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Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization
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JECS-10739; No. of Pages 6
Journal of the European Ceramic Society xxx (2016) xxx–xxx
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Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization Shi Xiao, Shaomei Xiu, Bo Shen, Jiwei Zhai ∗ Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China
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Article history: Received 15 April 2016 Received in revised form 22 June 2016 Accepted 23 June 2016 Available online xxx Keywords: Microwave sintering Glass-ceramics Microstructure Charge-discharge
a b s t r a c t The potassium strontium niobate boroaluminosilicate (KSN-BAS) glass-ceramics were prepared through microwave sintering. The effects of crystallization time on the microstructure and dielectric properties of the KSN-BAS glass-ceramics were investigated. The XRD results exhibited that an impure phase AlNbO4 was devitrified from the glass matrix as the crystallization time increased. The microstructure showed that the grain size increased as the crystallization time prolonged. Crystal boundary activation energy of the KSN-BAS glass-ceramics was performed by impedance analysis. The dielectric constant and electric breakdown strength indicated a trend of increasing at first and then reducing with the increase of the crystallization time. When the crystallization time was 10 min, the optimization of dielectric constant of 102.0 and breakdown strength of 1410.81 kV/cm were obtained. The maximum of theoretical energy storage density can reach a value of 8.99 J/cm3 . For pulsed power applications, the discharge efficiency and power density were evaluated by RLC circuit. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the development of the electronic equipment and pulse technology, there is a very urgent need to search new energy storage materials in possession of high energy density, high energy efficiency and power density [1]. At present, the research of energy storage materials applied in capacitors are mainly focused on dielectric materials, such as ferroelectric ceramics, antiferroelectric ceramics and polymer dielectrics owing to their inherent advantages like high dielectric constant, power density, long service life and short charging time etc [2,3]. However, their own shortcomings also restrict the improvement of energy storage density. It is still a challenging task to enhance the energy density in dielectric energy storage materials [4]. In order to improve the energy storage ability, many meaningful researches have been performed to develop new energy storage materials. The ferroelectric glass-ceramics materials are considered as one of the most potential candidates owing to their combination of high permittivity crystallites and high breakdown strength glass matrix. The previous researches have also confirmed that
∗ Corresponding author. E-mail address: [email protected] (J. Zhai).
the energy storage density of the ferroelectric glass-ceramics was indeed higher than that of the other materials, such as ferroelectric ceramic and polymer dielectrics [1–4]. In addition, glass-ceramics can provide a dense microstructure (almost zero porosity), and the phase structure and grain size can be adjusted by the controlled devitrification processing [5]. However, in the conventional heat treatment of glass-ceramics, the thermal energy is generally transferred from the surface of material to inside due to thermal gradients through convection, conduction, and radiation, which might appear that the nucleation and growth of crystal were preferentially formed in the surface of the devitrified sample [6,7]. Now, a variety of processing techniques have been utilized to modify the microstructure of ferroelectric glass-ceramics, such as liquid phase sintering, hot pressed sintering, and sol-gel method [8,9]. The microwave heat treatment has been widely used in the preparation of material because of its unique heating mode [6]. In contrast to the conventional furnaces, the microwave heat treatment depends on the transfer of electromagnetic energy. Moreover, the microwaves can penetrate materials and heat can be generated throughout the volume of the material. It is possible to achieve rapid and uniform heating in the preparation of materials [10]. Based on the aforementioned advantages, the performance of the materials prepared by microwave sintering could be superior to that of the materials via traditional heat treatment. Wang et al. found that
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Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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microwave heat treatment could effectively eliminate dendritic crystal structure in the BST-based glass-ceramics, resulting in the increase of the breakdown strength [11]. In addition, microwave sintering can enhance mechanical properties of the materials which could be advantageous to follow-up processing [12]. In spite of many advantages reflected in the microwave sintering, the absorption of electromagnetic field has great difference in the varying materials. So it is important to choose an appropriate condition to prepare the glass-ceramic materials by microwave heat treatment. Xiao et al. reported that the energy storage density could be increased by optimizing the crystallization temperature [13]. Zhang et al. found that the value of conductivity exhibited a rising trend as the crystallization time increased, which demonstrated a better temperature-sensitive behavior for the crystal phase [14]. But the reports about the effect crystallization time on the microstructure and dielectric properties of the glass-ceramic processed by microwave heat treatment are very rarely. For energy-storage dielectric material, besides a high energy density, the research of charge-discharge efficiency and the speed of charge-discharge are also required for the applications of high energy storage capacitors [15]. Xu et al. found that half of the stored charge can be released under 10 ns in the PLZT antiferroelectric ceramic by the RLC measuring circuit [16]. In this study, the K2 O-SrO-Nb2 O5 -B2 O3 -Al2 O3 -SiO2 (KSN-BAS) glass was crystallized by microwave route to obtain the glass-ceramics. Effects of the crystallization time on the microstructure, dielectric properties and charge-discharge properties of the glass-ceramics were investigated, and the optimized crystallization process to improve the energy storage density of the KSN-based glass-ceramic was presented.
