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Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics Juan Xie, Hua Hao, Hanxing Liu n, Zhonghua Yao, Zhe Song, Lin Zhang, Qi Xu, Jinqiang Dai, Minghe Cao State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2016 Received in revised form 30 March 2016 Accepted 7 May 2016
SrSnxTi(1 x)O3 (x¼ 0, 0.01, 0.03, 0.05, 0.07) dielectric ceramics were fabricated by the solid state reaction method. Significant refinement of grain size and improved resistivity were observed with the addition of Sn, accounting for effectively enhanced dielectric breakdown strength, beneficial for the energy storage applications. Impedance analysis was employed to calculate the conductivities of grain and grain boundary and resistance ratios (Rgb/Rg) of grain boundary to grain. The grain boundary effect was believed to dominate the modified macroscopic performance, which was confirmed by the complex impedance analysis. The optimal properties were achieved for samples with x ¼0.05, exhibiting a charge energy density of 1.1 J/cm3 and an energy efficiency of 87%. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: SrSnxTi(1 x)O3 Dielectric relaxation Energy density
1. Introduction SrTiO3 (ST) based ceramics are considered promising materials for electrical energy storage applications, due to their unique physical properties, such as high dielectric constant, low dielectric loss, relatively high dielectric breakdown strength ( 200 kV/cm) and favorable electric filed stability. For linear dielectrics including SrTiO3, the discharged energy storage density is mainly controlled by the dielectric constant and dielectric breakdown strength, as W = 1/2εr ε0 E2, indicating the pronounced contribution of dielectric breakdown strength to the energy storage density [1–3]. Numerous attempts have been explored to further improve the energy storage properties of ST-based ceramics [4–18]. Among them, doping is considered an effective approach for tailoring the dielectric properties by different metal ions, such as Ba2 þ , Bi3 þ , and trivalent rare earth (RE3 þ ) [4–7]. Patil et al. [4] and Chen et al. [5] respectively reported a significant increase of dielectric constant by the incorporation of Ba2 þ /Bi3 þ in SrTiO3. However, the increase of dielectric constant is generally achieved at the expense of breakdown strength. With respect to the equilibrium point between dielectric constant and breakdown strength, appropriate additives could effectively inhibit the grain growth and then increase the Eb [9,17] induced by the enhanced proportion of grain boundary in ceramic [19]. While limited researches focused on the n
Corresponding author. E-mail address: [email protected] (H. Liu).
effects of Sn doping on microstructures and energy storage properties for SrTiO3 ceramics. SrSnO3 exhibits a large band gap where the valence band is made up from O2 : 2p orbital separated from a conduction band (CB) of hybridized Sn: 5s/O2 : 2p by a forbidden band (Eg) exceeding 3 eV [20]. In this study, Sn was introduced into SrTiO3 matrix for optimized energy storage properties.
2. Experimental The SrSnxTi1 xO3(x ¼0, 0.01, 0.03, 0.05, 0.07) polycrystalline ceramics were prepared by conventional solid state reaction method with analytical reagent grade powders of SrCO3 (4 99.0%), TiO2( 499.0%) and SnO2(4 99.5%). After ball-milled in alcohol with zirconium media for 24 h, the slurry was dried, and then calcined in air at 1150 °C for 2 h. The calcined powders were ballmilled again for secondary grinding. Pellets with 12 mm in diameter and 1 mm in thickness were uniaxially pressed at 150 MPa using 5% PVA binder and slowly heated at 600 °C for 2 h to burn out the binder. The samples were sintered at 1450 °C for 2 h in air. Density measurement was carried out using the Archimedes method. The relative densities of all the sintered samples are above 97%. X-ray diffraction (XRD) measurement was employed at room temperature for phase structural analysis by a diffractometer (X’Pert PRO, PANalytical, Holland) using Cu Kα radiation. The microstructure was observed by field-emission scanning electron
http://dx.doi.org/10.1016/j.ceramint.2016.05.042 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i
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Fig. 1. XRD patterns of SrSnxTi1 xO3 ceramics with x ¼ 0–0.07. (a) 10–80° (b) 39–41°.
