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Dielectric relaxation behavior and energy storage properties in SrTiO3 ceramics with trace amounts of ZrO2 additives
Author's Accepted Manuscript
Dielectric relaxation behavior and energy storage properties in SrTiO3 ceramics with trace amounts of ZrO2 additives Zhijian Wang, Minghe Cao, Zhonghua Yao, Zhe Song, Guangyao Li, Wei Hu, Hua Hao, Hanxing Liu
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)00870-0 http://dx.doi.org/10.1016/j.ceramint.2014.05.147 CERI8683
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
6 May 2014 30 May 2014 30 May 2014
Cite this article as: Zhijian Wang, Minghe Cao, Zhonghua Yao, Zhe Song, Guangyao Li, Wei Hu, Hua Hao, Hanxing Liu, Dielectric relaxation behavior and energy storage properties in SrTiO3 ceramics with trace amounts of ZrO2 additives, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.05.147 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dielectric relaxation behavior and energy storage properties in SrTiO3 ceramics with trace amounts of ZrO2 additives Zhijian Wang, Minghe Cao*, Zhonghua Yao, Zhe Song, Guangyao Li, Wei Hu, Hua Hao, Hanxing Liu State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China Corresponding author. Tel.: +86 27 87885813; Fax: +86 27 87885813 E-mail address: [email protected] (M. Cao) Abstract SrTiO3 ceramics with trace amounts of ZrO2 additives were prepared via the solid state reaction route. The effects of ZrO2 concentrations on microstructures, dielectric and energy storage properties of SrTiO3 ceramics were investigated. The grain size of the ceramics decreases with increasing ZrO2 concentration. Dielectric relaxation behaviors were observed in dielectric loss versus temperature plots and studied by the measurements of the activation energies, which indicated the behaviors were due to the doubly ionized oxygen vacancies and the space-charge polarization. An energy density of 1.62 J/cm3 and a breakdown strength of 289 kV/cm were achieved for SrTiO3 ceramics with 0.4 mol% ZrO2 addition, which were higher than those of pure SrTiO3 ceramics. Keywords: B. Microstructure; C. Dielectric relaxation; Energy storage; D. SrTiO3 ceramic; ZrO2 1. Introduction 1
As quantum paraelectrics, SrTiO3 (ST) ceramics with high dielectric constant (~290), low dielectric loss (< 0.01), high breakdown strength (~200 kV/cm) and favorable bias stability are widely required for the application of high energy storage density dielectrics. It is well known that the energy storage density grows linearly with the dielectric constant and quadratically with the breakdown strength (BDS). Many methods have been used to improve the energy storage density, such as enhancing the dielectric constant and/or the BDS. Though doping by other cations with small or big ionic radius could increase the dielectric constant of the ST-based ceramics, such as Ba2+, Mn2+, Bi3+, and trivalent rare earth (RE3+) [1-5], the BDS sharply decreases due to the changed structure and increased defect. It is well known that high BDS makes a more pronounced contribution to the energy density [6]. Generally, the BDS is mainly affected by several factors, such as porosity [7], grain size [8,9], secondary phase [10], temperature [11], charge injection [12] and interfacial polarization [13]. Both high relative density and homogeneously small grains are beneficial to the BDS. Many methods have been used to improve the microstructure, such as additives [14-18] (for example, glasses and/or oxides), chemical preparation [14] and new sintering technology. Among them, use of additives is the simplest and most effective way. Despite having a high BDS, glasses have a low dielectric constant which is not beneficial to the energy density of the ST-based ceramics [19]. In many studies, it has been proved that the addition of some oxides could effectively inhibit the grain growth and increase the BDS [20], but the dielectric constant decreases due to the low dielectric constant of the oxides. 2
In this work, ZrO2 was selected as the additive, because Zr4+ ions in ST can stabilize the charge of Ti and suppress the oxygen dissociation when sintered at high temperature [21], meanwhile, some Zr4+ ions will be deposited at grain boundaries to inhibit the grain growth [22]. In order not to make the dielectric constant lower, trace amounts of ZrO2 additives were determined and a desired result was obtained. 2. Experimental procedure Powders of analytical reagent grade, comprising SrCO3 (> 99.0 %), TiO2 (> 99.0 %) and ZrO2 (> 99.0 %), were used as the starting materials and mixed according to the composition, ST + x mol% ZrO2 (x = 0, 0.2, 0.4 and 0.6) (short as STZ0, STZ2, STZ4, STZ6, respectively). After ball-milled in alcohol for 24 hours, the slurry was dried, then calcined in a closed environment at 1150 ºC for 2 hours. The calcined powder was ball-milled and dried again to obtain homogeneous powder. Pellets of 12 mm in diameter and about 0.5 mm in thickness were uniaxially pressed at 200 MPa using 5% PVA binder and slowly heated at 600 ºC for 2 h to burn out the binder. The samples were sintered at 1350 ºC for 2 h in air. The relative densities of all the sintered samples are more than 97%. The samples
