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Improved discharged energy density for niobate-based B2O3 system glass–ceramics by CeO2 addition
Materials Letters 136 (2014) 302–305
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improved discharged energy density for niobate-based B2O3 system glass–ceramics by CeO2 addition Guohua Chen n, Jun Song, Xiaoling Kang, Changlai Yuan, Changrong Zhou School of Materials Science and Engineering, Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 June 2014 Accepted 12 August 2014 Available online 21 August 2014
Strontium barium niobate-based borate system glass–ceramics with CeO2 addition were prepared via a controlled crystallization method. And the structural, dielectric and ferroelectric properties were investigated. The results showed that the moderate addition of CeO2 optimized the microstructure and improved permittivity, breakdown strength and discharged energy density of the glass–ceramics. A maximum discharged energy density of 1.95 J/cm3 was achieved in the glass–ceramics with 0.5 mol% CeO2, which was improved by 2.1 times as compared with that of the pure glass ceramics (0.95 J/cm3). & 2014 Elsevier B.V. All rights reserved.
Keywords: Ceramics Dielectrics Electrical properties
1. Introduction The dielectric materials with high discharged energy density (DED) have attracted great attention due to their significant role in driving miniaturization, realizing high volumetric efficiency and cost reduction in power electronic and pulse power applications [1]. Generally, the DED of a dielectric material is equal to the integral RP U d ¼ Prmax E dP, where E is the strength of electric field and P is the polarization. Pmax and Pr represent the maximum and remanent polarization from the P–E hysteresis loop [2]. Therefore, besides enhancing the breakdown strength (BDS), enlarging the polarization difference between Pmax and Pr is another key factor in achieving a high DED. In general, the application of ceramic dielectrics in high voltage was limited by several factors, such as the lower BDS caused by interior defect of pores during the sintering route and their high remanent polarization leading to a very small polarization difference in the P–E loop. Thus, ceramic dielectrics were usually selected as the medium voltage capacitors because of their extremely high permittivity and more porosity [3]. In contrast, glass–ceramic dielectrics having high permittivity provided by the ferroelectric grains, high BDS from the near pore-free microstructure of the glassy matrices and high DED [4] can be considered as a promising candidate in the fields of high-voltage and high-energy density applications [5]. There have been a large number of investigations of glass–ceramic dielectrics containing ferroelectric phases [6–8]. In our previous studies, we found that the changes in the Sr/Ba ratio (Sr/Ba ratio¼ x) in xSrO–(32 x) BaO–32Nb2O5–36B2O3 system (mol%, x¼8.0, 11.2, 14.4, 17.6, 20.8, and 24.0) dramatically affected the microstructure and n
Corresponding author. Tel.: þ 86 7732291957; fax: þ 86 7732191903. E-mail addresses: [email protected], [email protected] (G. Chen).
http://dx.doi.org/10.1016/j.matlet.2014.08.067 0167-577X/& 2014 Elsevier B.V. All rights reserved.
dielectric properties. When x¼14.4, the glass–ceramic with the composition of 14.4SrO–17.6BaO–32Nb2O5–36B2O3 has the maximum theoretical energy-storage density of 5.71 J/cm3 and actual energy density of 1.01 J/cm3 calculated from P–E hysteresis loops under applied electric field of 600 kV/cm [8]. Moreover, we also discovered that adding 4.4 mol% CeO2 to the glass–ceramics obviously decreased the activation energy of crystallization and enhanced the dielectric constant as well as BDS remarkably in the strontium barium niobate-based SiO2 system [5]. To our knowledge, up to now, there is no report about the effect of CeO2 addition on microstructure and energy-storage density in the strontium barium niobate-based B2O3 system glass–ceramics. In this work, attempts were made to achieve further improvement in the microstructure, dielectric properties and DED in 14.4SrO–17.6BaO–32Nb2O5–36B2O3 system glass–ceramics by adding CeO2.
