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Dielectric properties and charge–discharge behaviors in niobate glass ceramics for energy-storage applications
Journal of Alloys and Compounds 617 (2014) 418–422
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Dielectric properties and charge–discharge behaviors in niobate glass ceramics for energy-storage applications Shuangxi Xue a,b, Shaohui Liu a, Wenqin Zhang a, Jinwen Wang a, Linjiang Tang a, Bo Shen a, Jiwei Zhai a,⇑ a b
Functional Materials Research Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, China Department of Physics and Electronic Engineering, Taizhou University, Taizhou 318000, China
a r t i c l e
i n f o
Article history: Received 15 July 2014 Accepted 2 August 2014 Available online 10 August 2014 Keywords: Dielectric properties Dielectric breakdown strength Glass–ceramics
a b s t r a c t A series of niobate glass ceramics with varying glass content have been prepared via controlled-crystallization route, and the dielectric properties, the breakdown characteristics and charge–discharge behaviors have been investigated. The correlation between the dielectric breakdown performance and the activation energy was studied by the measurements of the dielectric breakdown strength and impedance spectroscopy. The charge–discharge efficiency was investigated using a charge–discharge system with high-speed capacitor discharge circuit device. It was found that the dielectric breakdown strength and the discharge efficiency strongly depend on the interface polarization based on the results of complex impedance analysis. With the increase of the glass content, the breakdown strength (BDS) and the charge–discharge efficiency show a clear increasing trend, when the content of glass was 60%, the typical G60 sample shows a high BDS of 1300 kV/cm and a high charge–discharge efficiency of 92.5%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction High density energy storage materials and the related devices have attracted increasing attention since the energy crisis became more serious in the new century. There are growing needs for energy storage devices that are high energy density, low cost, and environmentally friendly, for power electronics and pulsed power applications. In order to reach a high energy density, characteristics such as high dielectric constant, high breakdown strength and low dielectric loss for materials are needed. The traditional ferroelectric ceramic has been used for capacitors due to their high dielectric constants, but the low breakdown strength (BDS) limits its further application [1–4]. In the search for new high energy density materials, a significant challenge is to increase dielectric constant while maintaining high breakdown strength. Glass–ceramic dielectrics have been considered as the most promising candidate for energy storage capacitors, because they have high dielectric constant (from the precipitated crystalline phases) and high breakdown strength (due to the pore-free inherence) [5–9]. Different types of ferroelectric glass–ceramics such as (Ba, Sr)TiO3 and (Ba, Sr, Pb)TiO3 have been widely studied [10–14]. The dielectric constant of ferroelectric strongly varies with temperature
⇑ Corresponding author. Tel.: +86 21 65980544; fax: +86 21 65985179. E-mail address: [email protected] (J. Zhai). http://dx.doi.org/10.1016/j.jallcom.2014.08.006 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
near the Curie temperature. As the Curie temperature is approached, the dielectric constant increases rapidly as dipole orientation in the direction of the field can be more extensive. The dielectric constant decreases rapidly above the Curie temperature as the phase has become paraelectric phase. Thus, (Ba, Sr)TiO3 based ferroelectric capacitors with Curie temperatures near ambient temperature suffer unacceptable dielectric constant changes with small temperature shifts. For example, BaTiO3 has a Curie temperature of 120 °C; the dielectric constant shifts from 1250 at room temperature to 1750 at about 100 °C [15,16]. Niobate-based glass–ceramics is a promising candidate for overcoming the temperature variation of the dielectric constant due to their high Curie temperature which significantly higher than room temperature. For example, the Curie temperature of single crystal orthorhombic Ba2NaNb5O15 (BNN) is 600 °C [17]. In our recent work (unpublished), lead-free (4BaO–Na2O– 5Nb2O5)–SiO2 is a promising system in which Ba2NaNb5O15 as the main crystalline phase has high dielectric constants and high Curie temperature (about 600 °C), however, the dielectric breakdown strength was still not high enough. Alkali-free barium boroaluminosilicate glass has been proved in experiments with ultra high dielectric breakdown field strength. Smith found SchottAF45 glass (SchottAF-45; Schott Technologies Inc. The approximate bulk composition with 63% SiO2, 12% BaO, 16% B2O3 and 9% Al2O3 has remarkably high DC dielectric breakdown strength (12 MV/cm) [18]. In this work, barium boroaluminosilicate glass
S. Xue et al. / Journal of Alloys and Compounds 617 (2014) 418–422
with the same composition with SchottAF-45 glass was used as amorphous matrix, (40% BaO, 10% Na2O, 50% Nb2O5)–(63% SiO2, 12% BaO, 16% B2O3, 9% Al2O3) glass–ceramics (BNN) with varying glass content were prepared through the traditional glass preparation process and crystallization route. Many investigations on glass–ceramics with high dielectric constant or enhanced BDS have been reported. Considering the application of pulsed power capacitors, the breakdown characteristics and charge–discharge behavior of the glass–ceramics (such as the discharge efficiency, the power density, and the energy density) were important parameters to evaluate the capacitors. However, there is much less information available about the charge–discharge behavior of BNN glass– ceramics, the discharge behavior of the BNN based glass–ceramics were evaluated using a high-speed capacitor discharge circuit device designed by ourselves.
