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Study of the dielectric responses of Eu-doped CaCu3Ti4O12
Journal of Alloys and Compounds 699 (2017) 278e282
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Study of the dielectric responses of Eu-doped CaCu3Ti4O12 M. Li*, Q. Liu, C.X. Li Department of Applied Physics, Xi'an University of Technology, Xi'an 710054, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 December 2016 Received in revised form 28 December 2016 Accepted 30 December 2016 Available online 1 January 2017
We reported the effect of europium (Eu) on the dielectric properties of CaCu3Ti4O12 (CCTO). The experimental results show that the doping of Eu on a calcium (Ca) site improves the dielectric loss and decreases the dielectric constant of CCTO. To understand the underlying reasons for the depressed dielectric response of Eu-doped CCTO, scanning electron microscopy, high-temperature dielectric measurement, and complex impedance measurement were carried out. The DC bias was also used in the measurements. The results show that the depressed dielectric responses of Eu-doped CCTO may be related to the lack of the oxygen vacancies in the samples. Furthermore, the absence of the electrode contact response may be another factor responsible for the decreased dielectric constant. © 2017 Elsevier B.V. All rights reserved.
Keywords: CaCu3Ti4O12 Dielectric constant Oxygen vacancy Electrode contact response
1. Introduction CaCu3Ti4O12 (CCTO) with a high dielectric constant and weak temperature dependence has attracted much attention in recent years due to its potential applications in capacitors and transient voltage suppression devices [1e3]. Theoretical calculations reveal that CCTO provides dielectric constants only in the range of 40e50 [4]. Impedance spectroscopy study on CCTO ceramics shows that they are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. Therefore, the giant dielectric constant of CCTO is attributed to an internal barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure [5e8]. The semiconductivity of the grains may arise from a small amount of oxygen loss during ceramic processing in air. On cooling, limited reoxidation can produce thin insulating layers on the outer surfaces of pellets or along the grain boundaries [5,9]. Therefore, the oxygen vacancies play an important role in the dielectric behavior of CCTO [10e14]. The electrode contact response has been suggested to contribute to the high dielectric constant as well [15e17]. However, the dielectric loss of CCTO is rather high (0.1 at 1 kHz) from an application viewpoint. Improvements are still needed to facilitate the device implementations. Among various methods, doping is a common and an effective way to improve the properties of the materials. Considering that substitution of a trivalent rare
* Corresponding author. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.jallcom.2016.12.422 0925-8388/© 2017 Elsevier B.V. All rights reserved.
earth ion to the Ca site will create 1/3 vacancies on the Ca site to achieve charge neutrality, doping of rare elements may cause some changes in the dielectric response of CCTO. In this study, we report the effects of Eu-doping on the dielectric responses of CCTO. Doping of Eu on the Ca site was found to improve the dielectric loss of CCTO. However, the dielectric constant decreases. Scanning electron microscopy (SEM), high-temperature dielectric measurement, and complex impedance measurement were carried out. The possible reasons for the depressed dielectric response were discussed.