2. Experimental procedure The nominal composition of the glass–ceramics was 14.4SrO– 17.6BaO–32Nb2O5–36B2O3–xCeO2 (mol%, where x¼ 0–2). Powders of analytical reagent grade comprising SrCO3, BaCO3, Nb2O5, H3BO3, and CeO2 were used as starting materials. The wellmixed powders were melted in a corundum crucible in air at 1350 1C for 30 min and then quenched by pouring into a preheated stainless-steel mold. The as-quenched glass sheets were annealed at 550 1C for 10 h to remove residual stresses. Then the glass samples were heated in air at 700–850 1C for 3 h to make glass–ceramics. X-ray diffraction (XRD) (Model-D8 Advance, Bruker, Germany) and field emission scanning electron microscopy (FE-SEM, Model
G. Chen et al. / Materials Letters 136 (2014) 302–305
S-4800, Hitachi, Japan) were used to investigate the phase evolution and microstructure of the crystallized samples. For microstructure observation, the glass–ceramic specimens were polished and etched with 1 wt% HF solution, and platinum film was sputtered on surface of one side. For electrical measurements, the glass–ceramic samples were polished by using an automatic
303
polishing machine (Model UNIPOL-802, Shenyang Kejing Autoinstrument Company, China) to achieve parallel, smooth faces, and silver electrodes of 10 mm in diameter were sputtered on both faces through magnetic sputtering. The measurements of dielectric property, BDS and P–E hysteresis loops were performed according to the literature [8].
Fig. 1. XRD patterns (a) and SEM images (b) of glass–ceramics added with various contents of CeO2 addition heated at 850 1C for 3 h. Inset shows enlarged XRD pattern at 2θ of 461.
304
G. Chen et al. / Materials Letters 136 (2014) 302–305
3. Results and discussion Fig. 1(a) shows the XRD profiles of the glass–ceramics with CeO2 addition. The main crystalline phase Sr0.5Ba0.5Nb2O6 (SBN, JCPDS#39-0265) with tetragonal tungsten bronze (T.T.B.) structure and secondary phase SrNb2O6 (SN, JCPDS#37-1039) with orthorhombic structure were formed for CeO2-undoped sample. All the other compositions could be indexed based on SBN phase. The addition of CeO2 inhibits the formation of SN, which indicates that adding CeO2 lowers the activation energy of crystallization and enhances the precipitation of SBN crystals [5]. With the increase of CeO2, the diffraction peaks slightly shift to a higher angle (Inset in Fig. 1). The ionic radius of Ce3 þ (1.07 Å, 6CN) is smaller than that of Sr2 þ (1.12 Å, 6CN) or Ba2 þ (1.34 Å, 6CN) [9], So, Ce3 þ is prone to occupy both A-sites in T.T.B structure where Sr2 þ occupies. Therefore, the substitution of Ce3 þ for Sr2 þ leads to the decrease of the unit cell volume, which causes the increasing diffraction angle. SEM images of the samples are shown in Fig. 1(b). The un-doped glass–ceramic exhibits long rod-like feature of crystal grains and more porosity. Glass–ceramic with 0.5 mol% CeO2 possesses more uniform and dense microstructure with fine grains of 50–100 nm (Fig. 1(b)). However, with further increase to 2 mol% CeO2, the shape of grains changes from rod-like to short-acicular accompanying more porosity. Fig. 2 depicts the dielectric constant and loss tangent and Weibull plots of DBS of glass–ceramics with various contents of CeO2 addition. With the increase in CeO2 contents, the dielectric constant first increases and then decreases, and the maximum permittivity of 141 is achieved for 0.5 mol% CeO2 added glass– ceramics (Fig. 2(a)). From Fig. 1(a), this trend that the intensities of
700 750 800 850
C C C C
0mol% 0.5mol% 1mol% 1.5 mol% 2 mol%
0mol% = kV/cm 0.5mol% kV/cm
the main diffraction peaks first increase and then decrease with increasing contents of CeO2 can be observed. The strongest diffraction peaks occur as CeO2 ¼ 0.5 mol%. This means that adding small quantity of CeO2 (0.5 mol%) can promote the precipitation of Sr0.5Ba0.5Nb2O6. However, the addition of overfull CeO2 (40.5 mol%) may inhibit the formation of Sr0.5Ba0.5Nb2O6 [5]. The ionic polarizability of Ce3 þ ( 6.15 Å3) is bigger than that of Sr2 þ (4.24 Å3) [10], so the dielectric constant (εr) increases with adding moderate amounts of CeO2 as shown in Fig. 2(a). Hence, the dielectric constant is most likely related to the amounts of Sr0.5Ba0.5Nb2O6 crystal phases, the inner polarization enhanced by CeO2 doping and uniform dense