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Abnormal dielectric behaviors in Mn-doped CaCu3Ti4O12 ceramics and their response mechanism
Materials Science and Engineering B 177 (2012) 1773–1776
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Abnormal dielectric behaviors in Mn-doped CaCu3 Ti4 O12 ceramics and their response mechanism Yuan-Hua Lin a,∗ , Wei Deng a,b , Wei Xu c , Yong Liu a,d , Dongliang Chen c , Xiaoli Zhang c , Ce-Wen Nan a a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China d School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China b c
a r t i c l e
i n f o
Article history: Received 8 April 2012 Received in revised form 29 June 2012 Accepted 15 August 2012 Available online 30 August 2012 Keywords: CaCu3 Ti4 O12 High dielectric Activation energy
a b s t r a c t Mn-doped CaCu3 Ti4 O12 (CCTO) polycrystalline ceramics have been prepared by the conventional solid state sintering. Our results indicate that 10% Mn doping can decrease the dielectric permittivity in CaCu3 Ti4 O12 by about 2 orders of magnitude (from 104 to 102 ). The grain and grain boundary activation energies show an obvious increase from 0.054 eV to 0.256 eV, and decrease from 0.724 eV to 0.258 eV with increasing the Mn doping concentration, respectively, which may be caused by the variation of Cu and Ti valence states in the CCTO samples evidenced by the X-ray absorption spectra. The similar grain and grain boundary activation energies result in invalidation of the internal boundary layer capacitance effect for the 10% Mn-doped CCTO sample, and thus result in the dramatic decrease of dielectric permittivity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction CaCu3 Ti4 O12 (CCTO) has a distorted, complex cubic perovskitelike structure with a large unit cell, and shows a giant dielectric permittivity of ∼104 over a wide temperature range without any structural phase transition [1,2]. The temperature independence and giant dielectric constant of CCTO have attracted many researchers’ attention. Such special physical features and Ba/Pbfree make it promising applications for the microelectronic devices such as capacitors and memories. As for the origin of the giant dielectric behavior, some researchers proposed that it may be attributed to local dipole moments associated with off-center displacement of Ti ions [3,4]. The fluctuations of lattice-distortion induced dipoles in nanosize domains result in the giant dielectric constant. On the other hand, extrinsic effects, such as the interfacial polarization effect, internal boundary layer capacitance (IBLC) effect considering the insulating grain boundaries and semiconducting grains structure have also been proposed to understand the origin of the giant dielectric constant [5–7]. Besides the giant dielectric constant, the non-ohmic varistor of CCTO has also been observed by some researchers [8–10]. Although considerable efforts have been performed to understand the origin of the giant dielectric behavior of CCTO, it still remains controversial.
∗ Corresponding author. E-mail address: [email protected] (Y.-H. Lin). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.021
In this work, we prepared Mn-doped CCTO polycrystalline ceramics, and observed that only 10% Mn doping can suppress the dielectric permittivity of CaCu3 Ti4 O12 by nearly 2 orders of magnitude (from 104 to 102 ). Our results indicate that Mn doping can decrease the activation energy of grain boundary and increase the activation energy of grain, which results in abrupt decrease of dielectric constant. Our X-ray absorption spectra reveal that this variation of electric behavior may be ascribed to the tunable valence state of Ti, Mn and Cu in the grain and grain boundary. 2. Experiment procedure In order to understand this abnormal dielectric behavior, three kinds of Mn-doped CCTO ceramics with the nominal composition of CaCu3 Mnx Ti4−x O12 (x = 0, 0.05, and 0.10, abbreviated as CCTO, CCMTO-1, and CCMTO-2) were prepared by a solid-state reaction sintering method. MnO2 , CaCO3 , CuO, and TiO2 powders (analytical purity) were employed as the raw materials and milled for more than 10 h. The dried mixture powders were pre-sintered at 950 ◦ C for 5 h and then the pre-sintered powders were pressed to green pellets (12 mm in diameter) with polyvinyl alcohol binder. Finally, the pellets were sintered at 1100 ◦ C for 10 h in air. The phase compositions of these as-sintered samples were measured by the X-ray diffraction (XRD) equipment (Rigaku D-Max 3A, Cu K␣ radiation). The ceramic samples were polished and pasted by silver paste on both sides, and then treated at 600 ◦ C for 30 min to form the electrodes. The dielectric response was measured by using a HP 4192A gain phase analyzer at various frequencies at an oscillation voltage
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Y.-H. Lin et al. / Materials Science and Engineering B 177 (2012) 1773–1776
Mn, Cu and Ti K-edge XAS spectra have been performed using the IFEFFIT package [11].
