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MgO composite ceramics prepared by SPS
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Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS Pengrong Ren ∗ , Qian Wang, Shufeng Li, Gaoyang Zhao Shaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
a r t i c l e
i n f o
Article history: Received 26 April 2016 Received in revised form 16 November 2016 Accepted 9 December 2016 Available online xxx Keywords: BaTix Sn1 − x O3 Spark plasma sintering Energy storage Dielectric tunability Composite
a b s t r a c t (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO (BTS/MgO) composite ceramics were prepared by spark plasma sintering (SPS) technology. Phase constitution, microstructure, dielectric and electrical energy storage properties of BTS/MgO composite ceramics were investigated. The samples prepared by SPS had smaller grain size and presented layer-plate substructure. Dielectric permittivity and dielectric loss of BTS/MgO composite ceramics decreased significantly with the content of MgO increasing, and dielectric tunability maintained a relatively high value (>45%). Meanwhile, the dielectric breakdown strength was improved when addition of MgO in BTS matrix, which resulted in a significant improvement of energy storage density. The high dielectric breakdown strength of 190 kV/cm, energy storage density of 0.5107 J/cm3 and energy storage efficiency of 92.11% were obtained in 90 wt.% BaTi0.85 Sn0.15 O3 –10 wt.% MgO composite ceramics. Therefore, BTS/MgO composites with good tunable dielectric properties and electrical energy storage properties could be exploited for energy storage and phase shifter device applications. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As an intermediate step to versatile, clean and efficient use of energy, energy storage has received worldwide concern and increasing research interest [1]. Materials for energy storage cover a wide range of materials such as lithium ion batteries, fuel cells, fly wheels, electrostatic capacitors, and electrochemical capacitors [2]. Due to their high dielectric permittivity, low loss and low temperature coefficient of permittivity, dielectric materials used for energy storage applications have been receiving intensive attention from research and development to industrialization. The required energy storage properties usually meet as follows: large saturated polarization (Ps), small remnant polarization (Pr), and high electric breakdown field strength (BDS) [3,4]. Recently, paraelectric ceramics have been aroused great interests due to its high energy storage efficiency, wide operation temperature and high stability. In order to improve the energy storage density and energy conversion efficiency, various attempts have been made. Adding metallic oxide as the second phase in the matrix has drawn increasing interest because metallic oxide can significantly reduce the dielectric loss and enhance the electric breakdown field strength. Some chemical
∗ Corresponding author. E-mail address: [email protected] (P. Ren).
additives such as glass, MgO and Al2 O3 have been investigated to improve the maximum energy storage density [5–13]. For example, Zhang et al. [13] have successfully prepared BST/MgO composites with an enhanced energy storage density of 1.14 J/cm3 . In addition, the block ceramic is rarely totally densified, but defects (e.g. pores) are ineluctable. This situation may result in severe breakdown strength degradation and ultimately failure of devices, which affects the capability for storing high density of energy. It has been experimentally verified that the densification could be enhanced in various materials and chemical reactions at interface could be suppressed by using spark plasma sintering (SPS) technique [14–23]. Therefore, high energy storage density could be achieved in bulk materials by enhancing their electric breakdown field strength through better process to prepare a flaw-free sample with full density. Gheorghe Aldica [24] used SPS method for a fast pressing sintering process of BaTi0.87 Sn0.13 O3 powders. Final ceramics had a density of about ∼99% of the theoretical one. In order to further improve the energy storage density of BST/MgO composite, Huang [25] prepared Ba0.4 Sr0.6 TiO3 /MgO composite by SPS and obtained highest energy storage density of 1.50 J/cm3 and high energy storage efficiency of 88.5%. Barium stannate titanate (BaTi1 − x Snx O3 , BTS) ceramics are a solid solution system composed of ferroelectric barium titanate and non-ferroelectric barium stannate. Both of them are of perovskite structures with an ABO3 formula. The character of the ferroelec-
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Please cite this article in press as: P. Ren, et al., Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.016
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tric phase transition in BTS strongly depends on the Sn-level x. The observation of ferroelectric domains below the permittivity maximum temperature provides evidence of ferroelectric long-range order for x ≤ 0.13, whereas indication for the relaxor state of BTS is obtained for compositions x ≥ 2 [26]. The crystallographic phase structures evolve from tetragonal, orthorhombic, rhombohedral to cubic with the increasing tin composition at room temperature. Although this material is one of the earliest prototypes of diffused phase transition studies [27], it has riveted many research interests due to its abnormal dielectric properties and strong dielectric nonlinearity recently [28–30]. The permittivity is very high and as well temperature as bias field sensitive [31–34]. The composition x = 0.15 consists of a mixture of cubic and tetragonal phases, and the dielectric permittivity at room temperature attains a maximum value, over 5000 [35–37]. Close to room temperature the composition x = 0.15 shows a strong temperature variation of permittivity, induced by the proximity of their ferroelectric–paraelectric phase transitions [38]. Therefore, BTS shows interesting ferroelectric properties and is used for capacitors, relaxors and sensors. As far as we know, however, energy storage properties of BTS-based composite ceramics have not been studied. In this work, we reported energy storage properties of (100-x) wt.% BaTi0.85 Ti0.15 O3 –x wt.% MgO (BTS/MgO) (x = 0, 3, 5, 10) composite ceramics prepared by SPS. The effects of MgO content and SPS technology on structure and energy storage properties of BTS were investigated. As a result, our findings not only expand the spectrum of room-temperature lead-free energy storage materials for future applications but also may serve as a guide for revealing other energy storage material alternatives by SPS. 2. Experimental BaTi0.85 Sn0.15 O3 powders were prepared by the conventional solid-state reaction method with high-purity powders of BaCO3 (99.9%), TiO2 (99.9%) and SnO2 (99.9%). The stoichiometric mixtures were ball milled with zirconia balls in ethanol for 12 h. After drying, they were then calcined at 1150 ◦ C for 3 h to obtain a perovskite phase. Then, MgO were used as the raw materials. The
Fig. 1. XRD patterns of (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics: (a) BTS; (b) BTSM3; (c) BTSM5 and (d) BTSM10.