microstructure [5,10]. The variation in loss tangent can be associated with the strengthening relaxation effect caused by Ce3 þ doping during a polarization process [11]. As a whole, the values of dielectric loss keep very low level. The maximum of average BDS can be achieved for glass–ceramic with 0.5 mol% CeO2, which is attributed to its optimized uniform dense fine-grain microstructure shown in Fig. 1(b). Fig. 3(a) shows the P–E hysteresis loops for the glass–ceramics. The electric field applied for all samples is 600 kV/cm apart from the breakdown field. The polarization shows a remarkable increase and then slight decreases with increment contents of CeO2, which is agreement with the dielectric constant as shown in Fig. 2(a). The value of remanent polarization increases from 0.44 to 0.97 μC/cm2 and then decreases to 0.60 μC/cm2 with increase contents of CeO2. The DED of the glass ceramics is shown in
1mol%
kV/cm
0 mol% 0.5mol% 1 mol% 1.5mol% 2 mol%
1.5mol% kV/cm 2mol% kV/cm
Fig. 2. Dielectric constant and loss tangent (a) and Weibull plots of DBS (b) of the glass–ceramics with various contents of CeO2 additives.
Fig. 3. P–E hysteresis loops (a) and discharged energy density (b) of the CeO2added glass–ceramics heat treated at 850 1C for 3 h as a function of the electric field applied.
G. Chen et al. / Materials Letters 136 (2014) 302–305
Fig. 3(b). It can be seen that the DED undergoes an increase and then decreases with CeO2 addition. The pure glass–ceramic has a relatively low DED of 0.94 J/cm3. The DED reaches its maximum of 1.95 J/cm3 for 0.5 mol% CeO2 added glass–ceramics. It can be attributed to following factors: first, the addition of CeO2 enhances the BDS to which the optimized structure contributes. Second, the polarization difference between maximum and remanent polarization is enlarged by adding CeO2.
305
Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC no. 51162002), Science and Technology Project of Guangxi Returned Personnel (Contract no. 2012-250) and Patent Project of Guangxi Department of Education (ZL2014015).
References 4. Conclusion A series of strontium barium niobate-based B2O3 system glass– ceramics with CeO2 addition were successfully prepared by the conventional melting method. By adding moderate contents of CeO2, the microstructures and dielectric properties are improved significantly. Typically, the glass–ceramic with 0.5 mol% CeO2 shows enhanced BDS of 1092.5 kV/mm and DED of 1.95 J/cm3, suggesting the as-prepared glass–ceramics could be a promising candidate for high-energy density capacitors.
[1] Shen ZY, Hu QG, Li YM, Wang ZM, Luo WQ, Hong Y, et al. J Am Ceram Soc 2013;96:2551. [2] Chu BJ, Zhou X, Ren KL, Neese B, Lin M, Wang Q, et al. Science 2006;313:334. [3] Fletcher NH, Hilton AD, Ricketts BW. J Phys D: Appl Phys 1996;29:253. [4] Gorzkowski EP, Pan MJ, Bender BA, Wu CCM. J Am Ceram Soc 2008;91:1065. [5] Liu TY, Chen GH, Song J, Yuan CL. Ceram Int 2013;39:5553. [6] Wang JW, Tang LJ, Shen B, Zhai JW. Ceram Int 2014;40:2261. [7] Zeng YK, Qin XY, Jiang SL, Zhang GZ, Zhang L. J Am Ceram Soc 2011;94:469. [8] Song J, Chen GH, Yuan CL, Yang Y. Mater Lett 2014;117:7. [9] Shannon RD, Prewit CT. Acta Crystallogr B 1969;25:925. [10] Shannon RD. J Appl Phys 1993;73:348. [11] Cole MW, Nothwang WD, Hubbard C, Ngo E, Ervin M. J Appl Phys 2003;93:9218.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improved discharged energy density for niobate-based B2O3 system glass–ceramics by CeO2 addition Guohua Chen n, Jun Song, Xiaoling Kang, Changlai Yuan, Changrong Zhou School of Materials Science and Engineering, Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 June 2014 Accepted 12 August 2014 Available online 21 August 2014
Strontium barium niobate-based borate system glass–ceramics with CeO2 addition were prepared via a controlled crystallization method. And the structural, dielectric and ferroelectric properties were investigated. The results showed that the moderate addition of CeO2 optimized the microstructure and improved permittivity, breakdown strength and discharged energy density of the glass–ceramics. A maximum discharged energy density of 1.95 J/cm3 was achieved in the glass–ceramics with 0.5 mol% CeO2, which was improved by 2.1 times as compared with that of the pure glass ceramics (0.95 J/cm3). & 2014 Elsevier B.V. All rights reserved.