Fig. 2. XPS spectrum of the Ti 2p peak of the SrSnxTi1 xO3 ceramic (x ¼0, x¼ 0.05). Table 1 Structure parameters and dielectric properties of SrSnxTi1 xO3 ceramics at room temperature. Compositon (x)
Structure
εr (1 kHz)
tanδ (1 kHz)
0 0.01 0.03 0.05 0.07
Cubic Cubic Cubic Cubic Cubic
300 264 257 239 268
0.008 0.005 0.004 0.003 0.005
microscope (Quanta 450 FEG, FEI, USA). Dielectric properties, complex impedances and ac conductivities were measured with a precision impedance analyzer (Agilent 4980A, Agilent, USA). The dielectric breakdown strength and P–E hysteresis loops were examined at room temperature using a Radiant precision workstation (Radiant RT66A) based on the Sawyer-Tower circuit at 10 Hz. The energy density was estimated from the P–E curves, by integrating the area enclosed within the polarization axis and the discharged curve. 3. Results and discussion Fig. 1(a) shows room-temperature XRD patterns, which reveals
the presence of the single cubic perovskite phase for all the SrSnxTi1 xO3 ceramics, while no obvious secondary phases were observed. The total incorporation of Sn into SrTiO3 perovskite lattice can be confirmed by the shift of peaks (111) toward lower diffraction angles with increased Sn, being believed to be attributed to the larger unit cell parameter induced by the partial replacement of Ti4 þ (ionic radius 0.0605 nm) by Sn4 þ (ionic radius 0.069 nm) in octahedral sites, as shown in Fig. 1(b). X-ray photoelectron patterns of the titanium 2p spectrum of x¼ 0 and x ¼0.05 samples are displayed in Fig. 2. The binding energies of Ti2p3/2 and 2p1/2 peaks do not shifted by Sn doping, indicating the insignificant effect of Sn incorporation on Ti valence. A summary of the dielectric data for all samples at room temperature and 1 kHz is presented in Table 1. All samples show a low dielectric loss (less than 1%) accompanied by medium dielectric constant. For samples with smaller amount of Sn (x r0.05), the decreased dielectric constant and loss can be explained in terms of the decreased polarization owing to the tightening reaction of Sn4 þ in the oxygen octahedrons [21,22]. With further increasing Sn, increased bulk defects contribute to the enhanced dielectric constant. As shown in Fig. 3(a) and (c), almost no obvious change was found for dielectric constant with increasing temperature in the temperature region r200 °C, but extensively depends on the frequency at higher temperature. Compared to undoped SrTiO3, SrSn0.05Ti0.95O3 shows greatly reduced dielectric constant in the high temperature and slightly changed properties at low temperature. Fig. 3(b) and (d) illustrates the temperature dependence of the relaxation frequency fr in the temperature range below 500 °C in terms of ln fr versus 1000/T based on the Arrhenius law for SrTiO3 and SrSn0.05Ti0.95O3, according to
fr =f0 exp ( − Ea/(kB T )
(1)
where fr corresponds to the characteristic tanδ peaks, f0 is the relaxation frequency at an infinite temperature, Ea is the activation energy for the dielectric relaxation, and kB is the Boltzmann constant. The values of the activation energy Ea for all the samples (other fitting value not shown here) were found to locate from 0.91 eV to 0.96 eV, being good agreement with that of Bi-doped SrTiO3 ceramics [5], indicating that the dielectric relaxation results from the double ionization of oxygen vacancies or its related
Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i
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Fig. 3. Temperature dependence of dielectric constant and dielectric loss at various frequencies for SrSnxTi1 xO3 ceramics: (a) x ¼0; (b) x ¼0.05. The inset of (b), (d) shows the temperature dependence of the relaxation frequency fr with ln fr versus 1000/T function.
