IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
vol. 62, no. 4,
April
2015
609
The Effect of Grain Boundary on the Energy Storage Properties of (Ba0.4Sr0.6)TiO3 Paraelectric Ceramics by Varying Grain Sizes Zhe Song, Hanxing Liu, Hua Hao, Shujun Zhang, Minghe Cao, Zhonghua Yao, Zhijian Wang, Wei Hu, Yatong Shi, and Biyang Hu Abstract—(Ba0.4Sr0.6)TiO3 (BST) ceramics with various grain sizes (0.3–3.4 μm) were synthesized by the oxalate coprecipitation method and prepared by plasma activated sintering and conventional solid-state sintering process. The effect of grain boundary on the energy storage properties and the dielectric relaxation characteristics of BST paraelectric ceramics (Curie point ≈ −67°C) with various grain sizes were investigated. The dielectric breakdown strength (simplified as BDS) is obviously improved and then deteriorated with decreasing grain size, accounting for the energy density variation. The enhancement of interfacial polarization at grain boundary layers has a negative effect on the BDS, leading to the decreased values for samples with grain size smaller than 0.7 µm. In addition, the insulation effect of grain boundary barriers was discussed based on the complex impedance spectroscopy analysis, which was found to play a dominant role in controlling the BDS with coarser grain size. Among them, the sharply decreased BDS for BST with grain size of 1.8 µm was believed to be attributed to the combination of lower grain boundary density and higher interfacial polarization, due to the significant increase of oxygen vacancies at higher sintering temperature.
I. Introduction
E
lectrical energy storage dielectrics play an important role in mobile electronic devices, pulse power systems, hybrid electric vehicles, etc. [1], [2]. Among them, (Ba1−xSrx)TiO3 (x = 0–1) (BST) attracts considerable attention due to its high power density and good electrical reliability [3]. The Curie point (Tc) and phase structure, together with the macroscopic properties, of BST can be controlled by the mole fraction of Sr2+, which can be tailored for specific applications [4]. With the continuous demand for miniaturization of electronic components, size-dependent properties have
Manuscript received December 8, 2014; accepted January 18, 2015. This work was supported by the Key Program of Natural Science Foundation of China (No. 50932004), the International Science and Technology Cooperation Program of China (2011DFA52680), the Natural Science Foundation of China (No. 51102189, No. 51372191), the Program for New Century Excellent Talents in University (No. NCET-11-0685), and the Fundamental Research Funds for the Central Universities (2013YB-012). Z. Song, H. X. Liu, H. Hao, M. Cao, Z. Yao, Z. Wang, W. Hu, Y. Shi, and B. Hu are with the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China (e-mail: [email protected]). S. Zhang is with the Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA. DOI http://dx.doi.org/10.1109/TUFFC.2014.006927
been of great interest in many types of electronic ceramics including BST [5]–[7]. In our previous work, the grain size effect on the dielectric properties of (Ba0.4Sr0.6)TiO3 ceramics with paraelectric phase was discussed [8]. It was found that the grain boundary density changes with grain size playing an important role in energy storage performance. In addition to the grain boundary density, the dielectric breakdown strength (BDS) was also found to be strongly dependent on the interfacial polarization at grain boundary interfaces [9], [10], which has a negative effect on the dielectric breakdown strength, being more evident with smaller grain size [11]. Thus, in this study, (Ba0.4Sr0.6)TiO3 (BST) ceramics with uniform fine grain sizes (0.3–3.4 μm) were synthesized by oxalate co-precipitation method and prepared by plasma-activated sintering (PAS) and the conventional solid-state sintering process. In addition to the energy storage properties, the evolution of grain boundary density and the interfacial polarization effects on dielectric breakdown strength for BST with different grain sizes, and the relationship between the two effects were investigated to provide further information for the role of grain boundary as well as grain size effect on the energy storage properties of BST paraelectric ceramics. II. Experiment The (Ba0.4Sr0.6)TiO3 fine powders (the mean particle size is about 50 nm) were synthesized by oxalate co-precipitation method with high purity commercial powders of C4H6BaO4 (99%), C4H6O4Sr·0.5H2O (99%), C16H36O4Ti (98%), and C2H2O4·2H2O (99.5%). The oxalate was dissolved into ethanol as the precipitant reagents. A designed amount of tetrabutyl titanate was slowly added under stirring condition to form a transparent solution. The aqueous solution of barium acetate and strontium acetate were then added according to the nominal composition to form precipitates. After that, an appropriate amount of aqueous ammonia was dripped into the mixture to adjust the pH value to 3.0. After stirring at 55°C for 2 h, the precursor precipitates were filtered and dried. The precursors were calcined at 850°C for 2 h in air and then pressed to form disk-shaped samples at 150 MPa. To prepare dense ceramics with various grain sizes, the green pellets were sintered at 1300°C to 1380°C for 2 to 4 h by
0885–3010 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
610
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
Fig. 1. XRD patterns of BST ceramics under different sintering conditions.
