T_m in BZT-BZN ceramics with the increase of y.

Table 3 ΔT_rs and T_m at different frequencies for (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics

3.4 Diffuse phase transition

In order to describe the broadening in ε_r-T curve, the full-width of half-maximum (FWHW) of ε_r-T curve at 1 kHz is calculated here. The FWHW increases from ~47 °C (from 69 to 116 °C) for pure Ba(Zr_0.1Ti_0.9)O₃ sample to ~94 °C (from -14 to 80 °C) for 0.98Ba(Zr_0.1Ti_0.9)O₃-0.02Ba(Zn_1/3Nb_2/3)- O₃ sample and ~181 °C (from -111 to 70 °C) for the 0.96Ba(Zr_0.1Ti_0.9)O₃-0.04Ba(Zn_1/3Nb_2/3)O₃ sample. With increasing y, the Curie peak of (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics becomes more broadened, suggesting a diffuse phase transition around T_m. Besides the flattening of the dielectric constant (ε_r) versus temperature (T) curve previously mentioned, the diffuse phase transition is also characterized by a huge separation in temperature between the maximum of the real (dielectric constant) and imaginary (dielectric loss) parts of the dielectric spectrum, as indicated in Fig. 3.

The modified Curie-Weiss law is utilized to describe the diffuseness of the ferroelectric phase transition as well [21]:

(2)

where γ and C′ are assumed to be constants. The parameter γ (commonly between 1 and 2) describes the characteristic of the phase transition. When γ equals 1, a normal Curie-Weiss law is obtained. At the other extreme (γ=2), a complete diffuse phase transition is described. The parameters γ can be determined by the slope after linear fitting the data through ln(1/ε_r-1/ε_r,max)-ln(T-T_m). The plots of ln(1/ε_r-1/ε_r,max) as a function of ln(T-T_m) for (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics are shown in Fig. 4. The γ value increases as the y increases, clearly indicating that the (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics exhibit an enhanced diffuse ferroelectric-para- electric phase transition behavior with the increase of Ba(Zn_1/3Nb_2/3)O₃ content.

Fig. 4 Plots of ln(1/ε_r-1/ε_r,max) as function of ln(T-T_m) for (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics at 1 kHz

Since there is no frequency dispersion of ε_r and T_m, a normal ferroelectric behavior accompanied by diffuse phase transition is obtained for pure Ba(Zr_0.1Ti_0.9)O₃ ceramic. However, Ba(Zr_0.1Ti_0.9)O₃ ceramics with high Ba(Zn_1/3Nb_2/3)O₃ content exhibit the typical relaxor-like ferroelectric behavior with a strong frequency dispersion of ε_r at Tm and a strong diffuse phase transition. Moreover, the diffuse transition behavior and the frequency dispersion are enhanced with increasing Ba(Zn_1/3Nb_2/3)O₃ content, implying a composition-induced relaxor ferroelectric behavior.

3.5 Dielectric properties

Due to the above-mentioned intensified diffuse phase transformation, the temperature stability of permittivity is expected to be improved. For characterizing this property, the permittivity variation (Δε_r/ε_r) (from -30 to 85 °C and from -55 to 125 °C) at 1 kHz determined using the following equation is calculated:

(3)

where ε_r,RT is the relative dielectric constant at room temperature (25 °C) and ε_r,T is the relative dielectric constant at any other temperature.

The ε_r,RT, ε_r,max, Δε_r/ε_r and the dielectric loss at room temperature (tan δ_RT) are collected in Table 4. It is obvious that the ε_r,RT and the tan δ_RT increase initially and then decrease as y increases. The 0.96Ba(Zr_0.1Ti_0.9)O₃-0.04Ba(Zn_1/3Nb_2/3)O₃ sample exhibits moderate relative dielectric constant and dielectric loss at room temperature. The ε_r,max is suppressed from 8948 to 1611 as y increases from 0 to 0.05, which is attributed to the complex occupation of aliovalent ions in B sites of perovskite structure. The Δε_r/ε_r of Ba(Zr_0.1Ti_0.9)O₃ is also suppressed after being combined with Ba(Zn_1/3Nb_2/3)O₃, approaching the X7R standard (Δε_r/ε_r ≤±15% from -55 to 125 °C) [21] according to the Electronic Industries Association (EIA) as y increases. Although the present BZT-BZN ceramics still do not satisfy the X7R Standard, some of them are suitable for the Y5V (Δε_r/ε_r≤22%-(-82%) from -30 to 85 °C) ceramic capacitor applications.

The T_m of BZT-BZN ceramics decreases significantly with the increase of Ba(Zn_1/3Nb_2/3)O₃ content (see Table 3), indicating that the Ba(Zn_1/3Nb_2/3)O₃, actually the lattice deformation resulting from the B site occupation of Zn²⁺ and Nb⁵⁺ ions in BZT ceramics, can remarkably influence the ferroelectric-paraelectric phase transition temperature of present system.