crystalline were
structure
examined
by
and X-ray
microstructure
of
the
sintered
diffraction (XRD, PANalytical X’Pert
PRO) and scanning electron microscopy (SEM, JSM-7100F) measurements, respectively. Dielectric properties, complex impedances and ac conductivities were measured with a precision impedance analyzer (Agilent E4980A). The BDS and polarization versus electric field (P-E) hysteresis loop were determined by a DC bias 3
source on a ferroelectric test system (Radiant RT66A), and the energy density was estimated from the P-E curves, by integrating the area enclosed within the polarization axis and the discharged curve. For each composition, at least 8 samples were measured to obtain the average BDS and all of the samples were polished to about 0.3 mm in thickness for the BDS and P-E measurements. 3. Results and Discussion The XRD patterns of all the ceramics sintered at 1350 ºC are shown in Fig. 1. Though ZrO2 exhibits different crystal structure from that of ST and may form solid solution and/or secondary phase with ST, only a typical cubic symmetry was detected by XRD analysis for all compositions without any secondary phase because of the trace and/or instrument errors. All the XRD patterns were identical to the STZ0 (pure ST) ceramic. No systematic variation of the lattice parameter was observed. However, the microstructures of the ceramics changed obviously when ZrO2 was added. Fig. 2 shows the SEM micrographs of all the ceramics sintered at 1350 ºC for 2 h and reveals high density in microstructures for all the samples, and obvious differences in grain size and distribution. The average grain size as a function of ZrO2 concentration is shown in Fig. 3. The average grain size for the pure ST ceramic is about 2.3 μm， and decreases with increasing ZrO2 concentration to minimum value of approximately 1.3 μm for the sample with 0.6 mol% ZrO2 addition. Meanwhile with the increase of the ZrO2 concentration, the grain distribution gradually becomes uniform (Fig. 2), which is similar to other materials with ZrO2 additives due to the accumulation of ZrO2 at grain boundaries which restrains the grain growth [22,23]. 4
The room-temperature dielectric properties of all the ceramics are shown in Table 1, which displays that each sample has a low dielectric loss (< 0.01) as well as a medium dielectric constant. The variations of dielectric constant and dielectric loss with temperature at some selected frequencies for pure ST ceramics (STZ0) and STZ4 ceramics are shown in Fig. 4, which reveals obvious differences between them. For pure ST ceramics, the dielectric constant decreases gradually up to a certain temperature and increases rapidly with increasing temperature. Similar phenomenon of the dielectric constant was observed in STZ4 ceramics, as shown in the inset of Fig. 4(b). The dielectric loss of pure ST ceramics seems to remain constant firstly and then increases rapidly at a certain temperature with increasing temperature. However, for the STZ4 ceramics, the dielectric loss remains constant up to a certain temperature and increases rapidly with the increase of temperature, and the dielectric loss-temperature curves show one set of relaxation peaks, which were not observed in that of pure ST ceramics at the same temperature range. The position of the peak shifts to higher temperature with increasing frequency. Another set of relaxation peaks for the STZ4 ceramics were detected above 400 ºC. Results similar to STZ4 ceramics were observed in other samples (STZ2 and STZ6 ceramics). The inset of Fig. 4(d) shows the temperature dependence of the relaxation frequency f r in STZ4 ceramics in the temperature range below 400 ºC with ln f r vs 1000/T function, where f r is obtained from the position of the loss peak in tan δ versus temperature plots of STZ4 ceramics. The solid line is the best-fit curve
of the equation, 5
f r = f 1′ exp[ −Ea /( k BT )]
(1)
where f 1′ is the relaxation frequency at an infinite temperature, Ea is the activation energy for the dielectric relaxation, and k B is the Boltzmann constant. The value of the activation energy ( Ea ) is 1.09 eV, which is good agreement with that of Bi-doped SrTiO3 ceramics [3] at the same temperature range, indicating that the dielectric relaxation results from the doubly ionized oxygen vacancies. Fig. 5 shows the frequency dependence of ac conductivity ( σ ac ) for the pure ST and STZ4 ceramics measured at different temperatures. The σ ac indicates a dispersion that shifts to higher frequency with increasing temperature, and it decreases with decreasing frequency. The σ ac almost saturates to a constant value at low frequencies, which is approximately equal to the dc conductivity ( σ dc ). The variation of σ dc with 1/T can be fitted with the Arrhenius law,
σ dc = σ 0 exp( −Econd / k BT )
(2)