Keywords: Ceramics Dielectrics Electrical properties
1. Introduction The dielectric materials with high discharged energy density (DED) have attracted great attention due to their significant role in driving miniaturization, realizing high volumetric efficiency and cost reduction in power electronic and pulse power applications [1]. Generally, the DED of a dielectric material is equal to the integral RP U d ¼ Prmax E dP, where E is the strength of electric field and P is the polarization. Pmax and Pr represent the maximum and remanent polarization from the P–E hysteresis loop [2]. Therefore, besides enhancing the breakdown strength (BDS), enlarging the polarization difference between Pmax and Pr is another key factor in achieving a high DED. In general, the application of ceramic dielectrics in high voltage was limited by several factors, such as the lower BDS caused by interior defect of pores during the sintering route and their high remanent polarization leading to a very small polarization difference in the P–E loop. Thus, ceramic dielectrics were usually selected as the medium voltage capacitors because of their extremely high permittivity and more porosity [3]. In contrast, glass–ceramic dielectrics having high permittivity provided by the ferroelectric grains, high BDS from the near pore-free microstructure of the glassy matrices and high DED [4] can be considered as a promising candidate in the fields of high-voltage and high-energy density applications [5]. There have been a large number of investigations of glass–ceramic dielectrics containing ferroelectric phases [6–8]. In our previous studies, we found that the changes in the Sr/Ba ratio (Sr/Ba ratio¼ x) in xSrO–(32 x) BaO–32Nb2O5–36B2O3 system (mol%, x¼8.0, 11.2, 14.4, 17.6, 20.8, and 24.0) dramatically affected the microstructure and n
Corresponding author. Tel.: þ 86 7732291957; fax: þ 86 7732191903. E-mail addresses: [email protected], [email protected] (G. Chen).
http://dx.doi.org/10.1016/j.matlet.2014.08.067 0167-577X/& 2014 Elsevier B.V. All rights reserved.
dielectric properties. When x¼14.4, the glass–ceramic with the composition of 14.4SrO–17.6BaO–32Nb2O5–36B2O3 has the maximum theoretical energy-storage density of 5.71 J/cm3 and actual energy density of 1.01 J/cm3 calculated from P–E hysteresis loops under applied electric field of 600 kV/cm [8]. Moreover, we also discovered that adding 4.4 mol% CeO2 to the glass–ceramics obviously decreased the activation energy of crystallization and enhanced the dielectric constant as well as BDS remarkably in the strontium barium niobate-based SiO2 system [5]. To our knowledge, up to now, there is no report about the effect of CeO2 addition on microstructure and energy-storage density in the strontium barium niobate-based B2O3 system glass–ceramics. In this work, attempts were made to achieve further improvement in the microstructure, dielectric properties and DED in 14.4SrO–17.6BaO–32Nb2O5–36B2O3 system glass–ceramics by adding CeO2.
