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The dielectric properties and microstructure of BaTiO3 ceramics with ZnO–Nb2O5 composite addition
Journal of Alloys and Compounds 646 (2015) 748e752
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The dielectric properties and microstructure of BaTiO3 ceramics with ZnOeNb2O5 composite addition Yan Yan, Chao Ning, Zongzi Jin, Haoran Qin, Wenting Luo, Gang Liu* Faculty of Materials and Energy, Southwest University, Chongqing 400715, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2015 Received in revised form 21 May 2015 Accepted 23 May 2015 Available online 20 June 2015
ZnNb2O6 powders were prepared by solid state reaction method and doped into BT. The doped cation ions diffused into the BT crystal lattice, inducing diffuse phase transition and leading to broadened and flattened dielectric constant peaks especially in the low-temperature zone. When the doping concentration was below 6 wt.%, ceramics showed regular microstructures; when it reached 7.5wt.%, the Ba2Ti5O12 phase was found and it showed flake structure. With further increase of ZnNb2O6, the flake structure disappeared and the grain size markedly decreased. The decreased grain size was very effective to improve the breakdown strength. © 2015 Elsevier B.V. All rights reserved.
Keywords: Dielectric properties Composite Barium titanate Microstructure
1. Introduction Energy storage materials have received a lot attention in the past decades, due to their wide applications in many fields including hybrid vehicles, movable power source et al. [1e3]. However, because of the environmental concern and energy crisis, these materials are required to have lead-free nature [4] and higher energy storage density [5]. Generally, large dielectric constant and high electrical breakdown strength are the essential elements to obtain high energy storage density [6,7]. Among different kinds of dielectric materials, lead-free ceramics exhibit very high dielectric constant but relatively low breakdown strengths (BDS). A lot of efforts have hence been made to improve this limit, such as improving densification conditions [8,9], optimizing microstructure [10], adding glass [11], and so on. The addition of glass is an effective way to improve the BDS among the methods mentioned above, and a variety of glass materials, for example, BaOeSiO2eB2O3 [11], Al2O3eSiO2 [12], BaOeB2O3eSiO2 eNa2CO3eK2CO3 [13] et al., have already been employed to the dielectric ceramics. Barium Titanate (BT) is an important lead-free dielectric ceramic which is widely used in energy storage capacitors due to its very high dielectric constant. The addition with various additives to BT
* Corresponding author. E-mail address: [email protected] (G. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.05.260 0925-8388/© 2015 Elsevier B.V. All rights reserved.
ceramics has already been extensively studied [9,14,15]. Recently, ZnNb2O6 has been reported to be an additive which can greatly improve the BDS of BT [8]. However, few reports can be found about the relations between the dielectric properties and refined microstructures in ZnNb2O6-doped BT based lead-free ceramics. Therefore, from this point of view, we try to investigate and explain the effect of ZnNb2O6 on dielectric properties and microstructures in the current paper. 2. Experimental Commercial BT (99.9%, Shandong Sinocera Functional Material Co. Ltd., Dongying, China), ZnO (99%, Sinopharm Chemical Reagent Co.Ltd., China), and Nb2O5 powders (99.99%, Sinopharm Chemical Reagent Co. Ltd., China) were selected as the starting raw materials. ZnNb2O6 powders were prepared by the conventional solid state reaction method. ZnO and Nb2O5 powders were mixed in ethanol and ball-milled with zirconia milling media in a sealable PE bottle for 24 h. After drying, the above mixture was pressed into pellets under 5 MPa and calcined at 1150 C in air for 4 h. The calcined pellets were crushed and ball-milled again for another 24 h to obtain fine ZnNb2O6 powders. Various amounts of ZnNb2O6 (1.5, 3, 6, 7.5, 9 wt.%, based on BT power) were mixed with BT powders, and the corresponding ceramics were abbreviated as BZNT1 to BZNT5. After ball milling for 24 h and drying, the mixed powders and 3 wt.% PVA (based on the powder weight) were mixed well, sieved through a 300 mm-sieve, and uniaxially pressed into small pellets