raw materials were weighed according to the following chemical composition: (100-x) wt% BaTi0.85 Sr0.15 O3 –x wt% MgO (BTS/MgO) (x = 0, 3, 5, 10; abbreviated as BTS, BTSM3, BTSM5, BTSM10, respectively), and then ball milled in ethyl alcohol using zirconia balls for 12 h. Furthermore, the powders were placed in a graphite die. BTS and BTS/MgO composites were sintered at 1175 ◦ C and 1250 ◦ C for 5 min under a vacuum of 6 Pa with an SPS apparatus (LABOX330, Sinter Land, Japan) at the heating rate of 100 ◦ C/min. During the sintering, an applied mechanical uniaxial pressure of 30 MPa was applied to the sample. After sintering, the BTS/MgO composite ceramics all reached 99% of the theoretical density. All the spark plasma sintered samples were thermally treated at 1100 ◦ C for 2 h in air to remove the carbon contamination. In order to make a contrast, BTS/MgO composite ceramics were also prepared by using the conventional solid-state-reaction process and the samples were sintered at 1350–1450 ◦ C for 2 h in air. The crystalline phases of sintered samples were characterized by X-ray powder diffraction (XRD; D/Max2550VB+/PC, Rigaku, Tokyo, Japan) using Cu-K␣1 radiation with linear position-sensitive detector. The microstructures and the elemental distribution were observed from the thermal treated surfaces (1100–1200 ◦ C,
Fig. 2. SEM micrographs on thermal-treated surfaces of (100-x) wt.% BaTi0.85 Sr0.15 O3 –x wt.% MgO ceramics detected by signal of SE2: (a) BTS; (b) BTSM3; (c) BTSM5 and (d) BTSM10.
Please cite this article in press as: P. Ren, et al., Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.016
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30 min) with field emission scanning electron microscopy (SEM; JEOL-6700F, Japan Electron Co., Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDX; FeatureMax, Oxford Instruments, Oxfordshire, UK) operating at 20 kV. The average grain size of the sample is determined by Nanomeasurer 1.2 software. In order to characterize electric properties, the sintered pellets were polished, coated with silver electrodes, and sintered at 600 ◦ C for 30 min to form a metal–insulator–metal (MIM) capacitor. The dielectric properties of the ceramics were evaluated with a precision LCR meter (E4294A, Agilent, Santa Clara, CA, USA) associated with a temperature controller (TP94, Linkam, Surrey, U.K.) in a broad range of temperature (−100 ◦ C–300 ◦ C) with a cooling rate of 3 ◦ C/min and frequency (100 Hz–1 MHz) at a signal level of 0.5 V/mm. A ferroelectric tester (TF2000, aixACCT, Aachen, Germany) was used for P-E loops measurement on a sample with a thickness of 0.5 mm. The dielectric tunability properties were measured at 25 ◦ C using an automatic component analyzer (TH2818, Tonghui, Changzhou, China) at 10 kHz. A blocking circuit was adopted to protect the analyzer from bias voltages. External bias field was applied in steps of 1 kV/cm.
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Fig. 3. SEM–EDX line scans of BTSM10. (a) SEM image displays the position of the line scan and (b) EDX plot shows the relative elemental content along the line scan.
3. Results Fig. 1 shows XRD patterns of BTS/MgO composite ceramics sintered at 1175–1250 ◦ C by SPS. It can be seen that the composites consist of only two phases of BTS and MgO. All the samples exhibit pure pseudo-cubic phase because of the obvious (200) peaks. As the content of MgO increases, few (200) peak shifts to the lower angle, which are probably due to the reason that the fast sinter-
ing rate, short sintering periods and low sintering temperature are carried out in the SPS process, which totally or at least partially suppresses the substitution of Ti4+ by Mg2+ and the reaction between BTS and MgO. Howerever, in BTS/MgO composites prepared by conventional sintering method, the (200) peak always shifts to the lower angle because of partial substitution of Ti4+ (r = 0.061 nm) by Mg2+ (r = 0.072 nm) that causes the lattice expansion and an addi-
Fig. 4. Temperature dependence of dielectric permittivity and dielectric loss at various frequencies for (100-x) wt% BaTi0.85 Sn0.15 O3 –x wt% MgO ceramics: (a) (b) BTS; (c) (d) BTSM3; (e) (f) BTSM5 and (g) (h) BTSM10.
