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Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics
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Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics Chunli Diao a,b, Hanxing Liu a,n, Hua Hao a, Minghe Cao a, Zhonghua Yao a a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Department of Physics, Henan University, Kaifeng 475004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 April 2016 Received in revised form 27 April 2016 Accepted 28 April 2016
SiO2-added barium strontium titanate ceramics Ba0.4Sr0.6TiO3-xwt%SiO2 (x ¼ 0, 0.5, 1, 3, BSTSx) were prepared via a traditional solid state reaction method. The effect of SiO2 additive on the microstructure, dielectric response and energy storage properties was investigated. The results confirmed that with the increase of SiO2 additive, diffuse phase transition arises and the dielectric constant decreases. An equivalent circuit model and Arrhenius law were used to calculate the activation energy of grain and grain boundary, which indicated that the dielectric relaxation at high temperature was caused by oxygen vacancy. While appropriate SiO2 additive led to improve the breakdown strength, further increase of SiO2 deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: dielectric constant of 1002, dielectric loss of 0.45%, energy density of 0.86 J/cm3 and energy storage efficiency of 79% at 134 kV/cm. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Dielectric properties Ba0.4Sr0.6TiO3 ceramics Energy storage SiO2 additive
1. Introduction Capacitors with high power density and energy density have attracted much attention recently due to the increase of requirements for electrical devices in hybrid electric vehicles, mobile electronics and different forms of pulsed power technology [1–4]. The recoverable energy storage density Ureco of a capacitor can be calculated from the polarization-electric field (P–E) hysteresis loops according to the following equation:
Ureco =
∫P
Pmax
Edp
r
(1)
where E is the applied electrical field, P is the polarization, Pmax is the maximum polarization and Pr is the remnant polarization. It is indicated that high energy storage density depends on high applied electrical field and large difference between Pmax and Pr. Because of high breakdown strength (BDS) and energy storage density, polymer capacitors are usually applied in high-power output applications. However, polymers are used in a limited range for their low dielectric constant and poor thermal stability [5]. Ceramic capacitors are a potential candidate as energy storage devices for their high power density, excellent mechanical and thermal reliability, but their breakdown strength is low. Many efforts have been made to improve the breakdown strength of n
Corresponding author. E-mail address: [email protected] (H. Liu).
ceramic capacitors for energy storage, such as oxide doping [6], chemical coating [7], glass additive [8], spark plasma sintering [9], microwave sintering [10], etc. Among them, glass additive is a simple and economical method to achieve. Xu and Zhang et al. reported that different silicate glass additives were added to improve the energy storage density of ceramic capacitors [11,12]. Compared with silicate glass, SiO2 shows high energy bandgap, excellent electrical insulation and low dielectric loss. It can be expected that SiO2 additive could also improve the breakdown strength for energy storage in ceramics. Zhao et al. reported that SiO2 addition led to a great enhancement in the BDS of (1 x) SrTiO3-xSiO2 ceramics for energy storage [13]. (Ba1 xSrx)TiO3 (x ¼0 1) ceramics are a widely used lead-free dielectric material, with controllable Curie temperature (Tc) and dielectric properties by barium strontium ratio, which can be tailored for specific applications [14]. Among these, (Ba0.4Sr0.6)TiO3 (BST) is a promising energy storage material with paraelectric phase at room temperature, moderate dielectric constant (41000), low dielectric loss and good dielectric stability under DC bias electric field. The energy storage of BST ceramics have been researched by several groups [8–10]. However, there have been no reports on the energy storage of SiO2 added BST ceramics yet. In this work, the effect of SiO2 additive on the microstructure, dielectric response and energy storage performance of (Ba0.4Sr0.6) TiO3 ceramics were investigated.