Y. Yan et al. / Journal of Alloys and Compounds 646 (2015) 748e752
with 10 mm in diameter and 2.2 mm in thickness under a pressure of 100 MPa. After debinding, the small pellets were sintered in air at the temperature of 1280 C for 2 h. Silver paste was added onto both side of the pellets and sintered at 800 C for 10 min. The pellets for dielectric measurement were around 8.8 mm in diameter and 2 mm in thickness. Crystallographic and phase analyses for the sintered samples were performed using an X-ray diffractometer (7000, Shimadazu, Japan) with monochromatic Cu Ka radiation. The microstructures of the sintered samples were studied by scanning electron microscopy (JSM 7000, Jeol, Tokyo, Japan). Energy dispersive X-ray (EDX) spectroscopy was done using an Oxford Inca EDX system to evaluate the chemical composition of the samples. The temperature-dependent dielectric properties were measured at 1 kHz, 10 kHz, and 100 kHz using a LCR metre (HP4980, Agilent, Palo Alto, CA) connected with an environmental chamber with a heating rate of 3 C/min from 30 C to 200 C. The polarization electric field hysteresis loops were measured at 1 Hz at room temperature using a TF Analyser 2000 (aix ACCT) ferroelectric test system. The dielectric break-down strength was measured in silicon oil using an instrument (Nanjing Entai Electronics Co. Ltd., China) under DC voltage. The circular samples for breakdown strength test were around 2 mm in thickness. The voltage was added at the ramp around 1 kV/s until the break down occurred. At least 4 samples were utilized for each test to obtain an average value. 3. Results and discussion Fig. 1 shows the XRD patterns of BT and BZNT ceramics. It can be seen that the major phase of all samples was tetragonal BaTiO3. With the addition of ZnNb2O6 from 1.5 wt.% to 6 wt.%, there was no second phase detected. Principally, the site of incorporation for dopant ions in the BaTiO3 perovskite structure may be predicted by considering the corresponding ionic radii, and the Goldschmidt tolerance factor [16]. Incorporating the ionic radii of Ba2þ(1.35 Å), Ti4þ(0.68 Å), Zn2þ(0.74 Å), and Nb5þ(0.70 Å), the tolerance factor t of B-site (Ti) substitution in this doping is close to 1, therefore, B site substituting is more likely to happen. When ZnNb2O6 was doped into BT, the Zn2þ and Nb5þ ions would substitute the Ti4þ through 5þ 4þ forming the composite cation of ðZn2þ due to the similar 1=3 Nb2=3 Þ 4þ ionic radius to that of Ti , rather than the doping behaviours simply as accepter (Zn2þ) or donor (Nb5þ). So, this is very similar to the doping behaviour of composite formed by Ni2þ and Nb5þ in BT
Fig. 1. XRD spectrum of BT and doped BT ceramics.
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ceramics [17]. Furthermore, the valence of the composite cation of 5þ 4þ ðZn2þ was equal to Ti4þ. The corresponding substitution 1=3 Nb2=3 Þ reaction can be shown as below:
3BaO þ ZnO þ Nb2 O5 ¼ 3BaBa þZnTi þ2NbTi þ9Oo 00
(1)
The composite ZnNb2O6 may introduce certain defect types as shown above, resulting in higher solubility in BT ceramics. However, with further increase of ZnNb2O6 content (7.5 and 9 wt.%), other phases were detected. For example, Ba2Ti5O12 was found in the BZNT4 sample; Ba2Ti5O12 and ZnNb2O6 were detected in BZNT5 ceramics. This is mainly because the doping concentration may exceed the solubility limit in BT. Moreover, it should be noted that the diffraction peaks shift slightly to smaller angles with increasing the content of ZnNb2O6. Fig. 2 shows the temperature-dependant dielectric properties of BT and BZNT ceramics. It can be seen in Fig. 2(a) that the BT ceramic showed very sharp dielectric constant peak around 130 C, which is determined by the ferroelectriceparaelectric transition at Curie point. With increasing the doping concentration of ZnNb2O6 (Fig. 2(b)e(f)), the dielectric constant peaks were markedly broadened, and also became flatter at low temperature zone. The above phenomenon is attributed to the chemically inhomogeneous structure in the doped BaTiO3, which is also well known as the 5þ 4þ coreeshell structure [18]. The composite