2. Experimental procedure The nominal composition of the glass–ceramics was 14.4SrO– 17.6BaO–32Nb2O5–36B2O3–xCeO2 (mol%, where x¼ 0–2). Powders of analytical reagent grade comprising SrCO3, BaCO3, Nb2O5, H3BO3, and CeO2 were used as starting materials. The wellmixed powders were melted in a corundum crucible in air at 1350 1C for 30 min and then quenched by pouring into a preheated stainless-steel mold. The as-quenched glass sheets were annealed at 550 1C for 10 h to remove residual stresses. Then the glass samples were heated in air at 700–850 1C for 3 h to make glass–ceramics. X-ray diffraction (XRD) (Model-D8 Advance, Bruker, Germany) and field emission scanning electron microscopy (FE-SEM, Model
G. Chen et al. / Materials Letters 136 (2014) 302–305
S-4800, Hitachi, Japan) were used to investigate the phase evolution and microstructure of the crystallized samples. For microstructure observation, the glass–ceramic specimens were polished and etched with 1 wt% HF solution, and platinum film was sputtered on surface of one side. For electrical measurements, the glass–ceramic samples were polished by using an automatic
303
polishing machine (Model UNIPOL-802, Shenyang Kejing Autoinstrument Company, China) to achieve parallel, smooth faces, and silver electrodes of 10 mm in diameter were sputtered on both faces through magnetic sputtering. The measurements of dielectric property, BDS and P–E hysteresis loops were performed according to the literature [8].
Fig. 1. XRD patterns (a) and SEM images (b) of glass–ceramics added with various contents of CeO2 addition heated at 850 1C for 3 h. Inset shows enlarged XRD pattern at 2θ of 461.
304
G. Chen et al. / Materials Letters 136 (2014) 302–305
3. Results and discussion Fig. 1(a) shows the XRD profiles of the glass–ceramics with CeO2 addition. The main crystalline phase Sr0.5Ba0.5Nb2O6 (SBN, JCPDS#39-0265) with tetragonal tungsten bronze (T.T.B.) structure and secondary phase SrNb2O6 (SN, JCPDS#37-1039) with orthorhombic structure were formed for CeO2-undoped sample. All the other compositions could be indexed based on SBN phase. The addition of CeO2 inhibits the formation of SN, which indicates that adding CeO2 lowers the activation energy of crystallization and enhances the precipitation of SBN crystals [5]. With the increase of CeO2, the diffraction peaks slightly shift to a higher angle (Inset in Fig. 1). The ionic radius of Ce3 þ (1.07 Å, 6CN) is smaller than that of Sr2 þ (1.12 Å, 6CN) or Ba2 þ (1.34 Å, 6CN) [9], So, Ce3 þ is prone to occupy both A-sites in T.T.B structure where Sr2 þ occupies. Therefore, the substitution of Ce3 þ for Sr2 þ leads to the decrease of the unit cell volume, which causes the increasing diffraction angle. SEM images of the samples are shown in Fig. 1(b). The un-doped glass–ceramic exhibits long rod-like feature of crystal grains and more porosity. Glass–ceramic with 0.5 mol% CeO2 possesses more uniform and dense microstructure with fine grains of 50–100 nm (Fig. 1(b)). However, with further increase to 2 mol% CeO2, the shape of grains changes from rod-like to short-acicular accompanying more porosity. Fig. 2 depicts the dielectric constant and loss tangent and Weibull plots of DBS of glass–ceramics with various contents of CeO2 addition. With the increase in CeO2 contents, the dielectric constant first increases and then decreases, and the maximum permittivity of 141 is achieved for 0.5 mol% CeO2 added glass– ceramics (Fig. 2(a)). From Fig. 1(a), this trend that the intensities of
700 750 800 850
C C C C
0mol% 0.5mol% 1mol% 1.5 mol% 2 mol%
0mol% = kV/cm 0.5mol% kV/cm