2. Material and methods Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6 and 0.9) powders were synthesized via the conventional solid-state reaction method. High-purity CaCO3, CuO, TiO2, and Eu2O3 were weighed according to the stoichiometric ratios and mixed thoroughly in an agate mortar. The mixed powders were calcined in air at 1000 C for 12 h and at 1100 C for 24 h with an intermediate grinding. The calcined samples were milled and pressed into pellets of 5 mm in diameter and approximately 0.8 mm in thickness. Then, the pellets were sintered in air at 1100 C for 12 h. X-ray powder diffraction (XRD) data were recorded on an x-ray diffractometer (PANalytical X'pert PRO, the Netherlands). The surface morphologies of the samples were obtained using SEM (FEI, XL-30). To measure the dielectric properties, silver electrodes were painted on the samples' surfaces. Dielectric and complex impedance data were collected using an Agilent-4294A impedance analyzer with an AC voltage of 0.5 V. The
M. Li et al. / Journal of Alloys and Compounds 699 (2017) 278e282
measurements were performed between 300 and 473 K over the frequency range of 40 Hze2 MHz and DC bias range of 0e20 V. 3. Results and discussion The XRD patterns of pure and Eu-doped CCTO are shown in Fig. 1. All the diffraction peaks can be exactly assigned to the standard data of CCTO (PDF#75-2188), and no impurities were detected. Therefore, the doped Eu ions do not cause any significant changes to the crystal structure. Notably, the diffraction peaks have a slight shift toward high angles with the increasing Eu content (see the inset of Fig. 1), which is due to the different ionic radii of Eu3þ (94.7 pm), and vacancies existed on the Ca site. XRD patterns show that Eu3þ enters into the lattice and substitutes for Ca2þ. The microstructures of pure and Eu-doped CCTO are examined using SEM technique (see Fig. 2). The grain sizes of CCTO are inhomogeneous, consisting of a few very large grains (larger than 40 mm in size) isolated by fine grains (approximately 10 mm in size). Moreover, the grains of CCTO are irregular in shape. However, the grain sizes largely decrease by the doping of Eu. The largest grains of the Eu-doped samples are smaller than 10 mm in size. For the samples of x ¼ 0.1, 0.3, the grains are inhomogeneous in size and irregular in shape, similar to CCTO. When x increases, the sizes of grain trend toward homogeneousness, and the shape of the grain becomes cubic. This indicates that the microstructure of CCTO is largely changed by the doping of Eu. The frequency-dependent dielectric responses of pure and Eudoped CCTO measured at room temperature are shown in Fig. 3a. This indicates that the doping of Eu improves the loss tangent of CCTO. However, the dielectric constant is depressed by the doping of Eu. Specifically, the dielectric constants of the samples decrease as x increases. The dielectric constants and loss tangents of Ca1xEu2/3xCu3Ti4O12 are 9600 and 0.142 (x ¼ 0), 2900 and 0.050 (x ¼ 0.9), 2500 and 0.031 (x ¼ 0.3), 2200 and 0.037 (x ¼ 0.6), and 2000 and 0.030 (x ¼ 0.9), respectively. It is apparent that the doping of Eu reduces the loss tangent by more than 64%. Although the dielectric constant also decreases, it still maintains a high magnitude of 103. The temperature-dependent dielectric responses of pure and Eu-doped CCTO were also investigated. The results are shown in Fig. 3b. The samples of x ¼ 0, 0.1, and 0.3 were found to exhibit good temperature-independent dielectric constants below 360 K. Above 360 K, the dielectric constants of the three samples
Fig. 1. XRD patterns for Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6 and 0.9). The inset is the expanded view of the XRD patterns in the 2q range of 48e51.
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rapidly increase with the increasing temperature. The large increases in dielectric constants are accompanied with loss tangent peaks, indicating the high-temperature relaxation existing in the three samples. For the samples of x ¼ 0.6 and 0.9, the dielectric constants only slowly increase as the temperature increases, and no peaks were found in the loss tangents plots. Thus, the two samples do not exhibit high-temperature dielectric relaxation. The increments of dielectric constants in the temperature range of 300e473 K are about 370% (x ¼ 0), 410% (x ¼ 0.1), 280% (x ¼ 0.3), 150% (x ¼ 0.6), and 200% (x ¼ 0.9), respectively. In order to explore the observed electrical responses of pure and Eu-doped samples, the DC bias is applied to the measurements. Fig. 4 shows the frequency-dependent dielectric responses with response to DC bias. For the samples of x ¼ 0, 0.1, and 0.3, the lowfrequency dielectric constants increase as the DC bias increases, and the increments of the low-frequency dielectric constants gradually decrease with the increasing x. However, for the samples of x ¼ 0.6 and x ¼ 0.9, the dielectric constants do not show any changes as the DC bias increases. That the low-frequency dielectric constant of CCTO increases with the increasing DC bias indicates the existence of electrode contact response in the sample [16]. Therefore, the results indicate that the doping of Eu depresses the electrode contact response of CCTO. When x increases to a value larger than 0.6, the