419
Fig. 1 shows XRD patterns of the as-devitrified samples heated at 1100 °C for 2 h. By comparing the XRD patterns with interna-
tional centre for diffraction data (ICDD), two phases of tetragonal tungsten bronze (T.T.B.) and cubic perovskite (C.P.) were indexed, the crystalline phase with T.T.B. structure is related to Ba2NaNb5O15 and the C.P. structure is related to NaNbO3. The dielectric constant of the as-devitrified samples as a function of temperature is shown in Fig. 2. All samples shows stable dielectric constant over the temperature range of 150 °C to 150 °C. The dielectric constant of the as-devitrified samples decreases with the increase of the content of glass phase, which is attributed to the lower dielectric constant. Dielectric breakdown data are commonly interpreted via Weibull plots due to the inherently statistical nature of failure and its mediation through a variety of intrinsic and extrinsic factors. The Weibull distribution was described in detail elsewhere [19]. A 2-parameters Weibull distribution should be a line on the coordinate axis. The slope of the line is the shape parameter b, and the intercept on the x-axis is ln E. Where E is the BDS. Fig. 3 illustrates the Weibull distributions of the BDS for the as-devitrified G30–G60 samples annealed at 1100 °C for 2 h. In our case, as can be seen in Fig. 3, all the plots show a relatively good linearity and the values of shape parameter b for the four samples all are higher than eight, thus a valid comparison of characteristic BDS is possible. As shown in Fig. 3, all samples exhibit high BDS value and the BDS value of G60 is significantly higher than that of other samples. With the increase of glass content, the BDS increased to 1300 kV/cm for the G60 sample. It is well known that electrical properties of glass–ceramics are strongly affected by their microstructure. As a non-destructive analysis technique, an impedance spectrum that accurately reflects the microstructure features is usually the best tool to analyze their electrical properties. In this study, impedance analysis is introduced to analysis the electrical properties of the samples. The samples are measured at different temperatures to get a series of Cole–Cole images and Z 00 spectroscopic plots. Fig. 4 illustrates impedance spectra of the as-devitrified G30 sample measured at different temperatures, the inset shows the impedance Z 00 vs the measuring frequency. Below 520 °C the semicircles cannot be observed because of the extra high resistivity of this glass ceramic system. It can be seen that the impedance semicircles became smaller and the relaxation frequency at the extreme point of Z 00 spectroscopic plots increased with the increasing of temperature. Details of the impedance analysis and the activation energy (Ea) can be found elsewhere [2,20]. The reciprocal value of measurement frequency at the extreme point of each semicircle represents their relaxation time s of the interfacial polarization process at a certain measurement temperature. The ln s 1000/T plots can be obtained from Fig. 4, which obey Arrhenius relationship:
Fig. 1. XRD patterns of the as-devitrified samples heated at 1100 °C for 2 h. (a) NaNbO3(ICDD: 01-074-2441), (b) Ba2NaNb5O15 (ICDD: 01-086-0739).
Fig. 2. Temperature dependence of dielectric constant of G30–G60 samples annealed at 1100 °C for 2 h.