3. Result and discussion
Fig. 1. Dielectric behaviors of various Mn-doped CCTO samples at room temperature.
of 0.1 V in the frequency range from 100 Hz to 10 MHz. These measurements were performed in the temperature range from 130 K to 500 K. Each measured temperature was kept constant with an accuracy of ±1 K. The X-ray absorption near edge structure (XANES) spectra were collected at 1W1B of Beijing synchrotron radiation facility, running with the electron energy as 2.5 GeV and electric current between 160 and 250 mA. The Mn, Ti and Cu K-edge X-ray absorption structure (XAS) spectra are collected in a fluorescence mode using Lytle detector filled with mixed Ar/N2 gas at room temperature. The normalization and background subtraction of all the
Fig. 1 shows the room temperature dielectric response of various CCMTO samples between 100 Hz and 5 MHz. The results indicate that the Mn doping has significant effects on the dielectric properties of CCTO at room temperature. Obviously, the dielectric constants of CCMTO-2 ceramic sample decrease to about 102 at 1 kHz, which is only about 1% high as the typical dielectric constant of pure CCTO ceramic. Why does the dielectric permittivity of these CCMTO ceramics show large differences before and after Mn doping? Our previous EDX results revealed that the Mn4+ ions can enter into both of the grain and grain boundary regions, which can lead to the various compositions of the grain/grain boundary and related electrical characteristics. These may inhibit the double Schottky potential barrier at the grain boundaries [12]. Thus the Schottky barriers at grain boundaries are declined consequently the value of IBLC becomes smaller, resulting in lower permittivity. As previously reported [13,14], during the heating process, Cu2+ tended to be unstable and reduced to Cu+ in the pure CCTO system. In order to maintain the charge balance, the ionic formula of CCTO can be expressed as Ca2+ (Cu1−3x 2+ Cu2x + Tix 4+ )3 Ti4 4+ O12 . During the cooling process, Cu+ can be oxidized to Cu2+ , therefore, electrons move into the Ti 3d band and finally form Ca2+ (Cu1−x 2+ Tix 4+ )3 (Ti6x 3+ Ti4−6x 4+ )O12 . As Mn doped in the CCTO system, this variation of valence state of Cu
Fig. 2. Decomposition analysis for the Cu K XANES of CCMTO samples.
Fig. 3. Decomposition analysis for the Mn K XANES of CCMTO samples.
Y.-H. Lin et al. / Materials Science and Engineering B 177 (2012) 1773–1776
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Fig. 4. Decomposition analysis for the Ti K XANES of CCMTO samples.
and Ti will also result in the changes of Mn4+ /Mn3+ in the grain and grain boundary. In order to further reveal the variation of valence state of Mn, Cu and Ti ions in these Mn-doped CCTO samples, we performed XANES spectra measurements for these three CCMTO ceramic samples. Fig. 2 shows the decomposition analysis for the Cu K XANES of CCMTO samples into these of CCTO, Cu2 O (Cu+ ), and CuO (Cu2+ ). It seems that the fitted curves well reproduce the experimental spectra. With increasing the Mn concentration, the ratio of Cu+ component to the total amount of the components increases obviously (from 23.3% to 58.7% as denoted in Fig. 2). Considering the Mn substitution of Ti sites in the nominal composition of CaCu3 Mnx Ti4−x O12 samples, the changes of Cu valence states will lead to the variations of Mn and Ti valence state for charge balance in the CCMTO system. Therefore, we also performed the Mn and Ti K XANES spectra of CCMTO as shown in Figs. 3 and 4. It can be seen that the Mn3+ component increases with increasing Mn concentration (from 13.9% to 20.0% as denoted in Fig. 3). Although we cannot obtain the good fitted curves of Ti K XANES spectra shown in Fig. 4, the results can also illustrate that the Ti4+ concentration changes greatly in these XANES spectra. These variations of the valence state of Cu, Mn and Ti will have an influence on the electrical behaviors of grain and grain boundary regions in the bulk polycrystalline ceramics. Normally, the temperature dependence of impedance spectroscopy (IS) combined with dielectric modulus M*–f plots can be desirable for analysis of these grain and grain boundary characters. M*-modulus is defined as following equation [15,16]: M ∗ (ω) =
ε (ω) + iε (ω) ε (ω)2 + ε (ω)2
= M (ω) + iM (ω)
typical M –f plots of CCMTO-1 at various measured temperatures. The activation energy of grain and grain boundary can be evaluated by the Arrhenius equation [18]:
ln = −
Ea 1 · + ln 0 kB T
where Ea , kB , and T are the activation energy, Boltzmann constant, and absolute temperature, respectively. Fig. 6 shows the fitted Arrhenius plots of grain (a) and grain boundary phase (b). The results show that the activation energy of grain for Mn-doped CCTO specimen was increased from 0.054 eV of CCTO to 0.256 eV of CCMTO-2. At the same time, the activation energy of grain boundary was also decreased from 0.724 eV of CCTO to 0.258 eV of CCMTO-2. In other words, the semiconduction of grain and the resistance of grain boundary decrease with the increase of the content of Mn. Therefore, the similar grain and boundary activation energies for CCMTO-2 sample make the invalidation of the internal boundary layer capacitance effect. Therefore, the dielectric constant of Mn-doped CCTO ceramics was dramatically decreased in this research.