electrode contact response disappears. The complex impedance plots of pure and Eu-doped CCTO measured at room temperature are presented in Fig. 5. Semicircular arcs with nonzero intercepts on the Z-axis were observed for all the samples. The diameters of the low-frequency arcs correspond to the grain boundary resistances, whereas the high-frequency intercepts with the Z-axis (see the inset of Fig. 5) correspond to the grain resistances. It is apparent that all the samples are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. As the x increases, both the grain resistances and grain boundary resistances increase. It is known that the semiconductivity of grains may arise from a small amount of oxygen loss during ceramic processing in the air. On cooling, limited reoxidation along the grain boundaries must occur to produce insulating layers. Since the doping of Eu increases both the grain and grain boundary resistance of CCTO, it can be deduced that the doping of Eu decreases the content of oxygen vacancies both in grains and at grain boundary. Furthermore, the high-temperature dielectric relaxation of CCTO is related to the oxygen vacancies or the oxygen vacancy-related point defect located at the grain boundary [18,19]. The depressed hightemperature dielectric relaxation also indicates the decrease of oxygen vacancies in the Eu-doped CCTO. Furthermore, oxygen vacancy plays an important role in the grain growth of CCTO [13]. The increased oxygen vacancies at the grain boundary can promote more grain growth [20]. Thus, the Eu-doped samples with low oxygen vacancies will exhibit smaller grain size, in accordance with our experimental results. Therefore, we can infer that the doping of Eu decreases the content of oxygen vacancies and thus depresses the grain growth. According to the internal barrier layer capacitor model, the effective dielectric constant εeff can be represented by the following equation [21e23] as εeff ¼ εr(tg/tgb), where tg is the grain average size, tgb is the average thickness of the grain boundary layer, and εr is the relative dielectric constant of the grain boundary layer. Therefore, the sample with a larger grain size will exhibit a higher dielectric constant, in accordance with our experimental results that Eu-doped samples with small grain sizes exhibit relatively low dielectric constants. Therefore, the decreased grain size may be one of the important reasons for the decreased dielectric constants of the Eu-doped samples. As discussed above, the decreased grain size may due to the decreased oxygen vacancies in Eu-doped samples.
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Fig. 2. The SEM micrograph of Ca1-xEu2/3xCu3Ti4O12: (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.3, (d) x ¼ 0.6, and (e) x ¼ 0.9.
Fig. 3. The frequency-dependent dielectric responses measured at room temperature (a) and temperature-dependent dielectric responses measured at 1 kHz (b) of Ca1-xEu2/ (x ¼ 0, 0.1, 0.3, 0.6, and 0.9).
3xCu3Ti4O12
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Fig. 4. The frequency-dependent dielectric constants of Ca1-xEu2/3xCu3Ti4O12 under the different DC biases: (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.3, (d) x ¼ 0.6, and (e) x ¼ 0.9. The measurements were carried out at room temperature.
Thus, it can be inferred that the lack of oxygen vacancies prevent the grain growth and thus decrease the dielectric constants of Eudoped CCTO. On the other hand, oxygen vacancies in CCTO will create carriers which contribute to the dielectric polarization [10]. Thus, the lack of oxygen vacancies in Eu-doped CCTO will decrease the oxygen vacancies-related dielectric polarization, which also induce the decrease of the dielectric constants to some extent. In any case, the decreased dielectric constants of Eu-doped CCTO are related to the low concentrations of oxygen vacancies. As regards the reasons for the deficiency of oxygen vacancies in Eu-doped samples, this may be due to the different bond strengths of Eu-O and Ca-O.
In addition, the contact effects at the electrode/sample interfaces contributes partly to the high dielectric constant of CCTO ceramic [16,24,25]. The CCTO pellets with Ag electrodes coated on the surfaces can be equivalent to two metaleinsulatoresemiconductor (MIS) structures connected in series [16,24]. The total capacitance of the two MIS structures first decreases and then increases with increasing DC bias [16]. Since the shift voltages are smaller than 5 V, we can only observes the monotonically increasing of the low-frequency dielectric constants in Fig. 4. We found that the electrode contact response gradually disappears as the x increases. Since the electrode contact response contributes partly to the total dielectric constant of CCTO, the
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References
Fig. 5. The complex impedance plots of Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6, and 0.9) measured at room temperature. The inset shows the expanded view of the highfrequency impedance data close to the origin.
absence of electrode contact response may be another reason for the decreased the dielectric constant of Eu-doped CCTO. Further investigations are needed to explore the reasons for the disappearance of the electrode contact response.