Please cite this article in press as: P. Ren, et al., Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.016
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Fig. 5. The dependence of dielectric permittivity vs. electric field measured at 10 kHz and 25 ◦ C for (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics: (a) BTS; (b) BTSM3; (c) BTSM5 and (d) BTSM10.
tional phase of MgTiO3 was observed due to the reaction between Ba0.3 Sr0.6 Ca0.1 TiO3 and MgO [39]. SEM micrographs of the surface of the annealed pellets are illustrated in Fig. 2. Few cracks and pores can be observed in all the samples and well dense microstructure is obtained. The average grain sizes of BTS, BTSM3, BTSM5 and BTSM10 are 925, 409, 473 and 432 nm, respectively. This result indicates, as expected, that the short sintering period is an essential factor to obtain fine-grained BTS/MgO ceramics by the SPS process. More interestingly, each sample presents a layer-plate substructure, especially for BTSM10, as shown in Fig. 2 (d). The corresponding EDX elemental line map (Fig. 3) confirms that MgO is present in the matrix, and there is nearly no diffusion occurred between BTS and MgO. Fig. 4 shows temperature dependence of dielectric permittivity (ε) and dielectric loss (tanı) of BTS/MgO ceramics at various frequencies over −100 to 300 ◦ C. Compared with Curie temperature of pure BTS (23 ◦ C), Curie temperatures of the BTS/MgO composites shift to about −6 ◦ C at 10 kHz. At the room temperature, dielectric permittivity of BTS/MgO composites is about 15658, 2131, 1916 and 1586 at 10 kHz for BTS, BTSM3, BTSM5 and BTSM10, respectively, while dielectric loss is 0.02674, 0.01228, 0.01213 and 0.00987, respectively. It can be seen that the addition of MgO obviously leads to a significant decrease in dielectric permittivity and dielectric loss of BTS/MgO ceramics, which is due to that MgO has a lower permittivity and depresses the dielectric permittivity and dielectric loss of the composites. Besides, quite frequency stability of dielectric permittivity, low conductivity and low dielectric loss at lower frequencies are obtained in BTS/MgO composites, which is beneficial to tunable dielectric applications. The dependence of dielectric permittivity vs. electric field measured at 10 kHz and 25 ◦ C are shown in Fig. 5. As we can seen, the dielectric permittivity of all the samples decreases with the increase of applied electric field and has a rapid reduce at lower electric field. What’s more, we adopted a multi-polarization mech-
anism model and the following equation to interpret the field dependence of the dielectric permittivity [40]: ε (E) = εr (0) /{1 + [ε0 εr (0)3 E 2 ]}
1/3
+ (P0 x/ε0 )[cosh(Ex)]−2
(3)
where P0 , the effective polarization of one cluster; L, the cluster size; kB , the Boltzmann constant; T, the temperature and x = P0 L3 /kB T. The first term represents Johnson contribution which comes from the “intrinsic” lattice phonon polarization, and the second represents Langevin contribution which comes from some “extrinsic” polarization contributions, such as nanometer polar clusters, domain wall motions and phase boundary polarization [40]. The fitted experimental data with a statistical correlation of 0.999 are shown in Fig. 5. For all the samples, Langevin term gives more contribution to the nonlinear dielectric behavior at lower electric field, while Johnson term at higher electric field. Furthermore, it can be found that the contribution of Langevin term decreases after addition of MgO in the BTSn matrix, which might be ascribed to the reason that the existence of the second phase of has an effect on reduction of the grain size, therefore leading to the decreasing contribution of polar clusters in BST matrix on the nonlinear dielectric behavior. Fig. 6 illustrates tunability of the BTS/MgO ceramics at 10 kHz and room temperature as a function of electric field. Tunability of all the samples increases with the applied electric field while gradually decreases with the addition of MgO. The tunability of BTS, BTSM3, BTSM5 and BTSM10 composite is 78.1%, 52.9%, 50.2% and 45.8% at 25 ◦ C and 10 kHz under the electric field of 30 kV/cm. The value of tunability is much larger than other barium titanate (BaTiO3 )-based composites, such as Ba0.6 Sr0.4 TiO3 /MgO [41] BaZr0.25 Ti0.75 O3 /MgO [42] and Ba0.6 Sr0.4 TiO3 /BaZn6 Ti6 O19 [43] composites. Fig. 7 presents P–E hysteresis loops of BTS/MgO ceramics measured at ambient temperature with a triangular wave form at 20 Hz. Fig. 5(a) shows P–E hysteresis loops of BTS/MgO ceramics prepared by SPS. The electric filed is applied at the direction parallel to the uniaxial stress in SPS process. As shown in Fig. 7(a), the dielectric breakdown strength of BTS is about 105 kV/cm. At
Please cite this article in press as: P. Ren, et al., Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.016
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Fig. 6. The tunability at 10 kHz and room temperature as a function of the electric field for (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics.
