- Email: [email protected]
Temperature stability of Zn2SnO4-doped BaTiO3 ceramics
Accepted Manuscript Temperature stability of Zn2SnO4-doped BaTiO3 ceramics Yapeng Zhou, Liben Li, Guozhong Zang, Hualiang Cao, Xiaofei Wang PII: DOI: Reference:
S0925-8388(15)00666-0 http://dx.doi.org/10.1016/j.jallcom.2015.03.008 JALCOM 33593
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
30 November 2014 8 February 2015 1 March 2015
Please cite this article as: Y. Zhou, L. Li, G. Zang, H. Cao, X. Wang, Temperature stability of Zn2SnO4-doped BaTiO3 ceramics, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.03.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Temperature stability of Zn2SnO4-doped BaTiO3 ceramics Yapeng Zhou, Liben Li∗, Guozhong Zang, Hualiang Cao, Xiaofei Wang School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, People’s Republic of China.
Abstract: Zn2SnO4-doped BaTiO3 ceramics were prepared as dielectric material though a conventional solid state reaction method. The room temperature dielectric constant was improved by Zn2SnO4 doping, with the appearance of diffuse phase transition and reduced dielectric loss. More importantly, the temperature stability of BaTiO3 ceramics (△C/C25 ℃) was modified by varying doping contents. The temperature dependence of sample with 2% doping contents satisfied the Electronic Industries Alliance Y5V temperature characteristic specification and was near the X7R specification. The SEM micrographs indicate that the BaTiO3 solid solution has a homogeneous fine-grained microstructure with grain size <1 μm and X-ray diffraction pattern suggest that the sample with 2% Zn2SnO4 doping contents have a tetragonal phase at RT. DPT was resulted from element substitution of Ti4+ with Sn4+, as lattice parameters increasing under inter-grain stress. Results of impedance spectroscopic analysis suggest the presence of two relaxation mechanisms. One was originated from the dipolar polarization in the grain, and the other was
∗Corresponding author at: School of Physics and Engineering, Henan University of Science and Technology, Luoyang, 471023, P. R. China. Tel.: +86-0379-65626260. Email: [email protected]
related with the hopping charge carriers of oxygen vacancy in the grain boundary. The dielectric loss over a wide temperature range was modified by the activation energies of the two type relaxation mechanisms. Keywords: Electroceramics； Dielectric properties；Temperature stability； BaTiO3 PACS: 65.40.-b, 77.22.Gm, 74.62.Dh 1. Introduction BaTiO3 is a superior dielectric material with high dielectric constant, ferroelectricity and fine positive-temperature coefficients. This material is widely used in manufacture of multilayer ceramic capacitors. Pure BaTiO3 ceramics has a high sintering temperature of approximately 1300 ℃ and a considerable grain size of dozens micron. The relative dielectric constant at room temperature (RT) is approximately 1500-2000, and numerous pores result in large dielectric loss of 5%[1,2]. However, BaTiO3 application is greatly limited by the strong dielectric peak derived from the first phase transition. This characteristic imply the necessary to improve temperature stability, regulate the grain size distribution, and reduce the dielectric loss of BaTiO3 by doping proper metal oxides, in the end, the electrical properties of BaTiO3 ceramics should satisfy the standard specified by the Electronic Industries Alliance (EIA). The two major factors that affecting the BaTiO3 dielectric properties are micro-structural homogeneity[2-4] and distribution of chemical
elements[5,6]. The use of ultrafine BaTiO3 powders as raw material could yield ceramics with a relative constant of around 4000[4]. Dy2O3 and ZnO[1,7] are generally adopted to suppress the grain growth of BaTiO3, improve the RT dielectric constant, and reduce dielectric loss for reason that the porosity coalescence and removal was promoted during sintering process. ZrO2, Nb2O5, and SnO2[8-10] are usually adopted by single doping or codoping method. The element substitution effect occurred in B-site of perovskite lattice could result in reducing of Curie temperature and diffuse phase transition (DPT) on temperature dependence. Most importantly, the temperature stability of BaTiO3 ceramics would be remarkably enhanced. In the previous work[11], we have performed Zn2SnO4-doped BaTiO3 ceramics and got something outstanding results. In details, pre-synthesized BaTiO3 and Zn2SnO4 powders were adopted as raw materials to synthesize Zn2SnO4-doped BaTiO3 (ZS-BT) ceramics with different Zn2SnO4 doping contents. In this paper, we intend to deeply discuss the temperature stability influenced by doping Zn2SnO4, based on the issues of microscopic evolution, crystal lattice parameter variations and impedance spectroscopic analysis. It is mainly because zinc and tin are two excellent addition oxides, i.e., zinc could suppress the BaTiO3 grain