cation of ðZn2þ 1=3 Nb2=3 Þ 4þ ions diffused into the BT crystal lattice replacing Ti ions and formed the chemically inhomogeneous structure. This inhomogeneity in composition would induce diffuse phase transition, finally leading to the broadened and flattened dielectric constant peaks. It should be noted that the addition of ZnNb2O6 composite was effective to flatten the dielectric constant peak at low-temperature zone. Moreover, with the increase of ZnNb2O6 composite content, the dielectric constant decreased markedly from about 4000 to 1200 (at 40 C). Since the increase of the inhomogeneous structure or the existence of second phase decreased the dielectric constant greatly. The P-E hysteresis loops for BT and ZnNb2O6 composite doped BT ceramics are exhibited in Fig. 3. The pure BT ceramic showed a typical ferroelectric hysteresis loop [19]. With increasing the addition amount of ZnNb2O6 composite, the remanent polarization (Pr) values decreased greatly, especially when the doping concentration increased from zero to 1.5 wt.%. Moreover, when the ZnNb2O6 composite content was beyond 6.0 wt.%, samples showed very slim P-E loops. Fig. 4 shows the SEM photos of ZnNb2O6 composite doped BT. When the doping concentration was below 6 wt.%, all the samples including BZNT1, BZNT2 and BZNT3 showed regular microstructures with relatively homogeneous grain size. Moreover, the microstructure became denser with increasing the doping content, which can be seen from Fig. 4(a), (b), and Fig. 4(c). With further increase of ZnNb2O6 content, the morphology of the grains changed markedly. Flake structures surrounded by the fine grains were found in the BZNT4 sample, as shown in Fig. 4(d). However, with more addition of ZnNb2O6, as shown in Fig. 4(e), the flake structures disappeared, and one interesting microstructure appeared. It can be seen that this special structure was constituted by the interval fine and coarse grains. To make sure the detailed reason, EDX analysis at different zones was carried out and the corresponding results were shown in Table 1. The concentration of Ti at Site C was higher than that of Ba, indicating a Ti-rich second phase present in this sample. Incorporating the XRD result in Fig. 1, this Ti-rich phase should be the Ba2Ti5O12 phase. With further increasing the addition of ZnNb2O6, elements of Zn and Nb were detected (Site E and F). The concentrations of Zn and Nb at Site E were much higher than those at Site F
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Y. Yan et al. / Journal of Alloys and Compounds 646 (2015) 748e752
Fig. 2. Temperature-dependent dielectric properties of pure BT and ZnNb2O6 composite doped BT. (a) BT, (b) BZNT1, (c) BZNT2, (d) BZNT3, (e) BZNT4, (f) BZNT5.
respectively. This is in good agreement with the XRD result that ZnNb2O6 could be detected in BZNT6 sample. The ZnNb2O6erich phase showed relatively large grains, as shown in Fig. 4(d), while the phase with low ZnNb2O6 concentration showed fine grains. Therefore, the ZnNb2O6 phase was very effective to reduce the size of the grains surrounded it. The breakdown strengths of BT and doped-BT ceramics were also tested and the results are shown in Fig. 5. When the addition of ZnNb2O6 was below 6 wt.%, the breakdown strength increased with
the increase of doping concentration. This is mainly because the addition of ZnNb2O6 composite is very effective to reduce the grain size and leads to denser microstructures. However, BZNT4 sample showed much lower breakdown strength compared with other doped samples. This is possibly attributed to the presence of Ba2Ti5O12 phase that is harmful for improve the breakdown strength. With the further addition of ZnNb2O6, the breakdown strength increased markedly (around 61 kV/cm) due to the variety of finer grains obtained in the microstructure.
Y. Yan et al. / Journal of Alloys and Compounds 646 (2015) 748e752
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Table 1 Elements distribution of different sites in BZNT4 and BZNT5 ceramics. Elements
OK Ti K Zn L Nb L Ba L
Atomic percentage (%) Site C
Site D
Site E
Site F
58.87 23.08 — — 18.05
67.66 14.16 — — 18.18
70.22 8.37 2.25 4.25 14.91
62.61 15.30 1.38 2.31 18.40
Note: — presents the elements that were not detected.