2. Experimental procedure The KSA-BAS glass was fabricated from well-mixed powders of 15K2 CO3 , 15SrCO3 , 30Nb2 O5 , 32SiO2 , 4Al2 O3 , and 4B2 O3 (mol.%). The as-dried powders were melted at 1550 ◦ C for 2 h in an alumina crucible. Then the glass lava was quickly poured into the preheated copper mold at 600 ◦ C for 6 h to remove residual stress. The as-annealed glass was cut into piece-shaped samples with about 1.5 mm thickness and then the glass samples were crystalized by microwave heat treatment (HAMiLab-C1500, SYNOTHERM, 2.45 GHz) at 900 ◦ C for 5 min, 10 min 15 min 30 min and 2 h, respectively. In addition, some glasses named C-2 h were heated to 900 ◦ C for 2 h in a conventional box-type furnace as a comparison sample. X-ray diffraction (XRD) (D/max2550V, Rigaku, Japan) with Cu K␣ radiation at room temperature was used to determine the crystallographic structure of the glass-ceramics heated via different crystallization time. The microstructure was observed by a field scanning electron microcopy (SEM) (HITACHI S-4700, Japan). The dielectric constant and loss tangent of the glass-ceramic specimens were measured by using a LCR meter (Agilent E4980A, USA) under a frequency of 10 kHz and in the temperature range of −50 ◦ C to 120 ◦ C. The measurement of DC breakdown strength of all the glass-ceramic samples was performed by using a voltagewithstand testing source (ET2671B, Entai, Nanjing, China) at room temperature. A DC voltage ramp of about 0.2 kV/s was applied to the samples until the dielectric breakdown occurred. In the measurement process of breakdown strength, at least 15 testing samples were ground into about 0.07–0.1 mm thickness and immersed in silicon oil to prevent arcing. Complex impedance spectra were measured via the LCR meter (Agilent E4284, USA) in the frequencies range from 150 Hz to 2 MHz and a wide temperature range of 340 ◦ C–420 ◦ C with an AC electric field of 4 V/mm. The chargedischarge properties of the KSN-BAS glass-ceramic capacitors were detected with a specially designed, high-speed capacitor discharge
resistance, inductance, and capacitance (RLC) load circuit designed by ourselves. 3. Results and discussion Fig. 1 shows XRD patterns of the KSN-BAS glass-ceramics prepared by the microwave heating route under different crystallization time. It is noticed that the relative intensity of the crystal diffraction peaks is gradually increased with the increase of crystallization time, which implies that the content of crystal phases is increased. Compared with PDF cards, the crystalline phase KSr2 Nb5 O15 (JCPDS# 34-0108) with a tetragonal tungsten bronze structure is observed as the desired dielectric phase in all annealed samples [17]. But a second AlNbO4 (JCPDS# 41-0347) phase is observed as the crystallization time exceeds 10 min. Meanwhile, the peak strength of AlNbO4 phase becomes much stronger when the crystallization time increases. The results exhibit that the excessive soaking time is advantageous to the precipitation of impure phase. So choosing an appropriate crystallization time could restrict the precipitation of some unnecessary crystal phases. It is well known that the dielectric constant of the glass-ceramic depends on the type of crystal phases, crystallinity, interfacial polarization and the grain size [18,19]. Temperature dependence of dielectric constant and dielectric loss of the KSN-BAS glassceramics crystallized via microwave heat treatment at different soaking time are displayed in Fig. 2(a). All the dielectric constant could maintain excellent stability in the temperature range from −50 ◦ C to 120 ◦ C. The dielectric constant and dielectric loss as a function of crystallization time measured at a frequency of 10 kHz and a temperature of 25 ◦ C is shown in Fig. 2(b). The value of dielectric constant varies in the range of 66.2–102.0 at room temperature. The dielectric constant of the KSN-BAS glass-ceramic samples is presented a tendency of increasing as the crystallization time increases from 5 min to 10 min. This can be explained that a single crystal phase KSr2 Nb5 O15 with high dielectric constant (r ∼ 450) was precipitated from the parent matrixes. A maximum dielectric constant reaches up to 102.0 in the sample with a crystallization time of 10 min. But an opposite trend between the dielectric constant and the crystallization time is observed when the soaking time exceeds 10 min, which is ascribed the decrease of the KSr2 Nb5 O15 concentration and the increase of AlNbO4 phase of low dielectric constant (r ∼ 4) [20]. The results can be certified by the analysis of XRD patterns (shown in Fig. 1). For the dielectric loss, the value of dielectric loss of all the specimens varies in the range between
Fig. 1. XRD patterns of the glass-ceramic samples prepared via microwave heat treatment with different soaking time.
Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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Fig. 2. (a) Temperature dependence of dielectric constant and dielectric loss of the samples (at the measurement frequency of 10 kHz); (b) Dielectric constant and loss as a function of the crystallization time for the annealed samples.