http://dx.doi.org/10.1016/j.ceramint.2016.04.169 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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2. Experimental procedure (Ba0.4Sr0.6)TiO3-xwt%SiO2 (x ¼0, 0.5, 1, 3; abbreviated as BSTSx) ceramics were prepared using a traditional solid state method. The stoichiometric mixtures of analytical reagent grade powder BaCO3, SrCO3, TiO2 and SiO2 were weighed and mixed. After ball-milled for 24 h, the slurry was dried and calcined at 1150 °C for 2 h. The calcined powder was pressed into disks, using polyvinyl alcohol as a binder. After burning out the binder at 600 °C, the pellets were sintered at 1330 °C for 2 h and cooled naturally. The phase identification were identified by an X-ray diffractometer (XRD) with CuKα radiation (X’Pert PRO, PANalytical, Holland) and microstructure characterization was done using a field emission scanning electron microscopy (FE-SEM, Quanta 450 FEG, FEI, USA). To measure the electrical properties, silver paste was applied to the surfaces of the samples and fired at 800 °C for 20 min as electrodes. Dielectric response and complex impedance spectra were measured using an Agilent E4980A impedance analyzer equipped with a thermostat. The samples were polished with a thickness of 0.3 mm and the polarization-electric field (P–E) hysteresis loops were examined using a Radiant precision workstation (HVI0403–239) based on the Sawyer-Tower circuit at 1 Hz.
3. Results and discussion Fig. 1 shows the XRD patterns of BSTSx ceramics at the room temperature. It was observed that for the ceramics with x ¼0–1, a typical cubic perovskite structure was identified by comparing with standard powder diffraction file database within the scope of the instrument measuring precision. Nevertheless, for BSTS3 ceramics, small amount of impurity phase were detected besides the cubic perovskite structure. The peak position shifts slightly with SiO2 content. Through the refinement of XRD patterns, the cell parameters of the samples with x ¼0, 0.5, 1, 3 are 3.9439 (70.0002) Å, 3.9432 ( 70.0002) Å, 3.9420 (70.0003) Å, 3.9380 (70.0004) Å, respectively. The ionic radius of Si4 þ (0.40 Å, C. N.¼6) is smaller than that of Ti4 þ (0.605 Å, C. N.¼6) [15]. It could be concluded that Si ions occupy at least two positions in BSTMx ceramics: some Si enter into the lattice of BST and occupy the Ti site; some Si exist outside the lattice, which is based on the decrease of cell parameters of BSTMx ceramics and the appearance of impurity in BSTM3 ceramics. The SEM surface micrographs of BSTSx ceramics are displayed in Fig. 2. The microstructure of the pure BST ceramics is dense, and
Fig. 1. XRD patterns of BSTSx ceramics.
the size of angular blocky grains is about 3–10 μm. Compared with BST ceramics, the grain size changes unobviously, however, the grain shape of SiO2-added BST ceramics becomes rounded. Besides a small amount of SiO2 additive entering into the crystal lattice, some SiO2 probably formed glass phase in grain boundaries. With the increase of SiO2 additive, the densification of the ceramics decreases, which may be due to the difference of the physical properties between BST and SiO2. Moreover, second phase was observed in BSTS3 ceramics. This phenomenon is consistent with the results of XRD. Fig. 3a shows the temperature dependence of dielectric constant εr and dielectric loss tanδ of BSTSx ceramics measured at 1 kHz. A peak of dielectric constant at the low temperature corresponding to tetragonal-cubic phase transition was observed, meaning that BSTSx ceramics belong to paraelectric phase at the room temperature. The temperature of dielectric peak, namely Curie temperature Tc, is 71 °C for pure BST ceramics. The dielectric peak was suppressed after SiO2 additive, and not observed in BSTS3 ceramics, which suggested that SiO2 additive could lead to a tendency toward a diffuse phase transition associated with the internal stress between grains [16]. εr and tanδ demonstrate excellent thermal stability from the room temperature to 250 °C. In the loss response, two sets of peak were observed, described as peak 1 and peak 2. Peak 1 is caused by the ferroelectric–paraelectric phase transition discussed above. Peak 2 is associated with Maxwell–Wagner effect, which is similar to some perovskite oxides such as CaCu3Ti4O12, K0.5Na0.5NbO3, and BaTiO3 based ceramics [17–19]. For ceramic materials, the conductivity of insulating grain boundaries is different from that of semiconductor bulk grains, which causes charge accumulation at the grain boundaries. Accordingly, Maxwell–Wagner polarization occurs, relating to interfacial polarizations [20]. The enhancement of dielectric constant at high temperature may also be attributed to the Maxwell– Wagner effect. The dielectric properties of BSTSx ceramics are listed in Table 1. It was observed from Table 1 that after SiO2 addition, the dielectric constant decreases because of the low dielectric constant of SiO2, and dielectric loss increases from 0.0028 to 0.012. Due to the obvious effect of SiO2 additive on the grain shape (Fig. 2), it was indicated that the difference of electrical properties between grains and grain boundary increases. To separate the contribution of grains and grain boundaries, Curie–Weiss fitting was applied above Tc [21]:
εr = C /(T − T0)
(2)
where C is the Curie–Weiss constant and T0 is the Curie–Weiss temperature. The fitting deviation from the experimental plots shows the grain-boundary effect. Fig. 3b shows the Curie–Weiss fitting of BSTSx ceramics at 1 kHz. It was confirmed that with the increase of SiO2 additive, Curie–Weiss fitting deviation becomes obvious, demonstrating that SiO2 additive has a main effect on the electrical properties of grain boundary. Fig. 4 shows the complex impedance spectra of BSTS0.5 and BSTS3 ceramics measured at 20 Hz–2 MHz from 375 °C to 475 °C. For BSTS0.5 ceramics, two semicircular plots with different radii were observed (Fig. 4(a)), which are usually considered that the smaller semicircular corresponds the grain response and the larger one presents the contribution of grain boundaries. The impedance spectra of BST and BST1 ceramics are similar to that of BSTS0.5 ceramics. However, only a semicircular was observed in BSTS3 ceramics. For all the samples at high frequency, the plots tend to zero, indicating the high-frequency parts of the spectra are associated with bulk response. Obviously, the semicircular was depressed slightly and asymmetric (inset 1 of Fig. 4(b)), implying more than one contributing mode. In order to resolve the
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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Fig. 2. SEM surface micrographs of BSTx ceramics.
Fig. 3. (a) Temperature dependence of dielectric properties for BSTSx ceramics at 1 kHz; (b) Simulation of Curie–Weiss law for BSTSx ceramics above Tc.
Table 1 Dielectric properties of BSTSx ceramics at room temperature and 1 kHz.
Table 2 Activation energy of BSTSx ceramics for grain and grain boundary response.
Samples
BST
BSTS0.5
BSTS1
BSTS3
Samples
BST
BSTS0.5
BSTS1
BSTS3
εr tanδ
1132 0.0028
1002 0.0045
982 0.0059
659 0.012
Ea,g (eV) Ea, gb (eV)
1.05 1.10
0.98 0.91
0.99 0.94
0.97 0.99
impedance response, normalized imaginary parts of impedance Z″/Z″max and electric modulus M″/M″max as a function of frequency were shown in the inset 2 of Fig. 4(b). Only a single peak was
characterized for Z″/Z″max and M″/M″max plots, respectively. Moreover, an obvious distance exists between the two peak positions at the same temperature, suggesting two separate response
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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Fig. 4. (a) Complex impedance spectra of BSTS0.5 ceramics, insets are the equivalent circuit model and the enlarged high-frequency terminals of spectra; (b) complex impedance spectra of BSTS3 ceramics, inset 1 is the enlarged high-frequency terminals at 375 °C and inset 2 is the normalized impedance Z″/Z″max and electric modulus M″/M″max as a function of frequency.
modes contributing to the impedance spectra, owing to Z″ and M″ emphasizing different electric features [22]. Based on this consideration, an equivalent circuit model composed of two parallel RQ elements connected in series (inset in Fig. 4a) was used to analyze the impedance spectra. Here, R is resistance and Q refers to constant phase element (CPE). The two response processes are attributable to the contributions of grain and grain-boundary, correspondingly. The relaxation frequency of a response process frelax, corresponding to a specific RQ element, can be determined according to the following equation [23]:
frelax = (RQ )1/ n /2π
(3)
where n is the relaxation distribution parameter and n is between 1 and 2. n ¼1 corresponds to an ideal capacitor, and n¼ 2 refers to an ideal resistor. The relaxation frequency data were fitted to an Arrhenius relation [24]:
frelax = f0 exp( − Ea/kBT )
(4)
where f0, Ea and kB are pre-exponential factor, activation energy for relaxation and Boltzmann constant, respectively. Fig. 5 shows the plots of lnfrelax vs. 1000/T of BSTSx ceramics. The plots generally satisfy linear relation, with the R2 values exceeding 0.997. Similar results were obtained for BST and BST1 samples. The fitting activation energy data for grain and grain boundary relaxation process (Ea,g and Ea,gb) through Arrhenius law are listed
Fig. 5. lnfrelax vs. 1000/T of BSTS0.5 and BSTS3 ceramics.