(1)
The peak frequency of M –M cole–cole plots are accompanied with dielectric relaxation frequency. Moreover, according to Eq. (1), the real and imaginary part of M* is expressed as:
M (ω) = −ωC0 Z = C0
M (ω) = ωC0 Z = ωC0
Rgb ω 1 + (ω)2 Rg +
Rgb 1 + (ω)2
(2)
(3)
Therefore, local resistivity of the grain was calculated by using C = 1/2M max , R = −1/ωmax C [15,17]. Then, the resistivity of the grain boundary can be extracted from impedance spectra. Fig. 5 shows a
(4)
Fig. 5. M –f plots of CCMTO-1 sample at various temperatures.
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Fig. 6. The fitted activation energies of (a) grain and (b) grain boundary of Mn-doped CCTO samples.
4. Conclusion In summary, we prepared Mn-doped CCTO ceramics and found that the Mn doping shows remarkable influences on the dielectric behaviors. With the Mn doping increasing, the dielectric constant drops from ∼104 to ∼102 . The XANES spectra reveal that valence states of Ti, Mn and Cu in the grain and grain boundary changes greatly, which lead to the difference of the activation energy of grain and grain boundary disappearing with increasing the Mn doping concentration, and thus show this abnormal dielectric behavior. Acknowledgments This work was supported by the Ministry of Science and Technology of China through a 973-Project under Grant No. 2009CB623303, NSF of China (50972068, 50921061 and 51025205), and NSF of Beijing (2092016) and Scientific Research Project of Tsinghua University No. 2009THZ08063. We also thank M. Kobayashi for the analysis of XANES spectra. References [1] A.P. Ramirez, M.A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, S.M. Shapiro, Solid State Communications 115 (2000) 217–220.
[2] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Science 293 (2001) 673–676. [3] L. Wu, Y. Zhu, S. Park, S. Shapiro, G. Shirane, J. Tafto, Physical Review B 71 (2005) 014118. [4] R.K. Grubbs, E.L. Venturini, P.G. Clem, J.J. Richardson, B.A. Tuttle, G.A. Samara, Physical Review B 72 (2005) 104111. [5] D.C. Sinclair, T.B. Adams, F.D. Morrison, A.R. West, Applied Physics Letters 80 (2002) 2153. [6] T.B. Adams, D.C. Sinclair, A.R. West, Physical Review B 73 (2006) 094124. [7] Y.-H. Lin, J. Cai, M. Li, C.-W. Nan, J. He, Applied Physics Letters 88 (2006) 172902. [8] S.-Y. Chung, I.-D. Kim, S.-J.L. Kang, Nature Materials 3 (2004) 774–778. [9] M.A. Ramirez, P.R. Bueno, J.A. Varela, E. Longo, Applied Physics Letters 89 (2006) 212102. [10] Y.-H. Lin, J. Cai, M. Li, C.-W. Nan, J. He, Applied Physics 103 (2008) 074111. [11] B. Ravel, M. Newville, Journal of Synchrotron Radiation 2 (2005) 537–541. [12] J. Cai, Y.-H. Lin, B. Cheng, C.-W. Nan, J. He, Y. Wu, X.M. Chen, Applied Physics Letters 91 (2007) 252905. [13] J. Li, M.A. Subramanian, H.D. Rosenfeld, C.Y. Jones, B.H. Toby, A.W. Sleight, Chemistry of Materials 25 (2004) 5223–5255. [14] T. Fang, L. Mei, Journal of the American Ceramic Society 90 (2007) 638–640. [15] M. Li, A. Feteira, D.C. Sinclair, A.R. West, Applied Physics Letters 88 (2006) 232903. [16] G.M. Tsangaris, G.C. Psarras, N. Koulumbi, Journal of Materials Science 33 (1998) 2027–2037. [17] M. Guo, T. Wu, T. Liu, S. Wang, X. Zhao, Journal of Applied Physics 99 (2006) 124113. [18] D.L. Sun, A.Y. Wu, S.T. Yin, Journal of the American Ceramic Society 91 (2008) 169–173.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Abnormal dielectric behaviors in Mn-doped CaCu3 Ti4 O12 ceramics and their response mechanism Yuan-Hua Lin a,∗ , Wei Deng a,b , Wei Xu c , Yong Liu a,d , Dongliang Chen c , Xiaoli Zhang c , Ce-Wen Nan a a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China d School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China b c