2. Experimental procedures The BNN glass–ceramics used in this study were prepared from the well-mixed powders of analytical reagent grade BaCO3, NaCO3, Nb2O5, B2O3, Al2O3 to achieve the different glass compositions, represented by a chemical formula (100 x) (40BaO–10Na2O–50Nb2O5)–x(63SiO2–12BaO–16B2O3–9Al2O3) (x = 30, 40, 50, 60) (mole fraction). When the glass content was 30%, 40%, 50%, 60%, the corresponding sample names were G30, G40, G50, G60. Analytical reagent grade powders of Na2CO3, BaCO3, Nb2O5, SiO2, B2O3 and Al2O3 were ball mixed for 24 h in a high density polyethylene bottle with ethanol as milling media for homogenous mixing. After drying at 100 °C for 12 h, the powders were placed in an alumina crucible and melted at 1500 °C for 3 h to obtain the homogeneous liquid and the liquid was manually poured into a preheated copper mold to form glass bulks followed by quickly placed in an annealing oven at 550 °C to reduce the residual stress. The transparent glasses were cut into 6 mm 6 mm 1 mm sheets, and annealed in air by heating at 3 °C/min to 1100 °C for 2 h. X-ray diffraction (D8 Advance, Bruker AXS, Germany) were used to investigate the phase evolution. The temperature dependences of dielectric constant were measured using a LCR meter (AgilentE4980A) at the frequency of 1 MHz and in the temperature range from 50 °C to 100 °C. The DC dielectric breakdown strength (BDS) measurement was performed using a voltage-withstand testing device (ET2671B, Entai, Nanjing, China) at room temperature. A DC voltage ramp of about 1 kV/s was applied to the specimens until the dielectric breakdown occurred. At least 8 specimens were used for each composition during BDS testing. All samples were ground into about 0.15 mm thickness and immersed in silicone oil to prevent surface flashover. The impedance data were measured by a LCR meter (Agilent E4980A) over frequencies from 150 Hz to 2 MHz in a temperature range of 400–540 °C with an AC electric field of 4 V/mm. The charge–discharge behavior of the BNN based glass–ceramics were evaluated using a high-speed capacitor discharge circuit device designed by ourselves.
3. Results and discussion
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S. Xue et al. / Journal of Alloys and Compounds 617 (2014) 418–422
Fig. 3. Weibull plots of dielectric break down strength of the as-devitrified samples.
Fig. 5. Relaxation time as a function of 1000/T for the as-devitrified samples. The solid lines are fitting curves through the data.
Fig. 4. Impedance spectra of the as-devitrified G30, G40, G50, G60 sample measured at different temperature. Inset: Z00 spectroscopic plots at various temperature.
Fig. 6. Dielectric BDS and activation energy Ea for the as-devitrified samples.
ln s ¼
1 Ea þ ln s0 KBT
where Ea is the activation energy for the relaxation process, KB is the Boltzmann constant and T is the absolute temperature. The values of Ea for the samples can be calculated from the slope of the function between relaxation time (the reciprocal value of the relaxation frequency) and measuring temperature, which is shown in Fig. 5. In this study, the associated relaxation processes originated from space charge at grain boundaries. In our measurement condition the activation energy Ea corresponds the relaxation of space charges, it reflects the interfacial mobility and a higher value of Ea means bad charge flowing behavior, and results in the accumulation of charge at the interface, which leads to a low BDS value. As seen from Fig. 5, the Ea values for the samples were 1.19 eV (G30), 1.20 eV (G40), 1.17 eV (G50), 1.09 eV (G60). The BDS and the activation energy Ea for the glass ceramics were shown in Fig. 6. As the content of glass phase increase from 30% to 50%, the BDS value increases sharply and then increases gradually when the content of glass increase from 50% to 60%. The resistance of glass phase is far higher than that of the ceramic phase, the increase of glass phase will decrease the conductivity of the glass ceramic system, which will result in the high BDS value. Furthermore, the glass will restrain the growth of crystalline grain during the crystallization process, small and uniform particle size will be helpful to improve BDS. Consequently, it is easy to
understand the BDS value increases with the content of glass. However, with the reduction of the glass content, more crystallized particles will be formed from the residual glass and the amount of interfaces will increase. Because of the significant difference in dielectric constant of ferroelectric phase and glass, large amounts of bulk charges will accumulate at the interface, which leads to the increase of the activation energy Ea. Considering the application of pulsed power capacitors, the discharge behavior is critical and must be characterized to calculate the power density of the energy storage capacitors. The discharge behavior of the samples was measured using a specially designed, high-speed capacitor discharge circuit, in which the discharged energy was measured from a load resistor (RL) in series with the sample. As shown in Fig. 7, a vacuum switch was introduced to control the charge and discharge process, the switch was triggered through a DC pulse voltage source. The samples were firstly charged at a specified DC voltage source and then discharged across a load resistor (RL = 100 KX) in series with a resistor (RS = 100 MX), at the same time, the variety of voltage drop UL can be monitored through a voltage monitor device. The discharge performance of the sample was studied by analyzing the variety of voltage drop UL. As we know, the discharge time of capacitors is directly proportional to the value of RC (T / RC). We choose large resistance values of RL and RS to obtain the long discharge time so that the voltage monitor can capture the voltage drop signal UL. The UL value was calculated with the following equation:
S. Xue et al. / Journal of Alloys and Compounds 617 (2014) 418–422
Fig. 7. Schematic circuit diagrams of charging and discharging measurement.