the conventional solid-state sintering process. To prepare ceramics with finer grain size, PAS technology (PAS-III 15T-10P-50, Elenix Ltd., Kanagawa, Japan) was chosen as a supplemental process in this study. The samples were heated (200°C/min) to 950°C and 1000°C with a uniaxial load pressure of 30 MPa in vacuum condition. After holding for 3 min, the samples were cooled at a rate of 200°C/ min. Finally, the PAS samples were polished to 0.3 mm in thickness and then annealed in air at a temperature of 750°C and 800°C (depending on the sintering temperature) for 10 h to relieve the residual stresses and eliminate excess oxygen vacancies. The conventionally sintered ceramics were also polished to 0.3 mm and all samples with 0.3 mm in thickness and ~9.5 mm in diameter were then coated with fire-on silver electrodes with a diameter of 5 mm for electrical measurements.
vol. 62, no. 4,
April
2015
Density measurement was carried out using the Archimedes method. XRD analysis was performed on the sintered pellets using Cu Kα radiation (X’Pert PRO, PANalytical, Almelo, The Netherlands). Microstructure and grain size were observed on polished and etched surface of the sintered samples by a field-emission scanning electron microscope (FE-SEM; Quanta 450 FEG, FEI, Hillsboro, OR, USA). Average grain size of BST was statistically calculated by using Image Pro Plus software (Media Cybernetics Inc., Rockville, MD, USA), which was simulated by the average length of passing through the centroid of as many grains as possible (the amounts were reported below). The complex impedance characteristics were measured using a precision impedance analyzer (Agilent 4980 A, Agilent, Santa Clara, CA, USA). For the dielectric breakdown strength and P-E hysteresis loops measurements, samples were tested in silicon oil and at room temperature using a Radiant precision workstation (Radiant RT66A, Radiant Technologies Inc., Albuquerque, NM, USA) based on the Sawyer-Tower circuit at 10 Hz. 6 to 7 pellets were used, respectively, for each sample during the testing. The energy storage density γ was evaluated by integrating the area between the polarization axis and the discharge curve of the P-E hysteresis loops, which is given by the following equation: Pmax
γ =
∫
EdP, (1)
0
where P and E are the polarization and electric field, Pmax is the maximum polarization at the highest applied electric field [12]. The plotted P-E hysteresis loops in this work were selected from the samples with moderate BDS values among the tested pellets as representatives.
Fig. 2. SEM images of BST ceramics with different sintering conditions as well as grain sizes with their standard deviations: (a) 950°C for 3 min (PAS), 0.3 ± 0.04 μm, (b) 1000°C for 3 min (PAS), 0.4 ± 0.08 μm, (c) 1300°C for 2 h, 0.7 ± 0.19 μm, (d) 1320°C for 2 h, 0.9 ± 0.20 μm, (e) 1340°C for 2 h, 1.1 ± 0.27 μm, (f) 1360°C for 2 h, 1.2 ± 0.19 μm, (g) 1380°C for 2 h, 1.8 ± 0.50 μm, (h) 1380°C for 4 h, 3.4 ± 1.00 μm.
song et al.: effect of grain boundary on the energy storage properties of bst paraelectric ceramics
611
Fig. 3. Grain size distributions and the relative standard deviations statistically calculated from different numbers of grains on their SEM images, respectively: (a) 950°C for 3 min (PAS), more than 100 grains, (b) 1000°C for 3 min (PAS), more than 100 grains, (c) 1300°C for 2 h, more than 80 grains, (d) 1320°C for 2 h, more than 80 grains, (e) 1340°C for 2 h, more than 50 grains, (f) 1360°C for 2 h, more than 50 grains, (g) 1380°C for 2 h, more than 30 grains, (h) 1380°C for 4 h, more than 20 grains. RSD = relative standard deviation.