where σ 0 is the pre-exponential factor, Econd is the activation energy, and k B is the Boltzmann constant. As shown in Fig. 5(a) and (b), the conduction activation energies for the pure ST and STZ4 ceramics at elevated temperatures are calculated with the value of 1.31 and 1.27 eV, respectively. The two values are comparable to 1.07-1.31 eV, which is the conduction activation energy of La-doped PbTiO3 ceramics [24] at the same temperature range, indicating that the dielectric relaxations of the pure ST and STZ4 ceramics are due to the space-charge polarization in which the free carriers are stored at the two dielectric electrode interfaces. Fig. 6 shows the Weibull distribution of the BDS for ST ceramics with different 6
ZrO2 concentrations. This distribution has been found to be most appropriate for the BDS analysis and is described in detail elsewhere [25,26]. The plot is described by:
Xi = ln( Ei ) 1 )) 1 − Pi
Yi = ln(ln( Pi =
(3) (4)
i
(5)
n +1
where Xi
and Yi are two parameters in Weibull distribution function, Ei is
specific breakdown voltage of each specimen in the experiments, Pi is probability for dielectric breakdown, n is the sum of specimens of each sample, and i
is serial
number of specimen. The samples were arranged in ascending order of BDS values so that:
E1 ≤ E2 ≤ " ≤ Ei ≤ " ≤ En
(6)
According to the Weibull distribution equation, there is a linear relationship between Xi and Yi , where slope is the Weibull modulus m relating to a range of BDS, and the intercept is equal to − mln( Eb ) reflecting the magnitude of BDS. As can be seen in Fig. 6, all the samples fit well with the Weibull distribution, and the Weibull modulus m increases and then decreases with increase in ZrO2 concentration. The value of BDS was obtained from the intercept of each line and plotted in Fig. 3. Pure ST ceramics have an average BDS of about 201 kV/cm. When the ZrO2 concentration increases, the BDS increases rapidly to 289 kV/cm for the sample with 0.4 mol% ZrO2 addition and then decreases to 285 kV/cm for the sample with 0.6 mol% ZrO2 addition. The energy densities at the highest breakdown strengths are 7
shown in Table 2, which shows the same tendency of changes with the BDS for the energy density. A highest energy density with the value of 1.62 J/cm3 was obtained at the sample with 0.4 mol% ZrO2 addition. The complex impedance spectra measured at 440 ºC for the STZ ceramics are shown in Fig. 7. Two arcs with a strong overlap were observed from the complex impedance plots of the pure ST ceramics due to similar relaxation time constant of grain and grain-boundary relaxation processes. However, when ZrO2 was added, the complex impedance plots displayed two obvious arcs, as shown in the inset (a) of Fig. 7. All these implied that ZrO2 which was added to ST ceramics could significantly enhance the grain boundary characterization. All the impedance plots could be modeled on an equivalent circuit based on two parallel (resistor, R and capacitor, C ) elements connected in series. For such a circuit, each RC element ideally gives rise to semicircular arc in complex impedance plane and represents grain phases or grain-boundary phases. The complex impedance plots of all the samples could be simulated by two serial equivalent circuits ( R − C ), as shown in the inset (b) of Fig. 7, where the circuit elements Rg − Cg and Rgb − Cgb represent the contributions from the grain at high frequencies and grain boundary at low frequencies, respectively. The Rgb /( Rgb+Rg ) ratios at 440 ºC vs. ZrO2 concentration for STZ ceramics are shown in Fig. 3. Compared with pure ST ceramic, ZrO2 causes the obvious increase in
Rgb /( Rgb+Rg ) ratios, which increase with the ZrO2 concentration and are all greater than 90%. Generally, the BDS which plays an important role in the improvement of the 8
energy density often influenced by several internal parameters (relative density, grain size and grain boundary nature) and external parameters (sample thickness, sample area, and electrode configuration). We have kept the same external parameters for the BDS measurements to avoid the contribution of external effects on energy storage properties. In this work, these ceramics could be described by the double-layer dielectric model to illustrate the distribution of electric field. When a dc voltage was applied to the ceramic, because of that Rg << Rgb ，almost the entire field strength fallen across the grain boundary region after a certain time, and the grain could be regarded as virtually field free. We have introduced Rgb /( Rgb+Rg ) ratio to evaluate the amount of the electric field strength the grain boundary can withstand, which are shown in Fig. 3. Generally, the smaller grain size, the more grain boundaries and the larger Rgb /( Rgb+Rg ) ratio. Thus, as seen from Fig. 3, the samples with trace amounts of ZrO2 additives had large Rgb /( Rgb+Rg ) ratios due to the decreased grain sizes, and then had large breakdown strengths. A further increase in the ZrO2 concentration from 0.4 to 0.6 mol% led to a small decrease in the BDS which could be attributed to the increased dielectric loss (Table 1). 4. Conclusion The SrTiO3 ceramics with trace amounts of ZrO2 additives were prepared by solid-state method. The microstructures, dielectric relaxation behaviors and energy storage properties of the ceramics were investigated. The results show that with the ZrO2 concentration increasing, each sample has a low dielectric loss (< 0.01) with a medium dielectric constant and displays dielectric relaxation behaviors at high 9