4. Conclusion The dielectric responses of Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6, and 0.9) were investigated. The results show that the doping of Eu improves the loss tangent of CCTO. Although the dielectric constant decreases, it still maintains a high magnitude of 103. The SEM measurements reveal that the doping of Eu largely changes the microstructure of CCTO. The average grain size decreases, and the size distribution trends toward homogeneousness because of the Eu doping. The high-temperature dielectric measurements show that the high-temperature relaxation of CCTO was largely depressed by doping of Eu. The complex impedance measurements indicate that both the grain and grain boundary resistances increase with the increasing x. We speculate that the depressed dielectric response of CCTO may be related to the lack of the oxygen vacancies in the Eu-doped samples. Moreover, we found that the electrode contact response gradually disappears as the x increases. This may be another factor responsible for the decreased dielectric constant of Eu-doped CCTO.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant number11304244).
[1] M.A. Subramanian, D. Li, N. Duan, B.A. Reisner, A.W. Sleight, High dielectric constant in ACu3Ti4O12 and ACu3Ti3FeO12 phases, J. Solid State Chem. 151 (2000) 323e325. [2] C.H. Kim, Y.H. Jang, S.J. Seo, C.H. Song, J.Y. Son, Y.S. Yang, J.H. Cho, Effect of Mn doping on the temperature-dependent anomalous giant dielectric behavior of CaCu3Ti4O12, Phys. Rev. B 85 (2012) 245210. [3] J.Y. Li, X.T. Zhao, F. Gu, S.T. Li, Defects and dc electrical degradation in CaCu3Ti4O12 ceramics: role of oxygen vacancy migration, Appl. Phys. Lett. 100 (2012) 202905. [4] L. He, J.B. Neaton, M.H. Cohen, D. Vanderbilt, First-principles study of the structure and lattice dielectric response of CaCu3Ti4O12, Phys. Rev. B 65 (2002) 214112. [5] D.C. Sinclair, T.B. Adams, F.D. Morrison, A.R. West, CaCu3Ti4O12: one-step internal barrier layer capacitor, Appl. Phys. Lett. 80 (2002) 2153e2155. [6] T.T. Fang, C.P. Liu, Evidence of the internal domains for inducing the anomalously high dielectric constant of CaCu3Ti4O12, Chem. Mater. 17 (2005) 5167e5171. [7] J.L. Zhang, P. Zheng, C.L. Wang, M.L. Zhao, J.C. Li, J.F. Wang, Dielectric dispersion of CaCu3Ti4O12 ceramics at high temperatures, Appl. Phys. Lett. 87 (2005) 142901. [8] D. Capsoni, M. Bini, V. Massarotti, G. Chiodelli, M.C. Mozzatic, C.B. Azzoni, Role of doping and CuO segregation in improving the giant permittivity of CaCu3Ti4O12, J. Solid State Chem. 177 (2004) 4494e4500. [9] T.B. Adams, D.C. Sinclair, A.R. West, Giant barrier layer capacitance effects in CaCu3Ti4O12 ceramics, Adv. Mater. 14 (2002) 1321e1323. [10] C.C. Wang, L.W. Zhang, Oxygen-vacancy-related dielectric anomaly in CaCu3Ti4O12: post-sintering annealing studies, Phys. Rev. B 74 (2006) 024106. [11] X.J. Luo, Y.S. Liu, C.P. Yang, S.S. Chen, S.L. Tang, K. Barner, Oxygen vacancy related defect dipoles in CaCu3Ti4O12: detected by electron paramagnetic resonance spectroscopy, J. Eur. Ceram. Soc. 35 (2015) 2073e2081. [12] Z. Li, P. Zhao, Z. Wang, X.H. Xue, Z.T. Li, H.Q. Fan, Dielectric behavior associated with the synergetic microstructure and oxygen vacancies in CaCu3Ti4O12 ceramics, J. Mater. Sci. Mater. Electron 27 (2016) 7327e7334. [13] A.F. Anderson, D.N.B. Vinícius, O.O. Marcelo, V. Damjan, S. Matja z, L. Elson, , Role of oxygen on the phase stability and microstructure S. Danilo, A.V. Jose evolution of CaCu3Ti4O12 ceramics, J. Eur. Ceram. Soc. 37 (2017) 129e136. [14] Q. Zheng, H.Q. Fan, C.B. Long, Microstructures and electrical responses of pure and chromium-doped CaCu3Ti4O12 ceramics, J. Alloys Compd. 