III. Results and Discussion A. Phase Characterization and Microstructure Observation Fig. 1 shows XRD patterns of the BST ceramics with different sintering conditions. The phase structure was found to maintain the same, where no obvious second phase was observed. For all samples, the split of (2 0 0) peaks corresponding to the tetragonal structure was not observed, revealing that all samples exhibit paraelectric phase with cubic perovskite structure. FE-SEM micrographs measured on polished and etched surface of BST ceramics are given in Fig. 2. As observed in the figures, all samples show uniform and homogeneous morphologies, without an intergranular secondary phase that may affect the macroscopic properties. The grain size distributions and the relative standard deviations for the samples are presented in Fig. 3, which were statistically calculated on SEM images by Image Pro Plus software for different numbers of grains. It was found that the average grain size changed from 0.3 to 3.4 μm with increasing
sintering temperatures (or holding time), showing a strong dependence on the sintering conditions, which can be regarded as an ideal micro scale range to illustrate the grain size-dependent properties effectively [13]. Table I gives the measured relative densities and average grain size for the as-sintered samples, marked as 1# to 8#, respectively. All the measured samples are similar in size (0.3 mm thick and ~9.5 mm diameter). The densities were found to be above 95% of the theoretical value, regardless of the sintering conditions. In this case, the variation of densities has minimal impact on BST properties with different grain sizes [14], thus the change of grain size is considered as the dominant factor affecting the macroscopic properties. B. Energy Storage Property Measurement The representative P-E hysteresis loops of (Ba0.4Sr0.6) TiO3 with various grain sizes achieved at different applied electric fields before their respective dielectric breakdown strength (maximum electric fields) are plotted in Fig. 4(a), where the corresponding maximum electric fields
TABLE I. Relative Densities and Average Grain Size of BST Ceramics. Samples 1# 2# 3# 4# 5# 6# 7# 8#
Sintering process 950°C, 1000°C, 1300°C, 1320°C, 1340°C, 1360°C, 1380°C, 1380°C,
3 3 2 2 2 2 2 4
min (PAS) min (PAS) h h h h h h
Average grain size (µm)
Relative density (%)
0.3 0.4 0.7 0.9 1.1 1.2 1.8 3.4
96.2 96.8 95.9 96.6 97.3 96.4 95.9 95.2
612
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
vol. 62, no. 4,
April
2015
TABLE II. Energy Storage Properties of BST Ceramics With Various Grain Sizes.
Samples 1# 2# 3# 4# 5# 6# 7# 8#
Average grain size (µm)
BDS (kV/cm)
Maximum polarization, Pmax (µC/cm2)
Energy density, γ (J/cm3)
0.3 0.4 0.7 0.9 1.1 1.2 1.8 3.4
154 191 197 181 168 158 112 108
11.0 13.4 16.9 15.6 15.0 14.1 11.6 11.4
0.63 0.94 1.30 1.10 0.91 0.90 0.54 0.49
can be more clearly observed in Fig. 4(b). The hysteresis characteristic of the samples with various grain sizes is also dependent on the applied electric field, in addition to the material properties. The nonlinear hysteresis features, analogous to ferroelectrics, were found to appear because of the polarization of space charges and the growth of polar nano-regions (PNRs) existing in paraelectric BST [8] under a high electric field. For samples with 1.8 and
3.4 μm grain sizes, on the contrary, linear hysteresis loops were obtained due to the relatively lower applied field. The parameters of energy storage performance can be evaluated from the hysteresis loops, as listed in Table II. With grain size decreasing, the dielectric breakdown strength was obviously improved and then deteriorated, accompanied by a similar trend as the Pmax, depending on the intrinsic polarization of materials as well as the applied electric field. A better dielectric breakdown strength and higher Pmax will favor a greater energy density. Sample with the grain size of 0.7 µm exhibited the maximum energy density, being on the order of 1.30 J/cm3 due to its highest dielectric breakdown strength (197 kV/cm) and relatively higher Pmax (16.9 µC/cm2). The significant variation of BDS for samples with smaller grain size was considered to be mainly associated with the grain boundaries. Dielectric ceramics can be regarded as a composite including grain cores with high permittivity and insulated grain boundary layers with low permittivity [15], [16]. The grain boundary, as a barrier layer to the charge migration at the boundary interface, plays an important role for the dielectric breakdown strength of BST ceramics [10]. C. The Effect of Grain Boundary on the Energy Storage Properties
Fig. 4. (a) The hysteresis loops of BST ceramics with various grain sizes. (b) The corresponding Emax values in the first quadrant.
