Ceramics International 42 (2016) 12796–12801

Contents lists available at ScienceDirect

Ceramics International

http://d0272-88

n CorrE-m

journal homepage: www.elsevier.com/locate/ceramint

Dielectric relaxation behavior and energy storage properties of Snmodified SrTiO3 based ceramics

Juan Xie, Hua Hao, Hanxing Liu n, Zhonghua Yao, Zhe Song, Lin Zhang, Qi Xu,Jinqiang Dai, Minghe CaoState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Material Science and Engineering,Wuhan University of Technology, Wuhan 430070, China

a r t i c l e i n f o

Article history:Received 25 March 2016Received in revised form30 March 2016Accepted 7 May 2016Available online 7 May 2016

Keywords:SrSnxTi(1�x)O3

Dielectric relaxationEnergy density

x.doi.org/10.1016/j.ceramint.2016.05.04242/& 2016 Elsevier Ltd and Techna Group S.r

esponding author.ail address: lhxhp@whut.edu.cn (H. Liu).

a b s t r a c t

SrSnxTi(1�x)O3 (x¼0, 0.01, 0.03, 0.05, 0.07) dielectric ceramics were fabricated by the solid state reactionmethod. Significant refinement of grain size and improved resistivity were observed with the addition ofSn, accounting for effectively enhanced dielectric breakdown strength, beneficial for the energy storageapplications. Impedance analysis was employed to calculate the conductivities of grain and grainboundary and resistance ratios (Rgb/Rg) of grain boundary to grain. The grain boundary effect was be-lieved to dominate the modified macroscopic performance, which was confirmed by the complex im-pedance analysis. The optimal properties were achieved for samples with x¼0.05, exhibiting a chargeenergy density of 1.1 J/cm3 and an energy efficiency of 87%.

& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

1. Introduction

SrTiO3 (ST) based ceramics are considered promising materialsfor electrical energy storage applications, due to their uniquephysical properties, such as high dielectric constant, low dielectricloss, relatively high dielectric breakdown strength (�200 kV/cm)and favorable electric filed stability. For linear dielectrics includingSrTiO3, the discharged energy storage density is mainly controlledby the dielectric constant and dielectric breakdown strength, as

ε ε= EW 1/2 r 02, indicating the pronounced contribution of di-

electric 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]. Amongthem, doping is considered an effective approach for tailoring thedielectric properties by different metal ions, such as Ba2þ , Bi3þ ,and trivalent rare earth (RE3þ) [4–7]. Patil et al. [4] and Chen et al.[5] respectively reported a significant increase of dielectric con-stant by the incorporation of Ba2þ/Bi3þ in SrTiO3. However, theincrease of dielectric constant is generally achieved at the expenseof breakdown strength. With respect to the equilibrium pointbetween dielectric constant and breakdown strength, appropriateadditives could effectively inhibit the grain growth and then in-crease the Eb [9,17] induced by the enhanced proportion of grainboundary in ceramic [19]. While limited researches focused on the

.l. All rights reserved.

effects of Sn doping on microstructures and energy storage prop-erties for SrTiO3 ceramics. SrSnO3 exhibits a large band gap wherethe valence band is made up from O2�: 2p orbital separated from aconduction band (CB) of hybridized Sn: 5s/O2�: 2p by a forbiddenband (Eg) exceeding 3 eV [20]. In this study, Sn was introduced intoSrTiO3 matrix for optimized energy storage properties.

2. Experimental

The SrSnxTi1�xO3(x¼0, 0.01, 0.03, 0.05, 0.07) polycrystallineceramics were prepared by conventional solid state reactionmethod with analytical reagent grade powders of SrCO3 (499.0%),TiO2(499.0%) and SnO2(499.5%). After ball-milled in alcoholwith zirconium media for 24 h, the slurry was dried, and thencalcined in air at 1150 °C for 2 h. The calcined powders were ball-milled again for secondary grinding. Pellets with 12 mm in dia-meter and �1 mm in thickness were uniaxially pressed at150 MPa using 5% PVA binder and slowly heated at 600 °C for 2 hto burn out the binder. The samples were sintered at 1450 °C for2 h in air. Density measurement was carried out using the Archi-medes method. The relative densities of all the sintered samplesare above 97%.