511 (2012) 90e94. [15] S.Y. Chung, I.D. Kim, S.J.L. Kang, Strong nonlinear currentevoltage behaviour in perovskite-derivative calcium copper titanate, Nat. Mater. 3 (2004) 774e778. [16] M. Li, D.F. Zhang, W.Y. Wang, G. Wang, X.L. Chen, The effects of grain boundary response and electrode contact response on the dielectric properties of CaCu3Ti4O12, J. Phys. D. Appl. Phys. 43 (2010) 295405. [17] M. Li, D.C. Sinclair, A.R. West, Extrinsic origins of the apparent relaxorlike behavior in CaCu3Ti4O12 ceramics at high temperatures: a cautionary tale, J. Appl. Phys. 109 (2011) 084106. [18] R.K. Grubbs, E.L. Venturini, P.G. Clem, J.J. Richardson, B.A. Tuttle, G.A. Samara, Dielectric and magnetic properties of Fe- and Nb-doped CaCu3Ti4O12, Phys. Rev. B 72 (2005) 104111. [19] N. Lei, X.M. Chen, Dielectric relaxations and formation mechanism of giant dielectric constant step in CaCu3Ti4O12 ceramics, Appl. Phys. Lett. 91 (2007) 122905. [20] F.A.R. Mohd, D.H. Sabar, A.A. Zainal, F.A. Mohd, J.M. Julie, Characterization of microstructures evolution on electrical responses of CaCu3Ti4O12 ceramics, J. King Saud. Univ. Eng. Sci. 28 (2016) 240e245. [21] L. Ni, X.M. Chen, X.Q. Liu, R.Z. Hou, Microstructure-dependent giant dielectric response in CaCu3Ti4O12 ceramics, Solid State Commun. 139 (2006) 45e50. [22] A.R. West, T.B. Adams, F.D. Morrison, D.C. Sinclair, Novel high capacitance materials: BaTiO3: La and CaCu3Ti4O12, J. Eur. Ceram. Soc. 24 (2004) 1439e1448. [23] W.X. Yuan, Z.K. Luo, C.D. Wang, Investigation on effects of CuO secondary phase on dielectric properties of CaCu3Ti4O12 ceramics, J. Alloys Comp. 562 (2013) 1e4. [24] M. Li, Z.J. Shen, M. Nygren, A. Feteira, D.C. Sinclair, A.R. West, Origin (s) of the apparent high permittivity in CaCu3Ti4O12 ceramics: clarification on the contributions from internal barrier layer capacitor and sample-electrode contact effects, J. Appl. Phys. 106 (2009) 104106. [25] J. Yang, M.R. Shen, L. Fang, The electrode/sample contact effects on the dielectric properties of the CaCu3Ti4O12 ceramic, Mater. Lett. 59 (2005) 3990e3993.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Study of the dielectric responses of Eu-doped CaCu3Ti4O12 M. Li*, Q. Liu, C.X. Li Department of Applied Physics, Xi'an University of Technology, Xi'an 710054, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 December 2016 Received in revised form 28 December 2016 Accepted 30 December 2016 Available online 1 January 2017
We reported the effect of europium (Eu) on the dielectric properties of CaCu3Ti4O12 (CCTO). The experimental results show that the doping of Eu on a calcium (Ca) site improves the dielectric loss and decreases the dielectric constant of CCTO. To understand the underlying reasons for the depressed dielectric response of Eu-doped CCTO, scanning electron microscopy, high-temperature dielectric measurement, and complex impedance measurement were carried out. The DC bias was also used in the measurements. The results show that the depressed dielectric responses of Eu-doped CCTO may be related to the lack of the oxygen vacancies in the samples. Furthermore, the absence of the electrode contact response may be another factor responsible for the decreased dielectric constant. © 2017 Elsevier B.V. All rights reserved.