in Table 2, which are close to previous reports for perovskites [25,26]. It was found that the Ea data are consistent with the typical value for migration of oxygen vacancies in perovskite oxides ( 1 eV), indicating that oxygen vacancy is the perdominant mobile charge carriers in grains and grain boundaries for all samples. For titanate ceramics, oxygen vacancies tend to form during sintering at high temperatures. When sintered at higher temperature, it is easy to lose the lattice oxygen and form oxygen vacancies. It was reported that appropriate SiO2 addition benefits the sintering process [27]. Compared with the pure BST ceramics, the sintering activation energy of SiO2-added BST ceramics is lower. When sintering at the same temperature for pure and SiO2-added BST ceramics, more oxygen vacancies could be generated in the latter. Accordingly, the concentration of oxygen vacancies in SiO2-added BST ceramics is higher than that in the pure BST ceramics. Therefore, when SiO2 was added, the concentration of defects increased, leading to the decrease of activation energy, which is consistent with the previous report [24]. Fig. 6 shows the P–E hysteresis loops of the samples with x¼0–1 obtained at maximum electric fields Emax (prior to dielectric breakdown strength) at 1 Hz. It was found that SiO2 additive led to improve the Emax, moreover, a small increase of SiO2 additive can improve the maximum polarization Pmax. Due to the compositional inhomogeneity caused by SiO2 additive, the migration of carriers was hindered, space charge increased, and accordingly, interface polarization enhanced. In addition, further increase of SiO2 additive could
Fig. 6. P–E hysteresis loops of BSTSx ceramics at 1 Hz.
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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References
Table 3 Energy storage properties of BSTSx ceramics. Samples
Emax (kV/cm)
Ureco (J/cm3)
η (%)
BST BSTS0.5 BSTS1
130 134 160
0.71 0.86 0.78
74 79 68
lower the densification and increase the leakage. Especially, normal P–E plots was not obtained in BSTS3 ceramics because of its high porosity and large leakage. The energy storage performance was caculated from the P–E loops in positive electric field regin. The recoverable energy storage density Ureco, was the integration between the discharge curve and polarization axis. The energy storage efficiency η is also an important parameter for the application of capacitors, which is expressed as follows:
η=
Ureco Ureco + Uloss
5
(5)
where the energy loss density Uloss is the integration of closed area of the hysteresis loops in the first quadrant. Table 3 shows the energy storage properties of the samples with x¼ 0–1. For pure BST ceramics, Ureco is 0.71 J/cm3 and η is 74% at 130 kV/cm. With the increase of SiO2 additive, the energy storage density and efficiency first increase and then decrease. The optimized energy storage performance was obtained for BSTS0.5 ceramics: the recoverable energy storage density of 0.86 J/cm3 and energy storage efficiency of 79% at 134 kV/cm. The energy storage density of BSTS0.5 ceramics is higher than that the previous report on glass-additive BST ceramics sintered with the same conventional sintering method (0.72 J/cm3) [8].
4. Conclusions Ba0.4Sr0.6TiO3 ceramics with SiO2 additive were prepared by a solid state reaction technique. XRD and SEM confirmed that a single perovskite phase for the samples with x¼ 0–1 and second phase appears in BSTS3 ceramics. With the increase of SiO2 additive, the dielectric constant decreases and loss increases, with a tendency toward a diffuse phase transition. The calculated relaxation activation data are 0.91–1.10 eV, which could be attributed to the migration of oxygen vacancies. While appropriate SiO2 additive led to the improvement of the breakdown strength, further increase of SiO2 deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: Ureco is 0.86 J/cm3 and η is 79% at 134 kV/cm, indicating that SiO2 additive is a cost-effective way to improve the energy storage performance of BST ceramics.
Acknowledgments This work was supported by the National Key Basic Research Program of China (973 Program, 2015CB654601), Natural Science Foundation of China (Nos. 51102189, 51372069, 21403056), International Science and Technology Cooperation Program of China (2011DFA52680), the Fundamental Research Funds for the Central Universities (WUT: 2014-IV-134) and Research Project of Scientific and Technological Department of Henan Province (162102310051).