growth
during
sintering
and
improve
the
homogeneity
micro-structure, while tin could easily replace titanium in B-site of the
perovskite lattice, which could result in DPT phenomenon. We assumed that pre-synthesized Zn2SnO4 powders could be uniformly distributed around BaTiO3 powder agglomerates by ball-milling process, thereby this special preparation method could provide uniform concentrations of addition sources for sintering process and desired dielectric properties such as temperature stability. 2. Experimental procedure The raw materials are analytically pure BaCO3 (98.5%), TiO2 (98%), SnO2 (99.5%) and ZnO (99.5%). Firstly, BaCO3 and TiO2 mixtures with mole ratio 1：1 are prepared by ball-milling process for 6 h using zirconium ball in alcohol, followed by drying, and then pressed into discs with 30 mm in diameter under 100MPa. The discs were calcined at 1100 ℃ for 2 h and then ground into BaTiO3 powders. Secondly, the process above was repeated again to obtain Zn2SnO4, i.e., ZnO and SnO2 mixtures with mole ratio 2：1 were prepared by ball-milling process, the drying powders was pressed into discs and calcined at 1000 ℃ for 2 h, and then ground into Zn2SnO4 powders. Finally, pre-synthesized BaTiO3 powders and Zn2SnO4 powders were mixed with various BaTiO3：Zn2SnO4 mole ratios of (1-x) mol：x mol, while x = 0.002, 0.004, 0.006, 0.008, 0.02, 0.04, 0.06, and 0.08 (named as ZS0.2%, ZS0.4%, ZS0.6%, ZS0.8%, ZS2%, ZS4%, ZS6%, and ZS8%, respectively). The above eight species were mixed by ball-milling process for 6 h in alcohol. Then, the slurry was
dried and pressed into discs with 15 mm in diameter and roughly 1 mm in thickness by 200 MPa pressure. The eight species discs were calcined at 1260 ℃ for 2 h, the obtained ceramics were polished, then coated with silver paste on the upper and bottom surfaces and fired at 600 ℃。 The JSM-5610LV scanning electron microscope (SEM) was used to characterize the fracture plane of samples. The crystallographic structure was investigated by X-ray diffraction (XRD) on a D8 ADVANCE diffractometer using Cu Kα radiation. The temperature and frequency dependence of the dielectric constant and loss were measured using the Agilent 4294A precision impedance analyzer in vacuum. Liquid nitrogen was used as cold source and a heater coil was used for heating process. 3. Results and discussion Zn2SnO4 significantly suppress the dielectric peaks and raise dielectric constant between room and Curie temperatures (Fig.1(a)). The dielectric peaks at Curie temperature present the DPT phenomenon. With increasing dopant contents, the temperature of the tetragonal phase to the cubic phase decrease. The Curie temperatures of sample ZS0.2% and ZS 0.4% were 121.9 ℃ and 117.3 ℃, respectively. Sample ZS2% has a temperature stability of -35% to 12% and dielectric loss is less than 4% between -25 ℃ and 125 ℃(Fig.1(b)), which is closed to the EIA-X7R specification. The temperature stability between -30 ℃ and 85 ℃ is from -21% to 9% while dielectric loss is less than 4.2%, and the dielectric
properties meet the requirement of EIA-Y5V. The variation of the Curie temperature may be originated from the element substitution of Ti4+ with Sn4+[10]. The dielectric peaks of ZS4% and ZS8% were severely suppressed, which result in poor temperature stability. In this study, the temperature dependence of dielectric loss was investigated, and whether two different dependences of temperature exist was explored. Different Zn2SnO4 doping contents were tested at two temperature regions, marked as regions A (25 ℃-100 ℃) and region B (100 ℃-200 ℃) (Fig. 1(b)). Results indicate that the dielectric loss of ZS0.2%、ZS0.4%、ZS0.6% and ZS2% increase with increasing Zn2SnO4 contents in region A, on the contrary the loss decrease with doping