X-ray diffraction (XRD) measurement was employed at roomtemperature for phase structural analysis by a diffractometer(X’Pert PRO, PANalytical, Holland) using Cu Kα radiation. The mi-crostructure was observed by field-emission scanning electron

Fig. 1. XRD patterns of SrSnxTi1�xO3 ceramics with x¼0–0.07. (a) 10–80° (b) 39–41°.

Fig. 2. XPS spectrum of the Ti 2p peak of the SrSnxTi1�xO3 ceramic (x¼0, x¼0.05).

Table 1Structure parameters and dielectric properties of SrSnxTi1�xO3 ceramics at roomtemperature.

Compositon (x) Structure εr (1 kHz) tanδ (1 kHz)

0 Cubic 300 0.0080.01 Cubic 264 0.0050.03 Cubic 257 0.0040.05 Cubic 239 0.0030.07 Cubic 268 0.005

J. Xie et al. / Ceramics International 42 (2016) 12796–12801 12797

microscope (Quanta 450 FEG, FEI, USA). Dielectric properties,complex impedances and ac conductivities were measured with aprecision impedance analyzer (Agilent 4980A, Agilent, USA). Thedielectric breakdown strength and P–E hysteresis loops were ex-amined at room temperature using a Radiant precision work-station (Radiant RT66A) based on the Sawyer-Tower circuit at10 Hz. The energy density was estimated from the P–E curves, byintegrating the area enclosed within the polarization axis and thedischarged 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 theSrSnxTi1�xO3 ceramics, while no obvious secondary phases wereobserved. The total incorporation of Sn into SrTiO3 perovskitelattice can be confirmed by the shift of peaks (111) toward lowerdiffraction angles with increased Sn, being believed to be attrib-uted to the larger unit cell parameter induced by the partial re-placement of Ti4þ (ionic radius 0.0605 nm) by Sn4þ (ionic radius0.069 nm) in octahedral sites, as shown in Fig. 1(b).

X-ray photoelectron patterns of the titanium 2p spectrum ofx¼0 and x¼0.05 samples are displayed in Fig. 2. The bindingenergies 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 tem-perature and 1 kHz is presented in Table 1. All samples show a lowdielectric loss (less than 1%) accompanied by medium dielectricconstant. For samples with smaller amount of Sn (xr0.05), thedecreased dielectric constant and loss can be explained in terms ofthe decreased polarization owing to the tightening reaction ofSn4þ in the oxygen octahedrons [21,22]. With further increasingSn, increased bulk defects contribute to the enhanced dielectricconstant.

As shown in Fig. 3(a) and (c), almost no obvious change wasfound for dielectric constant with increasing temperature in thetemperature region r200 °C, but extensively depends on thefrequency at higher temperature. Compared to undoped SrTiO3,SrSn0.05Ti0.95O3 shows greatly reduced dielectric constant in thehigh temperature and slightly changed properties at low tem-perature. Fig. 3(b) and (d) illustrates the temperature dependenceof the relaxation frequency fr in the temperature range below500 °C in terms of ln fr versus 1000/T based on the Arrhenius lawfor SrTiO3 and SrSn0.05Ti0.95O3, according to

= ( − ( ) ( )f f E k Texp / 1r a B0

where fr corresponds to the characteristic tanδ peaks, f0 is therelaxation frequency at an infinite temperature, Ea is the activationenergy for the dielectric relaxation, and kB is the Boltzmann con-stant. The values of the activation energy Ea for all the samples(other fitting value not shown here) were found to locate from0.91 eV to 0.96 eV, being good agreement with that of Bi-dopedSrTiO3 ceramics [5], indicating that the dielectric relaxation resultsfrom the double ionization of oxygen vacancies or its related

Fig. 3. Temperature dependence of dielectric constant and dielectric loss at various frequencies for SrSnxTi1�xO3 ceramics: (a) x¼0; (b) x¼0.05. The inset of (b), (d) showsthe temperature dependence of the relaxation frequency fr with ln fr versus 1000/T function.

J. Xie et al. / Ceramics International 42 (2016) 12796–1280112798

defect associations (like Ti4þ/Ti3þ-VO.).