4. Conclusions
Fig. 3. PeE hysteresis loops of BT and doped-BT ceramics obtained at room temperature.
ZnNb2O6 powders were prepared and doped into BaTiO3 ceramics in this study. When the addition of ZnNb2O6 was below 6 wt.%, there was no second phase detected. However, with further increase of doping concentration, Ba2Ti5O12 and ZnNb2O6 were detected due to the limited solubility in BT. The cation of
Fig. 4. SEM microstructures of the fractured surface of ZnNb2O6 composite-doped BT ceramics: (a) BZNT1, (b) BZNT2 (c) BZNT3, (d) BZNT4, (e) BZNT5.
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References
Fig. 5. Breakdown strengths of BT and doped-BT ceramics.
5þ 4þ ðZn2þ ions diffused into the BT crystal lattice replacing 1=3 Nb2=3 Þ Ti4þ ions and formed the chemically inhomogeneous structure inducing diffuse phase transition and finally leading to the broadened and flattened dielectric constant peaks, especially in the low temperature zone. The dielectric constant decreased markedly from about 4000 to 1200 (at 40 C) when the doping content increased from 1.5 wt.% to 9 wt.%. Moreover, when the ZnNb2O6 composite content was beyond 6.0 wt.%, ceramics showed very slim P-E loops. EDX analysis was in good agreement with the XRD patterns. The flake structure was the Ti-rich Ba2Ti5O12 phase in the sample with 7.5 wt.% doping content. The ZnNb2O6 phase was very effective to reduce the size of the grains surrounded it and greatly improve the breakdown strength.
Acknowledgements The work is supported by “Fundamental Research Funds for the Central Universities” (XDJK2014C112, XDJK2015C001 and XDJK20 14C008), and the National Natural Science Foundation of China(51402243); and is partly supported by Chongqing Scientific and Technological Projects (CSTC2012GGYS5001, CSTC2013 JCYJYS5002).
[1] D.I. Woodward, I.M. Reaney, G.Y. Yang, E.C. Dickey, Vacancy ordering in reduced barium titanate, Appl. Phys. Lett. 84 (2004) 4650e4652. [2] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater. 22 (8) (2010) E28eE62. [3] E. Karden, S. Ploumen, B. Fricke, T. Miller, K. Snyder, Energy storage devices for future hybrid electric vehicles, J. Power Sources 168 (1) (2007). SI 2e11. [4] D. Damjanovic, N. Klein, J. Li, V. Porokhonskyy, What can be expected from lead-free piezoelectric materials? Funct. Mater. Lett. 3 (1) (2010) 5e13. [5] X. Wei, H. Yan, T. Wang, Q. Hu, G. Viola, S. Grasso, Q. Jiang, L. Jin, Z. Xu, M.J. Reece, Reverse boundary layer capacitor model in glass/ceramic composites for energy storage applications, J. Appl. Phys. 113 (2) (2013) 024103. [6] B.J. Chu, X. Zhou, K.L. Ren, B. Neese, M.R. Lin, Q. Wang, F. Bauer, Q.M. Zhang, A dielectric polymer with high electric energy density and fast discharge speed, Science 313 (2006) 334e336. [7] H. Ogihara, C.A. Randall, S. Trolier-McKinstry, High-energy density capacitors utilizing 0.7 BaTiO3-0.3BiScO3 ceramics, J. Am. Ceram. Soc. 92 (8) (2009) 1719e1724. [8] T. Wang, X. Wei, Q. Hu, L. Jin, Z. Xu, Y. Feng, Effects of ZnNb2O6 addition on BaTiO3 ceramics for energy storage, Mater. Sci. Eng. B 178 (2013) 1081e1086. [9] G. Liu, Y. Jiang, T.W. Button, Low temperature sintering and dielectric properties of BaTiO3 ceramics incorporating nano-sized powders, Ferroelectrics 421 (2011) 72e81. [10] M. Cernea, B.S. Vasile, A. Boni, A. Iuga, Synthesis, structural characterization and dielectric properties of Nb doped BaTiO3/SiO2 core-shell heterostructure, J. Alloy. Compd. 587 (2014) 553e559. [11] Q. Zhang, L. Wang, J. Luo, Q. Tang, J. Du, Improved energy storage density in barium strontium titanate by addition of BaO-SiO2-B2O3 glass, J. Am. Ceram. Soc. 92 (2009) 1871e1873. [12] J.W. Wang, C. Xu, B. Shen, J.W. Zhai, Enhancing energy storage density of (Ba, Sr)TiO3 ceramic particles by coating with Al2O3 and SiO2, J. Mater. Sci. Mater. Electron 24 (2013) 3309e3314. [13] T. Wang, L. Jin, L.L. Shu, Q.Y. Hu, X.Y. Wei, Energy storage properties in Ba0.4Sr0.6TiO3 ceramics with addition of semi-conductive BaO-B2O3-SiO2Na2CO3-K2CO3 glass, J. Alloy. Compd. 