in region B. This phenomenon will be discussed in detail later. Distinguishing the microstructure of grain and grain boundary in ZS0.2% is difficult (Fig. 2(a)), while in ZS0.4%(Fig. 2(b)) the fracture plane is apparently rough and has fewer pores compared with sample ZS0.2%. Pores are dramatically reduced in sample ZS0.8% and a rough microstructure of grain and grain boundary is observed (Fig. 2(c)). In sample ZS2% the grain could be well separated from the grain boundary with grain size of roughly 0.5 μm -1 μm, and the pores vanish (Fig. 2(d)). Thus, doping Zn2SnO4 could improve grain size distribution, constrain grain growth, promote the porosity removal, and mostly importantly generate a homogeneous microstructure of fine-grained BaTiO3. This
phenomenon could be attributed to the Zn2SnO4 uniform distribution among the BaTiO3 powder agglomerates during ball-milling process, and zinc element plays a key role in inhibiting uncontrolled growth of BaTiO3 grain during sintering [1]. ZS2% exist in tetragonal phase at RT with second phase Zn2SnO4 existing(Fig. 3). The XRD results were analyzed using the software Jade 6.0[12]. The grain size range from 378 nm to 854 nm by calculating each diffraction peaks, and Zn2SnO4 has a huge grain size >1000 nm. The average stress of the different sizes grain was approximately 0.152% by considering the peak broadening which resulted from inter-grain stress. BaTiO3 exist in tetragonal phase in RT with P4mm point group, lattice parameters a=b=3.986 Å, c=4.026 Å and axial ratio c/a=1.01003. Ti4+ (ionic radius, 0.605 Å) substitution with Sn4+ (ionic radius, 0.69 Å) could result in lattice distortion, and the ZS2% lattice parameters were calculated as a=b=3.99839 Å (0.31% increase), c=4.02682 Å (0.00204% increase), and axial ratio c/a=1.00711 (8.4% decrease). The decrease of axial ratio lead to decrease of spontaneous polarization. Therefore, DPT appears around the dielectric peaks. Dielectric loss is closely related to the charge carriers in dielectric. The relaxation behavior of charge carriers responsed to alternating current could reflect the information of dielectric loss. Two different temperature dependence of dielectric loss exist over a wide temperature range (Fig.
1(b)) as mentioned previous, i.e. the dielectric loss of ZS0.2%、ZS0.4%、 ZS0.6% and ZS2% increase with increasing Zn2SnO4 contents in region A, on the contrary the dielectric loss decrease with increasing Zn2SnO4 contents in region B. The frequency dependence of complex impedance for sample ZS0.2% and ZS2% over a wide temperature range was measured. The data was converted into relative permittivity complex plane (Fig. 4(a) and Fig. 5(a)) and frequency dependence of tanδ (Fig. 4(b) and Fig. 5(b)) based on Eq. (1): ε* =
1 = ε ′ + iε ′′ iωC 0 Z *
(1)
Where ε* represent the complex dielectric constant, Z* is complex impedance and C0 is the vacuum capacitance of sample. Two types of relaxation behaviors were found from 40 Hz to 106 Hz, namely, a low-frequency relaxation (40 Hz-103 Hz) originated from the grain boundary and a high-frequency relaxation (104 Hz-106 Hz) originated from the grain[13-15]. The relaxation behavior originated from the thermal actived charge carriers could be fitted with the Arrhenius relationship as given in Eq. (2). The calculated results indicate that, for sample ZS0.2%, the high-frequency grain relaxation energy Qgrain is 0.5767 eV, and the low-frequency grain boundary relaxation energy Qgrain boundary
is 0.9908 eV. The corresponding values for sample ZS2% are
0.3622eV and 2.2824 eV. It is obvious that the grain relaxation activation
energy evidently decrease with increasing Zn2SnO4 contents, on the contrary the grain boundary relaxation activation energy increase with increasing Zn2SnO4 contents. f peak = A exp( −
Q ) kT
(2)