3.6 Ferroelectric properties

Figure 5 shows the ferroelectric hysteresis loops of (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics at room temperature. The P-E loops for samples with y=0 or 0.01 confirm the ferroelectric nature of pure Ba(Zr_0.1Ti_0.9)O₃ ceramics and BZT-BZN ceramics with low Ba(Zn_1/3Nb_2/3)O₃ contents. However, the almost linear P-E curves for samples with y=0.04 or 0.05 as illustrated in Fig. 5 indicate the paraelectric nature of Ba(Zr_0.1Ti_0.9)O₃ceramics with high Ba(Zn_1/3Nb_2/3)O₃ contents. This demonstrates that the (1-y)Ba- (Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics transit from ferroelectric to paraelectric phase at room temperature with increasing y, which consists with the result revealed by the decreasing of T_m.

The remnant polarization (2P_r) and coercive field (2E_c) of (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3- Nb_2/3)O₃ ceramics are given in Table 5. The remnant polarization decreases obviously with increasing Ba(Zn_1/3Nb_2/3)O₃ content. The coercive field of BZT-BZN ceramics initially decreases remarkably with the increase of y and then increases slightly. The P_r and E_c are commonly used for characterizing the ferroelectric properties and are strongly depended on grain size, phase transition and some other factors like defect concentration [10,22,23]. The ceramics with large grains prefer to exhibit larger P_r and lower E_c [24]. The smaller grains in ferroelectric ceramics restrict the formation of large ferroelectric domains and thus reduce the effective contribution to total polarization [25]. Also, it is more difficult for the reversal polarization of a ferroelectric domain inside a small grain than in a large one [22]. Therefore, the above-mentioned decrease of the average grain size with increasing y leads to the decrease of P_r as well as the increase of E_c. The increasing amount of paraelectric phase in BZT ceramics commonly results in a smaller P_r [26]. The ferroelectric-paraelectric phase transition in the present concerned BZT-BZN ceramics with increasing Ba(Zn_1/3Nb_2/3)O₃ content also contributes to the decrease of P_r and E_c. Obviously, the variation of E_c with increasing y value is determined by the competitive result between the decreasing grain size and the increasing amount of paraelectric phase.

Table 4 Dielectric parameters for (1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics at 1 kHz

Fig. 5 Ferroelectric hysteresis loops of (1-y)Ba-(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ ceramics at room temperature

Table 5 2P_r and 2E_c values for (1-y)Ba(Zr_0.1Ti_0.9)O₃- yBa(Zn_1/3Nb_2/3)O₃ ceramics

4 Conclusions

(1) The BZT-BZN ceramic is a single phase solid solution with perovskite structure as y increases up to 0.05.

(2) The average grain size of BZT-BZN ceramics decreases from 54.27 to 1.12 μm with the increase of Ba(Zn_1/3Nb_2/3)O₃ content.

(3) The T_m of BZT-BZN ceramics is shifted to lower temperature by increasing the Ba(Zn_1/3Nb_2/3)O₃ content. The permittivity variation is suppressed with increasing y. According to the increase of critical exponent γ from the modified Curie-Weiss law, the composition-induced diffuse phase transition behavior of BZT-BZN ceramics is enhanced with increasing Ba(Zn_1/3Nb_2/3)O₃ content.

(4) The relaxor-like ferroelectric behavior with a strong frequency dispersion of T_m and ε_r at Tm accompanied by a strong diffuse phase transition is found for BZT-BZN ceramics with high Ba(Zn_1/3Nb_2/3)O₃ content.

(5) Due to the decreased grain size and the increased amount of paraelectric phase, the remnant polarization of BZT-BZN ceramics decreases with increasing y, while the coercive field decreases remarkably and then increases with the increase of Ba(Zn_1/3Nb_2/3)O₃ content.

Acknowledgments

This work is sponsored by the National Demonstration Center for Experimental Materials Science and Engineering Education (Jiangsu University of Science and Technology, China). This work is also funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, China.

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(1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃陶瓷的介电性能及弛豫铁电行为

钟 鑫，张 晨，陈方旭，唐柱明，简 刚

江苏科技大学 材料科学与工程学院，镇江 212003

摘 要：采用传统固相法制备(1-y)Ba(Zr_0.1Ti_0.9)O₃-yBa(Zn_1/3Nb_2/3)O₃ (y=0~0.05)陶瓷，利用XRD、 SEM、电参数测试系统以及铁电测试仪研究其显微结构、介电性能及铁电行为。结果表明：随着y增大至0.05，该锆钛酸钡基陶瓷始终为单相钙钛矿结构，其平均晶粒尺寸随y的增大而减小。介电常数最大值ε_r,max在1 kHz下随y增大从8948显著降低至1611，且其铁电-顺电相变温度T_m亦随y增大从93 °C降低至-89 °C。该陶瓷成分诱导弥散相变随y增大而逐渐增强。高y取值下陶瓷表现出以T_m及铁电温区介电常数的强烈频率色散及弥散相变为典型特征的弛豫铁电行为。该陶瓷体系的剩余极化强度随y增大而降低，而矫顽场则在显著降低后有所提高。

关键词：锆钛酸钡；介电性能；弛豫铁电行为；钙钛矿；相变

(Edited by Bing YANG)

Corresponding author: Chen ZHANG; Tel: +86-511-84401184; Fax: +86-511-84407381; E-mail: czhang1981@hotmail.com

DOI: 10.1016/S1003-6326(20)65251-7