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0.006 and 0.023, which is lower level as compared with that of the conventional heat treatment processed glass-ceramics [21]. Obviously, the more continuous and homogeneous microstructure and the inherence pore-free of the glass-ceramic result in the lower dielectric loss. The scanning electron micrographs (SEM) of the KSN-BAS glassceramics are shown in Fig. 3. Apparently, the average grain size of the KSN-BAS sample annealed by the microwave heat treatment is increased as the crystallization time increases. Owing to the rapid and uniform heating via electromagnetic field, the microwave processed glass-ceramics has been possessed the continuous and homogeneous microstructure wherever in the long soaking time or short. However, for the named C-2 h sample, some tremendous grains are presented in the surface of the sample. For the process of traditional heating, the energy transferred from the surface to inside due to the thermal gradients, which results in the nucleation and growth of grains preference to the surface of sample. The distribution of glass phase and ceramic phase becomes uneven owing to the presence of abnormal grains, resulting in a great interface polarization effect between glass phase and crystal phase [22]. However, the continuous and homogeneous crystal phase is presented in the glass-ceramic samples crystallized by microwave route, which is ascribed to the rapid and uniform heating via microwave [10]. The results indicate that microwave processes can provide an efficiency way to improve the microstructure of the glass-ceramics. The distribution of the dielectric breakdown strength (BDS) was described by the Weibull function. The Weibull distribution was illustrated in detail elsewhere [23]. As shown in Fig. 4, all the plots of the glass-ceramic specimens show a relatively good linearity. A maximum BDS of 1410.81 kV/cm is obtained in the KSN-BAS glassceramic specimens at 10 min soaking time. As the crystallization time exceeds 10 min, the BDS is decreased instead. The previous researches reported that the BDS is connected with a lot of factors, such as the chemical composition, crystallinity, microstructure and interfacial polarization. For the glass-ceramics, the research of microstructure and interfacial polarization is an effective way to analyze the value of BDS. The crystal nucleus was slowly formed in parent glass matrix as the crystallization time increased, which broke the dense structure of original glass, resulting in the deterio-
Fig. 3. SEM micrographs of the as-annealed KSN-BAS glass-ceramics prepared by varying crystallization time.
Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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ing behavior of space charge. The Cole–Cole plots of the sample at the crystallization time of 10 min are given in Fig. 5(a). The results illustrate that the high-frequency intercepts of the spectra at the real axis approach zero, which corresponds to the bulk resistance, showing a decreasing trend with increasing temperatures [25]. The inset of Fig. 5(a) shows that the areas of the impedance semicircles become smaller as the measurement temperature increases, which indicates the decrease of the grain boundary resistance. This trend is correlated with the thermally activated motions of defect owing to Maxwell–Wagner interfacial relaxation arising from the abundant interface between the crystal phases and the glass matrix. The grain boundary activation energy (Ea ) is calculated by Arrhenius relationship: ln =
Fig. 4. Weibull plots of dielectric breakdown strength of the as-devitrified samples. Inset: the dielectric breakdown strength as a function of crystallization time for the as-devitrified samples.
ration of BDS. Meanwhile, some previous researches indicated that the large grain size went against the improvement of the BDS, which is consistent with the microstructure (as shown in Fig. 3) [24]. In addition, the impedance analysis was used to analyze the spread-
1 Ea + ln0 TkB
(1)
The corresponding parameters of the function were illustrated in detail elsewhere [26]. The associated relaxation processes originated from space charge at grain boundaries. Ea reflects the interfacial charge mobility and a lower value of Ea means better charge flowing behavior. As seen from Fig. 5(b), the points of ln versus 1/T exhibit good linearities and then the relaxation activation energy can be acquired by the slopes of the fitted lines. The Ea as a function of crystallization time of these KSN-BAS glass-ceramics is showed in Fig. 5(c). It is noted that the Ea increased as the crystallization time is prolonged. The measurement value of the activation
Fig. 5. (a) Impedance spectra of the as-devitrified KSN-BAS sample at the crystallization time of 10 min measured in the temperature range of 340 ◦ C–420 ◦ C; (b) Relaxation times as a function of 1000/T for KSN-BAS glass-ceramics annealed at 900 ◦ C with different crystallization time; (c) Crystallization time and activation energy Ea for the as-devitrified samples; (d) Frequency dependent conductivity for all the samples heated at different crystallization time measured at 400 ◦ C.
Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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Fig. 6. Schematic circuit diagram of the charge-discharge measurement.
energy Ea is corresponded to the relaxation of space charge, which leads to the effective traps energy and the interfacial charge mobility in the defects where polarizability is changed and where charge and energy localization can occur [27]. A high value of Ea means that the interface of the grains and glass can accumulate more charge, which decreases the breakdown strength (BDS). The conductivity of the glass-ceramic samples measured at different frequency and 400 ◦ C is shown in Fig. 5(d). When the glass is crystallized at short soaking time, the crystalline phase is distributed in the glass matrix so isolatedly that the charge carries have to pass through the high resistive glass phase for transfer, which leads to a relatively low conductivity. As the crystallization time increases, the crystalline phases are connected with each other, which creates an easier pass way for charge carriers [16]. A higher conductivity means that the insulativity of glass-ceramic is more lower, which results in a lower BDS [28]. So it is important to choose an appropriate crystallization time so that improves BDS. The energy storage density can be calculated by the formula = 1/20 r E2 , where ω is the energy storage density (J/cm3 ), 0 is the dielectric constant, r is the relative dielectric constant, E and is the breakdown strength [29]. The calculated energy storage densities of the glass–ceramics heated by varying soaking time are summarized in Table 1. The optimal theoretical energy storage density of 8.99 J/cm3 is achieved in the KSN-BAS glass-ceramic sample with a soaking time of 10 min. For the application of pulsed power capacitors, the charge-discharge behavior of the KSN-BAS glass-ceramic capacitors was investigated by an RLC circuit. The schematic circuit diagram of the charge-discharge measurement is displayed in Fig. 6. To acquire a nanosecond pulse signal, a current probe was employed in the inductance (L) coil for the charged and discharged measurement. The detailed measuring process here can be described as follows: first the sample was charged via the DC high voltage source (U) then the test sample as a power source discharged across a load resistance (R). At the same time, the discharge performance was analyzed through monitoring the electric current of the load resistance by special oscilloscope. At last, the electric current (i) as a function of time (t) was obtained. For the linear dielectrics, the C is independent of the electric field. The charge energy (W) can be calculated via the formula W = 12 CU 2 . In the process of discharge, the power density (P) and the discharge energy (Wc ) can be calculated by using the following formulas: i(t) = C
du dt
1 u (t) = C
(2)
∞ i (t) dt 0
(3)
Fig. 7. (a) The discharge energy of the KSN-BAS glass-ceramic specimens under the test electric field of 1000 V. (b) The power density as a function of time for the KSN-BAS glass-ceramic samples prepared by varying soaking time.