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addition of MgO. This can be attributed to the huge dielectric breakdown strength of MgO [44]. In order to study the preparation process and uniaxial stress on the influence of the surface morphology, dielectric and ferroelectric of samples, other two samples are also studied. One is BTSM10 prepared by conventional sintering; the other is BTSM10 prepared by SPS, but during electrical measurement the electric field was applied on the direction perpendicular to the uniaxial stress in SPS process. From Fig. 7(b), we can see the BTSM10 prepared by conventional sintering is broken down at only 63.5 kV/cm. Temperature dependence of dielectric permittivity at various frequencies is illustrated in the inset at the top left of Fig. 7(b). Dielectric permittivity (1594 at 25 ◦ C and 10 kHz) of BTSM10 prepared by conventional sintering is close to that of SPS-sintered sample. However, much difference is shown in microstructure between the samples prepared by SPS and conventional sintering. BTSM10 prepared by conventional sintering has large grain size (761 nm) and presents some pores, as shown in the inset at the right corner of Fig. 7(b), which might lead to its lower breakdown strength. Furthermore, P–E hysteresis loops of the SPS-sintered sample are measured in the cross-section direction, as shown in Fig. 7(c). In this case, the dielectric breakdown strength of BTSM10 is 130 kV/cm. Correspondingly, its dielectric permittivity is 1749 at 25 ◦ C, 10 kHz, as illustrated in the inset at the top left of Fig. 7(c). Meanwhile, dense microstructure is maintained at the cross-section direction, as shown in the inset at the right corner of Fig. 7(c). It should be noted that the sample used to measure in Fig. 7(a) and (c) is the same. However, the sample prepared by SPS has different values of breakdown strength in these two directions, parallel and perpendicular to the uniaxial stress during SPS process. This result suggests that the breakdown strength is much related to the microstructure (e.g., grain size, shape, etc.) other than the density of the sample. The cross-section microstructure of BTSM10 prepared by SPS is shown in the inset at the right corner of Fig. 7(c). Its average grain size is 305 nm, smaller than the grain size at the surface. This means more grain boundaries are present in the cross-section direction. It is well known that there are many defects at the grain boundaries [45]; therefore, the breakdown strength of the BTSM10 prepared by SPS is lower at the cross-section direction. Energy delivered to the capacitor cannot be released completely due to the inconformity of the charging and discharging paths in hysteretic P-E loops of BTS/MgO. Therefore, energy storage density and energy efficiency are important factors to bench mark dielectrics for use in energy storage devices. For practical applications, these two parameters should both be taken into consideration. Generally speaking, the energy storage density (W1 ) and the energy efficiency () are given by the following Eqs. (1) and (2), respectively:
Pmax
W1 =
EdP
(1)
Pr
Fig. 7. P–E hysteresis loops of (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics measured at ambient temperature with a triangular wave form at 20 Hz: (a) BTS/MgO sintered by SPS. The electric filed is applied at the direction parallel to the uniaxial pressure in SPS process. (b) BTSM10 prepared by conventional sintering, (c) BTSM10 sintered by SPS. The electric filed is applied at the direction perpendicular to the uniaxial pressure in SPS process. Inset: Temperature dependence of dielectric permittivity at various frequencies and SEM micrographs on thermal treated surfaces detected by signal SE2.
20 Hz, its saturation polarization is about 14.4 C/cm2 . Compared to BTS, the saturation polarization in BTS/MgO composite ceramics decreases, which reaches 10.7 C/cm2 , 10.5 and 9.08 C/cm2 for BTSM3, BTSM5 and BTSM10, respectively. The dielectric breakdown strength of BTS/MgO ceramics significantly increases when
=
W1 × 100% W1 + W2
(2)
where E is the applied electric field, Pr is the remnant polarization, Pmax is the maximum polarization corresponding to the maximum electric field [2]. In general, the energy storage density (W1 ) was evaluated by P-E loops and it is equal to integral of the area enclosed by the discharge curve and the polarization axis. W2 represents the energy loss density and it is equal to integral of the area enclosed by the charge and the discharge curve. The energy densities and energy efficiency calculated from P-E hysteresis loops are depicted in Fig. 8. The highest energy storage density and energy efficiency of BTS ceramic are 0.3466 J/cm3 and 90.8%, respectively. With increasing MgO concentration, the energy storage density of the BTS/MgO ceramics increases and
Please cite this article in press as: P. Ren, et al., Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.016
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Fig. 8. The calculated energy storage density (W1 ) and energy efficiency () of the (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics as a function of MgO fraction (x).
energy efficiency increases firstly then decreases. BTSM10 exhibits the highest energy storage density, 0.5107 J/cm3 , at the electric field of 190 kV/cm. Meanwhile, it maintains higher energy efficiency, 92.11%. Although the energy storage density is not as high as the data reported by Huang et al. [25] in Ba0.4 Sr0.6 TiO3 /MgO composite prepared by SPS, its energy efficiency is much higher. Thus, The BTS/MgO ceramics by SPS possess high energy storage density simultaneously with high energy efficiency. 4. Conclusions BTS/MgO composite ceramics were successfully prepared by SPS method. The results revealed that the fast sintering rate, short sintering periods and lower sintering temperature are carried out in the SPS process, which suppress the substitution of Ti4+ by Mg2+ and the reaction between BTS and MgO. With the increase of the content of MgO, the dielectric permittivity and dielectric loss of the BTS/MgO composite ceramics decrease significantly; meanwhile, the dielectric tunability still maintains a relatively high value. With the MgO content increasing, dielectric tunability of the BTS/MgO composite ceramics gradually decreases, which is ascribed to the reason that the second phase of MgO suppresses the grain growth, leading to the diminishment of the contribution of polar clusters in BTS matrix. In addition, due to excellent electrical breakdown strength of MgO and SPS sintering, the dielectric breakdown strength of BTS/MgO composite is greatly improved, which results in a significant improvement of energy storage density. The highest dielectric breakdown strength of 190 kV/cm, maximum energy storage density of 0.5107 J/cm3 and higher energy storage efficiency of 92.11% are obtained in 90 wt.% BaTi0.85 Sn0.15 O3 –10 wt.% MgO composite. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 51402234), Natural Science Basic Research Plan (No. 2015JQ5198, 2015JQ5142), International Cooperation Project (2013KW14-01) and Young Talent fund of University Association for Science and Technology (20150106) in Shaanxi Province, the doctoral starting fund (No. 101-211408) and New-Star of science and technology (101-256101511) of Xi’an University of Technology. References [1] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Eng. Mater. 22 (2010) 28–62. [2] Z.B. Shen, X.H. Wang, B.C. Luo, L.T. Li, BaTiO3 –BiYbO3 perovskite materials for energy storage applications, J. Mater. Chem. A 3 (2015) 18146–18153. [3] I. Burn, D.M. Smyth, Energy storage in ceramic dielectrics, J. Mater. Sci. 7 (1972) 339–343.