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Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics Chunli Diao a,b, Hanxing Liu a,n, Hua Hao a, Minghe Cao a, Zhonghua Yao a a b
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Department of Physics, Henan University, Kaifeng 475004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 April 2016 Received in revised form 27 April 2016 Accepted 28 April 2016
SiO2-added barium strontium titanate ceramics Ba0.4Sr0.6TiO3-xwt%SiO2 (x ¼ 0, 0.5, 1, 3, BSTSx) were prepared via a traditional solid state reaction method. The effect of SiO2 additive on the microstructure, dielectric response and energy storage properties was investigated. The results confirmed that with the increase of SiO2 additive, diffuse phase transition arises and the dielectric constant decreases. An equivalent circuit model and Arrhenius law were used to calculate the activation energy of grain and grain boundary, which indicated that the dielectric relaxation at high temperature was caused by oxygen vacancy. While appropriate SiO2 additive led to improve the breakdown strength, further increase of SiO2 deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: dielectric constant of 1002, dielectric loss of 0.45%, energy density of 0.86 J/cm3 and energy storage efficiency of 79% at 134 kV/cm. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Dielectric properties Ba0.4Sr0.6TiO3 ceramics Energy storage SiO2 additive
1. Introduction Capacitors with high power density and energy density have attracted much attention recently due to the increase of requirements for electrical devices in hybrid electric vehicles, mobile electronics and different forms of pulsed power technology [1–4]. The recoverable energy storage density Ureco of a capacitor can be calculated from the polarization-electric field (P–E) hysteresis loops according to the following equation:
Ureco =
∫P
Pmax
Edp
r
(1)
where E is the applied electrical field, P is the polarization, Pmax is the maximum polarization and Pr is the remnant polarization. It is indicated that high energy storage density depends on high applied electrical field and large difference between Pmax and Pr. Because of high breakdown strength (BDS) and energy storage density, polymer capacitors are usually applied in high-power output applications. However, polymers are used in a limited range for their low dielectric constant and poor thermal stability [5]. Ceramic capacitors are a potential candidate as energy storage devices for their high power density, excellent mechanical and thermal reliability, but their breakdown strength is low. Many efforts have been made to improve the breakdown strength of n
Corresponding author. E-mail address: [email protected] (H. Liu).
ceramic capacitors for energy storage, such as oxide doping [6], chemical coating [7], glass additive [8], spark plasma sintering [9], microwave sintering [10], etc. Among them, glass additive is a simple and economical method to achieve. Xu and Zhang et al. reported that different silicate glass additives were added to improve the energy storage density of ceramic capacitors [11,12]. Compared with silicate glass, SiO2 shows high energy bandgap, excellent electrical insulation and low dielectric loss. It can be expected that SiO2 additive could also improve the breakdown strength for energy storage in ceramics. Zhao et al. reported that SiO2 addition led to a great enhancement in the BDS of (1 x) SrTiO3-xSiO2 ceramics for energy storage [13]. (Ba1 xSrx)TiO3 (x ¼0 1) ceramics are a widely used lead-free dielectric material, with controllable Curie temperature (Tc) and dielectric properties by barium strontium ratio, which can be tailored for specific applications [14]. Among these, (Ba0.4Sr0.6)TiO3 (BST) is a promising energy storage material with paraelectric phase at room temperature, moderate dielectric constant (41000), low dielectric loss and good dielectric stability under DC bias electric field. The energy storage of BST ceramics have been researched by several groups [8–10]. However, there have been no reports on the energy storage of SiO2 added BST ceramics yet. In this work, the effect of SiO2 additive on the microstructure, dielectric response and energy storage performance of (Ba0.4Sr0.6) TiO3 ceramics were investigated.