P = u (t) · i (t)
(4)
Wc =
(5)
Pdt
Where C is a static capacitance of the sample and U is the charge voltage. The discharge energy (Wc ) of the glass-ceramic samples under the test electric field of 1000 V can be directly obtained from Fig. 7(a). In this study, the values of C, W and average Wc of all the test samples are summarized in Table 2. The average energy conversion efficiency () can be calculated by the formula = Wc /W. It can be indicated that the average of the samples increased from 75.00% to 80.72% as the crystallization time less than 15 min. Subsequently, the average reduced as the soaking time exceeded 15 min. The higher released charge means the larger efficiency. Currently, the unreleased charges are affected by several factors, such as interfacial polarization, charge carriers and hysteresis loss. For the glassceramics, owing to the great difference in dielectric constant between ferroelectric crystal and glass phase, the interfaces are polarized by the charging electric field, which produced the interfacial polarization charges. However, the interfacial polarization charges cannot be eliminated in the discharging process due to the long relaxation process of interfacial polarization. As a result, some charges stored in electrodes are trapped to offset the electric field caused by the interfacial polarization charges in the discharge process. Compared with the previous researches, the
Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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Table 1 Dielectric constant, breakdown strength and theoretical energy storage density of the as-devitrified glass-ceramics heated by microwave with different crystallization time. Crystallization time (min) 5 10 15 30 120 C-2h
Dielectric constant (10 kHz, 25 ◦ C) 88.8 102.0 92.3 66.2 72.3 103.0
± ± ± ± ± ±
Average Breakdown strength (kV/cm)
4.3 7.9 5.7 2.6 3.6 8.5
1287.15 1410.81 1248.76 1209.53 1151.14 1069.71
Theoretical energy storage density (, J/cm3 ) 6.51 8.99 6.37 4.29 4.24 5.21
± ± ± ± ± ±
0.32 0.69 0.39 0.17 0.21 0.43
Table 2 The charge-discharge performances of the KSN-BAS glass-ceramics samples prepared via varying crystallization time. Crystallization time (min) 5 10 15 30 120
Capacitance (pF) 65.6–66.4 89.5–90.1 99.6–100.5 104.5–105.8 55.3–56.7
Charged energy (10−5 J) 3.28–3.32 4.47–4.51 4.98–5.03 5.22–5.29 2.76–2.84
Average discharged energy (10−5 J)
Average power density (MW/cm3 )
Average efficiency (%)
2.49 3.55 4.06 3.83 2.07
0.85 1.40 0.57 1.01 0.47
75.00 78.71 80.72 72.40 72.89
of KSN-BAS glass-ceramic heated by the microwave is obvious superior to the glass-ceramic systems heated by conventional furnace and ferroelectric ceramics, resulting in lesser chargers trapped by the interfacial polarization charges in discharge process, which was consistent with the microstructure and the boundary activation energy (as shown in Figs. 3 and 5 (c)) [16,30]. Certainly, the phenomenon was also presented in the system of BST, BSN and BNN glass-ceramic [11,26,31]. The power density (P) calculated from the discharge curve is shown in Fig. 7(b) and demonstrates the maximum power density of the glass-ceramic can reach 1.40 MW/cm3 at 22 ns, which has great potential application in high-speed pulse capacitors [32–35]. 4. Conclusions The microstructure evolution and energy storage properties of the KSN-BAS glass-ceramics through microwave heat treatment with different crystallization time were investigated. Choosing a reasonable crystallization time to fabricate the KSN-BAS glassceramics could effectively restrain the precipitation of impure phase and improve the microstructure, which enhanced the dielectric constant and breakdown strength. When the crystallization time was 10 min, the optimization of dielectric constant of 102.0 and breakdown strength of 1410.81 kV/cm was obtained. The maximum of theoretical energy storage density could reach up to 8.99 J/cm3 . Considering the application of high-speed pulse capacitors, the energy conversion efficiency and power density was measured. The charge-discharge period of all the samples can reach nanosecond. A maximum discharge efficiency of 80.72% and the largest power density of 1.40 MW/cm3 were obtained in the samples prepared at the crystallization of 15 min and 10 min respectively, which has great potential application in pulse capacitors. Acknowledgments This research was supported by the Ministry of Sciences and Technology of China through the 973-project (grant No. 2015CB654601). The authors would also like to thank the micro analysis center at the Fudan University for SEM analysis.