Impedance analysis was introduced to study the effect of grain boundaries on the energy storage properties. The complex impedance spectra was measured at 350°C to 400°C and in the frequency range of 20 Hz to 2 MHz. The Cole-Cole plots are presented in Fig. 5. Two semicircular plots with different radii were observed in the complex impedance planes, in which the smaller-sized semicircle corresponds to the response from grains, whereas the larger-scaled one represents the contribution from grain boundaries. Fig. 5 shows that for all the samples, both the two impedance semicircles become smaller with increasing measuring temperatures, where the observed dielectric relaxation behaviors were considered to be correlated with the thermally activated defect motions in grains and grain boundary regions [17]. In the complex impedance plane, the low frequency intercept of the impedance data on the real axis, Z′, corresponds to the bulk resistance [18], showing a decreasing trend with increasing temperatures.
song et al.: effect of grain boundary on the energy storage properties of bst paraelectric ceramics
613
Fig. 5. The complex impedance images for different samples at the measuring temperatures of 350 to 400°C and the equivalent circuit proposed for fitting. Inset: the enlarged scale of high frequency area for each sample.
The reciprocal value of measurement frequency at the extreme point of each semicircle represents the relaxation time τ arising from different activated defects at a certain temperature. According to the Arrhenius relationship, the activation energy can be calculated from the slope using the following equation:
τ = τ 0 exp(−E a k BT), (2)
where τ is the relaxation time, τ0 is the pre-exponential term, Ea is the relaxation activation energy, kB is the Boltzmann constant, and T is the absolute temperature. The relaxation time of grains and grain boundaries can also be defined as
τ g = R g ⋅ C g, (3)
τ gb = R gb ⋅ C gb, (4)
where Rg, Rgb, Cg, and Cgb are the resistances and capacitances of grains and grain boundaries, respectively. The two semicircular arcs in the impedance spectrum can
be simulated by an equivalent circuit consisting of two parallel RC elements connected in series, related to the individual contribution from the grain and grain boundary [19], [20], as shown in the inset of Fig. 5. Based on the equivalent circuit, the resistances and capacitances of different electrical regions can be estimated by using the ZView software (Scribner Associates Inc., Charlottesville, VA, USA). Fig. 6 plots the lnτ as a function of measuring temperatures for BST ceramics. The activation energies corresponding to the grains (Ea,g) and grain boundaries (Ea,gb) can be obtained from the slopes of the solid lines being linearly fitted by the measured data. In this case, Ea,gb corresponds to the relaxation of space charges, characteristic of the energy barrier that impedes the spreading of charges at grain boundaries, whereas Ea,g represents the ionization behavior and motion of oxygen vacancies in grains. The grain size-dependent activation energies of grain boundaries and grains are illustrated in Fig. 7. Ea,gb was found to gradually decrease from 1.29 to 0.52 eV, whereas Ea,g increases from 0.58 to 1.05 eV, with grain size increasing, which are in good agreement with those reported for
614
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
vol. 62, no. 4,
April
2015
Fig. 7. The grain size-dependent activation energies of grain boundaries and grains for BST ceramics with various grain sizes.
Fig. 6. Fitting curves of relaxation time of grains (a) and grain boundaries (b) for BST ceramics as a function of the measuring temperatures of 350 to 400°C.
BaTiO3-based ceramics [21], [22]. Generally, the enhanced activation energies indicate an increase in the resistance for each component [23]. Thus for the samples with grain sizes of 0.3 and 3.4 µm, the maximum Ea,gb and Ea,g indicate the dominated insulation contribution from grain boundaries and grains, respectively. It is known that the activation energies are grain sizedependent, resulting from the varying grain/grain boundary density [10], [24]. Larger amounts of charges in grains or grain boundaries will lead to the obvious increase of the corresponding activation energy. With larger grain size, the variation of activation energies was considered to be induced by the higher grain density and lower grain boundary density. As an exception, for sample with the grain size of 1.8 µm, because of the significant increase of oxygen vacancies at higher sintering temperature (1380°C) [25], [26], higher values of both Ea,g and Ea,gb were observed, revealing the inferior charge migration behavior in grains and grain boundaries. Compared with sample 7#, activation energies of sample 8#, with longer time (4 h) holding at 1380°C, are in good agreement with other samples, indicating that the longer holding time leads to the continuous growth of grains with minimal change of the oxygen vacancies. The energy barrier, Ea,gb, was utilized for the characterization of interfacial polarization at grain boundary interfaces [9]. The values of activation energy corresponding to the interfacial polarization are in good agreement with those reported elsewhere [24]. Generally, the enhancement of interfacial polarization at grain boundary layers has a negative effect on the dielectric breakdown strength for BST with smaller grain size [27]. As a result of the electri-