temperatures, which are due to the doubly ionized oxygen vacancies and the space-charge polarization. With the ZrO2 concentration increasing, the grain size decreases and distribution becomes homogeneous, leading to the increase of the grain boundary thickness and the improvement of the breakdown strength. An energy density of 1.62 J/cm3 with a breakdown strength of 289 kV/cm was achieved for SrTiO3 ceramics with 0.4 mol% ZrO2 addition. Acknowledgments This work was supported by the Key program of Natural Science Foundation of China (No.50932004), International Science and Technology Cooperation Program of China (2011DFA52680), Natural Science Foundation of China (No.51102189, 51372191), the program for New Century Excellent Talents in University (No.NCET-11-0685), the Fundamental Research Funds for the Central Universities (2012-Ia-005), and the Fundamental Research Funds for the Central Universities (No.2012-IV-006) and (WUT:2014-IV-134). References [1] A. Tkach, P.M. Vilarinho, A.L. Kholkin, Nonlinear dc electric-field dependence of the dielectric permittivity and cluster polarization of Sr1-xMnxTiO3 ceramics, Journal of Applied Physics 101 (2007) 084110. [2] T. Wu, Y. Pu, P. Gao, D. Liu, Influence of Sr/Ba ratio on the energy storage properties and dielectric relaxation behaviors of strontium barium titanate ceramics, Journal of Materials Science: Materials in Electronics 24 (2013) 4105-4112. 10
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sintering on the breakdown strength of barium titanate, Journal of the American Ceramic Society 90 (2007) 1504-1510. [11] M. Touzin, D. Goeuriot, H.J. Fitting, C. Guerret-Piécourt, D. Juvé, D. Tréheux, Relationships between dielectric breakdown resistance and charge transport in alumina materials — Effects of the microstructure, Journal of the European Ceramic Society 27 (2007) 1193-1197. [12] J. Liebault, J. Vallayer, D. Goeuriot, D. Treheux, F. Thevenot, How the trapping of charges can explain the dielectric breakdown performance of alumina ceramics, Journal of the European Ceramic Society 21 (2001) 389-397. [13] J. Huang, Y. Zhang, T. Ma, H. Li, L. Zhang, Correlation between dielectric breakdown strength and interface polarization in barium strontium titanate glass ceramics, Applied Physics Letters 96 (2010) 042902. [14] Q. Xu, D. Zhan, D. Huang, H. Liu, W. Chen, F. Zhang, Effect of MgO-CaO-Al2O3-SiO2
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[17] Q.G. Chi, L. Gao, X. Wang, J.Q. Lin, J. Sun, Q.Q. Lei, Effects of Zr doping on the microstructures and dielectric properties of CaCu3Ti4O12 ceramics, Journal of Alloys and Compounds 559 (2013) 45-48. [18] G. Dong, S. Ma, J. Du, J. Cui, Dielectric properties and energy storage density in ZnO-doped
Ba0.3Sr0.7TiO3
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relaxation in perovskites, Physical Review B 49 (1994) 7868-7873. [25] A.L. Young, G.E. Hilmas, S.C. Zhang, R.W. Schwartz, Mechanical vs. electrical failure mechanisms in high voltage, high energy density multillayer ceramic capacitors, Journal of Materials Science 42 (2007) 5613-5619. [26] J. Chen, Y. Zhang, C. Deng, X. Dai, L. Li, Effect of the Ba/Ti Ratio on the Microstructures
and
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Tables Table 1. The room-temperature dielectric properties of STZ ceramics measured at 1 kHz STZ0
STZ2
STZ4
STZ6
εr
295
299
302
304
tan δ
0.0023
0.0056
0.0062
0.0085
Table 2. The energy density of STZ ceramics
Energy
STZ0
STZ2
STZ4
STZ6
0.70
1.35
1.62
1.59
density(J/cm3)
14
Figure Caption: Fig. 1 XRD patterns of the sintered STZ ceramics measured at room temperature. Fig. 2 SEM micrographs of STZ ceramics sintered at 1350 ºC for 2 h. (a) STZ0 (Pure ST), (b) STZ2, (c) STZ4, (d) STZ6. Fig. 3 The average grain size, Rgb /( Rgb+Rg ) ratio and breakdown strength of the STZ ceramics as a function of ZrO2 concentration. Fig. 4 The variations of dielectric constant and dielectric loss with temperature at some selected frequencies for the STZ0 (Pure ST) and STZ4 ceramics. The inset of Fig. 4(b) shows the enlarged view of Fig. 4(b). The inset of Fig. 4(d) shows the temperature dependence of the relaxation frequency f r in STZ4 ceramics with ln f r vs 1000/T function. Fig. 5 The frequency dependence of ac conductivity ( σ ac ) for the STZ0 and STZ4 ceramics measured at different temperatures. (a) STZ0 (Pure ST), (b) STZ4. The insets of Fig. 5(a) and (b) show the Arrhenius plots of temperature dependence of dc conductivity for the STZ0 and STZ4 ceramics, respectively. Fig. 6 Weibull distribution of the BDS for STZ ceramics. Fig. 7 The complex impedance spectra measured at 440 ºC for the STZ ceramics. Inset (a) shows the enlarged view of Fig. 7. Inset (b) shows an equivalent circuit for the STZ ceramics.