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Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization Shi Xiao, Shaomei Xiu, Bo Shen, Jiwei Zhai ∗ Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China
a r t i c l e
i n f o
Article history: Received 15 April 2016 Received in revised form 22 June 2016 Accepted 23 June 2016 Available online xxx Keywords: Microwave sintering Glass-ceramics Microstructure Charge-discharge
a b s t r a c t The potassium strontium niobate boroaluminosilicate (KSN-BAS) glass-ceramics were prepared through microwave sintering. The effects of crystallization time on the microstructure and dielectric properties of the KSN-BAS glass-ceramics were investigated. The XRD results exhibited that an impure phase AlNbO4 was devitrified from the glass matrix as the crystallization time increased. The microstructure showed that the grain size increased as the crystallization time prolonged. Crystal boundary activation energy of the KSN-BAS glass-ceramics was performed by impedance analysis. The dielectric constant and electric breakdown strength indicated a trend of increasing at first and then reducing with the increase of the crystallization time. When the crystallization time was 10 min, the optimization of dielectric constant of 102.0 and breakdown strength of 1410.81 kV/cm were obtained. The maximum of theoretical energy storage density can reach a value of 8.99 J/cm3 . For pulsed power applications, the discharge efficiency and power density were evaluated by RLC circuit. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the development of the electronic equipment and pulse technology, there is a very urgent need to search new energy storage materials in possession of high energy density, high energy efficiency and power density [1]. At present, the research of energy storage materials applied in capacitors are mainly focused on dielectric materials, such as ferroelectric ceramics, antiferroelectric ceramics and polymer dielectrics owing to their inherent advantages like high dielectric constant, power density, long service life and short charging time etc [2,3]. However, their own shortcomings also restrict the improvement of energy storage density. It is still a challenging task to enhance the energy density in dielectric energy storage materials [4]. In order to improve the energy storage ability, many meaningful researches have been performed to develop new energy storage materials. The ferroelectric glass-ceramics materials are considered as one of the most potential candidates owing to their combination of high permittivity crystallites and high breakdown strength glass matrix. The previous researches have also confirmed that
∗ Corresponding author. E-mail address: [email protected] (J. Zhai).
the energy storage density of the ferroelectric glass-ceramics was indeed higher than that of the other materials, such as ferroelectric ceramic and polymer dielectrics [1–4]. In addition, glass-ceramics can provide a dense microstructure (almost zero porosity), and the phase structure and grain size can be adjusted by the controlled devitrification processing [5]. However, in the conventional heat treatment of glass-ceramics, the thermal energy is generally transferred from the surface of material to inside due to thermal gradients through convection, conduction, and radiation, which might appear that the nucleation and growth of crystal were preferentially formed in the surface of the devitrified sample [6,7]. Now, a variety of processing techniques have been utilized to modify the microstructure of ferroelectric glass-ceramics, such as liquid phase sintering, hot pressed sintering, and sol-gel method [8,9]. The microwave heat treatment has been widely used in the preparation of material because of its unique heating mode [6]. In contrast to the conventional furnaces, the microwave heat treatment depends on the transfer of electromagnetic energy. Moreover, the microwaves can penetrate materials and heat can be generated throughout the volume of the material. It is possible to achieve rapid and uniform heating in the preparation of materials [10]. Based on the aforementioned advantages, the performance of the materials prepared by microwave sintering could be superior to that of the materials via traditional heat treatment. Wang et al. found that
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Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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microwave heat treatment could effectively eliminate dendritic crystal structure in the BST-based glass-ceramics, resulting in the increase of the breakdown strength [11]. In addition, microwave sintering can enhance mechanical properties of the materials which could be advantageous to follow-up processing [12]. In spite of many advantages reflected in the microwave sintering, the absorption of electromagnetic field has great difference in the varying materials. So it is important to choose an appropriate condition to prepare the glass-ceramic materials by microwave heat treatment. Xiao et al. reported that the energy storage density could be increased by optimizing the crystallization temperature [13]. Zhang et al. found that the value of conductivity exhibited a rising trend as the crystallization time increased, which demonstrated a better temperature-sensitive behavior for the crystal phase [14]. But the reports about the effect crystallization time on the microstructure and dielectric properties of the glass-ceramic processed by microwave heat treatment are very rarely. For energy-storage dielectric material, besides a high energy density, the research of charge-discharge efficiency and the speed of charge-discharge are also required for the applications of high energy storage capacitors [15]. Xu et al. found that half of the stored charge can be released under 10 ns in the PLZT antiferroelectric ceramic by the RLC measuring circuit [16]. In this study, the K2 O-SrO-Nb2 O5 -B2 O3 -Al2 O3 -SiO2 (KSN-BAS) glass was crystallized by microwave route to obtain the glass-ceramics. Effects of the crystallization time on the microstructure, dielectric properties and charge-discharge properties of the glass-ceramics were investigated, and the optimized crystallization process to improve the energy storage density of the KSN-based glass-ceramic was presented.