defect associations (like Ti4 þ /Ti3 þ -VO.). Fig. 4 shows the fresh-fractured SEM micrographs of the SrSnxTi1 xO3 ceramics. All the samples show highly dense and homogeneous morphologies, while obvious change in grain size and its distribution were observed. The Sn content x enriched small grains surrounded by coarse grains until x o0.05. This composition-dependent microstructure was considered to demonstrate the inhibiting effect of the homovalently-substituted solute on grain growth [23]. The sample with x ¼0.05 exhibits more homogeneous microstructure and smallest grain size (the average grain size o 2 mm). However, abnormal grain growth was observed with further increased SnO2. Fig. 5 shows the frequency dependent ac conductivity (sac) for SrTiO3 and SrSn0.05Ti0.95O3 ceramics in the temperature range of 350–500 °C. Lower sac was observed in SrSn0.05Ti0.95O3 ceramics as compared to pure SrTiO3. The sac tends to be saturated at low frequencies, which is approximately equal to the dc conductivity (sdc). The lnsdc versus 1000/T was found to obey the Arrhenius relationship, in terms of
⎞ ⎛E σdc =σ 0 exp ⎜ cond ⎟ ⎝ kB T ⎠
(2)
where s0 is the pre-exponential factor, Econd is the activation energy, and kB is the Boltzmann constant. The activation energies of pure SrTiO3 and SrSn0.05Ti0.95O3 ceramics are 1.31 eV and 1.09 eV, respectively, indicating the dielectric relaxation behavior is caused by the space charge polarization by the free carriers at the dielectric-electrode interfaces [19]. Fig. 6(a) displays the complex impedance spectra measured at
400 °C for different samples. Two distinct arcs were observed in the patterns, which can be represented by a parallel RC element [24]. The equivalent circuit (shown in the inset of Fig. 6(a)) for the SrSnxTi1 xO3 ceramic system is consisted of two parallel resistorcapacitor (RC) elements connected in series. The semi-circle in the low frequency range corresponds to the grain boundary response and the smaller arc in the high frequency characters the bulk response. Impedance data of SrSnxTi1 xO3 specimens were measured at every 25 °C interval from 350 to 475 °C. The conductivities [25] of grain and grain boundary were extracted from the fitting of these impedance data using Eqs. (3) and (4).
ρg =Rg A/L
ρgb =Rgb Cgb ρg /(Rg Cg )
(ρg =1/σg )
(ρgb=1/σgb )
(3)
(4)
where A is the area and L is the thickness of the sample. The variation of conductivity of grain and grain boundary for all compositions as a function of temperature is illustrated in Fig. 6 (b). The conductivity of the grain is approximately five orders of magnitude higher than that of the grain boundary. Considering the difference in the electrical properties between grain and grain boundary, the redistribution of external electrical field can be described by [26]
Egb=
σg ( dgb+dg ) σgb dg +σg dgb
× E avg
(5)
Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i
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Fig. 4. The fresh-fractured SEM micrographs of the SrSnxTi1-xO3 ceramics specimens with x ¼(a) 0, (b) 0.01, (c) 0.03, (d) 0.05, (e) 0.07.
Fig. 5. The frequency dependence of ac conductivity (sac) for the pure SrTiO3 and SrSn0.05Ti0.95O3 ceramics.