Keywords: CaCu3Ti4O12 Dielectric constant Oxygen vacancy Electrode contact response
1. Introduction CaCu3Ti4O12 (CCTO) with a high dielectric constant and weak temperature dependence has attracted much attention in recent years due to its potential applications in capacitors and transient voltage suppression devices [1e3]. Theoretical calculations reveal that CCTO provides dielectric constants only in the range of 40e50 [4]. Impedance spectroscopy study on CCTO ceramics shows that they are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. Therefore, the giant dielectric constant of CCTO is attributed to an internal barrier layer capacitance (IBLC) instead of an intrinsic property associated with the crystal structure [5e8]. The semiconductivity of the grains may arise from a small amount of oxygen loss during ceramic processing in air. On cooling, limited reoxidation can produce thin insulating layers on the outer surfaces of pellets or along the grain boundaries [5,9]. Therefore, the oxygen vacancies play an important role in the dielectric behavior of CCTO [10e14]. The electrode contact response has been suggested to contribute to the high dielectric constant as well [15e17]. However, the dielectric loss of CCTO is rather high (0.1 at 1 kHz) from an application viewpoint. Improvements are still needed to facilitate the device implementations. Among various methods, doping is a common and an effective way to improve the properties of the materials. Considering that substitution of a trivalent rare
* Corresponding author. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.jallcom.2016.12.422 0925-8388/© 2017 Elsevier B.V. All rights reserved.
earth ion to the Ca site will create 1/3 vacancies on the Ca site to achieve charge neutrality, doping of rare elements may cause some changes in the dielectric response of CCTO. In this study, we report the effects of Eu-doping on the dielectric responses of CCTO. Doping of Eu on the Ca site was found to improve the dielectric loss of CCTO. However, the dielectric constant decreases. Scanning electron microscopy (SEM), high-temperature dielectric measurement, and complex impedance measurement were carried out. The possible reasons for the depressed dielectric response were discussed.
2. Material and methods Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6 and 0.9) powders were synthesized via the conventional solid-state reaction method. High-purity CaCO3, CuO, TiO2, and Eu2O3 were weighed according to the stoichiometric ratios and mixed thoroughly in an agate mortar. The mixed powders were calcined in air at 1000 C for 12 h and at 1100 C for 24 h with an intermediate grinding. The calcined samples were milled and pressed into pellets of 5 mm in diameter and approximately 0.8 mm in thickness. Then, the pellets were sintered in air at 1100 C for 12 h. X-ray powder diffraction (XRD) data were recorded on an x-ray diffractometer (PANalytical X'pert PRO, the Netherlands). The surface morphologies of the samples were obtained using SEM (FEI, XL-30). To measure the dielectric properties, silver electrodes were painted on the samples' surfaces. Dielectric and complex impedance data were collected using an Agilent-4294A impedance analyzer with an AC voltage of 0.5 V. The
M. Li et al. / Journal of Alloys and Compounds 699 (2017) 278e282
measurements were performed between 300 and 473 K over the frequency range of 40 Hze2 MHz and DC bias range of 0e20 V. 3. Results and discussion The XRD patterns of pure and Eu-doped CCTO are shown in Fig. 1. All the diffraction peaks can be exactly assigned to the standard data of CCTO (PDF#75-2188), and no impurities were detected. Therefore, the doped Eu ions do not cause any significant changes to the crystal structure. Notably, the diffraction peaks have a slight shift toward high angles with the increasing Eu content (see the inset of Fig. 1), which is due to the different ionic radii of Eu3þ (94.7 pm), and vacancies existed on the Ca site. XRD patterns show that Eu3þ enters into the lattice and substitutes for Ca2þ. The microstructures of pure and Eu-doped CCTO are examined using SEM technique (see Fig. 2). The grain sizes of CCTO are inhomogeneous, consisting of a few very large grains (larger than 40 mm in size) isolated by fine grains (approximately 10 mm in size). Moreover, the grains of CCTO are irregular in shape. However, the grain sizes largely decrease by the doping of Eu. The largest grains of the Eu-doped samples are smaller than 10 mm in size. For the samples of x ¼ 0.1, 0.3, the grains are inhomogeneous in size and irregular in shape, similar to CCTO. When x increases, the sizes of grain trend toward homogeneousness, and the shape of the grain becomes cubic. This indicates that the microstructure of CCTO is largely changed by the doping of Eu. The frequency-dependent dielectric responses of pure and Eudoped CCTO measured at room temperature are shown in Fig. 3a. This indicates that the doping of Eu improves the loss tangent of CCTO. However, the dielectric constant is depressed by the doping of Eu. Specifically, the dielectric constants of the samples decrease as x increases. The dielectric constants and loss tangents of Ca1xEu2/3xCu3Ti4O12 are 9600 and 0.142 (x ¼ 0), 2900 and 0.050 (x ¼ 0.9), 2500 and 0.031 (x ¼ 0.3), 2200 and 0.037 (x ¼ 0.6), and 2000 and 0.030 (x ¼ 0.9), respectively. It is apparent that the doping of Eu reduces the loss tangent by more than 64%. Although the dielectric constant also decreases, it still maintains a high magnitude of 103. The temperature-dependent dielectric responses of pure and Eu-doped CCTO were also investigated. The results are shown in Fig. 3b. The samples of x ¼ 0, 0.1, and 0.3 were found to exhibit good temperature-independent dielectric constants below 360 K. Above 360 K, the dielectric constants of the three samples