cal difference between grains and grain boundaries, the spreading of local charge carriers is impeded, being accumulated at the grain boundary interfaces, leading to the occurrence of interfacial polarization, accounting for the lower dielectric breakdown strength. An obvious increase of Ea,gb, corresponding to the intensity of interfacial polarization, was observed for samples with grain size of
vol. 62, no. 4,
April
2015
609
The Effect of Grain Boundary on the Energy Storage Properties of (Ba0.4Sr0.6)TiO3 Paraelectric Ceramics by Varying Grain Sizes Zhe Song, Hanxing Liu, Hua Hao, Shujun Zhang, Minghe Cao, Zhonghua Yao, Zhijian Wang, Wei Hu, Yatong Shi, and Biyang Hu Abstract—(Ba0.4Sr0.6)TiO3 (BST) ceramics with various grain sizes (0.3–3.4 μm) were synthesized by the oxalate coprecipitation method and prepared by plasma activated sintering and conventional solid-state sintering process. The effect of grain boundary on the energy storage properties and the dielectric relaxation characteristics of BST paraelectric ceramics (Curie point ≈ −67°C) with various grain sizes were investigated. The dielectric breakdown strength (simplified as BDS) is obviously improved and then deteriorated with decreasing grain size, accounting for the energy density variation. The enhancement of interfacial polarization at grain boundary layers has a negative effect on the BDS, leading to the decreased values for samples with grain size smaller than 0.7 µm. In addition, the insulation effect of grain boundary barriers was discussed based on the complex impedance spectroscopy analysis, which was found to play a dominant role in controlling the BDS with coarser grain size. Among them, the sharply decreased BDS for BST with grain size of 1.8 µm was believed to be attributed to the combination of lower grain boundary density and higher interfacial polarization, due to the significant increase of oxygen vacancies at higher sintering temperature.
I. Introduction
E
lectrical energy storage dielectrics play an important role in mobile electronic devices, pulse power systems, hybrid electric vehicles, etc. [1], [2]. Among them, (Ba1−xSrx)TiO3 (x = 0–1) (BST) attracts considerable attention due to its high power density and good electrical reliability [3]. The Curie point (Tc) and phase structure, together with the macroscopic properties, of BST can be controlled by the mole fraction of Sr2+, which can be tailored for specific applications [4]. With the continuous demand for miniaturization of electronic components, size-dependent properties have
Manuscript received December 8, 2014; accepted January 18, 2015. This work was supported by the Key Program of Natural Science Foundation of China (No. 50932004), the International Science and Technology Cooperation Program of China (2011DFA52680), the Natural Science Foundation of China (No. 51102189, No. 51372191), the Program for New Century Excellent Talents in University (No. NCET-11-0685), and the Fundamental Research Funds for the Central Universities (2013YB-012). Z. Song, H. X. Liu, H. Hao, M. Cao, Z. Yao, Z. Wang, W. Hu, Y. Shi, and B. Hu are with the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China (e-mail: [email protected]). S. Zhang is with the Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA. DOI http://dx.doi.org/10.1109/TUFFC.2014.006927
been of great interest in many types of electronic ceramics including BST [5]–[7]. In our previous work, the grain size effect on the dielectric properties of (Ba0.4Sr0.6)TiO3 ceramics with paraelectric phase was discussed [8]. It was found that the grain boundary density changes with grain size playing an important role in energy storage performance. In addition to the grain boundary density, the dielectric breakdown strength (BDS) was also found to be strongly dependent on the interfacial polarization at grain boundary interfaces [9], [10], which has a negative effect on the dielectric breakdown strength, being more evident with smaller grain size [11]. Thus, in this study, (Ba0.4Sr0.6)TiO3 (BST) ceramics with uniform fine grain sizes (0.3–3.4 μm) were synthesized by oxalate co-precipitation method and prepared by plasma-activated sintering (PAS) and the conventional solid-state sintering process. In addition to the energy storage properties, the evolution of grain boundary density and the interfacial polarization effects on dielectric breakdown strength for BST with different grain sizes, and the relationship between the two effects were investigated to provide further information for the role of grain boundary as well as grain size effect on the energy storage properties of BST paraelectric ceramics. II. Experiment The (Ba0.4Sr0.6)TiO3 fine powders (the mean particle size is about 50 nm) were synthesized by oxalate co-precipitation method with high purity commercial powders of C4H6BaO4 (99%), C4H6O4Sr·0.5H2O (99%), C16H36O4Ti (98%), and C2H2O4·2H2O (99.5%). The oxalate was dissolved into ethanol as the precipitant reagents. A designed amount of tetrabutyl titanate was slowly added under stirring condition to form a transparent solution. The aqueous solution of barium acetate and strontium acetate were then added according to the nominal composition to form precipitates. After that, an appropriate amount of aqueous ammonia was dripped into the mixture to adjust the pH value to 3.0. After stirring at 55°C for 2 h, the precursor precipitates were filtered and dried. The precursors were calcined at 850°C for 2 h in air and then pressed to form disk-shaped samples at 150 MPa. To prepare dense ceramics with various grain sizes, the green pellets were sintered at 1300°C to 1380°C for 2 to 4 h by
0885–3010 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
610
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
Fig. 1. XRD patterns of BST ceramics under different sintering conditions.