http://dx.doi.org/10.1016/j.ceramint.2016.04.169 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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2. Experimental procedure (Ba0.4Sr0.6)TiO3-xwt%SiO2 (x ¼0, 0.5, 1, 3; abbreviated as BSTSx) ceramics were prepared using a traditional solid state method. The stoichiometric mixtures of analytical reagent grade powder BaCO3, SrCO3, TiO2 and SiO2 were weighed and mixed. After ball-milled for 24 h, the slurry was dried and calcined at 1150 °C for 2 h. The calcined powder was pressed into disks, using polyvinyl alcohol as a binder. After burning out the binder at 600 °C, the pellets were sintered at 1330 °C for 2 h and cooled naturally. The phase identification were identified by an X-ray diffractometer (XRD) with CuKα radiation (X’Pert PRO, PANalytical, Holland) and microstructure characterization was done using a field emission scanning electron microscopy (FE-SEM, Quanta 450 FEG, FEI, USA). To measure the electrical properties, silver paste was applied to the surfaces of the samples and fired at 800 °C for 20 min as electrodes. Dielectric response and complex impedance spectra were measured using an Agilent E4980A impedance analyzer equipped with a thermostat. The samples were polished with a thickness of 0.3 mm and the polarization-electric field (P–E) hysteresis loops were examined using a Radiant precision workstation (HVI0403–239) based on the Sawyer-Tower circuit at 1 Hz.
3. Results and discussion Fig. 1 shows the XRD patterns of BSTSx ceramics at the room temperature. It was observed that for the ceramics with x ¼0–1, a typical cubic perovskite structure was identified by comparing with standard powder diffraction file database within the scope of the instrument measuring precision. Nevertheless, for BSTS3 ceramics, small amount of impurity phase were detected besides the cubic perovskite structure. The peak position shifts slightly with SiO2 content. Through the refinement of XRD patterns, the cell parameters of the samples with x ¼0, 0.5, 1, 3 are 3.9439 (70.0002) Å, 3.9432 ( 70.0002) Å, 3.9420 (70.0003) Å, 3.9380 (70.0004) Å, respectively. The ionic radius of Si4 þ (0.40 Å, C. N.¼6) is smaller than that of Ti4 þ (0.605 Å, C. N.¼6) [15]. It could be concluded that Si ions occupy at least two positions in BSTMx ceramics: some Si enter into the lattice of BST and occupy the Ti site; some Si exist outside the lattice, which is based on the decrease of cell parameters of BSTMx ceramics and the appearance of impurity in BSTM3 ceramics. The SEM surface micrographs of BSTSx ceramics are displayed in Fig. 2. The microstructure of the pure BST ceramics is dense, and
Fig. 1. XRD patterns of BSTSx ceramics.
the size of angular blocky grains is about 3–10 μm. Compared with BST ceramics, the grain size changes unobviously, however, the grain shape of SiO2-added BST ceramics becomes rounded. Besides a small amount of SiO2 additive entering into the crystal lattice, some SiO2 probably formed glass phase in grain boundaries. With the increase of SiO2 additive, the densification of the ceramics decreases, which may be due to the difference of the physical properties between BST and SiO2. Moreover, second phase was observed in BSTS3 ceramics. This phenomenon is consistent with the results of XRD. Fig. 3a shows the temperature dependence of dielectric constant εr and dielectric loss tanδ of BSTSx ceramics measured at 1 kHz. A peak of dielectric constant at the low temperature corresponding to tetragonal-cubic phase transition was observed, meaning that BSTSx ceramics belong to paraelectric phase at the room temperature. The temperature of dielectric peak, namely Curie temperature Tc, is 71 °C for pure BST ceramics. The dielectric peak was suppressed after SiO2 additive, and not observed in BSTS3 ceramics, which suggested that SiO2 additive could lead to a tendency toward a diffuse phase transition associated with the internal stress between grains [16]. εr and tanδ demonstrate excellent thermal stability from the room temperature to 250 °C. In the loss response, two sets of peak were observed, described as peak 1 and peak 2. Peak 1 is caused by the ferroelectric–paraelectric phase transition discussed above. Peak 2 is associated with Maxwell–Wagner effect, which is similar to some perovskite oxides such as CaCu3Ti4O12, K0.5Na0.5NbO3, and BaTiO3 based ceramics [17–19]. For ceramic materials, the conductivity of insulating grain boundaries is different from that of semiconductor bulk grains, which causes charge accumulation at the grain boundaries. Accordingly, Maxwell–Wagner polarization occurs, relating to interfacial polarizations [20]. The enhancement of dielectric constant at high temperature may also be attributed to the Maxwell– Wagner effect. The dielectric properties of BSTSx ceramics are listed in Table 1. It was observed from Table 1 that after SiO2 addition, the dielectric constant decreases because of the low dielectric constant of SiO2, and dielectric loss increases from 0.0028 to 0.012. Due to the obvious effect of SiO2 additive on the grain shape (Fig. 2), it was indicated that the difference of electrical properties between grains and grain boundary increases. To separate the contribution of grains and grain boundaries, Curie–Weiss fitting was applied above Tc [21]:
εr = C /(T − T0)
(2)
where C is the Curie–Weiss constant and T0 is the Curie–Weiss temperature. The fitting deviation from the experimental plots shows the grain-boundary effect. Fig. 3b shows the Curie–Weiss fitting of BSTSx ceramics at 1 kHz. It was confirmed that with the increase of SiO2 additive, Curie–Weiss fitting deviation becomes obvious, demonstrating that SiO2 additive has a main effect on the electrical properties of grain boundary. Fig. 4 shows the complex impedance spectra of BSTS0.5 and BSTS3 ceramics measured at 20 Hz–2 MHz from 375 °C to 475 °C. For BSTS0.5 ceramics, two semicircular plots with different radii were observed (Fig. 4(a)), which are usually considered that the smaller semicircular corresponds the grain response and the larger one presents the contribution of grain boundaries. The impedance spectra of BST and BST1 ceramics are similar to that of BSTS0.5 ceramics. However, only a semicircular was observed in BSTS3 ceramics. For all the samples at high frequency, the plots tend to zero, indicating the high-frequency parts of the spectra are associated with bulk response. Obviously, the semicircular was depressed slightly and asymmetric (inset 1 of Fig. 4(b)), implying more than one contributing mode. In order to resolve the
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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Fig. 2. SEM surface micrographs of BSTx ceramics.
Fig. 3. (a) Temperature dependence of dielectric properties for BSTSx ceramics at 1 kHz; (b) Simulation of Curie–Weiss law for BSTSx ceramics above Tc.
Table 1 Dielectric properties of BSTSx ceramics at room temperature and 1 kHz.
Table 2 Activation energy of BSTSx ceramics for grain and grain boundary response.
Samples
BST
BSTS0.5
BSTS1
BSTS3
Samples
BST
BSTS0.5
BSTS1
BSTS3
εr tanδ
1132 0.0028
1002 0.0045
982 0.0059
659 0.012
Ea,g (eV) Ea, gb (eV)
1.05 1.10
0.98 0.91
0.99 0.94
0.97 0.99
impedance response, normalized imaginary parts of impedance Z″/Z″max and electric modulus M″/M″max as a function of frequency were shown in the inset 2 of Fig. 4(b). Only a single peak was
characterized for Z″/Z″max and M″/M″max plots, respectively. Moreover, an obvious distance exists between the two peak positions at the same temperature, suggesting two separate response
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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Fig. 4. (a) Complex impedance spectra of BSTS0.5 ceramics, insets are the equivalent circuit model and the enlarged high-frequency terminals of spectra; (b) complex impedance spectra of BSTS3 ceramics, inset 1 is the enlarged high-frequency terminals at 375 °C and inset 2 is the normalized impedance Z″/Z″max and electric modulus M″/M″max as a function of frequency.
modes contributing to the impedance spectra, owing to Z″ and M″ emphasizing different electric features [22]. Based on this consideration, an equivalent circuit model composed of two parallel RQ elements connected in series (inset in Fig. 4a) was used to analyze the impedance spectra. Here, R is resistance and Q refers to constant phase element (CPE). The two response processes are attributable to the contributions of grain and grain-boundary, correspondingly. The relaxation frequency of a response process frelax, corresponding to a specific RQ element, can be determined according to the following equation [23]:
frelax = (RQ )1/ n /2π
(3)
where n is the relaxation distribution parameter and n is between 1 and 2. n ¼1 corresponds to an ideal capacitor, and n¼ 2 refers to an ideal resistor. The relaxation frequency data were fitted to an Arrhenius relation [24]:
frelax = f0 exp( − Ea/kBT )
(4)
where f0, Ea and kB are pre-exponential factor, activation energy for relaxation and Boltzmann constant, respectively. Fig. 5 shows the plots of lnfrelax vs. 1000/T of BSTSx ceramics. The plots generally satisfy linear relation, with the R2 values exceeding 0.997. Similar results were obtained for BST and BST1 samples. The fitting activation energy data for grain and grain boundary relaxation process (Ea,g and Ea,gb) through Arrhenius law are listed
Fig. 5. lnfrelax vs. 1000/T of BSTS0.5 and BSTS3 ceramics.