UL ¼ U
RL RL þ RS
ð1Þ
where U represents the charging voltage. During the discharging process, the power density (P) and the discharge energy (Wd) of the capacitor can be calculated with the following equation:
P¼
U 2L RL þ RS RL RL
Wd ¼
Z
Pdt
ð2Þ
ð3Þ
Typical discharged voltage–time (UL–t) curve of the G30 sample under 30 kV/cm is shown in Fig. 8. The UL value with a rapid increase and decrease process is observed, which means the charge and discharge process of the sample. The discharge energy density is obtained through analyzing the UL–t curve. The discharge energy density as a function of time is presented in the inset. The G30 sample exhibits an energy density of 0.0097 J/cm3 under a low field of 30 kV/cm. Furthermore, the discharged energy density increased with the square of E, which suggests that, by increasing the field, a much higher energy density can be achieved before the field reaches BDS. The discharge energy density of the G30, G40, G50 and G60 can be analyzed using the homothetic method. The discharge energy of the samples is the product of the discharge energy density and the volume. However, the discharged energy density measured in glass ceramics exhibited inconsistency with the
421
calculation result from their dielectric constant and dielectric breakdown strength. This problem is attributed to interfacial polarization. In the ferroelectric glass ceramics, the dispersed ferroelectric phase is typically structurally incompatible with the host glass. In addition, the dielectric constant of the ferroelectric ceramic phase and glass is vastly different. Low energy storage density was attributed to the building up of local charge and ions at the interface. The charge energy value (WC) calculated using the formula W C ¼ 12 CU 2 , where C is the capacitance of the sample and U is the charging voltage. For practical applications, it is desirable to not only have a high energy density, but to also maintain a high discharge–charge efficiency (g, the discharge energy/charge energy), since the energy losses in the capacitor leads to heating and, consequently, to detrimental effects on the performance and reliability of the capacitors [21,22]. The discharge performance such as the discharge energy and the efficiency was summarized in Table 1. Fig. 9 shows the activation energy Ea and efficiency for the asdevitrified G30–G60 samples annealed at 1000 °C for 2 h. As shown in Fig. 9, with the increase of the content of glass, an opposite trend is observed between the activation energy Ea and efficiency g. As shown in the former work, the relaxation activation energy Ea obtained from the complex impedance analysis reflects the interfacial mobility in defects where polarizability is modified and where charge and energy localization can occur, which will lead to the low released energy density and efficiency. A higher value of Ea means bad charge flowing behavior, which results in the accumulation of charge at the interface, and leads to low released energy density and efficiency. Therefore, the energy-storage mechanism as well as the efficiency can be qualitatively understood on the base of variation of the activation energy. The activation energy for the G30, G40, G50, G60 samples was 1.19, 1.20, 1.17, and 1.09 eV, respectively. As shown in Fig. 9, with the change of the compositions, an opposite trend is observed between the efficiency and the activation energy Ea. In the specific testing condition, the charge energy density is certain, higher unreleased energy density means lower efficiency. Furthermore, with the increase of glass phase content, the number of crystal particles will be reduced, as a result, the interface number and interfacial area between the crystallized phases and the residual glass also reduced. This
Fig. 8. Typical discharged voltage–time (VL–t) curve and discharged energy density (inset) of the G30 sample under 30 kV/cm.
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Table 1 The discharge performance including the discharge energy and the efficiency for the as-devitrified G30–G60 samples. Sample G30 G40 G50 G60
Capacitance (pF) 391 172 60 24
Discharged energy (J)
Calculated charge energy (J)
4
4
1.73 10 7.70 105 2.76 105 1.11 105
1.96 10 8.60 105 3.00 105 1.20 105
Efficiency (%) 88.3 89.5 92.0 92.5
interface properties within the ferroelectric glass ceramics. With the increase of the glass content, the breakdown strength (BDS) and the charge–discharge efficiency show a clear increasing trend, the typical G60 sample shows a high BDS of 1300 kV/cm and a high charge–discharge efficiency of 92.5%. Acknowledgment The authors would like to acknowledge the support from National Key Fundamental Research Program (2009CB623302). References
Fig. 9. The activation energy Ea and efficiency for the as-devitrified G30–G60 samples.