Fig. 1. XRD patterns for Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6 and 0.9). The inset is the expanded view of the XRD patterns in the 2q range of 48e51.
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rapidly increase with the increasing temperature. The large increases in dielectric constants are accompanied with loss tangent peaks, indicating the high-temperature relaxation existing in the three samples. For the samples of x ¼ 0.6 and 0.9, the dielectric constants only slowly increase as the temperature increases, and no peaks were found in the loss tangents plots. Thus, the two samples do not exhibit high-temperature dielectric relaxation. The increments of dielectric constants in the temperature range of 300e473 K are about 370% (x ¼ 0), 410% (x ¼ 0.1), 280% (x ¼ 0.3), 150% (x ¼ 0.6), and 200% (x ¼ 0.9), respectively. In order to explore the observed electrical responses of pure and Eu-doped samples, the DC bias is applied to the measurements. Fig. 4 shows the frequency-dependent dielectric responses with response to DC bias. For the samples of x ¼ 0, 0.1, and 0.3, the lowfrequency dielectric constants increase as the DC bias increases, and the increments of the low-frequency dielectric constants gradually decrease with the increasing x. However, for the samples of x ¼ 0.6 and x ¼ 0.9, the dielectric constants do not show any changes as the DC bias increases. That the low-frequency dielectric constant of CCTO increases with the increasing DC bias indicates the existence of electrode contact response in the sample [16]. Therefore, the results indicate that the doping of Eu depresses the electrode contact response of CCTO. When x increases to a value larger than 0.6, the electrode contact response disappears. The complex impedance plots of pure and Eu-doped CCTO measured at room temperature are presented in Fig. 5. Semicircular arcs with nonzero intercepts on the Z-axis were observed for all the samples. The diameters of the low-frequency arcs correspond to the grain boundary resistances, whereas the high-frequency intercepts with the Z-axis (see the inset of Fig. 5) correspond to the grain resistances. It is apparent that all the samples are electrically heterogeneous and consist of semiconducting grains with insulating grain boundaries. As the x increases, both the grain resistances and grain boundary resistances increase. It is known that the semiconductivity of grains may arise from a small amount of oxygen loss during ceramic processing in the air. On cooling, limited reoxidation along the grain boundaries must occur to produce insulating layers. Since the doping of Eu increases both the grain and grain boundary resistance of CCTO, it can be deduced that the doping of Eu decreases the content of oxygen vacancies both in grains and at grain boundary. Furthermore, the high-temperature dielectric relaxation of CCTO is related to the oxygen vacancies or the oxygen vacancy-related point defect located at the grain boundary [18,19]. The depressed hightemperature dielectric relaxation also indicates the decrease of oxygen vacancies in the Eu-doped CCTO. Furthermore, oxygen vacancy plays an important role in the grain growth of CCTO [13]. The increased oxygen vacancies at the grain boundary can promote more grain growth [20]. Thus, the Eu-doped samples with low oxygen vacancies will exhibit smaller grain size, in accordance with our experimental results. Therefore, we can infer that the doping of Eu decreases the content of oxygen vacancies and thus depresses the grain growth. According to the internal barrier layer capacitor model, the effective dielectric constant εeff can be represented by the following equation [21e23] as εeff ¼ εr(tg/tgb), where tg is the grain average size, tgb is the average thickness of the grain boundary layer, and εr is the relative dielectric constant of the grain boundary layer. Therefore, the sample with a larger grain size will exhibit a higher dielectric constant, in accordance with our experimental results that Eu-doped samples with small grain sizes exhibit relatively low dielectric constants. Therefore, the decreased grain size may be one of the important reasons for the decreased dielectric constants of the Eu-doped samples. As discussed above, the decreased grain size may due to the decreased oxygen vacancies in Eu-doped samples.