Eg=
σgb ( dgb+dg ) σgb dg +σg dgb
× E avg
(6)
Egb, Eg, and Eavg are the electric field strength corresponding to the grain boundary, the grain and the average value, respectively. sg is observed much greater than sgb from Fig. 6(b), revealing that
Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i
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Fig. 6. (a) The complex impedance spectra measured at 400 °C for SrSnxTi1 xO3. Inset shows the enlarged view; inset of (a) The equivalent circuit for the SrSnxTi1 xO3 ceramic system; (b) Conductivities of the grain and grain boundary as a function of 1000/T for the SrSnxTi1 xO3 ceramics; (c) Rgb/Rg and Breakdown strength Eb dependence of Sn content x; (d) Breakdown strength Eb, recharge Energy density W and energy efficiency η as a function of Sn content x.
the grain boundary plays a more preponderant role in breakdown performance of specimens than grain. Generally, the refinement of grain size results in the increase of grain boundaries amount and also the Rgb/Rg ratio. As shown in Fig. 6(c), the enhancement of breakdown strength can be ascribed to the increase of the Rgb/Rg ratio when x r0.05, while further increased Sn dopants lead to a decrease of Eb. Fig. 6(d) shows the variation of recharged energy density W and energy efficiency η with Sn contents. Both W and η gradually increase and then reduce with increasing Sn from 0.05 to 0.1, being dominated by the enhanced dielectric breakdown strength. The optimal electric breakdown strength of 25.5 kV/mm was achieved for SrSnxTi1 xO3 at x ¼0.05, accounting for a superior charge energy density of 1.1 J/cm3 and energy storage efficiency of 86.03%.
4. Conclusions The phase structure, microstructure and energy storage properties of SrSnxTi1 xO3 ceramics were investigated in this study. A low dielectric loss r1% with medium dielectric constant was observed for all samples. Obviously, Sn doping inhibits the grain growth and increases the grain boundary amount, beneficial for energy storage properties. Optimal charge energy density 1.1 J/cm3 and an energy efficiency of 87% accompanied by enhanced breakdown strength of 25.5 kV/mm were achieved for SrSn0.05Ti0.95O3. Acknowledgements. This work was supported by National Natural Science Foundation of China (No. 51372191), the National Key Basic Research Program of China (973 Program) (No. 2015CB654601) and International Science and Technology Cooperation Program of China
(2011DFA52680).
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Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics Juan Xie, Hua Hao, Hanxing Liu n, Zhonghua Yao, Zhe Song, Lin Zhang, Qi Xu, Jinqiang Dai, Minghe Cao State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2016 Received in revised form 30 March 2016 Accepted 7 May 2016
SrSnxTi(1 x)O3 (x¼ 0, 0.01, 0.03, 0.05, 0.07) dielectric ceramics were fabricated by the solid state reaction method. Significant refinement of grain size and improved resistivity were observed with the addition of Sn, accounting for effectively enhanced dielectric breakdown strength, beneficial for the energy storage applications. Impedance analysis was employed to calculate the conductivities of grain and grain boundary and resistance ratios (Rgb/Rg) of grain boundary to grain. The grain boundary effect was believed to dominate the modified macroscopic performance, which was confirmed by the complex impedance analysis. The optimal properties were achieved for samples with x ¼0.05, exhibiting a charge energy density of 1.1 J/cm3 and an energy efficiency of 87%. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: SrSnxTi(1 x)O3 Dielectric relaxation Energy density
1. Introduction SrTiO3 (ST) based ceramics are considered promising materials for electrical energy storage applications, due to their unique physical properties, such as high dielectric constant, low dielectric loss, relatively high dielectric breakdown strength ( 200 kV/cm) and favorable electric filed stability. For linear dielectrics including SrTiO3, the discharged energy storage density is mainly controlled by the dielectric constant and dielectric breakdown strength, as W = 1/2εr ε0 E2, indicating the pronounced contribution of dielectric breakdown strength to the energy storage density [1–3]. Numerous attempts have been explored to further improve the energy storage properties of ST-based ceramics [4–18]. Among them, doping is considered an effective approach for tailoring the dielectric properties by different metal ions, such as Ba2 þ , Bi3 þ , and trivalent rare earth (RE3 þ ) [4–7]. Patil et al. [4] and Chen et al. [5] respectively reported a significant increase of dielectric constant by the incorporation of Ba2 þ /Bi3 þ in SrTiO3. However, the increase of dielectric constant is generally achieved at the expense of breakdown strength. With respect to the equilibrium point between dielectric constant and breakdown strength, appropriate additives could effectively inhibit the grain growth and then increase the Eb [9,17] induced by the enhanced proportion of grain boundary in ceramic [19]. While limited researches focused on the n
Corresponding author. E-mail address: [email protected] (H. Liu).