reduction causes lower content of space charges at the interfaces and subsequent decrease in the interfacial polarization. Thus, the unreleased energy density decreased and the efficiency improved, the typical G60 sample shows a high charge–discharge efficiency of 92.5%. 4. Conclusion The breakdown characteristics and discharge behaviors of (100 x) (40BaO–10Na2O–50Nb2O5)–x(63SiO2–12BaO–16B2O3– 9Al2O3) (x = 30, 40, 50, 60) niobate glass ceramics have been investigated based on the measurement of BDS, complex impedance spectrum and discharged voltage–time curve. Impedance analysis suggests that there is an opposite trend between the dielectric BDS and the activation energy Ea. Furthermore, a homologous trend between the activation energy Ea and discharge efficiency was observed. Experimental results demonstrate that the breakdown characteristics and discharge behaviors are related to the
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Dielectric properties and charge–discharge behaviors in niobate glass ceramics for energy-storage applications Shuangxi Xue a,b, Shaohui Liu a, Wenqin Zhang a, Jinwen Wang a, Linjiang Tang a, Bo Shen a, Jiwei Zhai a,⇑ a b
Functional Materials Research Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, China Department of Physics and Electronic Engineering, Taizhou University, Taizhou 318000, China
a r t i c l e
i n f o
Article history: Received 15 July 2014 Accepted 2 August 2014 Available online 10 August 2014 Keywords: Dielectric properties Dielectric breakdown strength Glass–ceramics
a b s t r a c t A series of niobate glass ceramics with varying glass content have been prepared via controlled-crystallization route, and the dielectric properties, the breakdown characteristics and charge–discharge behaviors have been investigated. The correlation between the dielectric breakdown performance and the activation energy was studied by the measurements of the dielectric breakdown strength and impedance spectroscopy. The charge–discharge efficiency was investigated using a charge–discharge system with high-speed capacitor discharge circuit device. It was found that the dielectric breakdown strength and the discharge efficiency strongly depend on the interface polarization based on the results of complex impedance analysis. With the increase of the glass content, the breakdown strength (BDS) and the charge–discharge efficiency show a clear increasing trend, when the content of glass was 60%, the typical G60 sample shows a high BDS of 1300 kV/cm and a high charge–discharge efficiency of 92.5%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction High density energy storage materials and the related devices have attracted increasing attention since the energy crisis became more serious in the new century. There are growing needs for energy storage devices that are high energy density, low cost, and environmentally friendly, for power electronics and pulsed power applications. In order to reach a high energy density, characteristics such as high dielectric constant, high breakdown strength and low dielectric loss for materials are needed. The traditional ferroelectric ceramic has been used for capacitors due to their high dielectric constants, but the low breakdown strength (BDS) limits its further application [1–4]. In the search for new high energy density materials, a significant challenge is to increase dielectric constant while maintaining high breakdown strength. Glass–ceramic dielectrics have been considered as the most promising candidate for energy storage capacitors, because they have high dielectric constant (from the precipitated crystalline phases) and high breakdown strength (due to the pore-free inherence) [5–9]. Different types of ferroelectric glass–ceramics such as (Ba, Sr)TiO3 and (Ba, Sr, Pb)TiO3 have been widely studied [10–14]. The dielectric constant of ferroelectric strongly varies with temperature
⇑ Corresponding author. Tel.: +86 21 65980544; fax: +86 21 65985179. E-mail address: [email protected] (J. Zhai). http://dx.doi.org/10.1016/j.jallcom.2014.08.006 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
near the Curie temperature. As the Curie temperature is approached, the dielectric constant increases rapidly as dipole orientation in the direction of the field can be more extensive. The dielectric constant decreases rapidly above the Curie temperature as the phase has become paraelectric phase. Thus, (Ba, Sr)TiO3 based ferroelectric capacitors with Curie temperatures near ambient temperature suffer unacceptable dielectric constant changes with small temperature shifts. For example, BaTiO3 has a Curie temperature of 120 °C; the dielectric constant shifts from 1250 at room temperature to 1750 at about 100 °C [15,16]. Niobate-based glass–ceramics is a promising candidate for overcoming the temperature variation of the dielectric constant due to their high Curie temperature which significantly higher than room temperature. For example, the Curie temperature of single crystal orthorhombic Ba2NaNb5O15 (BNN) is 600 °C [17]. In our recent work (unpublished), lead-free (4BaO–Na2O– 5Nb2O5)–SiO2 is a promising system in which Ba2NaNb5O15 as the main crystalline phase has high dielectric constants and high Curie temperature (about 600 °C), however, the dielectric breakdown strength was still not high enough. Alkali-free barium boroaluminosilicate glass has been proved in experiments with ultra high dielectric breakdown field strength. Smith found SchottAF45 glass (SchottAF-45; Schott Technologies Inc. The approximate bulk composition with 63% SiO2, 12% BaO, 16% B2O3 and 9% Al2O3 has remarkably high DC dielectric breakdown strength (12 MV/cm) [18]. In this work, barium boroaluminosilicate glass
S. Xue et al. / Journal of Alloys and Compounds 617 (2014) 418–422
with the same composition with SchottAF-45 glass was used as amorphous matrix, (40% BaO, 10% Na2O, 50% Nb2O5)–(63% SiO2, 12% BaO, 16% B2O3, 9% Al2O3) glass–ceramics (BNN) with varying glass content were prepared through the traditional glass preparation process and crystallization route. Many investigations on glass–ceramics with high dielectric constant or enhanced BDS have been reported. Considering the application of pulsed power capacitors, the breakdown characteristics and charge–discharge behavior of the glass–ceramics (such as the discharge efficiency, the power density, and the energy density) were important parameters to evaluate the capacitors. However, there is much less information available about the charge–discharge behavior of BNN glass– ceramics, the discharge behavior of the BNN based glass–ceramics were evaluated using a high-speed capacitor discharge circuit device designed by ourselves.