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Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS Pengrong Ren ∗ , Qian Wang, Shufeng Li, Gaoyang Zhao Shaanxi Province Key Laboratory for Electrical Materials and Infiltration Technology, School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China
a r t i c l e
i n f o
Article history: Received 26 April 2016 Received in revised form 16 November 2016 Accepted 9 December 2016 Available online xxx Keywords: BaTix Sn1 − x O3 Spark plasma sintering Energy storage Dielectric tunability Composite
a b s t r a c t (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO (BTS/MgO) composite ceramics were prepared by spark plasma sintering (SPS) technology. Phase constitution, microstructure, dielectric and electrical energy storage properties of BTS/MgO composite ceramics were investigated. The samples prepared by SPS had smaller grain size and presented layer-plate substructure. Dielectric permittivity and dielectric loss of BTS/MgO composite ceramics decreased significantly with the content of MgO increasing, and dielectric tunability maintained a relatively high value (>45%). Meanwhile, the dielectric breakdown strength was improved when addition of MgO in BTS matrix, which resulted in a significant improvement of energy storage density. The high dielectric breakdown strength of 190 kV/cm, energy storage density of 0.5107 J/cm3 and energy storage efficiency of 92.11% were obtained in 90 wt.% BaTi0.85 Sn0.15 O3 –10 wt.% MgO composite ceramics. Therefore, BTS/MgO composites with good tunable dielectric properties and electrical energy storage properties could be exploited for energy storage and phase shifter device applications. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction As an intermediate step to versatile, clean and efficient use of energy, energy storage has received worldwide concern and increasing research interest [1]. Materials for energy storage cover a wide range of materials such as lithium ion batteries, fuel cells, fly wheels, electrostatic capacitors, and electrochemical capacitors [2]. Due to their high dielectric permittivity, low loss and low temperature coefficient of permittivity, dielectric materials used for energy storage applications have been receiving intensive attention from research and development to industrialization. The required energy storage properties usually meet as follows: large saturated polarization (Ps), small remnant polarization (Pr), and high electric breakdown field strength (BDS) [3,4]. Recently, paraelectric ceramics have been aroused great interests due to its high energy storage efficiency, wide operation temperature and high stability. In order to improve the energy storage density and energy conversion efficiency, various attempts have been made. Adding metallic oxide as the second phase in the matrix has drawn increasing interest because metallic oxide can significantly reduce the dielectric loss and enhance the electric breakdown field strength. Some chemical
∗ Corresponding author. E-mail address: [email protected] (P. Ren).
additives such as glass, MgO and Al2 O3 have been investigated to improve the maximum energy storage density [5–13]. For example, Zhang et al. [13] have successfully prepared BST/MgO composites with an enhanced energy storage density of 1.14 J/cm3 . In addition, the block ceramic is rarely totally densified, but defects (e.g. pores) are ineluctable. This situation may result in severe breakdown strength degradation and ultimately failure of devices, which affects the capability for storing high density of energy. It has been experimentally verified that the densification could be enhanced in various materials and chemical reactions at interface could be suppressed by using spark plasma sintering (SPS) technique [14–23]. Therefore, high energy storage density could be achieved in bulk materials by enhancing their electric breakdown field strength through better process to prepare a flaw-free sample with full density. Gheorghe Aldica [24] used SPS method for a fast pressing sintering process of BaTi0.87 Sn0.13 O3 powders. Final ceramics had a density of about ∼99% of the theoretical one. In order to further improve the energy storage density of BST/MgO composite, Huang [25] prepared Ba0.4 Sr0.6 TiO3 /MgO composite by SPS and obtained highest energy storage density of 1.50 J/cm3 and high energy storage efficiency of 88.5%. Barium stannate titanate (BaTi1 − x Snx O3 , BTS) ceramics are a solid solution system composed of ferroelectric barium titanate and non-ferroelectric barium stannate. Both of them are of perovskite structures with an ABO3 formula. The character of the ferroelec-
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tric phase transition in BTS strongly depends on the Sn-level x. The observation of ferroelectric domains below the permittivity maximum temperature provides evidence of ferroelectric long-range order for x ≤ 0.13, whereas indication for the relaxor state of BTS is obtained for compositions x ≥ 2 [26]. The crystallographic phase structures evolve from tetragonal, orthorhombic, rhombohedral to cubic with the increasing tin composition at room temperature. Although this material is one of the earliest prototypes of diffused phase transition studies [27], it has riveted many research interests due to its abnormal dielectric properties and strong dielectric nonlinearity recently [28–30]. The permittivity is very high and as well temperature as bias field sensitive [31–34]. The composition x = 0.15 consists of a mixture of cubic and tetragonal phases, and the dielectric permittivity at room temperature attains a maximum value, over 5000 [35–37]. Close to room temperature the composition x = 0.15 shows a strong temperature variation of permittivity, induced by the proximity of their ferroelectric–paraelectric phase transitions [38]. Therefore, BTS shows interesting ferroelectric properties and is used for capacitors, relaxors and sensors. As far as we know, however, energy storage properties of BTS-based composite ceramics have not been studied. In this work, we reported energy storage properties of (100-x) wt.% BaTi0.85 Ti0.15 O3 –x wt.% MgO (BTS/MgO) (x = 0, 3, 5, 10) composite ceramics prepared by SPS. The effects of MgO content and SPS technology on structure and energy storage properties of BTS were investigated. As a result, our findings not only expand the spectrum of room-temperature lead-free energy storage materials for future applications but also may serve as a guide for revealing other energy storage material alternatives by SPS. 2. Experimental BaTi0.85 Sn0.15 O3 powders were prepared by the conventional solid-state reaction method with high-purity powders of BaCO3 (99.9%), TiO2 (99.9%) and SnO2 (99.9%). The stoichiometric mixtures were ball milled with zirconia balls in ethanol for 12 h. After drying, they were then calcined at 1150 ◦ C for 3 h to obtain a perovskite phase. Then, MgO were used as the raw materials. The
Fig. 1. XRD patterns of (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics: (a) BTS; (b) BTSM3; (c) BTSM5 and (d) BTSM10.