15
Figure 1
16
Figure 2
17
Figure 3
18
Figure 4
19
Figure 5
20
Figure 6
21
Figure 7
22
Dielectric relaxation behavior and energy storage properties in SrTiO3 ceramics with trace amounts of ZrO2 additives Zhijian Wang, Minghe Cao, Zhonghua Yao, Zhe Song, Guangyao Li, Wei Hu, Hua Hao, Hanxing Liu
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)00870-0 http://dx.doi.org/10.1016/j.ceramint.2014.05.147 CERI8683
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
6 May 2014 30 May 2014 30 May 2014
Cite this article as: Zhijian Wang, Minghe Cao, Zhonghua Yao, Zhe Song, Guangyao Li, Wei Hu, Hua Hao, Hanxing Liu, Dielectric relaxation behavior and energy storage properties in SrTiO3 ceramics with trace amounts of ZrO2 additives, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.05.147 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Dielectric relaxation behavior and energy storage properties in SrTiO3 ceramics with trace amounts of ZrO2 additives Zhijian Wang, Minghe Cao*, Zhonghua Yao, Zhe Song, Guangyao Li, Wei Hu, Hua Hao, Hanxing Liu State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China Corresponding author. Tel.: +86 27 87885813; Fax: +86 27 87885813 E-mail address: [email protected] (M. Cao) Abstract SrTiO3 ceramics with trace amounts of ZrO2 additives were prepared via the solid state reaction route. The effects of ZrO2 concentrations on microstructures, dielectric and energy storage properties of SrTiO3 ceramics were investigated. The grain size of the ceramics decreases with increasing ZrO2 concentration. Dielectric relaxation behaviors were observed in dielectric loss versus temperature plots and studied by the measurements of the activation energies, which indicated the behaviors were due to the doubly ionized oxygen vacancies and the space-charge polarization. An energy density of 1.62 J/cm3 and a breakdown strength of 289 kV/cm were achieved for SrTiO3 ceramics with 0.4 mol% ZrO2 addition, which were higher than those of pure SrTiO3 ceramics. Keywords: B. Microstructure; C. Dielectric relaxation; Energy storage; D. SrTiO3 ceramic; ZrO2 1. Introduction 1
As quantum paraelectrics, SrTiO3 (ST) ceramics with high dielectric constant (~290), low dielectric loss (< 0.01), high breakdown strength (~200 kV/cm) and favorable bias stability are widely required for the application of high energy storage density dielectrics. It is well known that the energy storage density grows linearly with the dielectric constant and quadratically with the breakdown strength (BDS). Many methods have been used to improve the energy storage density, such as enhancing the dielectric constant and/or the BDS. Though doping by other cations with small or big ionic radius could increase the dielectric constant of the ST-based ceramics, such as Ba2+, Mn2+, Bi3+, and trivalent rare earth (RE3+) [1-5], the BDS sharply decreases due to the changed structure and increased defect. It is well known that high BDS makes a more pronounced contribution to the energy density [6]. Generally, the BDS is mainly affected by several factors, such as porosity [7], grain size [8,9], secondary phase [10], temperature [11], charge injection [12] and interfacial polarization [13]. Both high relative density and homogeneously small grains are beneficial to the BDS. Many methods have been used to improve the microstructure, such as additives [14-18] (for example, glasses and/or oxides), chemical preparation [14] and new sintering technology. Among them, use of additives is the simplest and most effective way. Despite having a high BDS, glasses have a low dielectric constant which is not beneficial to the energy density of the ST-based ceramics [19]. In many studies, it has been proved that the addition of some oxides could effectively inhibit the grain growth and increase the BDS [20], but the dielectric constant decreases due to the low dielectric constant of the oxides. 2
In this work, ZrO2 was selected as the additive, because Zr4+ ions in ST can stabilize the charge of Ti and suppress the oxygen dissociation when sintered at high temperature [21], meanwhile, some Zr4+ ions will be deposited at grain boundaries to inhibit the grain growth [22]. In order not to make the dielectric constant lower, trace amounts of ZrO2 additives were determined and a desired result was obtained. 2. Experimental procedure Powders of analytical reagent grade, comprising SrCO3 (> 99.0 %), TiO2 (> 99.0 %) and ZrO2 (> 99.0 %), were used as the starting materials and mixed according to the composition, ST + x mol% ZrO2 (x = 0, 0.2, 0.4 and 0.6) (short as STZ0, STZ2, STZ4, STZ6, respectively). After ball-milled in alcohol for 24 hours, the slurry was dried, then calcined in a closed environment at 1150 ºC for 2 hours. The calcined powder was ball-milled and dried again to obtain homogeneous powder. Pellets of 12 mm in diameter and about 0.5 mm in thickness were uniaxially pressed at 200 MPa using 5% PVA binder and slowly heated at 600 ºC for 2 h to burn out the binder. The samples were sintered at 1350 ºC for 2 h in air. The relative densities of all the sintered samples are more than 97%. The samples