effects of Sn doping on microstructures and energy storage properties for SrTiO3 ceramics. SrSnO3 exhibits a large band gap where the valence band is made up from O2 : 2p orbital separated from a conduction band (CB) of hybridized Sn: 5s/O2 : 2p by a forbidden band (Eg) exceeding 3 eV [20]. In this study, Sn was introduced into SrTiO3 matrix for optimized energy storage properties.
2. Experimental The SrSnxTi1 xO3(x ¼0, 0.01, 0.03, 0.05, 0.07) polycrystalline ceramics were prepared by conventional solid state reaction method with analytical reagent grade powders of SrCO3 (4 99.0%), TiO2( 499.0%) and SnO2(4 99.5%). After ball-milled in alcohol with zirconium media for 24 h, the slurry was dried, and then calcined in air at 1150 °C for 2 h. The calcined powders were ballmilled again for secondary grinding. Pellets with 12 mm in diameter and 1 mm in thickness were uniaxially pressed at 150 MPa using 5% PVA binder and slowly heated at 600 °C for 2 h to burn out the binder. The samples were sintered at 1450 °C for 2 h in air. Density measurement was carried out using the Archimedes method. The relative densities of all the sintered samples are above 97%. X-ray diffraction (XRD) measurement was employed at room temperature for phase structural analysis by a diffractometer (X’Pert PRO, PANalytical, Holland) using Cu Kα radiation. The microstructure was observed by field-emission scanning electron
http://dx.doi.org/10.1016/j.ceramint.2016.05.042 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i
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Fig. 1. XRD patterns of SrSnxTi1 xO3 ceramics with x ¼ 0–0.07. (a) 10–80° (b) 39–41°.
Fig. 2. XPS spectrum of the Ti 2p peak of the SrSnxTi1 xO3 ceramic (x ¼0, x¼ 0.05). Table 1 Structure parameters and dielectric properties of SrSnxTi1 xO3 ceramics at room temperature. Compositon (x)
Structure
εr (1 kHz)
tanδ (1 kHz)
0 0.01 0.03 0.05 0.07
Cubic Cubic Cubic Cubic Cubic
300 264 257 239 268
0.008 0.005 0.004 0.003 0.005
microscope (Quanta 450 FEG, FEI, USA). Dielectric properties, complex impedances and ac conductivities were measured with a precision impedance analyzer (Agilent 4980A, Agilent, USA). The dielectric breakdown strength and P–E hysteresis loops were examined at room temperature using a Radiant precision workstation (Radiant RT66A) based on the Sawyer-Tower circuit at 10 Hz. The energy density was estimated from the P–E curves, by integrating the area enclosed within the polarization axis and the discharged curve. 3. Results and discussion Fig. 1(a) shows room-temperature XRD patterns, which reveals
the presence of the single cubic perovskite phase for all the SrSnxTi1 xO3 ceramics, while no obvious secondary phases were observed. The total incorporation of Sn into SrTiO3 perovskite lattice can be confirmed by the shift of peaks (111) toward lower diffraction angles with increased Sn, being believed to be attributed to the larger unit cell parameter induced by the partial replacement of Ti4 þ (ionic radius 0.0605 nm) by Sn4 þ (ionic radius 0.069 nm) in octahedral sites, as shown in Fig. 1(b). X-ray photoelectron patterns of the titanium 2p spectrum of x¼ 0 and x ¼0.05 samples are displayed in Fig. 2. The binding energies of Ti2p3/2 and 2p1/2 peaks do not shifted by Sn doping, indicating the insignificant effect of Sn incorporation on Ti valence. A summary of the dielectric data for all samples at room temperature and 1 kHz is presented in Table 1. All samples show a low dielectric loss (less than 1%) accompanied by medium dielectric constant. For samples with smaller amount of Sn (x r0.05), the decreased dielectric constant and loss can be explained in terms of the decreased polarization owing to the tightening reaction of Sn4 þ in the oxygen octahedrons [21,22]. With further increasing Sn, increased