the conventional solid-state sintering process. To prepare ceramics with finer grain size, PAS technology (PAS-III 15T-10P-50, Elenix Ltd., Kanagawa, Japan) was chosen as a supplemental process in this study. The samples were heated (200°C/min) to 950°C and 1000°C with a uniaxial load pressure of 30 MPa in vacuum condition. After holding for 3 min, the samples were cooled at a rate of 200°C/ min. Finally, the PAS samples were polished to 0.3 mm in thickness and then annealed in air at a temperature of 750°C and 800°C (depending on the sintering temperature) for 10 h to relieve the residual stresses and eliminate excess oxygen vacancies. The conventionally sintered ceramics were also polished to 0.3 mm and all samples with 0.3 mm in thickness and ~9.5 mm in diameter were then coated with fire-on silver electrodes with a diameter of 5 mm for electrical measurements.
vol. 62, no. 4,
April
2015
Density measurement was carried out using the Archimedes method. XRD analysis was performed on the sintered pellets using Cu Kα radiation (X’Pert PRO, PANalytical, Almelo, The Netherlands). Microstructure and grain size were observed on polished and etched surface of the sintered samples by a field-emission scanning electron microscope (FE-SEM; Quanta 450 FEG, FEI, Hillsboro, OR, USA). Average grain size of BST was statistically calculated by using Image Pro Plus software (Media Cybernetics Inc., Rockville, MD, USA), which was simulated by the average length of passing through the centroid of as many grains as possible (the amounts were reported below). The complex impedance characteristics were measured using a precision impedance analyzer (Agilent 4980 A, Agilent, Santa Clara, CA, USA). For the dielectric breakdown strength and P-E hysteresis loops measurements, samples were tested in silicon oil and at room temperature using a Radiant precision workstation (Radiant RT66A, Radiant Technologies Inc., Albuquerque, NM, USA) based on the Sawyer-Tower circuit at 10 Hz. 6 to 7 pellets were used, respectively, for each sample during the testing. The energy storage density γ was evaluated by integrating the area between the polarization axis and the discharge curve of the P-E hysteresis loops, which is given by the following equation: Pmax
γ =
∫
EdP, (1)
0
where P and E are the polarization and electric field, Pmax is the maximum polarization at the highest applied electric field [12]. The plotted P-E hysteresis loops in this work were selected from the samples with moderate BDS values among the tested pellets as representatives.
Fig. 2. SEM images of BST ceramics with different sintering conditions as well as grain sizes with their standard deviations: (a) 950°C for 3 min (PAS), 0.3 ± 0.04 μm, (b) 1000°C for 3 min (PAS), 0.4 ± 0.08 μm, (c) 1300°C for 2 h, 0.7 ± 0.19 μm, (d) 1320°C for 2 h, 0.9 ± 0.20 μm, (e) 1340°C for 2 h, 1.1 ± 0.27 μm, (f) 1360°C for 2 h, 1.2 ± 0.19 μm, (g) 1380°C for 2 h, 1.8 ± 0.50 μm, (h) 1380°C for 4 h, 3.4 ± 1.00 μm.
song et al.: effect of grain boundary on the energy storage properties of bst paraelectric ceramics
611
Fig. 3. Grain size distributions and the relative standard deviations statistically calculated from different numbers of grains on their SEM images, respectively: (a) 950°C for 3 min (PAS), more than 100 grains, (b) 1000°C for 3 min (PAS), more than 100 grains, (c) 1300°C for 2 h, more than 80 grains, (d) 1320°C for 2 h, more than 80 grains, (e) 1340°C for 2 h, more than 50 grains, (f) 1360°C for 2 h, more than 50 grains, (g) 1380°C for 2 h, more than 30 grains, (h) 1380°C for 4 h, more than 20 grains. RSD = relative standard deviation.