419
Fig. 1 shows XRD patterns of the as-devitrified samples heated at 1100 °C for 2 h. By comparing the XRD patterns with interna-
tional centre for diffraction data (ICDD), two phases of tetragonal tungsten bronze (T.T.B.) and cubic perovskite (C.P.) were indexed, the crystalline phase with T.T.B. structure is related to Ba2NaNb5O15 and the C.P. structure is related to NaNbO3. The dielectric constant of the as-devitrified samples as a function of temperature is shown in Fig. 2. All samples shows stable dielectric constant over the temperature range of 150 °C to 150 °C. The dielectric constant of the as-devitrified samples decreases with the increase of the content of glass phase, which is attributed to the lower dielectric constant. Dielectric breakdown data are commonly interpreted via Weibull plots due to the inherently statistical nature of failure and its mediation through a variety of intrinsic and extrinsic factors. The Weibull distribution was described in detail elsewhere [19]. A 2-parameters Weibull distribution should be a line on the coordinate axis. The slope of the line is the shape parameter b, and the intercept on the x-axis is ln E. Where E is the BDS. Fig. 3 illustrates the Weibull distributions of the BDS for the as-devitrified G30–G60 samples annealed at 1100 °C for 2 h. In our case, as can be seen in Fig. 3, all the plots show a relatively good linearity and the values of shape parameter b for the four samples all are higher than eight, thus a valid comparison of characteristic BDS is possible. As shown in Fig. 3, all samples exhibit high BDS value and the BDS value of G60 is significantly higher than that of other samples. With the increase of glass content, the BDS increased to 1300 kV/cm for the G60 sample. It is well known that electrical properties of glass–ceramics are strongly affected by their microstructure. As a non-destructive analysis technique, an impedance spectrum that accurately reflects the microstructure features is usually the best tool to analyze their electrical properties. In this study, impedance analysis is introduced to analysis the electrical properties of the samples. The samples are measured at different temperatures to get a series of Cole–Cole images and Z 00 spectroscopic plots. Fig. 4 illustrates impedance spectra of the as-devitrified G30 sample measured at different temperatures, the inset shows the impedance Z 00 vs the measuring frequency. Below 520 °C the semicircles cannot be observed because of the extra high resistivity of this glass ceramic system. It can be seen that the impedance semicircles became smaller and the relaxation frequency at the extreme point of Z 00 spectroscopic plots increased with the increasing of temperature. Details of the impedance analysis and the activation energy (Ea) can be found elsewhere [2,20]. The reciprocal value of measurement frequency at the extreme point of each semicircle represents their relaxation time s of the interfacial polarization process at a certain measurement temperature. The ln s 1000/T plots can be obtained from Fig. 4, which obey Arrhenius relationship:
Fig. 1. XRD patterns of the as-devitrified samples heated at 1100 °C for 2 h. (a) NaNbO3(ICDD: 01-074-2441), (b) Ba2NaNb5O15 (ICDD: 01-086-0739).
Fig. 2. Temperature dependence of dielectric constant of G30–G60 samples annealed at 1100 °C for 2 h.