617 (2014) 399e403. [14] H.P. Jeon, S.K. Lee, S.W. Kim, D.K. Choi, Effects of BaO-B2O3-SiO2 glass additive on densification and dielectric properties of BaTiO3 ceramics, Mater. Chem. Phys. 94 (2005) 185e189. [15] T.H. Song, C.A. Randall, Copper cofire X7R dielectrics and multilayer capacitors based on zinc borate fluxed barium titanate ceramic, J. Electroceram 10 (2003) 39e46. [16] R.H. Buttner, E.N. Maslen, Structural parameters and electron difference density in BaTiO3, Acta Cryst. B48 (1992) 764e769. [17] L.X. Li, R.X. Fu, Q.W. Liao, L.J. Ji, Doping behaviors of NiO and Nb2O5 in BaTiO3 and dielectric properties of BaTiO3-based X7R ceramics, Ceram. Int. 38 (2012) 1915e1920. [18] Y. Mizuno, T. Hagiwara, H. Kishi, Microstructural design of dielectrics for NiMLCC with ultra-thin active layers, J. Ceram. Soc. Jpn. 115 (1342) (2007) 360e364. [19] L. Jin, F. Li, S. Zhang, Decoding the fingerprint of ferroelectric loops: comprehension of the material properties, J. Am. Ceram. Soc. 97 (1) (2014) 1e27.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The dielectric properties and microstructure of BaTiO3 ceramics with ZnOeNb2O5 composite addition Yan Yan, Chao Ning, Zongzi Jin, Haoran Qin, Wenting Luo, Gang Liu* Faculty of Materials and Energy, Southwest University, Chongqing 400715, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2015 Received in revised form 21 May 2015 Accepted 23 May 2015 Available online 20 June 2015
ZnNb2O6 powders were prepared by solid state reaction method and doped into BT. The doped cation ions diffused into the BT crystal lattice, inducing diffuse phase transition and leading to broadened and flattened dielectric constant peaks especially in the low-temperature zone. When the doping concentration was below 6 wt.%, ceramics showed regular microstructures; when it reached 7.5wt.%, the Ba2Ti5O12 phase was found and it showed flake structure. With further increase of ZnNb2O6, the flake structure disappeared and the grain size markedly decreased. The decreased grain size was very effective to improve the breakdown strength. © 2015 Elsevier B.V. All rights reserved.
Keywords: Dielectric properties Composite Barium titanate Microstructure
1. Introduction Energy storage materials have received a lot attention in the past decades, due to their wide applications in many fields including hybrid vehicles, movable power source et al. [1e3]. However, because of the environmental concern and energy crisis, these materials are required to have lead-free nature [4] and higher energy storage density [5]. Generally, large dielectric constant and high electrical breakdown strength are the essential elements to obtain high energy storage density [6,7]. Among different kinds of dielectric materials, lead-free ceramics exhibit very high dielectric constant but relatively low breakdown strengths (BDS). A lot of efforts have hence been made to improve this limit, such as improving densification conditions [8,9], optimizing microstructure [10], adding glass [11], and so on. The addition of glass is an effective way to improve the BDS among the methods mentioned above, and a variety of glass materials, for example, BaOeSiO2eB2O3 [11], Al2O3eSiO2 [12], BaOeB2O3eSiO2 eNa2CO3eK2CO3 [13] et al., have already been employed to the dielectric ceramics. Barium Titanate (BT) is an important lead-free dielectric ceramic which is widely used in energy storage capacitors due to its very high dielectric constant. The addition with various additives to BT
* Corresponding author. E-mail address: [email protected] (G. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.05.260 0925-8388/© 2015 Elsevier B.V. All rights reserved.