crystalline were
structure
examined
by
and X-ray
microstructure
of
the
sintered
diffraction (XRD, PANalytical X’Pert
PRO) and scanning electron microscopy (SEM, JSM-7100F) measurements, respectively. Dielectric properties, complex impedances and ac conductivities were measured with a precision impedance analyzer (Agilent E4980A). The BDS and polarization versus electric field (P-E) hysteresis loop were determined by a DC bias 3
source on a ferroelectric test system (Radiant RT66A), and the energy density was estimated from the P-E curves, by integrating the area enclosed within the polarization axis and the discharged curve. For each composition, at least 8 samples were measured to obtain the average BDS and all of the samples were polished to about 0.3 mm in thickness for the BDS and P-E measurements. 3. Results and Discussion The XRD patterns of all the ceramics sintered at 1350 ºC are shown in Fig. 1. Though ZrO2 exhibits different crystal structure from that of ST and may form solid solution and/or secondary phase with ST, only a typical cubic symmetry was detected by XRD analysis for all compositions without any secondary phase because of the trace and/or instrument errors. All the XRD patterns were identical to the STZ0 (pure ST) ceramic. No systematic variation of the lattice parameter was observed. However, the microstructures of the ceramics changed obviously when ZrO2 was added. Fig. 2 shows the SEM micrographs of all the ceramics sintered at 1350 ºC for 2 h and reveals high density in microstructures for all the samples, and obvious differences in grain size and distribution. The average grain size as a function of ZrO2 concentration is shown in Fig. 3. The average grain size for the pure ST ceramic is about 2.3 μm， and decreases with increasing ZrO2 concentration to minimum value of approximately 1.3 μm for the sample with 0.6 mol% ZrO2 addition. Meanwhile with the increase of the ZrO2 concentration, the grain distribution gradually becomes uniform (Fig. 2), which is similar to other materials with ZrO2 additives due to the accumulation of ZrO2 at grain boundaries which restrains the grain growth [22,23]. 4
The room-temperature dielectric properties of all the ceramics are shown in Table 1, which displays that each sample has a low dielectric loss (< 0.01) as well as a medium dielectric constant. The variations of dielectric constant and dielectric loss with temperature at some selected frequencies for pure ST ceramics (STZ0) and STZ4 ceramics are shown in Fig. 4, which reveals obvious differences between them. For pure ST ceramics, the dielectric constant decreases gradually up to a certain temperature and increases rapidly with increasing temperature. Similar phenomenon of the dielectric constant was observed in STZ4 ceramics, as shown in the inset of Fig. 4(b). The dielectric loss of pure ST ceramics seems to remain constant firstly and then increases rapidly at a certain temperature with increasing temperature. However, for the STZ4 ceramics, the dielectric loss remains constant up to a certain temperature and increases rapidly with the increase of temperature, and the dielectric loss-temperature curves show one set of relaxation peaks, which were not observed in that of pure ST ceramics at the same temperature range. The position of the peak shifts to higher temperature with increasing frequency. Another set of relaxation peaks for the STZ4 ceramics were detected above 400 ºC. Results similar to STZ4 ceramics were observed in other samples (STZ2 and STZ6 ceramics). The inset of Fig. 4(d) shows the temperature dependence of the relaxation frequency f r in STZ4 ceramics in the temperature range below 400 ºC with ln f r vs 1000/T function, where f r is obtained from the position of the loss peak in tan δ versus temperature plots of STZ4 ceramics. The solid line is the best-fit curve
of the equation, 5
f r = f 1′ exp[ −Ea /( k BT )]
(1)