where A is the pre-exponential factor, k is Boltzmann constant, T is the temperature in Kelvin and Q is relaxation activation energy. The different charge carriers lead to different response mechanisms towards alternating current. The high-frequency relaxation behavior is assumed to be a Debye-type relaxation caused by dipoles in the grain with minor activation energy of 0.3 eV-0.5 eV[17,19]. For perovskite dielectric, the relaxation behavior in paraelectric phase caused by thermal activation is related to oxygen vacancy[16-18,20] with higher activation energy of 1 eV-2 eV. Therefore the low-frequency relaxation behavior could be related to the hopping charge carriers of oxygen vacancy[16,20] in the grain boundary. The dielectric loss was modified by relaxation activation energy [21] in different temperature regions. In temperature around RT as shown in region A, the dielectric loss measured at 1k Hz was mainly affected by dipoles in the grain. Consequently, the dielectric loss increase and the high-frequency relaxation activation energy decrease with increasing Zn2SnO4 contents. In paraelectric phase at high temperature as shown in region B, the dielectric loss measured at 1k Hz is mainly affected by
oxygen vacancy in the grain boundary. Consequently, dielectric loss decrease and the low-frequency relaxation activation energy increase with increasing Zn2SnO4 contents. 4. Conclusion Thus, the remarkable enhanced temperature stability of ZS-BT ceramics was attributed to the following reasons: 1. Zn2SnO4 addition result in a homogeneous fine-grained microstructure with grain size <1 μm, which promote porosity coalescence and removal, thereby increase the relative constant and reduce dielectric loss around room temperature. 2. Ti4+ substitution with Sn4+ during sintering result in distortion of the perovskite lattice. The ionic radius difference between two elements lead to the increase in lattice parameters a, b, and c, Thereby the relative constant were promoted by inner-stress of the grain. However, the decrease of the axial ratio c/a lead to the decrease of spontaneous polarization. Consequently, diffuse phase transition appear around the dielectric peaks. 2. The dielectric loss around RT was modified by the high-frequency relaxation activation energy generated by the dipoles in the grain. The dielectric loss increase with increasing Zn2SnO4 doping contents. Acknowledgement
The authors gratefully acknowledge the Education Department of Henan province, science and technology research projects: 14A480001, for financial support. References [1] A. C. Caballero, J. F. Fernandez, C. Moure, P. Duran, J. Eur. Ceram. Soc., 17(1997)513. [2] W. L. Luan, L. Gao, J. K. Guo, Ceram. Int., 25(1999)727. [3] X. H. Wang, R. Z. Chen, Z. L. Gui, L. T. Li, Mater. Sci. Eng. B, 99(2003)199. [4] P. Zheng, J. L. Zhang, Y. Q. Tan, C. L. Wang, Acta Mater., 60(2012)5022. [5] D. Hennings, G. Rosenstein, J. Am. Ceram. Soc., 67(1984)249. [6] A. C. Caballero, J. F. Fernandez, C. Moure, P. Duran, J. L. G. Fierro, J. Eur. Ceram. Soc., 17(1997)1223. [7] V. Paunovic, V. V. Mitic, Z. Prijic, Lj. Zivkovic, Ceram. Int., 40(2014)4277. [8] M. R. Nateghi, M. R. Barzegari, J. Alloys Compd., 452(2008)36. [9] M. Cernea, B. S. Vasile, A. Boni, A. Iuga, J. Alloys Compd., 587(2014)553. [10] X. Y. Wei, X. Yao, Mater. Sci. Eng. B, 137(2007)184. [11] Y. P. Zhou, F. Z. Zhou, G. Z. Zang, Y. Q. Wang, Y. H. Dong, L. B. Li, J. Funct. Mater., 7(2014)07039. [12] J. L. Ye, C. C. Wang, W. Ni, X. H. Sun, J. Alloys Compd., 617(2014)850. [13] A. Huanosta, A. R. West, J. Appl. Phys., 61(1987)5386. [14] N. Hirose, A. R. West, J. Am. Ceram. Soc., 79(1996)1633. [15] G. H. Cao, L. X. Feng, C. Wang, J. Phys. D: Appl. Phys., 40(2007)2899. [16] O. Bidault, P. Goux, M. Kchikech, M. Belkaoumi, M. Maglione, Phys. Rev. B: Condens. Matter, 49(1994)7868. [17] Chen Ang, Z. Yu, L. E. Cross, Phys. Rev. B: Condens. Matter, 62(2000)228. [18] X. Y. Wei, X. Wan, X. Yao, J. Electroceram., 21(2008)226. [19] D. W. Johnson, L. E. Cross, F. A. Hummel, J. Appl. Phys., 41(1970)2828. [20] R. Stumpe, D. Wagner, D. Bäuerle, Phys. stat. sol. (a), 75(1983)143.
[21] J. L. Li, F. Li, Y. Y. Zhuang, L. Jin, L. H. Wang, X. Y. Wei, Z. Xu, S. J. Zhang, J. Appl. Phys., 116(2014)074105.
Fig Caption FIG. 1. Temperature dependence of dielectric constant and dielectric loss: (a) Temperature dependence of εr at 1k Hz for ZS-BT ceramics, (b) Temperature dependence of tanδ at 1k Hz for ZS-BT ceramics.
FIG. 2. SEM micrograph of samples with varying Zn2SnO4 contents: (a) SEM micrograph of ZS0.2% at RT, (b) SEM micrograph of ZS0.4% at RT, (c) SEM micrograph of ZS0.8% at RT, (d) SEM micrograph of ZS2% at RT. FIG. 3. XRD pattern of ZS2% at RT.
FIG. 4. Dielectric relaxation behaviour of sample ZS0.2%: (a) Relative permittivity complex plane plot at various temperatures, (b) Frequency dependence of tanδ at various temperatures.