in Table 2, which are close to previous reports for perovskites [25,26]. It was found that the Ea data are consistent with the typical value for migration of oxygen vacancies in perovskite oxides ( 1 eV), indicating that oxygen vacancy is the perdominant mobile charge carriers in grains and grain boundaries for all samples. For titanate ceramics, oxygen vacancies tend to form during sintering at high temperatures. When sintered at higher temperature, it is easy to lose the lattice oxygen and form oxygen vacancies. It was reported that appropriate SiO2 addition benefits the sintering process [27]. Compared with the pure BST ceramics, the sintering activation energy of SiO2-added BST ceramics is lower. When sintering at the same temperature for pure and SiO2-added BST ceramics, more oxygen vacancies could be generated in the latter. Accordingly, the concentration of oxygen vacancies in SiO2-added BST ceramics is higher than that in the pure BST ceramics. Therefore, when SiO2 was added, the concentration of defects increased, leading to the decrease of activation energy, which is consistent with the previous report [24]. Fig. 6 shows the P–E hysteresis loops of the samples with x¼0–1 obtained at maximum electric fields Emax (prior to dielectric breakdown strength) at 1 Hz. It was found that SiO2 additive led to improve the Emax, moreover, a small increase of SiO2 additive can improve the maximum polarization Pmax. Due to the compositional inhomogeneity caused by SiO2 additive, the migration of carriers was hindered, space charge increased, and accordingly, interface polarization enhanced. In addition, further increase of SiO2 additive could
Fig. 6. P–E hysteresis loops of BSTSx ceramics at 1 Hz.
Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i
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References
Table 3 Energy storage properties of BSTSx ceramics. Samples
Emax (kV/cm)
Ureco (J/cm3)
η (%)
BST BSTS0.5 BSTS1
130 134 160
0.71 0.86 0.78
74 79 68
lower the densification and increase the leakage. Especially, normal P–E plots was not obtained in BSTS3 ceramics because of its high porosity and large leakage. The energy storage performance was caculated from the P–E loops in positive electric field regin. The recoverable energy storage density Ureco, was the integration between the discharge curve and polarization axis. The energy storage efficiency η is also an important parameter for the application of capacitors, which is expressed as follows:
η=
Ureco Ureco + Uloss
5
(5)
where the energy loss density Uloss is the integration of closed area of the hysteresis loops in the first quadrant. Table 3 shows the energy storage properties of the samples with x¼ 0–1. For pure BST ceramics, Ureco is 0.71 J/cm3 and η is 74% at 130 kV/cm. With the increase of SiO2 additive, the energy storage density and efficiency first increase and then decrease. The optimized energy storage performance was obtained for BSTS0.5 ceramics: the recoverable energy storage density of 0.86 J/cm3 and energy storage efficiency of 79% at 134 kV/cm. The energy storage density of BSTS0.5 ceramics is higher than that the previous report on glass-additive BST ceramics sintered with the same conventional sintering method (0.72 J/cm3) [8].
4. Conclusions Ba0.4Sr0.6TiO3 ceramics with SiO2 additive were prepared by a solid state reaction technique. XRD and SEM confirmed that a single perovskite phase for the samples with x¼ 0–1 and second phase appears in BSTS3 ceramics. With the increase of SiO2 additive, the dielectric constant decreases and loss increases, with a tendency toward a diffuse phase transition. The calculated relaxation activation data are 0.91–1.10 eV, which could be attributed to the migration of oxygen vacancies. While appropriate SiO2 additive led to the improvement of the breakdown strength, further increase of SiO2 deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: Ureco is 0.86 J/cm3 and η is 79% at 134 kV/cm, indicating that SiO2 additive is a cost-effective way to improve the energy storage performance of BST ceramics.
Acknowledgments This work was supported by the National Key Basic Research Program of China (973 Program, 2015CB654601), Natural Science Foundation of China (Nos. 51102189, 51372069, 21403056), International Science and Technology Cooperation Program of China (2011DFA52680), the Fundamental Research Funds for the Central Universities (WUT: 2014-IV-134) and Research Project of Scientific and Technological Department of Henan Province (162102310051).
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Please cite this article as: C. Diao, et al., Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.169i