raw materials were weighed according to the following chemical composition: (100-x) wt% BaTi0.85 Sr0.15 O3 –x wt% MgO (BTS/MgO) (x = 0, 3, 5, 10; abbreviated as BTS, BTSM3, BTSM5, BTSM10, respectively), and then ball milled in ethyl alcohol using zirconia balls for 12 h. Furthermore, the powders were placed in a graphite die. BTS and BTS/MgO composites were sintered at 1175 ◦ C and 1250 ◦ C for 5 min under a vacuum of 6 Pa with an SPS apparatus (LABOX330, Sinter Land, Japan) at the heating rate of 100 ◦ C/min. During the sintering, an applied mechanical uniaxial pressure of 30 MPa was applied to the sample. After sintering, the BTS/MgO composite ceramics all reached 99% of the theoretical density. All the spark plasma sintered samples were thermally treated at 1100 ◦ C for 2 h in air to remove the carbon contamination. In order to make a contrast, BTS/MgO composite ceramics were also prepared by using the conventional solid-state-reaction process and the samples were sintered at 1350–1450 ◦ C for 2 h in air. The crystalline phases of sintered samples were characterized by X-ray powder diffraction (XRD; D/Max2550VB+/PC, Rigaku, Tokyo, Japan) using Cu-K␣1 radiation with linear position-sensitive detector. The microstructures and the elemental distribution were observed from the thermal treated surfaces (1100–1200 ◦ C,
Fig. 2. SEM micrographs on thermal-treated surfaces of (100-x) wt.% BaTi0.85 Sr0.15 O3 –x wt.% MgO ceramics detected by signal of SE2: (a) BTS; (b) BTSM3; (c) BTSM5 and (d) BTSM10.
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30 min) with field emission scanning electron microscopy (SEM; JEOL-6700F, Japan Electron Co., Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDX; FeatureMax, Oxford Instruments, Oxfordshire, UK) operating at 20 kV. The average grain size of the sample is determined by Nanomeasurer 1.2 software. In order to characterize electric properties, the sintered pellets were polished, coated with silver electrodes, and sintered at 600 ◦ C for 30 min to form a metal–insulator–metal (MIM) capacitor. The dielectric properties of the ceramics were evaluated with a precision LCR meter (E4294A, Agilent, Santa Clara, CA, USA) associated with a temperature controller (TP94, Linkam, Surrey, U.K.) in a broad range of temperature (−100 ◦ C–300 ◦ C) with a cooling rate of 3 ◦ C/min and frequency (100 Hz–1 MHz) at a signal level of 0.5 V/mm. A ferroelectric tester (TF2000, aixACCT, Aachen, Germany) was used for P-E loops measurement on a sample with a thickness of 0.5 mm. The dielectric tunability properties were measured at 25 ◦ C using an automatic component analyzer (TH2818, Tonghui, Changzhou, China) at 10 kHz. A blocking circuit was adopted to protect the analyzer from bias voltages. External bias field was applied in steps of 1 kV/cm.
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Fig. 3. SEM–EDX line scans of BTSM10. (a) SEM image displays the position of the line scan and (b) EDX plot shows the relative elemental content along the line scan.
3. Results Fig. 1 shows XRD patterns of BTS/MgO composite ceramics sintered at 1175–1250 ◦ C by SPS. It can be seen that the composites consist of only two phases of BTS and MgO. All the samples exhibit pure pseudo-cubic phase because of the obvious (200) peaks. As the content of MgO increases, few (200) peak shifts to the lower angle, which are probably due to the reason that the fast sinter-
ing rate, short sintering periods and low sintering temperature are carried out in the SPS process, which totally or at least partially suppresses the substitution of Ti4+ by Mg2+ and the reaction between BTS and MgO. Howerever, in BTS/MgO composites prepared by conventional sintering method, the (200) peak always shifts to the lower angle because of partial substitution of Ti4+ (r = 0.061 nm) by Mg2+ (r = 0.072 nm) that causes the lattice expansion and an addi-
Fig. 4. Temperature dependence of dielectric permittivity and dielectric loss at various frequencies for (100-x) wt% BaTi0.85 Sn0.15 O3 –x wt% MgO ceramics: (a) (b) BTS; (c) (d) BTSM3; (e) (f) BTSM5 and (g) (h) BTSM10.