FIG. 5. Dielectric relaxation behaviour of sample ZS2%: (a) Relative permittivity complex plane plot at various temperatures, (b) Frequency dependence of tanδ at various temperatures.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
S0925-8388(15)00666-0 http://dx.doi.org/10.1016/j.jallcom.2015.03.008 JALCOM 33593
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
30 November 2014 8 February 2015 1 March 2015
Please cite this article as: Y. Zhou, L. Li, G. Zang, H. Cao, X. Wang, Temperature stability of Zn2SnO4-doped BaTiO3 ceramics, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.03.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Temperature stability of Zn2SnO4-doped BaTiO3 ceramics Yapeng Zhou, Liben Li∗, Guozhong Zang, Hualiang Cao, Xiaofei Wang School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471023, People’s Republic of China.
Abstract: Zn2SnO4-doped BaTiO3 ceramics were prepared as dielectric material though a conventional solid state reaction method. The room temperature dielectric constant was improved by Zn2SnO4 doping, with the appearance of diffuse phase transition and reduced dielectric loss. More importantly, the temperature stability of BaTiO3 ceramics (△C/C25 ℃) was modified by varying doping contents. The temperature dependence of sample with 2% doping contents satisfied the Electronic Industries Alliance Y5V temperature characteristic specification and was near the X7R specification. The SEM micrographs indicate that the BaTiO3 solid solution has a homogeneous fine-grained microstructure with grain size <1 μm and X-ray diffraction pattern suggest that the sample with 2% Zn2SnO4 doping contents have a tetragonal phase at RT. DPT was resulted from element substitution of Ti4+ with Sn4+, as lattice parameters increasing under inter-grain stress. Results of impedance spectroscopic analysis suggest the presence of two relaxation mechanisms. One was originated from the dipolar polarization in the grain, and the other was
∗Corresponding author at: School of Physics and Engineering, Henan University of Science and Technology, Luoyang, 471023, P. R. China. Tel.: +86-0379-65626260. Email: [email protected]
related with the hopping charge carriers of oxygen vacancy in the grain boundary. The dielectric loss over a wide temperature range was modified by the activation energies of the two type relaxation mechanisms. Keywords: Electroceramics； Dielectric properties；Temperature stability； BaTiO3 PACS: 65.40.-b, 77.22.Gm, 74.62.Dh 1. Introduction BaTiO3 is a superior dielectric material with high dielectric constant, ferroelectricity and fine positive-temperature coefficients. This material is widely used in manufacture of multilayer ceramic capacitors. Pure BaTiO3 ceramics has a high sintering temperature of approximately 1300 ℃ and a considerable grain size of dozens micron. The relative dielectric constant at room temperature (RT) is approximately 1500-2000, and numerous pores result in large dielectric loss of 5%[1,2]. However, BaTiO3 application is greatly limited by the strong dielectric peak derived from the first phase transition. This characteristic imply the necessary to improve temperature stability, regulate the grain size distribution, and reduce the dielectric loss of BaTiO3 by doping proper metal oxides, in the end, the electrical properties of BaTiO3 ceramics should satisfy the standard specified by the Electronic Industries Alliance (EIA). The two major factors that affecting the BaTiO3 dielectric properties are micro-structural homogeneity[2-4] and distribution of chemical
elements[5,6]. The use of ultrafine BaTiO3 powders as raw material could yield ceramics with a relative constant of around 4000[4]. Dy2O3 and ZnO[1,7] are generally adopted to suppress the grain growth of BaTiO3, improve the RT dielectric constant, and reduce dielectric loss for reason that the porosity coalescence and removal was promoted during sintering process. ZrO2, Nb2O5, and SnO2[8-10] are usually adopted by single doping or codoping method. The element substitution effect occurred in B-site of perovskite lattice could result in reducing of Curie temperature and diffuse phase transition (DPT) on temperature dependence. Most importantly, the temperature stability of BaTiO3 ceramics would be remarkably enhanced. In the previous work[11], we have performed Zn2SnO4-doped BaTiO3 ceramics and got something outstanding results. In details, pre-synthesized BaTiO3 and Zn2SnO4 powders were adopted as raw materials to synthesize Zn2SnO4-doped BaTiO3 (ZS-BT) ceramics with different Zn2SnO4 doping contents. In this paper, we intend to deeply discuss the temperature stability influenced by doping Zn2SnO4, based on the issues of microscopic evolution, crystal lattice parameter variations and impedance spectroscopic analysis. It is mainly because zinc and tin are two excellent addition oxides, i.e., zinc could suppress the BaTiO3 grain