ceramics has already been extensively studied [9,14,15]. Recently, ZnNb2O6 has been reported to be an additive which can greatly improve the BDS of BT [8]. However, few reports can be found about the relations between the dielectric properties and refined microstructures in ZnNb2O6-doped BT based lead-free ceramics. Therefore, from this point of view, we try to investigate and explain the effect of ZnNb2O6 on dielectric properties and microstructures in the current paper. 2. Experimental Commercial BT (99.9%, Shandong Sinocera Functional Material Co. Ltd., Dongying, China), ZnO (99%, Sinopharm Chemical Reagent Co.Ltd., China), and Nb2O5 powders (99.99%, Sinopharm Chemical Reagent Co. Ltd., China) were selected as the starting raw materials. ZnNb2O6 powders were prepared by the conventional solid state reaction method. ZnO and Nb2O5 powders were mixed in ethanol and ball-milled with zirconia milling media in a sealable PE bottle for 24 h. After drying, the above mixture was pressed into pellets under 5 MPa and calcined at 1150 C in air for 4 h. The calcined pellets were crushed and ball-milled again for another 24 h to obtain fine ZnNb2O6 powders. Various amounts of ZnNb2O6 (1.5, 3, 6, 7.5, 9 wt.%, based on BT power) were mixed with BT powders, and the corresponding ceramics were abbreviated as BZNT1 to BZNT5. After ball milling for 24 h and drying, the mixed powders and 3 wt.% PVA (based on the powder weight) were mixed well, sieved through a 300 mm-sieve, and uniaxially pressed into small pellets
Y. Yan et al. / Journal of Alloys and Compounds 646 (2015) 748e752
with 10 mm in diameter and 2.2 mm in thickness under a pressure of 100 MPa. After debinding, the small pellets were sintered in air at the temperature of 1280 C for 2 h. Silver paste was added onto both side of the pellets and sintered at 800 C for 10 min. The pellets for dielectric measurement were around 8.8 mm in diameter and 2 mm in thickness. Crystallographic and phase analyses for the sintered samples were performed using an X-ray diffractometer (7000, Shimadazu, Japan) with monochromatic Cu Ka radiation. The microstructures of the sintered samples were studied by scanning electron microscopy (JSM 7000, Jeol, Tokyo, Japan). Energy dispersive X-ray (EDX) spectroscopy was done using an Oxford Inca EDX system to evaluate the chemical composition of the samples. The temperature-dependent dielectric properties were measured at 1 kHz, 10 kHz, and 100 kHz using a LCR metre (HP4980, Agilent, Palo Alto, CA) connected with an environmental chamber with a heating rate of 3 C/min from 30 C to 200 C. The polarization electric field hysteresis loops were measured at 1 Hz at room temperature using a TF Analyser 2000 (aix ACCT) ferroelectric test system. The dielectric break-down strength was measured in silicon oil using an instrument (Nanjing Entai Electronics Co. Ltd., China) under DC voltage. The circular samples for breakdown strength test were around 2 mm in thickness. The voltage was added at the ramp around 1 kV/s until the break down occurred. At least 4 samples were utilized for each test to obtain an average value. 3. Results and discussion Fig. 1 shows the XRD patterns of BT and BZNT ceramics. It can be seen that the major phase of all samples was tetragonal BaTiO3. With the addition of ZnNb2O6 from 1.5 wt.% to 6 wt.%, there was no second phase detected. Principally, the site of incorporation for dopant ions in the BaTiO3 perovskite structure may be predicted by considering the corresponding ionic radii, and the Goldschmidt tolerance factor [16]. Incorporating the ionic radii of Ba2þ(1.35 Å), Ti4þ(0.68 Å), Zn2þ(0.74 Å), and Nb5þ(0.70 Å), the tolerance factor t of B-site (Ti) substitution in this doping is close to 1, therefore, B site substituting is more likely to happen. When ZnNb2O6 was doped into BT, the Zn2þ and Nb5þ ions would substitute the Ti4þ through 5þ 4þ forming the composite cation of ðZn2þ due to the similar 1=3 Nb2=3 Þ 4þ ionic radius to that of Ti , rather than the doping behaviours simply as accepter (Zn2þ) or donor (Nb5þ). So, this is very similar to the doping behaviour of composite formed by Ni2þ and Nb5þ in BT
Fig. 1. XRD spectrum of BT and doped BT ceramics.