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Fig. 2. The SEM micrograph of Ca1-xEu2/3xCu3Ti4O12: (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.3, (d) x ¼ 0.6, and (e) x ¼ 0.9.
Fig. 3. The frequency-dependent dielectric responses measured at room temperature (a) and temperature-dependent dielectric responses measured at 1 kHz (b) of Ca1-xEu2/ (x ¼ 0, 0.1, 0.3, 0.6, and 0.9).
3xCu3Ti4O12
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Fig. 4. The frequency-dependent dielectric constants of Ca1-xEu2/3xCu3Ti4O12 under the different DC biases: (a) x ¼ 0, (b) x ¼ 0.1, (c) x ¼ 0.3, (d) x ¼ 0.6, and (e) x ¼ 0.9. The measurements were carried out at room temperature.
Thus, it can be inferred that the lack of oxygen vacancies prevent the grain growth and thus decrease the dielectric constants of Eudoped CCTO. On the other hand, oxygen vacancies in CCTO will create carriers which contribute to the dielectric polarization [10]. Thus, the lack of oxygen vacancies in Eu-doped CCTO will decrease the oxygen vacancies-related dielectric polarization, which also induce the decrease of the dielectric constants to some extent. In any case, the decreased dielectric constants of Eu-doped CCTO are related to the low concentrations of oxygen vacancies. As regards the reasons for the deficiency of oxygen vacancies in Eu-doped samples, this may be due to the different bond strengths of Eu-O and Ca-O.
In addition, the contact effects at the electrode/sample interfaces contributes partly to the high dielectric constant of CCTO ceramic [16,24,25]. The CCTO pellets with Ag electrodes coated on the surfaces can be equivalent to two metaleinsulatoresemiconductor (MIS) structures connected in series [16,24]. The total capacitance of the two MIS structures first decreases and then increases with increasing DC bias [16]. Since the shift voltages are smaller than 5 V, we can only observes the monotonically increasing of the low-frequency dielectric constants in Fig. 4. We found that the electrode contact response gradually disappears as the x increases. Since the electrode contact response contributes partly to the total dielectric constant of CCTO, the
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References
Fig. 5. The complex impedance plots of Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6, and 0.9) measured at room temperature. The inset shows the expanded view of the highfrequency impedance data close to the origin.
absence of electrode contact response may be another reason for the decreased the dielectric constant of Eu-doped CCTO. Further investigations are needed to explore the reasons for the disappearance of the electrode contact response.
4. Conclusion The dielectric responses of Ca1-xEu2/3xCu3Ti4O12 (x ¼ 0, 0.1, 0.3, 0.6, and 0.9) were investigated. The results show that the doping of Eu improves the loss tangent of CCTO. Although the dielectric constant decreases, it still maintains a high magnitude of 103. The SEM measurements reveal that the doping of Eu largely changes the microstructure of CCTO. The average grain size decreases, and the size distribution trends toward homogeneousness because of the Eu doping. The high-temperature dielectric measurements show that the high-temperature relaxation of CCTO was largely depressed by doping of Eu. The complex impedance measurements indicate that both the grain and grain boundary resistances increase with the increasing x. We speculate that the depressed dielectric response of CCTO may be related to the lack of the oxygen vacancies in the Eu-doped samples. Moreover, we found that the electrode contact response gradually disappears as the x increases. This may be another factor responsible for the decreased dielectric constant of Eu-doped CCTO.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant number11304244).
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