growth
during
sintering
and
improve
the
homogeneity
micro-structure, while tin could easily replace titanium in B-site of the
perovskite lattice, which could result in DPT phenomenon. We assumed that pre-synthesized Zn2SnO4 powders could be uniformly distributed around BaTiO3 powder agglomerates by ball-milling process, thereby this special preparation method could provide uniform concentrations of addition sources for sintering process and desired dielectric properties such as temperature stability. 2. Experimental procedure The raw materials are analytically pure BaCO3 (98.5%), TiO2 (98%), SnO2 (99.5%) and ZnO (99.5%). Firstly, BaCO3 and TiO2 mixtures with mole ratio 1：1 are prepared by ball-milling process for 6 h using zirconium ball in alcohol, followed by drying, and then pressed into discs with 30 mm in diameter under 100MPa. The discs were calcined at 1100 ℃ for 2 h and then ground into BaTiO3 powders. Secondly, the process above was repeated again to obtain Zn2SnO4, i.e., ZnO and SnO2 mixtures with mole ratio 2：1 were prepared by ball-milling process, the drying powders was pressed into discs and calcined at 1000 ℃ for 2 h, and then ground into Zn2SnO4 powders. Finally, pre-synthesized BaTiO3 powders and Zn2SnO4 powders were mixed with various BaTiO3：Zn2SnO4 mole ratios of (1-x) mol：x mol, while x = 0.002, 0.004, 0.006, 0.008, 0.02, 0.04, 0.06, and 0.08 (named as ZS0.2%, ZS0.4%, ZS0.6%, ZS0.8%, ZS2%, ZS4%, ZS6%, and ZS8%, respectively). The above eight species were mixed by ball-milling process for 6 h in alcohol. Then, the slurry was
dried and pressed into discs with 15 mm in diameter and roughly 1 mm in thickness by 200 MPa pressure. The eight species discs were calcined at 1260 ℃ for 2 h, the obtained ceramics were polished, then coated with silver paste on the upper and bottom surfaces and fired at 600 ℃。 The JSM-5610LV scanning electron microscope (SEM) was used to characterize the fracture plane of samples. The crystallographic structure was investigated by X-ray diffraction (XRD) on a D8 ADVANCE diffractometer using Cu Kα radiation. The temperature and frequency dependence of the dielectric constant and loss were measured using the Agilent 4294A precision impedance analyzer in vacuum. Liquid nitrogen was used as cold source and a heater coil was used for heating process. 3. Results and discussion Zn2SnO4 significantly suppress the dielectric peaks and raise dielectric constant between room and Curie temperatures (Fig.1(a)). The dielectric peaks at Curie temperature present the DPT phenomenon. With increasing dopant contents, the temperature of the tetragonal phase to the cubic phase decrease. The Curie temperatures of sample ZS0.2% and ZS 0.4% were 121.9 ℃ and 117.3 ℃, respectively. Sample ZS2% has a temperature stability of -35% to 12% and dielectric loss is less than 4% between -25 ℃ and 125 ℃(Fig.1(b)), which is closed to the EIA-X7R specification. The temperature stability between -30 ℃ and 85 ℃ is from -21% to 9% while dielectric loss is less than 4.2%, and the dielectric
properties meet the requirement of EIA-Y5V. The variation of the Curie temperature may be originated from the element substitution of Ti4+ with Sn4+[10]. The dielectric peaks of ZS4% and ZS8% were severely suppressed, which result in poor temperature stability. In this study, the temperature dependence of dielectric loss was investigated, and whether two different dependences of temperature exist was explored. Different Zn2SnO4 doping contents were tested at two temperature regions, marked as regions A (25 ℃-100 ℃) and region B (100 ℃-200 ℃) (Fig. 1(b)). Results indicate that the dielectric loss of ZS0.2%、ZS0.4%、ZS0.6% and ZS2% increase with increasing Zn2SnO4 contents in region A, on the contrary the loss decrease with doping in region B. This phenomenon will be discussed in detail later. Distinguishing the microstructure of grain and grain boundary in ZS0.2% is difficult (Fig. 2(a)), while in ZS0.4%(Fig. 2(b)) the fracture plane is apparently rough and has fewer pores compared with sample ZS0.2%. Pores are dramatically reduced in sample ZS0.8% and a rough microstructure of grain and grain boundary is observed (Fig. 2(c)). In sample ZS2% the grain could be well separated from the grain boundary with grain size of roughly 0.5 μm -1 μm, and the pores vanish (Fig. 2(d)). Thus, doping Zn2SnO4 could improve grain size distribution, constrain grain growth, promote the porosity removal, and mostly importantly generate a homogeneous microstructure of fine-grained BaTiO3. This