III. Results and Discussion A. Phase Characterization and Microstructure Observation Fig. 1 shows XRD patterns of the BST ceramics with different sintering conditions. The phase structure was found to maintain the same, where no obvious second phase was observed. For all samples, the split of (2 0 0) peaks corresponding to the tetragonal structure was not observed, revealing that all samples exhibit paraelectric phase with cubic perovskite structure. FE-SEM micrographs measured on polished and etched surface of BST ceramics are given in Fig. 2. As observed in the figures, all samples show uniform and homogeneous morphologies, without an intergranular secondary phase that may affect the macroscopic properties. The grain size distributions and the relative standard deviations for the samples are presented in Fig. 3, which were statistically calculated on SEM images by Image Pro Plus software for different numbers of grains. It was found that the average grain size changed from 0.3 to 3.4 μm with increasing
sintering temperatures (or holding time), showing a strong dependence on the sintering conditions, which can be regarded as an ideal micro scale range to illustrate the grain size-dependent properties effectively [13]. Table I gives the measured relative densities and average grain size for the as-sintered samples, marked as 1# to 8#, respectively. All the measured samples are similar in size (0.3 mm thick and ~9.5 mm diameter). The densities were found to be above 95% of the theoretical value, regardless of the sintering conditions. In this case, the variation of densities has minimal impact on BST properties with different grain sizes [14], thus the change of grain size is considered as the dominant factor affecting the macroscopic properties. B. Energy Storage Property Measurement The representative P-E hysteresis loops of (Ba0.4Sr0.6) TiO3 with various grain sizes achieved at different applied electric fields before their respective dielectric breakdown strength (maximum electric fields) are plotted in Fig. 4(a), where the corresponding maximum electric fields
TABLE I. Relative Densities and Average Grain Size of BST Ceramics. Samples 1# 2# 3# 4# 5# 6# 7# 8#
Sintering process 950°C, 1000°C, 1300°C, 1320°C, 1340°C, 1360°C, 1380°C, 1380°C,
3 3 2 2 2 2 2 4
min (PAS) min (PAS) h h h h h h
Average grain size (µm)
Relative density (%)
0.3 0.4 0.7 0.9 1.1 1.2 1.8 3.4
96.2 96.8 95.9 96.6 97.3 96.4 95.9 95.2
612
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
vol. 62, no. 4,
April
2015
TABLE II. Energy Storage Properties of BST Ceramics With Various Grain Sizes.
Samples 1# 2# 3# 4# 5# 6# 7# 8#
Average grain size (µm)
BDS (kV/cm)
Maximum polarization, Pmax (µC/cm2)
Energy density, γ (J/cm3)
0.3 0.4 0.7 0.9 1.1 1.2 1.8 3.4
154 191 197 181 168 158 112 108
11.0 13.4 16.9 15.6 15.0 14.1 11.6 11.4
0.63 0.94 1.30 1.10 0.91 0.90 0.54 0.49
can be more clearly observed in Fig. 4(b). The hysteresis characteristic of the samples with various grain sizes is also dependent on the applied electric field, in addition to the material properties. The nonlinear hysteresis features, analogous to ferroelectrics, were found to appear because of the polarization of space charges and the growth of polar nano-regions (PNRs) existing in paraelectric BST [8] under a high electric field. For samples with 1.8 and
3.4 μm grain sizes, on the contrary, linear hysteresis loops were obtained due to the relatively lower applied field. The parameters of energy storage performance can be evaluated from the hysteresis loops, as listed in Table II. With grain size decreasing, the dielectric breakdown strength was obviously improved and then deteriorated, accompanied by a similar trend as the Pmax, depending on the intrinsic polarization of materials as well as the applied electric field. A better dielectric breakdown strength and higher Pmax will favor a greater energy density. Sample with the grain size of 0.7 µm exhibited the maximum energy density, being on the order of 1.30 J/cm3 due to its highest dielectric breakdown strength (197 kV/cm) and relatively higher Pmax (16.9 µC/cm2). The significant variation of BDS for samples with smaller grain size was considered to be mainly associated with the grain boundaries. Dielectric ceramics can be regarded as a composite including grain cores with high permittivity and insulated grain boundary layers with low permittivity [15], [16]. The grain boundary, as a barrier layer to the charge migration at the boundary interface, plays an important role for the dielectric breakdown strength of BST ceramics [10]. C. The Effect of Grain Boundary on the Energy Storage Properties
Fig. 4. (a) The hysteresis loops of BST ceramics with various grain sizes. (b) The corresponding Emax values in the first quadrant.