2. Experimental procedures The BNN glass–ceramics used in this study were prepared from the well-mixed powders of analytical reagent grade BaCO3, NaCO3, Nb2O5, B2O3, Al2O3 to achieve the different glass compositions, represented by a chemical formula (100 x) (40BaO–10Na2O–50Nb2O5)–x(63SiO2–12BaO–16B2O3–9Al2O3) (x = 30, 40, 50, 60) (mole fraction). When the glass content was 30%, 40%, 50%, 60%, the corresponding sample names were G30, G40, G50, G60. Analytical reagent grade powders of Na2CO3, BaCO3, Nb2O5, SiO2, B2O3 and Al2O3 were ball mixed for 24 h in a high density polyethylene bottle with ethanol as milling media for homogenous mixing. After drying at 100 °C for 12 h, the powders were placed in an alumina crucible and melted at 1500 °C for 3 h to obtain the homogeneous liquid and the liquid was manually poured into a preheated copper mold to form glass bulks followed by quickly placed in an annealing oven at 550 °C to reduce the residual stress. The transparent glasses were cut into 6 mm 6 mm 1 mm sheets, and annealed in air by heating at 3 °C/min to 1100 °C for 2 h. X-ray diffraction (D8 Advance, Bruker AXS, Germany) were used to investigate the phase evolution. The temperature dependences of dielectric constant were measured using a LCR meter (AgilentE4980A) at the frequency of 1 MHz and in the temperature range from 50 °C to 100 °C. The DC dielectric breakdown strength (BDS) measurement was performed using a voltage-withstand testing device (ET2671B, Entai, Nanjing, China) at room temperature. A DC voltage ramp of about 1 kV/s was applied to the specimens until the dielectric breakdown occurred. At least 8 specimens were used for each composition during BDS testing. All samples were ground into about 0.15 mm thickness and immersed in silicone oil to prevent surface flashover. The impedance data were measured by a LCR meter (Agilent E4980A) over frequencies from 150 Hz to 2 MHz in a temperature range of 400–540 °C with an AC electric field of 4 V/mm. The charge–discharge behavior of the BNN based glass–ceramics were evaluated using a high-speed capacitor discharge circuit device designed by ourselves.
3. Results and discussion
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Fig. 3. Weibull plots of dielectric break down strength of the as-devitrified samples.
Fig. 5. Relaxation time as a function of 1000/T for the as-devitrified samples. The solid lines are fitting curves through the data.
Fig. 4. Impedance spectra of the as-devitrified G30, G40, G50, G60 sample measured at different temperature. Inset: Z00 spectroscopic plots at various temperature.
Fig. 6. Dielectric BDS and activation energy Ea for the as-devitrified samples.
ln s ¼
1 Ea þ ln s0 KBT
where Ea is the activation energy for the relaxation process, KB is the Boltzmann constant and T is the absolute temperature. The values of Ea for the samples can be calculated from the slope of the function between relaxation time (the reciprocal value of the relaxation frequency) and measuring temperature, which is shown in Fig. 5. In this study, the associated relaxation processes originated from space charge at grain boundaries. In our measurement condition the activation energy Ea corresponds the relaxation of space charges, it reflects the interfacial mobility and a higher value of Ea means bad charge flowing behavior, and results in the accumulation of charge at the interface, which leads to a low BDS value. As seen from Fig. 5, the Ea values for the samples were 1.19 eV (G30), 1.20 eV (G40), 1.17 eV (G50), 1.09 eV (G60). The BDS and the activation energy Ea for the glass ceramics were shown in Fig. 6. As the content of glass phase increase from 30% to 50%, the BDS value increases sharply and then increases gradually when the content of glass increase from 50% to 60%. The resistance of glass phase is far higher than that of the ceramic phase, the increase of glass phase will decrease the conductivity of the glass ceramic system, which will result in the high BDS value. Furthermore, the glass will restrain the growth of crystalline grain during the crystallization process, small and uniform particle size will be helpful to improve BDS. Consequently, it is easy to
understand the BDS value increases with the content of glass. However, with the reduction of the glass content, more crystallized particles will be formed from the residual glass and the amount of interfaces will increase. Because of the significant difference in dielectric constant of ferroelectric phase and glass, large amounts of bulk charges will accumulate at the interface, which leads to the increase of the activation energy Ea. Considering the application of pulsed power capacitors, the discharge behavior is critical and must be characterized to calculate the power density of the energy storage capacitors. The discharge behavior of the samples was measured using a specially designed, high-speed capacitor discharge circuit, in which the discharged energy was measured from a load resistor (RL) in series with the sample. As shown in Fig. 7, a vacuum switch was introduced to control the charge and discharge process, the switch was triggered through a DC pulse voltage source. The samples were firstly charged at a specified DC voltage source and then discharged across a load resistor (RL = 100 KX) in series with a resistor (RS = 100 MX), at the same time, the variety of voltage drop UL can be monitored through a voltage monitor device. The discharge performance of the sample was studied by analyzing the variety of voltage drop UL. As we know, the discharge time of capacitors is directly proportional to the value of RC (T / RC). We choose large resistance values of RL and RS to obtain the long discharge time so that the voltage monitor can capture the voltage drop signal UL. The UL value was calculated with the following equation:
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Fig. 7. Schematic circuit diagrams of charging and discharging measurement.