Fig. 4 shows the fresh-fractured SEM micrographs of theSrSnxTi1�xO3 ceramics. All the samples show highly dense andhomogeneous morphologies, while obvious change in grain sizeand its distribution were observed. The Sn content x enrichedsmall grains surrounded by coarse grains until xo0.05. Thiscomposition-dependent microstructure was considered to de-monstrate the inhibiting effect of the homovalently-substitutedsolute on grain growth [23]. The sample with x¼0.05 exhibitsmore homogeneous microstructure and smallest grain size (theaverage grain size o2 mm). However, abnormal grain growth wasobserved with further increased SnO2.

Fig. 5 shows the frequency dependent ac conductivity (sac) forSrTiO3 and SrSn0.05Ti0.95O3 ceramics in the temperature range of350–500 °C. Lower sac was observed in SrSn0.05Ti0.95O3 ceramicsas compared to pure SrTiO3. The sac tends to be saturated at lowfrequencies, which is approximately equal to the dc conductivity(sdc). The lnsdc versus 1000/T was found to obey the Arrheniusrelationship, in terms of

σ σ=( )

⎛⎝⎜

⎞⎠⎟

Ek

expT 2

dccond

B0

where s0 is the pre-exponential factor, Econd is the activation en-ergy, and kB is the Boltzmann constant. The activation energies ofpure SrTiO3 and SrSn0.05Ti0.95O3 ceramics are 1.31 eV and 1.09 eV,respectively, indicating the dielectric relaxation behavior is causedby the space charge polarization by the free carriers at the di-electric-electrode interfaces [19].

Fig. 6(a) displays the complex impedance spectra measured at

400 °C for different samples. Two distinct arcs were observed inthe patterns, which can be represented by a parallel RC element[24]. The equivalent circuit (shown in the inset of Fig. 6(a)) for theSrSnxTi1�xO3 ceramic system is consisted of two parallel resistor-capacitor (RC) elements connected in series. The semi-circle in thelow frequency range corresponds to the grain boundary responseand the smaller arc in the high frequency characters the bulkresponse.

Impedance data of SrSnxTi1�xO3 specimens were measured atevery 25 °C interval from 350 to 475 °C. The conductivities [25] ofgrain and grain boundary were extracted from the fitting of theseimpedance data using Eqs. (3) and (4).

σρ (ρ = ) ( )=R A L/ 1/ 3g g g g

σρ ρ ( ) (ρ = ) ( )=R C R C/ 1/ 4gb gb gb g g g gb gb

where A is the area and L is the thickness of the sample. Thevariation of conductivity of grain and grain boundary for allcompositions as a function of temperature is illustrated in Fig. 6(b). The conductivity of the grain is approximately five orders ofmagnitude higher than that of the grain boundary.

Considering the difference in the electrical properties betweengrain and grain boundary, the redistribution of external electricalfield can be described by [26]

( )σσ σ

=+

+×

( )

d d

d dE E

5g gb g

gb g g gbgb avg

Fig. 4. The fresh-fractured SEM micrographs of the SrSnxTi1-xO3 ceramics specimens with x¼(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.

J. Xie et al. / Ceramics International 42 (2016) 12796–12801 12799

( )σσ σ

=+

+×

( )

d d

d dE E

6gb gb g

gb g g gbg avg

Egb, Eg, and Eavg are the electric field strength corresponding tothe grain boundary, the grain and the average value, respectively.sg is observed much greater than sgb from Fig. 6(b), revealing that

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 dependenceof Sn content x; (d) Breakdown strength Eb, recharge Energy density W and energy efficiency η as a function of Sn content x.

J. Xie et al. / Ceramics International 42 (2016) 12796–1280112800

the grain boundary plays a more preponderant role in breakdownperformance of specimens than grain.