bulk defects contribute to the enhanced dielectric constant. As shown in Fig. 3(a) and (c), almost no obvious change was found for dielectric constant with increasing temperature in the temperature region r200 °C, but extensively depends on the frequency at higher temperature. Compared to undoped SrTiO3, SrSn0.05Ti0.95O3 shows greatly reduced dielectric constant in the high temperature and slightly changed properties at low temperature. Fig. 3(b) and (d) illustrates the temperature dependence of the relaxation frequency fr in the temperature range below 500 °C in terms of ln fr versus 1000/T based on the Arrhenius law for SrTiO3 and SrSn0.05Ti0.95O3, according to
fr =f0 exp ( − Ea/(kB T )
(1)
where fr corresponds to the characteristic tanδ peaks, f0 is the relaxation frequency at an infinite temperature, Ea is the activation energy for the dielectric relaxation, and kB is the Boltzmann constant. The values of the activation energy Ea for all the samples (other fitting value not shown here) were found to locate from 0.91 eV to 0.96 eV, being good agreement with that of Bi-doped SrTiO3 ceramics [5], indicating that the dielectric relaxation results from the double ionization of oxygen vacancies or its related
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Fig. 3. Temperature dependence of dielectric constant and dielectric loss at various frequencies for SrSnxTi1 xO3 ceramics: (a) x ¼0; (b) x ¼0.05. The inset of (b), (d) shows the temperature dependence of the relaxation frequency fr with ln fr versus 1000/T function.
defect associations (like Ti4 þ /Ti3 þ -VO.). Fig. 4 shows the fresh-fractured SEM micrographs of the SrSnxTi1 xO3 ceramics. All the samples show highly dense and homogeneous morphologies, while obvious change in grain size and its distribution were observed. The Sn content x enriched small grains surrounded by coarse grains until x o0.05. This composition-dependent microstructure was considered to demonstrate the inhibiting effect of the homovalently-substituted solute on grain growth [23]. The sample with x ¼0.05 exhibits more homogeneous microstructure and smallest grain size (the average grain size o 2 mm). However, abnormal grain growth was observed with further increased SnO2. Fig. 5 shows the frequency dependent ac conductivity (sac) for SrTiO3 and SrSn0.05Ti0.95O3 ceramics in the temperature range of 350–500 °C. Lower sac was observed in SrSn0.05Ti0.95O3 ceramics as compared to pure SrTiO3. The sac tends to be saturated at low frequencies, which is approximately equal to the dc conductivity (sdc). The lnsdc versus 1000/T was found to obey the Arrhenius relationship, in terms of
⎞ ⎛E σdc =σ 0 exp ⎜ cond ⎟ ⎝ kB T ⎠
(2)
where s0 is the pre-exponential factor, Econd is the activation energy, and kB is the Boltzmann constant. The activation energies of pure SrTiO3 and SrSn0.05Ti0.95O3 ceramics are 1.31 eV and 1.09 eV, respectively, indicating the dielectric relaxation behavior is caused by the space charge polarization by the free carriers at the dielectric-electrode interfaces [19]. Fig. 6(a) displays the complex impedance spectra measured at
400 °C for different samples. Two distinct arcs were observed in the patterns, which can be represented by a parallel RC element [24]. The equivalent circuit (shown in the inset of Fig. 6(a)) for the SrSnxTi1 xO3 ceramic system is consisted of two parallel resistorcapacitor (RC) elements connected in series. The semi-circle in the low frequency range corresponds to the grain boundary response and the smaller arc in the high frequency characters the bulk response. Impedance data of SrSnxTi1 xO3 specimens were measured at every 25 °C interval from 350 to 475 °C. The conductivities [25] of grain and grain boundary were extracted from the fitting of these impedance data using Eqs. (3) and (4).