where f 1′ is the relaxation frequency at an infinite temperature, Ea is the activation energy for the dielectric relaxation, and k B is the Boltzmann constant. The value of the activation energy ( Ea ) is 1.09 eV, which is good agreement with that of Bi-doped SrTiO3 ceramics [3] at the same temperature range, indicating that the dielectric relaxation results from the doubly ionized oxygen vacancies. Fig. 5 shows the frequency dependence of ac conductivity ( σ ac ) for the pure ST and STZ4 ceramics measured at different temperatures. The σ ac indicates a dispersion that shifts to higher frequency with increasing temperature, and it decreases with decreasing frequency. The σ ac almost saturates to a constant value at low frequencies, which is approximately equal to the dc conductivity ( σ dc ). The variation of σ dc with 1/T can be fitted with the Arrhenius law,
σ dc = σ 0 exp( −Econd / k BT )
(2)
where σ 0 is the pre-exponential factor, Econd is the activation energy, and k B is the Boltzmann constant. As shown in Fig. 5(a) and (b), the conduction activation energies for the pure ST and STZ4 ceramics at elevated temperatures are calculated with the value of 1.31 and 1.27 eV, respectively. The two values are comparable to 1.07-1.31 eV, which is the conduction activation energy of La-doped PbTiO3 ceramics [24] at the same temperature range, indicating that the dielectric relaxations of the pure ST and STZ4 ceramics are due to the space-charge polarization in which the free carriers are stored at the two dielectric electrode interfaces. Fig. 6 shows the Weibull distribution of the BDS for ST ceramics with different 6
ZrO2 concentrations. This distribution has been found to be most appropriate for the BDS analysis and is described in detail elsewhere [25,26]. The plot is described by:
Xi = ln( Ei ) 1 )) 1 − Pi
Yi = ln(ln( Pi =
(3) (4)
i
(5)
n +1
where Xi
and Yi are two parameters in Weibull distribution function, Ei is
specific breakdown voltage of each specimen in the experiments, Pi is probability for dielectric breakdown, n is the sum of specimens of each sample, and i
is serial
number of specimen. The samples were arranged in ascending order of BDS values so that:
E1 ≤ E2 ≤ " ≤ Ei ≤ " ≤ En
(6)
According to the Weibull distribution equation, there is a linear relationship between Xi and Yi , where slope is the Weibull modulus m relating to a range of BDS, and the intercept is equal to − mln( Eb ) reflecting the magnitude of BDS. As can be seen in Fig. 6, all the samples fit well with the Weibull distribution, and the Weibull modulus m increases and then decreases with increase in ZrO2 concentration. The value of BDS was obtained from the intercept of each line and plotted in Fig. 3. Pure ST ceramics have an average BDS of about 201 kV/cm. When the ZrO2 concentration increases, the BDS increases rapidly to 289 kV/cm for the sample with 0.4 mol% ZrO2 addition and then decreases to 285 kV/cm for the sample with 0.6 mol% ZrO2 addition. The energy densities at the highest breakdown strengths are 7
shown in Table 2, which shows the same tendency of changes with the BDS for the energy density. A highest energy density with the value of 1.62 J/cm3 was obtained at the sample with 0.4 mol% ZrO2 addition. The complex impedance spectra measured at 440 ºC for the STZ ceramics are shown in Fig. 7. Two arcs with a strong overlap were observed from the complex impedance plots of the pure ST ceramics due to similar relaxation time constant of grain and grain-boundary relaxation processes. However, when ZrO2 was added, the complex impedance plots displayed two obvious arcs, as shown in the inset (a) of Fig. 7. All these implied that ZrO2 which was added to ST ceramics could significantly enhance the grain boundary characterization. All the impedance plots could be modeled on an equivalent circuit based on two parallel (resistor, R and capacitor, C ) elements connected in series. For such a circuit, each RC element ideally gives rise to semicircular arc in complex impedance plane and represents grain phases or grain-boundary phases. The complex impedance plots of all the samples could be simulated by two serial equivalent circuits ( R − C ), as shown in the inset (b) of Fig. 7, where the circuit elements Rg − Cg and Rgb − Cgb represent the contributions from the grain at high frequencies and grain boundary at low frequencies, respectively. The Rgb /( Rgb+Rg ) ratios at 440 ºC vs. ZrO2 concentration for STZ ceramics are shown in Fig. 3. Compared with pure ST ceramic, ZrO2 causes the obvious increase in
Rgb /( Rgb+Rg ) ratios, which increase with the ZrO2 concentration and are all greater than 90%. Generally, the BDS which plays an important role in the improvement of the 8