Impedance analysis was introduced to study the effect of grain boundaries on the energy storage properties. The complex impedance spectra was measured at 350°C to 400°C and in the frequency range of 20 Hz to 2 MHz. The Cole-Cole plots are presented in Fig. 5. Two semicircular plots with different radii were observed in the complex impedance planes, in which the smaller-sized semicircle corresponds to the response from grains, whereas the larger-scaled one represents the contribution from grain boundaries. Fig. 5 shows that for all the samples, both the two impedance semicircles become smaller with increasing measuring temperatures, where the observed dielectric relaxation behaviors were considered to be correlated with the thermally activated defect motions in grains and grain boundary regions [17]. In the complex impedance plane, the low frequency intercept of the impedance data on the real axis, Z′, corresponds to the bulk resistance [18], showing a decreasing trend with increasing temperatures.
song et al.: effect of grain boundary on the energy storage properties of bst paraelectric ceramics
613
Fig. 5. The complex impedance images for different samples at the measuring temperatures of 350 to 400°C and the equivalent circuit proposed for fitting. Inset: the enlarged scale of high frequency area for each sample.
The reciprocal value of measurement frequency at the extreme point of each semicircle represents the relaxation time τ arising from different activated defects at a certain temperature. According to the Arrhenius relationship, the activation energy can be calculated from the slope using the following equation:
τ = τ 0 exp(−E a k BT), (2)
where τ is the relaxation time, τ0 is the pre-exponential term, Ea is the relaxation activation energy, kB is the Boltzmann constant, and T is the absolute temperature. The relaxation time of grains and grain boundaries can also be defined as
τ g = R g ⋅ C g, (3)
τ gb = R gb ⋅ C gb, (4)
where Rg, Rgb, Cg, and Cgb are the resistances and capacitances of grains and grain boundaries, respectively. The two semicircular arcs in the impedance spectrum can
be simulated by an equivalent circuit consisting of two parallel RC elements connected in series, related to the individual contribution from the grain and grain boundary [19], [20], as shown in the inset of Fig. 5. Based on the equivalent circuit, the resistances and capacitances of different electrical regions can be estimated by using the ZView software (Scribner Associates Inc., Charlottesville, VA, USA). Fig. 6 plots the lnτ as a function of measuring temperatures for BST ceramics. The activation energies corresponding to the grains (Ea,g) and grain boundaries (Ea,gb) can be obtained from the slopes of the solid lines being linearly fitted by the measured data. In this case, Ea,gb corresponds to the relaxation of space charges, characteristic of the energy barrier that impedes the spreading of charges at grain boundaries, whereas Ea,g represents the ionization behavior and motion of oxygen vacancies in grains. The grain size-dependent activation energies of grain boundaries and grains are illustrated in Fig. 7. Ea,gb was found to gradually decrease from 1.29 to 0.52 eV, whereas Ea,g increases from 0.58 to 1.05 eV, with grain size increasing, which are in good agreement with those reported for
614
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,
vol. 62, no. 4,
April
2015
Fig. 7. The grain size-dependent activation energies of grain boundaries and grains for BST ceramics with various grain sizes.
Fig. 6. Fitting curves of relaxation time of grains (a) and grain boundaries (b) for BST ceramics as a function of the measuring temperatures of 350 to 400°C.
BaTiO3-based ceramics [21], [22]. Generally, the enhanced activation energies indicate an increase in the resistance for each component [23]. Thus for the samples with grain sizes of 0.3 and 3.4 µm, the maximum Ea,gb and Ea,g indicate the dominated insulation contribution from grain boundaries and grains, respectively. It is known that the activation energies are grain sizedependent, resulting from the varying grain/grain boundary density [10], [24]. Larger amounts of charges in grains or grain boundaries will lead to the obvious increase of the corresponding activation energy. With larger grain size, the variation of activation energies was considered to be induced by the higher grain density and lower grain boundary density. As an exception, for sample with the grain size of 1.8 µm, because of the significant increase of oxygen vacancies at higher sintering temperature (1380°C) [25], [26], higher values of both Ea,g and Ea,gb were observed, revealing the inferior charge migration behavior in grains and grain boundaries. Compared with sample 7#, activation energies of sample 8#, with longer time (4 h) holding at 1380°C, are in good agreement with other samples, indicating that the longer holding time leads to the continuous growth of grains with minimal change of the oxygen vacancies. The energy barrier, Ea,gb, was utilized for the characterization of interfacial polarization at grain boundary interfaces [9]. The values of activation energy corresponding to the interfacial polarization are in good agreement with those reported elsewhere [24]. Generally, the enhancement of interfacial polarization at grain boundary layers has a negative effect on the dielectric breakdown strength for BST with smaller grain size [27]. As a result of the electri-
cal difference between grains and grain boundaries, the spreading of local charge carriers is impeded, being accumulated at the grain boundary interfaces, leading to the occurrence of interfacial polarization, accounting for the lower dielectric breakdown strength. An obvious increase of Ea,gb, corresponding to the intensity of interfacial polarization, was observed for samples with grain size of