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Fig. 5. The dependence of dielectric permittivity vs. electric field measured at 10 kHz and 25 ◦ C for (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics: (a) BTS; (b) BTSM3; (c) BTSM5 and (d) BTSM10.
tional phase of MgTiO3 was observed due to the reaction between Ba0.3 Sr0.6 Ca0.1 TiO3 and MgO [39]. SEM micrographs of the surface of the annealed pellets are illustrated in Fig. 2. Few cracks and pores can be observed in all the samples and well dense microstructure is obtained. The average grain sizes of BTS, BTSM3, BTSM5 and BTSM10 are 925, 409, 473 and 432 nm, respectively. This result indicates, as expected, that the short sintering period is an essential factor to obtain fine-grained BTS/MgO ceramics by the SPS process. More interestingly, each sample presents a layer-plate substructure, especially for BTSM10, as shown in Fig. 2 (d). The corresponding EDX elemental line map (Fig. 3) confirms that MgO is present in the matrix, and there is nearly no diffusion occurred between BTS and MgO. Fig. 4 shows temperature dependence of dielectric permittivity (ε) and dielectric loss (tanı) of BTS/MgO ceramics at various frequencies over −100 to 300 ◦ C. Compared with Curie temperature of pure BTS (23 ◦ C), Curie temperatures of the BTS/MgO composites shift to about −6 ◦ C at 10 kHz. At the room temperature, dielectric permittivity of BTS/MgO composites is about 15658, 2131, 1916 and 1586 at 10 kHz for BTS, BTSM3, BTSM5 and BTSM10, respectively, while dielectric loss is 0.02674, 0.01228, 0.01213 and 0.00987, respectively. It can be seen that the addition of MgO obviously leads to a significant decrease in dielectric permittivity and dielectric loss of BTS/MgO ceramics, which is due to that MgO has a lower permittivity and depresses the dielectric permittivity and dielectric loss of the composites. Besides, quite frequency stability of dielectric permittivity, low conductivity and low dielectric loss at lower frequencies are obtained in BTS/MgO composites, which is beneficial to tunable dielectric applications. The dependence of dielectric permittivity vs. electric field measured at 10 kHz and 25 ◦ C are shown in Fig. 5. As we can seen, the dielectric permittivity of all the samples decreases with the increase of applied electric field and has a rapid reduce at lower electric field. What’s more, we adopted a multi-polarization mech-
anism model and the following equation to interpret the field dependence of the dielectric permittivity [40]: ε (E) = εr (0) /{1 + [ε0 εr (0)3 E 2 ]}
1/3
+ (P0 x/ε0 )[cosh(Ex)]−2
(3)
where P0 , the effective polarization of one cluster; L, the cluster size; kB , the Boltzmann constant; T, the temperature and x = P0 L3 /kB T. The first term represents Johnson contribution which comes from the “intrinsic” lattice phonon polarization, and the second represents Langevin contribution which comes from some “extrinsic” polarization contributions, such as nanometer polar clusters, domain wall motions and phase boundary polarization [40]. The fitted experimental data with a statistical correlation of 0.999 are shown in Fig. 5. For all the samples, Langevin term gives more contribution to the nonlinear dielectric behavior at lower electric field, while Johnson term at higher electric field. Furthermore, it can be found that the contribution of Langevin term decreases after addition of MgO in the BTSn matrix, which might be ascribed to the reason that the existence of the second phase of has an effect on reduction of the grain size, therefore leading to the decreasing contribution of polar clusters in BST matrix on the nonlinear dielectric behavior. Fig. 6 illustrates tunability of the BTS/MgO ceramics at 10 kHz and room temperature as a function of electric field. Tunability of all the samples increases with the applied electric field while gradually decreases with the addition of MgO. The tunability of BTS, BTSM3, BTSM5 and BTSM10 composite is 78.1%, 52.9%, 50.2% and 45.8% at 25 ◦ C and 10 kHz under the electric field of 30 kV/cm. The value of tunability is much larger than other barium titanate (BaTiO3 )-based composites, such as Ba0.6 Sr0.4 TiO3 /MgO [41] BaZr0.25 Ti0.75 O3 /MgO [42] and Ba0.6 Sr0.4 TiO3 /BaZn6 Ti6 O19 [43] composites. Fig. 7 presents P–E hysteresis loops of BTS/MgO ceramics measured at ambient temperature with a triangular wave form at 20 Hz. Fig. 5(a) shows P–E hysteresis loops of BTS/MgO ceramics prepared by SPS. The electric filed is applied at the direction parallel to the uniaxial stress in SPS process. As shown in Fig. 7(a), the dielectric breakdown strength of BTS is about 105 kV/cm. At
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Fig. 6. The tunability at 10 kHz and room temperature as a function of the electric field for (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics.
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addition of MgO. This can be attributed to the huge dielectric breakdown strength of MgO [44]. In order to study the preparation process and uniaxial stress on the influence of the surface morphology, dielectric and ferroelectric of samples, other two samples are also studied. One is BTSM10 prepared by conventional sintering; the other is BTSM10 prepared by SPS, but during electrical measurement the electric field was applied on the direction perpendicular to the uniaxial stress in SPS process. From Fig. 7(b), we can see the BTSM10 prepared by conventional sintering is broken down at only 63.5 kV/cm. Temperature dependence of dielectric permittivity at various frequencies is illustrated in the inset at the top left of Fig. 7(b). Dielectric permittivity (1594 at 25 ◦ C and 10 kHz) of BTSM10 prepared by conventional sintering is close to that of SPS-sintered sample. However, much difference is shown in microstructure between the samples prepared by SPS and conventional sintering. BTSM10 prepared by conventional sintering has large grain size (761 nm) and presents some pores, as shown in the inset at the right corner of Fig. 7(b), which might lead to its lower breakdown strength. Furthermore, P–E hysteresis loops of the SPS-sintered sample are measured in the cross-section direction, as shown in Fig. 7(c). In this case, the dielectric breakdown strength of BTSM10 is 130 kV/cm. Correspondingly, its dielectric permittivity is 1749 at 25 ◦ C, 10 kHz, as illustrated in the inset at the top left of Fig. 7(c). Meanwhile, dense microstructure is maintained at the cross-section direction, as shown in the inset at the right corner of Fig. 7(c). It should be noted that the sample used to measure in Fig. 7(a) and (c) is the same. However, the sample prepared by SPS has different values of breakdown strength in these two directions, parallel and perpendicular to the uniaxial stress during SPS process. This result suggests that the breakdown strength is much related to the microstructure (e.g., grain size, shape, etc.) other than the density of the sample. The cross-section microstructure of BTSM10 prepared by SPS is shown in the inset at the right corner of Fig. 7(c). Its average grain size is 305 nm, smaller than the grain size at the surface. This means more grain boundaries are present in the cross-section direction. It is well known that there are many defects at the grain boundaries [45]; therefore, the breakdown strength of the BTSM10 prepared by SPS is lower at the cross-section direction. Energy delivered to the capacitor cannot be released completely due to the inconformity of the charging and discharging paths in hysteretic P-E loops of BTS/MgO. Therefore, energy storage density and energy efficiency are important factors to bench mark dielectrics for use in energy storage devices. For practical applications, these two parameters should both be taken into consideration. Generally speaking, the energy storage density (W1 ) and the energy efficiency () are given by the following Eqs. (1) and (2), respectively:
Pmax
W1 =
EdP
(1)
Pr
Fig. 7. P–E hysteresis loops of (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics measured at ambient temperature with a triangular wave form at 20 Hz: (a) BTS/MgO sintered by SPS. The electric filed is applied at the direction parallel to the uniaxial pressure in SPS process. (b) BTSM10 prepared by conventional sintering, (c) BTSM10 sintered by SPS. The electric filed is applied at the direction perpendicular to the uniaxial pressure in SPS process. Inset: Temperature dependence of dielectric permittivity at various frequencies and SEM micrographs on thermal treated surfaces detected by signal SE2.