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ceramics [17]. Furthermore, the valence of the composite cation of 5þ 4þ ðZn2þ was equal to Ti4þ. The corresponding substitution 1=3 Nb2=3 Þ reaction can be shown as below:
3BaO þ ZnO þ Nb2 O5 ¼ 3BaBa þZnTi þ2NbTi þ9Oo 00
(1)
The composite ZnNb2O6 may introduce certain defect types as shown above, resulting in higher solubility in BT ceramics. However, with further increase of ZnNb2O6 content (7.5 and 9 wt.%), other phases were detected. For example, Ba2Ti5O12 was found in the BZNT4 sample; Ba2Ti5O12 and ZnNb2O6 were detected in BZNT5 ceramics. This is mainly because the doping concentration may exceed the solubility limit in BT. Moreover, it should be noted that the diffraction peaks shift slightly to smaller angles with increasing the content of ZnNb2O6. Fig. 2 shows the temperature-dependant dielectric properties of BT and BZNT ceramics. It can be seen in Fig. 2(a) that the BT ceramic showed very sharp dielectric constant peak around 130 C, which is determined by the ferroelectriceparaelectric transition at Curie point. With increasing the doping concentration of ZnNb2O6 (Fig. 2(b)e(f)), the dielectric constant peaks were markedly broadened, and also became flatter at low temperature zone. The above phenomenon is attributed to the chemically inhomogeneous structure in the doped BaTiO3, which is also well known as the 5þ 4þ coreeshell structure [18]. The composite cation of ðZn2þ 1=3 Nb2=3 Þ 4þ ions diffused into the BT crystal lattice replacing Ti ions and formed the chemically inhomogeneous structure. This inhomogeneity in composition would induce diffuse phase transition, finally leading to the broadened and flattened dielectric constant peaks. It should be noted that the addition of ZnNb2O6 composite was effective to flatten the dielectric constant peak at low-temperature zone. Moreover, with the increase of ZnNb2O6 composite content, the dielectric constant decreased markedly from about 4000 to 1200 (at 40 C). Since the increase of the inhomogeneous structure or the existence of second phase decreased the dielectric constant greatly. The P-E hysteresis loops for BT and ZnNb2O6 composite doped BT ceramics are exhibited in Fig. 3. The pure BT ceramic showed a typical ferroelectric hysteresis loop [19]. With increasing the addition amount of ZnNb2O6 composite, the remanent polarization (Pr) values decreased greatly, especially when the doping concentration increased from zero to 1.5 wt.%. Moreover, when the ZnNb2O6 composite content was beyond 6.0 wt.%, samples showed very slim P-E loops. Fig. 4 shows the SEM photos of ZnNb2O6 composite doped BT. When the doping concentration was below 6 wt.%, all the samples including BZNT1, BZNT2 and BZNT3 showed regular microstructures with relatively homogeneous grain size. Moreover, the microstructure became denser with increasing the doping content, which can be seen from Fig. 4(a), (b), and Fig. 4(c). With further increase of ZnNb2O6 content, the morphology of the grains changed markedly. Flake structures surrounded by the fine grains were found in the BZNT4 sample, as shown in Fig. 4(d). However, with more addition of ZnNb2O6, as shown in Fig. 4(e), the flake structures disappeared, and one interesting microstructure appeared. It can be seen that this special structure was constituted by the interval fine and coarse grains. To make sure the detailed reason, EDX analysis at different zones was carried out and the corresponding results were shown in Table 1. The concentration of Ti at Site C was higher than that of Ba, indicating a Ti-rich second phase present in this sample. Incorporating the XRD result in Fig. 1, this Ti-rich phase should be the Ba2Ti5O12 phase. With further increasing the addition of ZnNb2O6, elements of Zn and Nb were detected (Site E and F). The concentrations of Zn and Nb at Site E were much higher than those at Site F
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Fig. 2. Temperature-dependent dielectric properties of pure BT and ZnNb2O6 composite doped BT. (a) BT, (b) BZNT1, (c) BZNT2, (d) BZNT3, (e) BZNT4, (f) BZNT5.
