J Mater Sci: Mater Electron DOI 10.1007/s10854-016-5552-8

Sintering temperature dependence of dielectric properties and energy-storage properties in (Ba,Zr)TiO3 ceramics Yan Zhang1,2 • Yaoyao Li1,2 • Haikui Zhu1,2 • Zhenxiao Fu3 • Qitu Zhang1,2

Received: 10 July 2016 / Accepted: 11 August 2016 Ó Springer Science+Business Media New York 2016

Abstract BaZr0.1Ti0.9O3 ceramics are prepared via the conventional solid state reaction method. The Zr4? ions have diffused into the BaTiO3 lattices to form a homogenous solid solution. We investigate the dielectric properties and energy storage density of BaZr0.1Ti0.9O3 ceramics at different sintering temperature. The temperature dependence of dielectric constant of BaZr0.1Ti0.9O3 ceramics illustrates the obvious relaxor phase transition characteristics. The polarization hysteresis loops P–E of ceramics sintered at 1260–1300 °C show slimmer comparable to that of ceramics sintered at 1240 °C, resulting in high energy storage density. Excellent dielectric properties and energy storage density are achieved in the BaZr0.1Ti0.9O3 ceramics sintered at 1260 °C for 2 h: er = 2998, tand = 0.007 and J = 0.5 J cm-3.

1 Introduction Materials with high dielectric constant, low dielectric loss, high electric breakdown strength and piezoelectric coefficients are very attractive for high energy density capacitors [1–5]. Lead-based perovskite ceramics have been gotten much attention and extensively study due to their excellent

& Qitu Zhang [email protected] 1

College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China

2

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, Jiangsu, China

3

Guangdong Fenghua Advanced Technology Company Limited, Zhaoqing 526020, Guangdong, China

dielectric properties and high energy storage density, and Pb(Zr,Ti)O3-based ceramics are widely used as piezoelectric actuators, sensors, and transducers [6, 7]. Lead-based ferroelectric PLZT thin films exhibit an energy density of *46 J cm-3 at 1.3 MV cm-1 and anti-ferroelectric Lamodified PbZrO3 thin films reach 60 MV m-1 breakdown strength with energy density *15 J cm-3 [8–11]. However, environmental issues have raised the need for nonhazardous materials with properties comparable to that of Pb-based materials for using in all kinds of device fabrications [12]. Therefore, lead-free perovskite ceramics have been investigated widely in recent years owing to the environmental protection concerns. Barium titanate (BaTiO3) and its solid solutions represent one of the most important dielectric and piezoelectric families [13, 14]. However, the application in a certain domain of pure BaTiO3 ceramics is limited because of the narrow working temperature-stable range and high dielectric loss [15]. The recently have published lead-free BZT–BCT, BST–BCT ceramics [16, 17]. For the BaZrxTi1-xO3 ceramics, after Zr4? ions diffuse into the BaTiO3 lattices, Zr4? ions replace Ti4? ions in the crystal lattice, Zr4? ions have good chemical stability with Ti4? ions, and the conduction by electron hopping between Ti4? ions and Ti3? ions would be decreased, thereby the dielectric loss is reducing. And then BaZrxTi1-xO3 can be adjusted to obtain a desired ferroelectric relaxor behavior by varying the stoichiometry [18–21]. Relaxor ferroelectrics have high Pmax, lower Prem and slender polarization hysteresis loop, which makes them promising candidate materials applied for the energy storage ceramic capacitors. There are few systematic reports about the effects of sintering temperature on the dielectric properties and energy storage density of the BaZrxTi1-xO3 ceramics. In the present work, we study and discuss in details the effects

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of sintering temperature on the dielectric properties and energy storage density of BaZr0.1Ti0.9O3 ceramics, which are synthesized by the solid state reaction method. The ceramics are sintered respectively at 1240, 1260, 1280 and 1300 °C for 2 h.

All the BaZr0.1Ti0.9O3 (BZT10) ceramic samples were fabricated by the conventional mixed-oxide method. High purity starting materials of barium titanate (BaTiO3), barium carbonate (BaCO3) and zirconium dioxide (ZrO2) were weighed in stoichiometric proportion, ball-milled in water, dried and then calcined at 1150 °C for 2 h. The obtained powders were mixed thoroughly with a binder of 7 wt% polyvinyl alcohol (PVA), and then were pressed at 100 MPa into disk-shaped samples with a diameter of 13.0 mm and thickness of about 3.0 mm, The BZT10 diskshaped samples were separately sintered at 1240, 1260, 1280 and 1300 °C for 2 h. X-ray diffraction patterns of samples were obtained with Cu Ka radiation (k = 0.1541 nm) over a 2h angle from 20° to 80° at room temperature. The surface morphologies of the samples were analyzed using scanning electron microscopy (SEM). Silver electrodes were painted on both sides of the polished samples and fired at 850 °C for 20 min. The dielectric constant (er) and dielectric loss (tand) of high energy storage ceramic capacitor materials were directly measured by 2611A type capacitance measuring instrument at 1 kHz. The electric field induced polarization (P-E) was measured by ferroelectric test system (P-PMF, Radiant). The energy storage performance was calculated according to the P-E results.