UL ¼ U
RL RL þ RS
ð1Þ
where U represents the charging voltage. During the discharging process, the power density (P) and the discharge energy (Wd) of the capacitor can be calculated with the following equation:
P¼
U 2L RL þ RS RL RL
Wd ¼
Z
Pdt
ð2Þ
ð3Þ
Typical discharged voltage–time (UL–t) curve of the G30 sample under 30 kV/cm is shown in Fig. 8. The UL value with a rapid increase and decrease process is observed, which means the charge and discharge process of the sample. The discharge energy density is obtained through analyzing the UL–t curve. The discharge energy density as a function of time is presented in the inset. The G30 sample exhibits an energy density of 0.0097 J/cm3 under a low field of 30 kV/cm. Furthermore, the discharged energy density increased with the square of E, which suggests that, by increasing the field, a much higher energy density can be achieved before the field reaches BDS. The discharge energy density of the G30, G40, G50 and G60 can be analyzed using the homothetic method. The discharge energy of the samples is the product of the discharge energy density and the volume. However, the discharged energy density measured in glass ceramics exhibited inconsistency with the
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calculation result from their dielectric constant and dielectric breakdown strength. This problem is attributed to interfacial polarization. In the ferroelectric glass ceramics, the dispersed ferroelectric phase is typically structurally incompatible with the host glass. In addition, the dielectric constant of the ferroelectric ceramic phase and glass is vastly different. Low energy storage density was attributed to the building up of local charge and ions at the interface. The charge energy value (WC) calculated using the formula W C ¼ 12 CU 2 , where C is the capacitance of the sample and U is the charging voltage. For practical applications, it is desirable to not only have a high energy density, but to also maintain a high discharge–charge efficiency (g, the discharge energy/charge energy), since the energy losses in the capacitor leads to heating and, consequently, to detrimental effects on the performance and reliability of the capacitors [21,22]. The discharge performance such as the discharge energy and the efficiency was summarized in Table 1. Fig. 9 shows the activation energy Ea and efficiency for the asdevitrified G30–G60 samples annealed at 1000 °C for 2 h. As shown in Fig. 9, with the increase of the content of glass, an opposite trend is observed between the activation energy Ea and efficiency g. As shown in the former work, the relaxation activation energy Ea obtained from the complex impedance analysis reflects the interfacial mobility in defects where polarizability is modified and where charge and energy localization can occur, which will lead to the low released energy density and efficiency. A higher value of Ea means bad charge flowing behavior, which results in the accumulation of charge at the interface, and leads to low released energy density and efficiency. Therefore, the energy-storage mechanism as well as the efficiency can be qualitatively understood on the base of variation of the activation energy. The activation energy for the G30, G40, G50, G60 samples was 1.19, 1.20, 1.17, and 1.09 eV, respectively. As shown in Fig. 9, with the change of the compositions, an opposite trend is observed between the efficiency and the activation energy Ea. In the specific testing condition, the charge energy density is certain, higher unreleased energy density means lower efficiency. Furthermore, with the increase of glass phase content, the number of crystal particles will be reduced, as a result, the interface number and interfacial area between the crystallized phases and the residual glass also reduced. This
Fig. 8. Typical discharged voltage–time (VL–t) curve and discharged energy density (inset) of the G30 sample under 30 kV/cm.
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Table 1 The discharge performance including the discharge energy and the efficiency for the as-devitrified G30–G60 samples. Sample G30 G40 G50 G60
Capacitance (pF) 391 172 60 24
Discharged energy (J)
Calculated charge energy (J)
4
4
1.73 10 7.70 105 2.76 105 1.11 105
1.96 10 8.60 105 3.00 105 1.20 105
Efficiency (%) 88.3 89.5 92.0 92.5
interface properties within the ferroelectric glass ceramics. With the increase of the glass content, the breakdown strength (BDS) and the charge–discharge efficiency show a clear increasing trend, the typical G60 sample shows a high BDS of 1300 kV/cm and a high charge–discharge efficiency of 92.5%. Acknowledgment The authors would like to acknowledge the support from National Key Fundamental Research Program (2009CB623302). References
Fig. 9. The activation energy Ea and efficiency for the as-devitrified G30–G60 samples.
reduction causes lower content of space charges at the interfaces and subsequent decrease in the interfacial polarization. Thus, the unreleased energy density decreased and the efficiency improved, the typical G60 sample shows a high charge–discharge efficiency of 92.5%. 4. Conclusion The breakdown characteristics and discharge behaviors of (100 x) (40BaO–10Na2O–50Nb2O5)–x(63SiO2–12BaO–16B2O3– 9Al2O3) (x = 30, 40, 50, 60) niobate glass ceramics have been investigated based on the measurement of BDS, complex impedance spectrum and discharged voltage–time curve. Impedance analysis suggests that there is an opposite trend between the dielectric BDS and the activation energy Ea. Furthermore, a homologous trend between the activation energy Ea and discharge efficiency was observed. Experimental results demonstrate that the breakdown characteristics and discharge behaviors are related to the
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