2. Experimental procedure The KSA-BAS glass was fabricated from well-mixed powders of 15K2 CO3 , 15SrCO3 , 30Nb2 O5 , 32SiO2 , 4Al2 O3 , and 4B2 O3 (mol.%). The as-dried powders were melted at 1550 ◦ C for 2 h in an alumina crucible. Then the glass lava was quickly poured into the preheated copper mold at 600 ◦ C for 6 h to remove residual stress. The as-annealed glass was cut into piece-shaped samples with about 1.5 mm thickness and then the glass samples were crystalized by microwave heat treatment (HAMiLab-C1500, SYNOTHERM, 2.45 GHz) at 900 ◦ C for 5 min, 10 min 15 min 30 min and 2 h, respectively. In addition, some glasses named C-2 h were heated to 900 ◦ C for 2 h in a conventional box-type furnace as a comparison sample. X-ray diffraction (XRD) (D/max2550V, Rigaku, Japan) with Cu K␣ radiation at room temperature was used to determine the crystallographic structure of the glass-ceramics heated via different crystallization time. The microstructure was observed by a field scanning electron microcopy (SEM) (HITACHI S-4700, Japan). The dielectric constant and loss tangent of the glass-ceramic specimens were measured by using a LCR meter (Agilent E4980A, USA) under a frequency of 10 kHz and in the temperature range of −50 ◦ C to 120 ◦ C. The measurement of DC breakdown strength of all the glass-ceramic samples was performed by using a voltagewithstand testing source (ET2671B, Entai, Nanjing, China) at room temperature. A DC voltage ramp of about 0.2 kV/s was applied to the samples until the dielectric breakdown occurred. In the measurement process of breakdown strength, at least 15 testing samples were ground into about 0.07–0.1 mm thickness and immersed in silicon oil to prevent arcing. Complex impedance spectra were measured via the LCR meter (Agilent E4284, USA) in the frequencies range from 150 Hz to 2 MHz and a wide temperature range of 340 ◦ C–420 ◦ C with an AC electric field of 4 V/mm. The chargedischarge properties of the KSN-BAS glass-ceramic capacitors were detected with a specially designed, high-speed capacitor discharge
resistance, inductance, and capacitance (RLC) load circuit designed by ourselves. 3. Results and discussion Fig. 1 shows XRD patterns of the KSN-BAS glass-ceramics prepared by the microwave heating route under different crystallization time. It is noticed that the relative intensity of the crystal diffraction peaks is gradually increased with the increase of crystallization time, which implies that the content of crystal phases is increased. Compared with PDF cards, the crystalline phase KSr2 Nb5 O15 (JCPDS# 34-0108) with a tetragonal tungsten bronze structure is observed as the desired dielectric phase in all annealed samples [17]. But a second AlNbO4 (JCPDS# 41-0347) phase is observed as the crystallization time exceeds 10 min. Meanwhile, the peak strength of AlNbO4 phase becomes much stronger when the crystallization time increases. The results exhibit that the excessive soaking time is advantageous to the precipitation of impure phase. So choosing an appropriate crystallization time could restrict the precipitation of some unnecessary crystal phases. It is well known that the dielectric constant of the glass-ceramic depends on the type of crystal phases, crystallinity, interfacial polarization and the grain size [18,19]. Temperature dependence of dielectric constant and dielectric loss of the KSN-BAS glassceramics crystallized via microwave heat treatment at different soaking time are displayed in Fig. 2(a). All the dielectric constant could maintain excellent stability in the temperature range from −50 ◦ C to 120 ◦ C. The dielectric constant and dielectric loss as a function of crystallization time measured at a frequency of 10 kHz and a temperature of 25 ◦ C is shown in Fig. 2(b). The value of dielectric constant varies in the range of 66.2–102.0 at room temperature. The dielectric constant of the KSN-BAS glass-ceramic samples is presented a tendency of increasing as the crystallization time increases from 5 min to 10 min. This can be explained that a single crystal phase KSr2 Nb5 O15 with high dielectric constant (r ∼ 450) was precipitated from the parent matrixes. A maximum dielectric constant reaches up to 102.0 in the sample with a crystallization time of 10 min. But an opposite trend between the dielectric constant and the crystallization time is observed when the soaking time exceeds 10 min, which is ascribed the decrease of the KSr2 Nb5 O15 concentration and the increase of AlNbO4 phase of low dielectric constant (r ∼ 4) [20]. The results can be certified by the analysis of XRD patterns (shown in Fig. 1). For the dielectric loss, the value of dielectric loss of all the specimens varies in the range between
Fig. 1. XRD patterns of the glass-ceramic samples prepared via microwave heat treatment with different soaking time.
Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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Fig. 2. (a) Temperature dependence of dielectric constant and dielectric loss of the samples (at the measurement frequency of 10 kHz); (b) Dielectric constant and loss as a function of the crystallization time for the annealed samples.