3 Results and discussion 3.1 Crystal structure and microstructures Figure 1 shows the typical XRD patterns of the BZT10 samples sintered at 1240, 1260, 1280 and 1300 °C for 2 h, respectively. It can be seen that all the characteristic peaks of the major phase can be assigned to the tetragonal phase of BaTiO3. But, the samples sintered at 1240 °C have any impurity phase, the impurity phase are (Ti, Zr)O2. Because the sintering temperature is too low, TiO2 and ZrO2 did not react completely, forming the impurity phase, the impurity phase is cannot be found obviously in the samples sintered above 1240 °C. The sintering temperature at 1260–1300 °C, the diffraction peaks of all the ceramics are indexed belonging to polycrystalline perovskite structure [22], which

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Fig. 1 XRD patterns of BZT10 ceramics sintered at 1240–1300 °C for 2 h

indicates that the B-site Zr4? ions have diffused into the BaTiO3 lattices and form a homogenous solid solution. Figure 2 shows the surface morphologies of the BZT10 ceramics as a function of the sintering temperature. There are several pores in samples sintered at 1240 and 1300 °C, but the densification of all the BZT10 ceramics are still relative high. It can be observed that the grain boundaries are clear, the average grain sizes of the sintered ceramic pellets are found in the range of 0.2–2 lm. When the sintering temperature increases from 1240 to 1300 °C, the average grain sizes increase slightly from 0.2–1 to 0.2–3 lm. This could be probably attribute to the high sintering temperature causing the abnormal grain growth. 3.2 Dielectric properties Figure 3 shows the dielectric constants, and dielectric losses of the samples as a function of sintering temperature. The date show that the dielectric constant (er) is the highest and dielectric loss (tand) is the lowest at 1260 °C, which is 2998 and 0.007, respectively. Figure 4 shows the variation of dielectric constant measured at 1 kHz as a function of temperature. The paraelectric-to-ferroelectric phase transition behavior, at about 100 °C which are lower than that of pure BaTiO3 (130 °C), is observed for the samples sintered at 1240–1300 °C temperatures. Furthermore, with the increasing sintering temperature, the Curie temperature (Tc) becomes more apparent. This result indicates the increase of ferroelectric phase, which is consistent with the polarization hysteresis loops P–E result. And the samples sintered at 1240–1280 °C generally have a flat dielectric constant response, especially, the dielectric constant of

J Mater Sci: Mater Electron Fig. 2 SEM micrographs of BZT10 ceramics with different sintering temperature: a 1240 °C, b 1260 °C, c 1280 °C, d 1300 °C

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Fig. 3 The dielectric properties of BZT10 ceramics sintered at various temperature

sintered at 1240 °C samples change slightly with the measurement temperature, which is most likely related to the small grain size and low crystallinity [23]. 3.3 Ferroelectric properties Figure 5 displays at higher applied fields, the polarization hysteresis loops P–E of BZT ceramics change obviously with sintering temperature. All the ferroelectric hysteresis loops P–E are obtained under the maximum electric field (30 kV cm-1) before the dielectric breakdown occurs. Polarization hysteresis loops P–E of BZT ceramics show obviously unsaturated. The main reasons are as follows:the

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Fig. 4 Variation of dielectric constant with temperature for the BZT10 samples sintered at different temperatures

sample surface is rough, it is easy to gather local charge, resulting in leakage current, the principle of instrument test and the upper and lower electrode interface may be asymmetric. When the sintering temperature is 1240 °C, the polarization hysteresis loop P–E shows plump. While the samples sintered at 1260–1300 °C, slender ferroelectric hysteresis P–E loops are obtained, attributing to the phase change from ferroelectric phase to the ferroelectric/antiferroelectric mixed phase in the BZT ceramics. With the increase in sintering temperature, the polarization hysteresis loops have a tendency of growing polarization values, indicating the enhanced ferroelectric properties

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Fig. 5 The P–E loops of the BZT10 ceramics with different sintering temperature: a 1240 °C, b 1260 °C, c 1280 °C, d 1300 °C

3.4 Energy storage density The energy storage density is calculated from P–E loop according to the formula [26] Z Pmax J¼ EdP ð1Þ

The results are shown in Fig. 6, which we plot calculate energy storage density as a function of sintering temperature. With the sintering temperature increasing, the energy 0.6

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[24, 25]. For example, the maximum polarization of the samples sintered at 1300 °C is 16.72 lC cm-2 under the electric field of 30 kv cm-1, which is 16 % higher than that of the samples sintered at 1240 °C (14.37 lC cm-2). The increase of polarization values could be probably due to enhanced grain growth and increased in domain size at higher sintering.

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where E is the strength of electric field and Pmax is the maximum electric polarization. Equation (1) suggests that high energy storage density can be obtained by huge difference between Pmax and Pr in one kind of material. Therefore, the ceramics possess higher energy storage density, its ferroelectric hysteresis P–E loops should be more ‘‘slanted’’ and ‘‘slender’’ [27].

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Fig. 6 The energy density measured from P–E loops as a function of sintering temperatures

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storage density of all samples is enhanced. And samples sintered at 1240 and 1300 °C have the lower values of energy storage density, which may be related to the leakage phenomenon and obviously unsaturated loops observed in the Fig. 4. At the same time, the Pr and electrical breakdown strength of the samples have a direct relationship for energy storage, many articles have reported that lower Pr and Ec are benefit for energy storage [28].

4 Conclusions Dielectric properties and energy storage density of the BaZr0.1Ti0.9O3 (BZT10) ceramics by the conventional solid state reaction method have been studied in the present work. The effects of sintering temperature on the dielectric properties of BZT10 ceramics for energy storage density capacitor applications have been systematically investigated. The ceramics sintered at 1260 °C possess excellent dielectric properties and energy storage density, er = 2998, tand = 0.007 and J = 0.5 J cm-3. The ceramics which the ferroelectric hysteresis loops P–E show more ‘‘slanted’’ and ‘‘slender’’ will possess higher energy storage density. Acknowledgments This work is supported by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (Project No. SKL201309SIC), as well as Science and Technology Projects of Guangdong Province (Project No. 2011A091103002). This work is partly supported by National Natural Science Foundation of China (51502132).

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