phenomenon could be attributed to the Zn2SnO4 uniform distribution among the BaTiO3 powder agglomerates during ball-milling process, and zinc element plays a key role in inhibiting uncontrolled growth of BaTiO3 grain during sintering [1]. ZS2% exist in tetragonal phase at RT with second phase Zn2SnO4 existing(Fig. 3). The XRD results were analyzed using the software Jade 6.0[12]. The grain size range from 378 nm to 854 nm by calculating each diffraction peaks, and Zn2SnO4 has a huge grain size >1000 nm. The average stress of the different sizes grain was approximately 0.152% by considering the peak broadening which resulted from inter-grain stress. BaTiO3 exist in tetragonal phase in RT with P4mm point group, lattice parameters a=b=3.986 Å, c=4.026 Å and axial ratio c/a=1.01003. Ti4+ (ionic radius, 0.605 Å) substitution with Sn4+ (ionic radius, 0.69 Å) could result in lattice distortion, and the ZS2% lattice parameters were calculated as a=b=3.99839 Å (0.31% increase), c=4.02682 Å (0.00204% increase), and axial ratio c/a=1.00711 (8.4% decrease). The decrease of axial ratio lead to decrease of spontaneous polarization. Therefore, DPT appears around the dielectric peaks. Dielectric loss is closely related to the charge carriers in dielectric. The relaxation behavior of charge carriers responsed to alternating current could reflect the information of dielectric loss. Two different temperature dependence of dielectric loss exist over a wide temperature range (Fig.
1(b)) as mentioned previous, i.e. the dielectric loss of ZS0.2%、ZS0.4%、 ZS0.6% and ZS2% increase with increasing Zn2SnO4 contents in region A, on the contrary the dielectric loss decrease with increasing Zn2SnO4 contents in region B. The frequency dependence of complex impedance for sample ZS0.2% and ZS2% over a wide temperature range was measured. The data was converted into relative permittivity complex plane (Fig. 4(a) and Fig. 5(a)) and frequency dependence of tanδ (Fig. 4(b) and Fig. 5(b)) based on Eq. (1): ε* =
1 = ε ′ + iε ′′ iωC 0 Z *
(1)
Where ε* represent the complex dielectric constant, Z* is complex impedance and C0 is the vacuum capacitance of sample. Two types of relaxation behaviors were found from 40 Hz to 106 Hz, namely, a low-frequency relaxation (40 Hz-103 Hz) originated from the grain boundary and a high-frequency relaxation (104 Hz-106 Hz) originated from the grain[13-15]. The relaxation behavior originated from the thermal actived charge carriers could be fitted with the Arrhenius relationship as given in Eq. (2). The calculated results indicate that, for sample ZS0.2%, the high-frequency grain relaxation energy Qgrain is 0.5767 eV, and the low-frequency grain boundary relaxation energy Qgrain boundary
is 0.9908 eV. The corresponding values for sample ZS2% are
0.3622eV and 2.2824 eV. It is obvious that the grain relaxation activation
energy evidently decrease with increasing Zn2SnO4 contents, on the contrary the grain boundary relaxation activation energy increase with increasing Zn2SnO4 contents. f peak = A exp( −
Q ) kT
(2)
where A is the pre-exponential factor, k is Boltzmann constant, T is the temperature in Kelvin and Q is relaxation activation energy. The different charge carriers lead to different response mechanisms towards alternating current. The high-frequency relaxation behavior is assumed to be a Debye-type relaxation caused by dipoles in the grain with minor activation energy of 0.3 eV-0.5 eV[17,19]. For perovskite dielectric, the relaxation behavior in paraelectric phase caused by thermal activation is related to oxygen vacancy[16-18,20] with higher activation energy of 1 eV-2 eV. Therefore the low-frequency relaxation behavior could be related to the hopping charge carriers of oxygen vacancy[16,20] in the grain boundary. The dielectric loss was modified by relaxation activation energy [21] in different temperature regions. In temperature around RT as shown in region A, the dielectric loss measured at 1k Hz was mainly affected by dipoles in the grain. Consequently, the dielectric loss increase and the high-frequency relaxation activation energy decrease with increasing Zn2SnO4 contents. In paraelectric phase at high temperature as shown in region B, the dielectric loss measured at 1k Hz is mainly affected by