20 Hz, its saturation polarization is about 14.4 C/cm2 . Compared to BTS, the saturation polarization in BTS/MgO composite ceramics decreases, which reaches 10.7 C/cm2 , 10.5 and 9.08 C/cm2 for BTSM3, BTSM5 and BTSM10, respectively. The dielectric breakdown strength of BTS/MgO ceramics significantly increases when
=
W1 × 100% W1 + W2
(2)
where E is the applied electric field, Pr is the remnant polarization, Pmax is the maximum polarization corresponding to the maximum electric field [2]. In general, the energy storage density (W1 ) was evaluated by P-E loops and it is equal to integral of the area enclosed by the discharge curve and the polarization axis. W2 represents the energy loss density and it is equal to integral of the area enclosed by the charge and the discharge curve. The energy densities and energy efficiency calculated from P-E hysteresis loops are depicted in Fig. 8. The highest energy storage density and energy efficiency of BTS ceramic are 0.3466 J/cm3 and 90.8%, respectively. With increasing MgO concentration, the energy storage density of the BTS/MgO ceramics increases and
Please cite this article in press as: P. Ren, et al., Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.016
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Fig. 8. The calculated energy storage density (W1 ) and energy efficiency () of the (100-x) wt.% BaTi0.85 Sn0.15 O3 –x wt.% MgO ceramics as a function of MgO fraction (x).
energy efficiency increases firstly then decreases. BTSM10 exhibits the highest energy storage density, 0.5107 J/cm3 , at the electric field of 190 kV/cm. Meanwhile, it maintains higher energy efficiency, 92.11%. Although the energy storage density is not as high as the data reported by Huang et al. [25] in Ba0.4 Sr0.6 TiO3 /MgO composite prepared by SPS, its energy efficiency is much higher. Thus, The BTS/MgO ceramics by SPS possess high energy storage density simultaneously with high energy efficiency. 4. Conclusions BTS/MgO composite ceramics were successfully prepared by SPS method. The results revealed that the fast sintering rate, short sintering periods and lower sintering temperature are carried out in the SPS process, which suppress the substitution of Ti4+ by Mg2+ and the reaction between BTS and MgO. With the increase of the content of MgO, the dielectric permittivity and dielectric loss of the BTS/MgO composite ceramics decrease significantly; meanwhile, the dielectric tunability still maintains a relatively high value. With the MgO content increasing, dielectric tunability of the BTS/MgO composite ceramics gradually decreases, which is ascribed to the reason that the second phase of MgO suppresses the grain growth, leading to the diminishment of the contribution of polar clusters in BTS matrix. In addition, due to excellent electrical breakdown strength of MgO and SPS sintering, the dielectric breakdown strength of BTS/MgO composite is greatly improved, which results in a significant improvement of energy storage density. The highest dielectric breakdown strength of 190 kV/cm, maximum energy storage density of 0.5107 J/cm3 and higher energy storage efficiency of 92.11% are obtained in 90 wt.% BaTi0.85 Sn0.15 O3 –10 wt.% MgO composite. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 51402234), Natural Science Basic Research Plan (No. 2015JQ5198, 2015JQ5142), International Cooperation Project (2013KW14-01) and Young Talent fund of University Association for Science and Technology (20150106) in Shaanxi Province, the doctoral starting fund (No. 101-211408) and New-Star of science and technology (101-256101511) of Xi’an University of Technology. References [1] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Eng. Mater. 22 (2010) 28–62. [2] Z.B. Shen, X.H. Wang, B.C. Luo, L.T. Li, BaTiO3 –BiYbO3 perovskite materials for energy storage applications, J. Mater. Chem. A 3 (2015) 18146–18153. [3] I. Burn, D.M. Smyth, Energy storage in ceramic dielectrics, J. Mater. Sci. 7 (1972) 339–343.
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Please cite this article in press as: P. Ren, et al., Energy storage density and tunable dielectric properties of BaTi0.85 Sn0.15 O3 /MgO composite ceramics prepared by SPS, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.016