the main diffraction peaks first increase and then decrease with increasing contents of CeO2 can be observed. The strongest diffraction peaks occur as CeO2 ¼ 0.5 mol%. This means that adding small quantity of CeO2 (0.5 mol%) can promote the precipitation of Sr0.5Ba0.5Nb2O6. However, the addition of overfull CeO2 (40.5 mol%) may inhibit the formation of Sr0.5Ba0.5Nb2O6 [5]. The ionic polarizability of Ce3 þ ( 6.15 Å3) is bigger than that of Sr2 þ (4.24 Å3) [10], so the dielectric constant (εr) increases with adding moderate amounts of CeO2 as shown in Fig. 2(a). Hence, the dielectric constant is most likely related to the amounts of Sr0.5Ba0.5Nb2O6 crystal phases, the inner polarization enhanced by CeO2 doping and uniform dense microstructure [5,10]. The variation in loss tangent can be associated with the strengthening relaxation effect caused by Ce3 þ doping during a polarization process [11]. As a whole, the values of dielectric loss keep very low level. The maximum of average BDS can be achieved for glass–ceramic with 0.5 mol% CeO2, which is attributed to its optimized uniform dense fine-grain microstructure shown in Fig. 1(b). Fig. 3(a) shows the P–E hysteresis loops for the glass–ceramics. The electric field applied for all samples is 600 kV/cm apart from the breakdown field. The polarization shows a remarkable increase and then slight decreases with increment contents of CeO2, which is agreement with the dielectric constant as shown in Fig. 2(a). The value of remanent polarization increases from 0.44 to 0.97 μC/cm2 and then decreases to 0.60 μC/cm2 with increase contents of CeO2. The DED of the glass ceramics is shown in
1mol%
kV/cm
0 mol% 0.5mol% 1 mol% 1.5mol% 2 mol%
1.5mol% kV/cm 2mol% kV/cm
Fig. 2. Dielectric constant and loss tangent (a) and Weibull plots of DBS (b) of the glass–ceramics with various contents of CeO2 additives.
Fig. 3. P–E hysteresis loops (a) and discharged energy density (b) of the CeO2added glass–ceramics heat treated at 850 1C for 3 h as a function of the electric field applied.
G. Chen et al. / Materials Letters 136 (2014) 302–305
Fig. 3(b). It can be seen that the DED undergoes an increase and then decreases with CeO2 addition. The pure glass–ceramic has a relatively low DED of 0.94 J/cm3. The DED reaches its maximum of 1.95 J/cm3 for 0.5 mol% CeO2 added glass–ceramics. It can be attributed to following factors: first, the addition of CeO2 enhances the BDS to which the optimized structure contributes. Second, the polarization difference between maximum and remanent polarization is enlarged by adding CeO2.
305
Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC no. 51162002), Science and Technology Project of Guangxi Returned Personnel (Contract no. 2012-250) and Patent Project of Guangxi Department of Education (ZL2014015).
References 4. Conclusion A series of strontium barium niobate-based B2O3 system glass– ceramics with CeO2 addition were successfully prepared by the conventional melting method. By adding moderate contents of CeO2, the microstructures and dielectric properties are improved significantly. Typically, the glass–ceramic with 0.5 mol% CeO2 shows enhanced BDS of 1092.5 kV/mm and DED of 1.95 J/cm3, suggesting the as-prepared glass–ceramics could be a promising candidate for high-energy density capacitors.
[1] Shen ZY, Hu QG, Li YM, Wang ZM, Luo WQ, Hong Y, et al. J Am Ceram Soc 2013;96:2551. [2] Chu BJ, Zhou X, Ren KL, Neese B, Lin M, Wang Q, et al. Science 2006;313:334. [3] Fletcher NH, Hilton AD, Ricketts BW. J Phys D: Appl Phys 1996;29:253. [4] Gorzkowski EP, Pan MJ, Bender BA, Wu CCM. J Am Ceram Soc 2008;91:1065. [5] Liu TY, Chen GH, Song J, Yuan CL. Ceram Int 2013;39:5553. [6] Wang JW, Tang LJ, Shen B, Zhai JW. Ceram Int 2014;40:2261. [7] Zeng YK, Qin XY, Jiang SL, Zhang GZ, Zhang L. J Am Ceram Soc 2011;94:469. [8] Song J, Chen GH, Yuan CL, Yang Y. Mater Lett 2014;117:7. [9] Shannon RD, Prewit CT. Acta Crystallogr B 1969;25:925. [10] Shannon RD. J Appl Phys 1993;73:348. [11] Cole MW, Nothwang WD, Hubbard C, Ngo E, Ervin M. J Appl Phys 2003;93:9218.