oxygen vacancy in the grain boundary. Consequently, dielectric loss decrease and the low-frequency relaxation activation energy increase with increasing Zn2SnO4 contents. 4. Conclusion Thus, the remarkable enhanced temperature stability of ZS-BT ceramics was attributed to the following reasons: 1. Zn2SnO4 addition result in a homogeneous fine-grained microstructure with grain size <1 μm, which promote porosity coalescence and removal, thereby increase the relative constant and reduce dielectric loss around room temperature. 2. Ti4+ substitution with Sn4+ during sintering result in distortion of the perovskite lattice. The ionic radius difference between two elements lead to the increase in lattice parameters a, b, and c, Thereby the relative constant were promoted by inner-stress of the grain. However, the decrease of the axial ratio c/a lead to the decrease of spontaneous polarization. Consequently, diffuse phase transition appear around the dielectric peaks. 2. The dielectric loss around RT was modified by the high-frequency relaxation activation energy generated by the dipoles in the grain. The dielectric loss increase with increasing Zn2SnO4 doping contents. Acknowledgement
The authors gratefully acknowledge the Education Department of Henan province, science and technology research projects: 14A480001, for financial support. References [1] A. C. Caballero, J. F. Fernandez, C. Moure, P. Duran, J. Eur. Ceram. Soc., 17(1997)513. [2] W. L. Luan, L. Gao, J. K. Guo, Ceram. Int., 25(1999)727. [3] X. H. Wang, R. Z. Chen, Z. L. Gui, L. T. Li, Mater. Sci. Eng. B, 99(2003)199. [4] P. Zheng, J. L. Zhang, Y. Q. Tan, C. L. Wang, Acta Mater., 60(2012)5022. [5] D. Hennings, G. Rosenstein, J. Am. Ceram. Soc., 67(1984)249. [6] A. C. Caballero, J. F. Fernandez, C. Moure, P. Duran, J. L. G. Fierro, J. Eur. Ceram. Soc., 17(1997)1223. [7] V. Paunovic, V. V. Mitic, Z. Prijic, Lj. Zivkovic, Ceram. Int., 40(2014)4277. [8] M. R. Nateghi, M. R. Barzegari, J. Alloys Compd., 452(2008)36. [9] M. Cernea, B. S. Vasile, A. Boni, A. Iuga, J. Alloys Compd., 587(2014)553. [10] X. Y. Wei, X. Yao, Mater. Sci. Eng. B, 137(2007)184. [11] Y. P. Zhou, F. Z. Zhou, G. Z. Zang, Y. Q. Wang, Y. H. Dong, L. B. Li, J. Funct. Mater., 7(2014)07039. [12] J. L. Ye, C. C. Wang, W. Ni, X. H. Sun, J. Alloys Compd., 617(2014)850. [13] A. Huanosta, A. R. West, J. Appl. Phys., 61(1987)5386. [14] N. Hirose, A. R. West, J. Am. Ceram. Soc., 79(1996)1633. [15] G. H. Cao, L. X. Feng, C. Wang, J. Phys. D: Appl. Phys., 40(2007)2899. [16] O. Bidault, P. Goux, M. Kchikech, M. Belkaoumi, M. Maglione, Phys. Rev. B: Condens. Matter, 49(1994)7868. [17] Chen Ang, Z. Yu, L. E. Cross, Phys. Rev. B: Condens. Matter, 62(2000)228. [18] X. Y. Wei, X. Wan, X. Yao, J. Electroceram., 21(2008)226. [19] D. W. Johnson, L. E. Cross, F. A. Hummel, J. Appl. Phys., 41(1970)2828. [20] R. Stumpe, D. Wagner, D. Bäuerle, Phys. stat. sol. (a), 75(1983)143.
[21] J. L. Li, F. Li, Y. Y. Zhuang, L. Jin, L. H. Wang, X. Y. Wei, Z. Xu, S. J. Zhang, J. Appl. Phys., 116(2014)074105.
Fig Caption FIG. 1. Temperature dependence of dielectric constant and dielectric loss: (a) Temperature dependence of εr at 1k Hz for ZS-BT ceramics, (b) Temperature dependence of tanδ at 1k Hz for ZS-BT ceramics.
FIG. 2. SEM micrograph of samples with varying Zn2SnO4 contents: (a) SEM micrograph of ZS0.2% at RT, (b) SEM micrograph of ZS0.4% at RT, (c) SEM micrograph of ZS0.8% at RT, (d) SEM micrograph of ZS2% at RT. FIG. 3. XRD pattern of ZS2% at RT.
FIG. 4. Dielectric relaxation behaviour of sample ZS0.2%: (a) Relative permittivity complex plane plot at various temperatures, (b) Frequency dependence of tanδ at various temperatures.
FIG. 5. Dielectric relaxation behaviour of sample ZS2%: (a) Relative permittivity complex plane plot at various temperatures, (b) Frequency dependence of tanδ at various temperatures.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5