respectively. This is in good agreement with the XRD result that ZnNb2O6 could be detected in BZNT6 sample. The ZnNb2O6erich phase showed relatively large grains, as shown in Fig. 4(d), while the phase with low ZnNb2O6 concentration showed fine grains. Therefore, the ZnNb2O6 phase was very effective to reduce the size of the grains surrounded it. The breakdown strengths of BT and doped-BT ceramics were also tested and the results are shown in Fig. 5. When the addition of ZnNb2O6 was below 6 wt.%, the breakdown strength increased with
the increase of doping concentration. This is mainly because the addition of ZnNb2O6 composite is very effective to reduce the grain size and leads to denser microstructures. However, BZNT4 sample showed much lower breakdown strength compared with other doped samples. This is possibly attributed to the presence of Ba2Ti5O12 phase that is harmful for improve the breakdown strength. With the further addition of ZnNb2O6, the breakdown strength increased markedly (around 61 kV/cm) due to the variety of finer grains obtained in the microstructure.
Y. Yan et al. / Journal of Alloys and Compounds 646 (2015) 748e752
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Table 1 Elements distribution of different sites in BZNT4 and BZNT5 ceramics. Elements
OK Ti K Zn L Nb L Ba L
Atomic percentage (%) Site C
Site D
Site E
Site F
58.87 23.08 — — 18.05
67.66 14.16 — — 18.18
70.22 8.37 2.25 4.25 14.91
62.61 15.30 1.38 2.31 18.40
Note: — presents the elements that were not detected.
4. Conclusions
Fig. 3. PeE hysteresis loops of BT and doped-BT ceramics obtained at room temperature.
ZnNb2O6 powders were prepared and doped into BaTiO3 ceramics in this study. When the addition of ZnNb2O6 was below 6 wt.%, there was no second phase detected. However, with further increase of doping concentration, Ba2Ti5O12 and ZnNb2O6 were detected due to the limited solubility in BT. The cation of
Fig. 4. SEM microstructures of the fractured surface of ZnNb2O6 composite-doped BT ceramics: (a) BZNT1, (b) BZNT2 (c) BZNT3, (d) BZNT4, (e) BZNT5.
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Y. Yan et al. / Journal of Alloys and Compounds 646 (2015) 748e752
References
Fig. 5. Breakdown strengths of BT and doped-BT ceramics.
5þ 4þ ðZn2þ ions diffused into the BT crystal lattice replacing 1=3 Nb2=3 Þ Ti4þ ions and formed the chemically inhomogeneous structure inducing diffuse phase transition and finally leading to the broadened and flattened dielectric constant peaks, especially in the low temperature zone. The dielectric constant decreased markedly from about 4000 to 1200 (at 40 C) when the doping content increased from 1.5 wt.% to 9 wt.%. Moreover, when the ZnNb2O6 composite content was beyond 6.0 wt.%, ceramics showed very slim P-E loops. EDX analysis was in good agreement with the XRD patterns. The flake structure was the Ti-rich Ba2Ti5O12 phase in the sample with 7.5 wt.% doping content. The ZnNb2O6 phase was very effective to reduce the size of the grains surrounded it and greatly improve the breakdown strength.
Acknowledgements The work is supported by “Fundamental Research Funds for the Central Universities” (XDJK2014C112, XDJK2015C001 and XDJK20 14C008), and the National Natural Science Foundation of China(51402243); and is partly supported by Chongqing Scientific and Technological Projects (CSTC2012GGYS5001, CSTC2013 JCYJYS5002).
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