ρg =Rg A/L
ρgb =Rgb Cgb ρg /(Rg Cg )
(ρg =1/σg )
(ρgb=1/σgb )
(3)
(4)
where A is the area and L is the thickness of the sample. The variation of conductivity of grain and grain boundary for all compositions as a function of temperature is illustrated in Fig. 6 (b). The conductivity of the grain is approximately five orders of magnitude higher than that of the grain boundary. Considering the difference in the electrical properties between grain and grain boundary, the redistribution of external electrical field can be described by [26]
Egb=
σg ( dgb+dg ) σgb dg +σg dgb
× E avg
(5)
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Fig. 4. The fresh-fractured SEM micrographs of the SrSnxTi1-xO3 ceramics specimens with x ¼(a) 0, (b) 0.01, (c) 0.03, (d) 0.05, (e) 0.07.
Fig. 5. The frequency dependence of ac conductivity (sac) for the pure SrTiO3 and SrSn0.05Ti0.95O3 ceramics.
Eg=
σgb ( dgb+dg ) σgb dg +σg dgb
× E avg
(6)
Egb, Eg, and Eavg are the electric field strength corresponding to the grain boundary, the grain and the average value, respectively. sg is observed much greater than sgb from Fig. 6(b), revealing that
Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i
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Fig. 6. (a) The complex impedance spectra measured at 400 °C for SrSnxTi1 xO3. Inset shows the enlarged view; inset of (a) The equivalent circuit for the SrSnxTi1 xO3 ceramic system; (b) Conductivities of the grain and grain boundary as a function of 1000/T for the SrSnxTi1 xO3 ceramics; (c) Rgb/Rg and Breakdown strength Eb dependence of Sn content x; (d) Breakdown strength Eb, recharge Energy density W and energy efficiency η as a function of Sn content x.
the grain boundary plays a more preponderant role in breakdown performance of specimens than grain. Generally, the refinement of grain size results in the increase of grain boundaries amount and also the Rgb/Rg ratio. As shown in Fig. 6(c), the enhancement of breakdown strength can be ascribed to the increase of the Rgb/Rg ratio when x r0.05, while further increased Sn dopants lead to a decrease of Eb. Fig. 6(d) shows the variation of recharged energy density W and energy efficiency η with Sn contents. Both W and η gradually increase and then reduce with increasing Sn from 0.05 to 0.1, being dominated by the enhanced dielectric breakdown strength. The optimal electric breakdown strength of 25.5 kV/mm was achieved for SrSnxTi1 xO3 at x ¼0.05, accounting for a superior charge energy density of 1.1 J/cm3 and energy storage efficiency of 86.03%.
4. Conclusions The phase structure, microstructure and energy storage properties of SrSnxTi1 xO3 ceramics were investigated in this study. A low dielectric loss r1% with medium dielectric constant was observed for all samples. Obviously, Sn doping inhibits the grain growth and increases the grain boundary amount, beneficial for energy storage properties. Optimal charge energy density 1.1 J/cm3 and an energy efficiency of 87% accompanied by enhanced breakdown strength of 25.5 kV/mm were achieved for SrSn0.05Ti0.95O3. Acknowledgements. This work was supported by National Natural Science Foundation of China (No. 51372191), the National Key Basic Research Program of China (973 Program) (No. 2015CB654601) and International Science and Technology Cooperation Program of China
(2011DFA52680).
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Please cite this article as: J. Xie, et al., Dielectric relaxation behavior and energy storage properties of Sn modified SrTiO3 based ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.05.042i