energy density often influenced by several internal parameters (relative density, grain size and grain boundary nature) and external parameters (sample thickness, sample area, and electrode configuration). We have kept the same external parameters for the BDS measurements to avoid the contribution of external effects on energy storage properties. In this work, these ceramics could be described by the double-layer dielectric model to illustrate the distribution of electric field. When a dc voltage was applied to the ceramic, because of that Rg << Rgb ，almost the entire field strength fallen across the grain boundary region after a certain time, and the grain could be regarded as virtually field free. We have introduced Rgb /( Rgb+Rg ) ratio to evaluate the amount of the electric field strength the grain boundary can withstand, which are shown in Fig. 3. Generally, the smaller grain size, the more grain boundaries and the larger Rgb /( Rgb+Rg ) ratio. Thus, as seen from Fig. 3, the samples with trace amounts of ZrO2 additives had large Rgb /( Rgb+Rg ) ratios due to the decreased grain sizes, and then had large breakdown strengths. A further increase in the ZrO2 concentration from 0.4 to 0.6 mol% led to a small decrease in the BDS which could be attributed to the increased dielectric loss (Table 1). 4. Conclusion The SrTiO3 ceramics with trace amounts of ZrO2 additives were prepared by solid-state method. The microstructures, dielectric relaxation behaviors and energy storage properties of the ceramics were investigated. The results show that with the ZrO2 concentration increasing, each sample has a low dielectric loss (< 0.01) with a medium dielectric constant and displays dielectric relaxation behaviors at high 9
temperatures, which are due to the doubly ionized oxygen vacancies and the space-charge polarization. With the ZrO2 concentration increasing, the grain size decreases and distribution becomes homogeneous, leading to the increase of the grain boundary thickness and the improvement of the breakdown strength. An energy density of 1.62 J/cm3 with a breakdown strength of 289 kV/cm was achieved for SrTiO3 ceramics with 0.4 mol% ZrO2 addition. Acknowledgments This work was supported by the Key program of Natural Science Foundation of China (No.50932004), International Science and Technology Cooperation Program of China (2011DFA52680), Natural Science Foundation of China (No.51102189, 51372191), the program for New Century Excellent Talents in University (No.NCET-11-0685), the Fundamental Research Funds for the Central Universities (2012-Ia-005), and the Fundamental Research Funds for the Central Universities (No.2012-IV-006) and (WUT:2014-IV-134). References [1] A. Tkach, P.M. Vilarinho, A.L. Kholkin, Nonlinear dc electric-field dependence of the dielectric permittivity and cluster polarization of Sr1-xMnxTiO3 ceramics, Journal of Applied Physics 101 (2007) 084110. [2] T. Wu, Y. Pu, P. Gao, D. Liu, Influence of Sr/Ba ratio on the energy storage properties and dielectric relaxation behaviors of strontium barium titanate ceramics, Journal of Materials Science: Materials in Electronics 24 (2013) 4105-4112. 10
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Tables Table 1. The room-temperature dielectric properties of STZ ceramics measured at 1 kHz STZ0
STZ2
STZ4
STZ6
εr
295
299
302
304
tan δ
0.0023
0.0056
0.0062
0.0085
Table 2. The energy density of STZ ceramics
Energy
STZ0
STZ2
STZ4
STZ6
0.70
1.35
1.62
1.59
density(J/cm3)
14
Figure Caption: Fig. 1 XRD patterns of the sintered STZ ceramics measured at room temperature. Fig. 2 SEM micrographs of STZ ceramics sintered at 1350 ºC for 2 h. (a) STZ0 (Pure ST), (b) STZ2, (c) STZ4, (d) STZ6. Fig. 3 The average grain size, Rgb /( Rgb+Rg ) ratio and breakdown strength of the STZ ceramics as a function of ZrO2 concentration. Fig. 4 The variations of dielectric constant and dielectric loss with temperature at some selected frequencies for the STZ0 (Pure ST) and STZ4 ceramics. The inset of Fig. 4(b) shows the enlarged view of Fig. 4(b). The inset of Fig. 4(d) shows the temperature dependence of the relaxation frequency f r in STZ4 ceramics with ln f r vs 1000/T function. Fig. 5 The frequency dependence of ac conductivity ( σ ac ) for the STZ0 and STZ4 ceramics measured at different temperatures. (a) STZ0 (Pure ST), (b) STZ4. The insets of Fig. 5(a) and (b) show the Arrhenius plots of temperature dependence of dc conductivity for the STZ0 and STZ4 ceramics, respectively. Fig. 6 Weibull distribution of the BDS for STZ ceramics. Fig. 7 The complex impedance spectra measured at 440 ºC for the STZ ceramics. Inset (a) shows the enlarged view of Fig. 7. Inset (b) shows an equivalent circuit for the STZ ceramics.
15
Figure 1
16
Figure 2
17
Figure 3
18
Figure 4
19
Figure 5
20
Figure 6
21
Figure 7
22