Generally, the refinement of grain size results in the increase ofgrain boundaries amount and also the Rgb/Rg ratio. As shown inFig. 6(c), the enhancement of breakdown strength can be ascribedto the increase of the Rgb/Rg ratio when xr0.05, while furtherincreased Sn dopants lead to a decrease of Eb. Fig. 6(d) shows thevariation of recharged energy density W and energy efficiency ηwith Sn contents. Both W and η gradually increase and then re-duce with increasing Sn from 0.05 to 0.1, being dominated by theenhanced dielectric breakdown strength. The optimal electricbreakdown strength of 25.5 kV/mmwas achieved for SrSnxTi1�xO3

at x¼0.05, accounting for a superior charge energy density of1.1 J/cm3 and energy storage efficiency of 86.03%.

4. Conclusions

The phase structure, microstructure and energy storage prop-erties of SrSnxTi1�xO3 ceramics were investigated in this study. Alow dielectric loss r1% with medium dielectric constant wasobserved for all samples. Obviously, Sn doping inhibits the graingrowth and increases the grain boundary amount, beneficial forenergy storage properties. Optimal charge energy density 1.1 J/cm3

and an energy efficiency of 87% accompanied by enhancedbreakdown strength of 25.5 kV/mm were achieved forSrSn0.05Ti0.95O3.

Acknowledgements.This work was supported by National Natural Science Foun-

dation of China (No. 51372191), the National Key Basic ResearchProgram of China (973 Program) (No. 2015CB654601) and Inter-national Science and Technology Cooperation Program of China

(2011DFA52680).

References

[1] I. Burn, D.M. Smyth, Energy storage in ceramic dielectrics, J. Mater. Sci. 7(1972) 339–343.

[2] G.X. Dong, S.W. Ma, J. Du, J.D. Cui, Dielectric properties and energy storagedensity in ZnO-doped Ba0.3Sr0.7TiO3 ceramics, Ceram. Int. 35 (2009)2069–2075.

[3] G.F. Zhang, H.X. Liu, Z.H. Yao, M.H. Cao, H. Hao, Effects of Ca doping on theenergy storage properties of (Sr, Ca)TiO3 paraelectric ceramics, J. Mater. Sci. 26(2015) 2726–2732.

[4] D.R. Patil, S.A. Lokare, R.S. Devan, S.S. Chougule, C.M. Kanamadi, Y.D. Kolekar, B.K. Chougule, Studies on electrical and dielectric properties of Ba1-xSrxTiO3,Mater. Chem. Phys. 104 (2007) 254–257.

[5] A. Chen, Y. Zhi, L.E. Cross, Oxygen-vacancy-related low-frequency dielectricrelaxation and electrical conduction in Bi:SrTiO3, Phys. Rev. B 62 (2000)228–236.

[6] Y. Zhi, A. Chen, L.E. Cross, Oxygen-vacancy-related dielectric anomalies in La:SrTiO3, Appl. Phys. Lett. 74 (1999) 3044–3046.

[7] Q.G. Hu, Z.Y. Shen, Y.M. Li, Z.M. Wang, W.Q. Luo, Z.X. Xie, Enhanced energystorage properties of dysprosium doped strontium titanate ceramics, Ceram.Int. 40 (2014) 2529–2534.

[8] Z.H. Wu, H.X. Liu, M.H. Cao, Z.Y. Shen, Z.H. Yao, H. Hao, D.B. Luo, Effect ofBaO-Al2O3-B2O3-SiO2 glass additive on densification and dielectric propertiesof Ba0.3Sr0.7TiO3 ceramics, J. Ceram. Soc. Jpn. 116 (2008) 345–349.

[9] L.X. Li, X.X. Yu, H.C. Cai, Q.W. Liao, Y.M. Han, Z.D. Gao, Preparation and di-electric properties of BaCu(B2O5)-doped SrTiO3-based ceramics for energystorage, Mater. Sci. Eng. B – Adv. 178 (2013) 1509–1514.

[10] J. Liebault, J. Vallayer, D. Goeuriot, D. Treheux, F. Thevenot, How the trappingof charges can explain the dielectric breakdown performance of aluminaceramic, J. Eur. Ceram. Soc. 21 (2001) 389–397.

[11] Z. Song, H.X. Liu, S.J. Zhang, Z.J. Wang, Y.T. Shi, H. Hao, M.H. Cao, Z.H. Yao, Z.Y. Yu, Effect of grain size on the energy storage properties of (Ba0.4Sr0.6)TiO3

paraelectric ceramics, J. Eur. Ceram. Soc. 34 (2014) 1209–1217.[12] B. Parija, S.K. Rout, L.S. Cavalcante, A.Z. Simões, S. Panigrahi, E. Longo, N.