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0.006 and 0.023, which is lower level as compared with that of the conventional heat treatment processed glass-ceramics [21]. Obviously, the more continuous and homogeneous microstructure and the inherence pore-free of the glass-ceramic result in the lower dielectric loss. The scanning electron micrographs (SEM) of the KSN-BAS glassceramics are shown in Fig. 3. Apparently, the average grain size of the KSN-BAS sample annealed by the microwave heat treatment is increased as the crystallization time increases. Owing to the rapid and uniform heating via electromagnetic field, the microwave processed glass-ceramics has been possessed the continuous and homogeneous microstructure wherever in the long soaking time or short. However, for the named C-2 h sample, some tremendous grains are presented in the surface of the sample. For the process of traditional heating, the energy transferred from the surface to inside due to the thermal gradients, which results in the nucleation and growth of grains preference to the surface of sample. The distribution of glass phase and ceramic phase becomes uneven owing to the presence of abnormal grains, resulting in a great interface polarization effect between glass phase and crystal phase [22]. However, the continuous and homogeneous crystal phase is presented in the glass-ceramic samples crystallized by microwave route, which is ascribed to the rapid and uniform heating via microwave [10]. The results indicate that microwave processes can provide an efficiency way to improve the microstructure of the glass-ceramics. The distribution of the dielectric breakdown strength (BDS) was described by the Weibull function. The Weibull distribution was illustrated in detail elsewhere [23]. As shown in Fig. 4, all the plots of the glass-ceramic specimens show a relatively good linearity. A maximum BDS of 1410.81 kV/cm is obtained in the KSN-BAS glassceramic specimens at 10 min soaking time. As the crystallization time exceeds 10 min, the BDS is decreased instead. The previous researches reported that the BDS is connected with a lot of factors, such as the chemical composition, crystallinity, microstructure and interfacial polarization. For the glass-ceramics, the research of microstructure and interfacial polarization is an effective way to analyze the value of BDS. The crystal nucleus was slowly formed in parent glass matrix as the crystallization time increased, which broke the dense structure of original glass, resulting in the deterio-
Fig. 3. SEM micrographs of the as-annealed KSN-BAS glass-ceramics prepared by varying crystallization time.
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ing behavior of space charge. The Cole–Cole plots of the sample at the crystallization time of 10 min are given in Fig. 5(a). The results illustrate that the high-frequency intercepts of the spectra at the real axis approach zero, which corresponds to the bulk resistance, showing a decreasing trend with increasing temperatures [25]. The inset of Fig. 5(a) shows that the areas of the impedance semicircles become smaller as the measurement temperature increases, which indicates the decrease of the grain boundary resistance. This trend is correlated with the thermally activated motions of defect owing to Maxwell–Wagner interfacial relaxation arising from the abundant interface between the crystal phases and the glass matrix. The grain boundary activation energy (Ea ) is calculated by Arrhenius relationship: ln =
Fig. 4. Weibull plots of dielectric breakdown strength of the as-devitrified samples. Inset: the dielectric breakdown strength as a function of crystallization time for the as-devitrified samples.
ration of BDS. Meanwhile, some previous researches indicated that the large grain size went against the improvement of the BDS, which is consistent with the microstructure (as shown in Fig. 3) [24]. In addition, the impedance analysis was used to analyze the spread-
1 Ea + ln0 TkB
(1)
The corresponding parameters of the function were illustrated in detail elsewhere [26]. The associated relaxation processes originated from space charge at grain boundaries. Ea reflects the interfacial charge mobility and a lower value of Ea means better charge flowing behavior. As seen from Fig. 5(b), the points of ln versus 1/T exhibit good linearities and then the relaxation activation energy can be acquired by the slopes of the fitted lines. The Ea as a function of crystallization time of these KSN-BAS glass-ceramics is showed in Fig. 5(c). It is noted that the Ea increased as the crystallization time is prolonged. The measurement value of the activation
Fig. 5. (a) Impedance spectra of the as-devitrified KSN-BAS sample at the crystallization time of 10 min measured in the temperature range of 340 ◦ C–420 ◦ C; (b) Relaxation times as a function of 1000/T for KSN-BAS glass-ceramics annealed at 900 ◦ C with different crystallization time; (c) Crystallization time and activation energy Ea for the as-devitrified samples; (d) Frequency dependent conductivity for all the samples heated at different crystallization time measured at 400 ◦ C.
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Fig. 6. Schematic circuit diagram of the charge-discharge measurement.
energy Ea is corresponded to the relaxation of space charge, which leads to the effective traps energy and the interfacial charge mobility in the defects where polarizability is changed and where charge and energy localization can occur [27]. A high value of Ea means that the interface of the grains and glass can accumulate more charge, which decreases the breakdown strength (BDS). The conductivity of the glass-ceramic samples measured at different frequency and 400 ◦ C is shown in Fig. 5(d). When the glass is crystallized at short soaking time, the crystalline phase is distributed in the glass matrix so isolatedly that the charge carries have to pass through the high resistive glass phase for transfer, which leads to a relatively low conductivity. As the crystallization time increases, the crystalline phases are connected with each other, which creates an easier pass way for charge carriers [16]. A higher conductivity means that the insulativity of glass-ceramic is more lower, which results in a lower BDS [28]. So it is important to choose an appropriate crystallization time so that improves BDS. The energy storage density can be calculated by the formula = 1/20 r E2 , where ω is the energy storage density (J/cm3 ), 0 is the dielectric constant, r is the relative dielectric constant, E and is the breakdown strength [29]. The calculated energy storage densities of the glass–ceramics heated by varying soaking time are summarized in Table 1. The optimal theoretical energy storage density of 8.99 J/cm3 is achieved in the KSN-BAS glass-ceramic sample with a soaking time of 10 min. For the application of pulsed power capacitors, the charge-discharge behavior of the KSN-BAS glass-ceramic capacitors was investigated by an RLC circuit. The schematic circuit diagram of the charge-discharge measurement is displayed in Fig. 6. To acquire a nanosecond pulse signal, a current probe was employed in the inductance (L) coil for the charged and discharged measurement. The detailed measuring process here can be described as follows: first the sample was charged via the DC high voltage source (U) then the test sample as a power source discharged across a load resistance (R). At the same time, the discharge performance was analyzed through monitoring the electric current of the load resistance by special oscilloscope. At last, the electric current (i) as a function of time (t) was obtained. For the linear dielectrics, the C is independent of the electric field. The charge energy (W) can be calculated via the formula W = 12 CU 2 . In the process of discharge, the power density (P) and the discharge energy (Wc ) can be calculated by using the following formulas: i(t) = C
du dt
1 u (t) = C
(2)
∞ i (t) dt 0
(3)
Fig. 7. (a) The discharge energy of the KSN-BAS glass-ceramic specimens under the test electric field of 1000 V. (b) The power density as a function of time for the KSN-BAS glass-ceramic samples prepared by varying soaking time.