C. Batista, Structure, microstructure and dielectric properties of100-x(Bi0.5Na0.5)TiO3-x[SrTiO3] composites ceramics, Appl. Phys. A 109 (2012)715–723.

[13] D. Zhan, Q. Xu, D.P. Huang, H.X. Liu, W. Chen, Dielectric responses of glass-

J. Xie et al. / Ceramics International 42 (2016) 12796–12801 12801

added Ba0.95Ca0.05Zr0.3Ti0.7O3 ceramics for energy storage capacitor, Physica B440 (2014) 67–72.

[14] J. Wang, L. Tang, B. Shen, J.W. Zhai, Property optimization of BST-based com-posite glass ceramics for energy-storage applications, Ceram. Int. 40 (2014)2261–2266.

[15] T. Wu, Y. Pu, K. Chen, Dielectric relaxation behavior and energy storageproperties in Ba0.4Sr0.6Zr0.15Ti0.85O3 ceramics with glass additive, Ceram. Int.39 (2013) 6787–6793.

[16] Q. Xu, D. Zhan, D.P. Huang, H.X. Liu, W. Chen, F. Zhang, Effect ofMgO-CaO-Al2O3-SiO2 glass additive on dielectric propertiesofBa0.95Sr0.05Zr0.2Ti0.8O3 ceramics, J. Alloy. Compd. 558 (2013) 77–83.

[17] Q. Xu, X.F. Zhang, Y.H. Huang, W. Chen, H.X. Liu, M. Chen, Effect of MgO onstructure and nonlinear dielectric properties of Ba0.6Sr0.4TiO3/MgO compositeceramics prepared from superfine powders, J. Alloy. Compd. 488 (2009)448–453.

[18] X.F. Zhang, Q. Xu, D. Zhan, H.X. Liu, W. Chen, D.P. Huang, Dielectric response ofMgO-added Ba0.6Sr0.4TiO3 ceramics under bias electric field: examination ofcontributing mechanisms, Physica B 410 (2013) 170–176.

[19] Z.J. Wang, M.H. Cao, Z.H. Yao, Z. Song, G.Y. Li, W. Hu, H. Hao, H.X. Liu, Dielectricrelaxation behavior and energy storage properties in SrTiO3 ceramics with

trace amounts of ZrO2 additives, Ceram. Int. 40 (2014) 14127–14132.[20] B. Hadjarab, A. Bouguelia, M. Trari, Synthesis, physical and photo electro-

chemical characterization of La-doped SrSnO3, J. Phys. Chem. Solids 68 (2007)1491–1499.

[21] L.X. Fu, L.Y. Zhang, X. Yao, Structural and dielectric properties ofBa0.80Sr0.20Ti1�xSnxO3 ceramics, J. Electroceram. 21 (2008) 561–564.

[22] N. Baskaran, H. Chang, Effect of Sn doping on the phase transformationproperties of ferroelectric BaTiO3, J. Mater. Sci. - Mater. Electron. 12 (2001)527–531.

[23] A. Ianculescu, D. Berger, L.P. Curecheriu, N. Drăgan, E. Vasile, Properties ofBa1-xSrxTiO3 ceramics prepared by the modified-Pechini method, Ferro-electrics 369 (2008) 22–34.

[24] D.C. Sinclair, A.R. West, Impedance and modulus spectroscopy of semi-conducting BaTiO3 showing positive temperature coefficient of resistance, J.Appl. Phys. 66 (1989) 3850–3856.

[25] Y. Sun, H.X. Liu, H. Hao, L. Zhang, S.J. Zhang, The role of Co in the BaTiO3 -Na0.5Bi0.5TiO3 based X9R ceramics, Ceram. Int. 41 (2015) 931–939.

[26] T. Wu, Y. Pu, K. Chen, Dielectric relaxation behavior and energy storageproperties in Ba0.4Sr0.6Zr0.15Ti0.85O3 ceramics with glass additives, Ceram. Int.39 (2013) 6787–6793.