P = u (t) · i (t)
(4)
Wc =
(5)
Pdt
Where C is a static capacitance of the sample and U is the charge voltage. The discharge energy (Wc ) of the glass-ceramic samples under the test electric field of 1000 V can be directly obtained from Fig. 7(a). In this study, the values of C, W and average Wc of all the test samples are summarized in Table 2. The average energy conversion efficiency () can be calculated by the formula = Wc /W. It can be indicated that the average of the samples increased from 75.00% to 80.72% as the crystallization time less than 15 min. Subsequently, the average reduced as the soaking time exceeded 15 min. The higher released charge means the larger efficiency. Currently, the unreleased charges are affected by several factors, such as interfacial polarization, charge carriers and hysteresis loss. For the glassceramics, owing to the great difference in dielectric constant between ferroelectric crystal and glass phase, the interfaces are polarized by the charging electric field, which produced the interfacial polarization charges. However, the interfacial polarization charges cannot be eliminated in the discharging process due to the long relaxation process of interfacial polarization. As a result, some charges stored in electrodes are trapped to offset the electric field caused by the interfacial polarization charges in the discharge process. Compared with the previous researches, the
Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044
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Table 1 Dielectric constant, breakdown strength and theoretical energy storage density of the as-devitrified glass-ceramics heated by microwave with different crystallization time. Crystallization time (min) 5 10 15 30 120 C-2h
Dielectric constant (10 kHz, 25 ◦ C) 88.8 102.0 92.3 66.2 72.3 103.0
± ± ± ± ± ±
Average Breakdown strength (kV/cm)
4.3 7.9 5.7 2.6 3.6 8.5
1287.15 1410.81 1248.76 1209.53 1151.14 1069.71
Theoretical energy storage density (, J/cm3 ) 6.51 8.99 6.37 4.29 4.24 5.21
± ± ± ± ± ±
0.32 0.69 0.39 0.17 0.21 0.43
Table 2 The charge-discharge performances of the KSN-BAS glass-ceramics samples prepared via varying crystallization time. Crystallization time (min) 5 10 15 30 120
Capacitance (pF) 65.6–66.4 89.5–90.1 99.6–100.5 104.5–105.8 55.3–56.7
Charged energy (10−5 J) 3.28–3.32 4.47–4.51 4.98–5.03 5.22–5.29 2.76–2.84
Average discharged energy (10−5 J)
Average power density (MW/cm3 )
Average efficiency (%)
2.49 3.55 4.06 3.83 2.07
0.85 1.40 0.57 1.01 0.47
75.00 78.71 80.72 72.40 72.89
of KSN-BAS glass-ceramic heated by the microwave is obvious superior to the glass-ceramic systems heated by conventional furnace and ferroelectric ceramics, resulting in lesser chargers trapped by the interfacial polarization charges in discharge process, which was consistent with the microstructure and the boundary activation energy (as shown in Figs. 3 and 5 (c)) [16,30]. Certainly, the phenomenon was also presented in the system of BST, BSN and BNN glass-ceramic [11,26,31]. The power density (P) calculated from the discharge curve is shown in Fig. 7(b) and demonstrates the maximum power density of the glass-ceramic can reach 1.40 MW/cm3 at 22 ns, which has great potential application in high-speed pulse capacitors [32–35]. 4. Conclusions The microstructure evolution and energy storage properties of the KSN-BAS glass-ceramics through microwave heat treatment with different crystallization time were investigated. Choosing a reasonable crystallization time to fabricate the KSN-BAS glassceramics could effectively restrain the precipitation of impure phase and improve the microstructure, which enhanced the dielectric constant and breakdown strength. When the crystallization time was 10 min, the optimization of dielectric constant of 102.0 and breakdown strength of 1410.81 kV/cm was obtained. The maximum of theoretical energy storage density could reach up to 8.99 J/cm3 . Considering the application of high-speed pulse capacitors, the energy conversion efficiency and power density was measured. The charge-discharge period of all the samples can reach nanosecond. A maximum discharge efficiency of 80.72% and the largest power density of 1.40 MW/cm3 were obtained in the samples prepared at the crystallization of 15 min and 10 min respectively, which has great potential application in pulse capacitors. Acknowledgments This research was supported by the Ministry of Sciences and Technology of China through the 973-project (grant No. 2015CB654601). The authors would also like to thank the micro analysis center at the Fudan University for SEM analysis.
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Please cite this article in press as: S. Xiao, et al., Microstructure evolution and energy storage properties of potassium strontium niobate boroaluminosilicate glass-ceramics by microwave crystallization, J Eur Ceram Soc (2016), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.06.044