a r t i c l e
i n f o
Article history: Received 8 April 2012 Received in revised form 29 June 2012 Accepted 15 August 2012 Available online 30 August 2012 Keywords: CaCu3 Ti4 O12 High dielectric Activation energy
a b s t r a c t Mn-doped CaCu3 Ti4 O12 (CCTO) polycrystalline ceramics have been prepared by the conventional solid state sintering. Our results indicate that 10% Mn doping can decrease the dielectric permittivity in CaCu3 Ti4 O12 by about 2 orders of magnitude (from 104 to 102 ). The grain and grain boundary activation energies show an obvious increase from 0.054 eV to 0.256 eV, and decrease from 0.724 eV to 0.258 eV with increasing the Mn doping concentration, respectively, which may be caused by the variation of Cu and Ti valence states in the CCTO samples evidenced by the X-ray absorption spectra. The similar grain and grain boundary activation energies result in invalidation of the internal boundary layer capacitance effect for the 10% Mn-doped CCTO sample, and thus result in the dramatic decrease of dielectric permittivity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction CaCu3 Ti4 O12 (CCTO) has a distorted, complex cubic perovskitelike structure with a large unit cell, and shows a giant dielectric permittivity of ∼104 over a wide temperature range without any structural phase transition [1,2]. The temperature independence and giant dielectric constant of CCTO have attracted many researchers’ attention. Such special physical features and Ba/Pbfree make it promising applications for the microelectronic devices such as capacitors and memories. As for the origin of the giant dielectric behavior, some researchers proposed that it may be attributed to local dipole moments associated with off-center displacement of Ti ions [3,4]. The fluctuations of lattice-distortion induced dipoles in nanosize domains result in the giant dielectric constant. On the other hand, extrinsic effects, such as the interfacial polarization effect, internal boundary layer capacitance (IBLC) effect considering the insulating grain boundaries and semiconducting grains structure have also been proposed to understand the origin of the giant dielectric constant [5–7]. Besides the giant dielectric constant, the non-ohmic varistor of CCTO has also been observed by some researchers [8–10]. Although considerable efforts have been performed to understand the origin of the giant dielectric behavior of CCTO, it still remains controversial.
∗ Corresponding author. E-mail address: [email protected] (Y.-H. Lin). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.021
In this work, we prepared Mn-doped CCTO polycrystalline ceramics, and observed that only 10% Mn doping can suppress the dielectric permittivity of CaCu3 Ti4 O12 by nearly 2 orders of magnitude (from 104 to 102 ). Our results indicate that Mn doping can decrease the activation energy of grain boundary and increase the activation energy of grain, which results in abrupt decrease of dielectric constant. Our X-ray absorption spectra reveal that this variation of electric behavior may be ascribed to the tunable valence state of Ti, Mn and Cu in the grain and grain boundary. 2. Experiment procedure In order to understand this abnormal dielectric behavior, three kinds of Mn-doped CCTO ceramics with the nominal composition of CaCu3 Mnx Ti4−x O12 (x = 0, 0.05, and 0.10, abbreviated as CCTO, CCMTO-1, and CCMTO-2) were prepared by a solid-state reaction sintering method. MnO2 , CaCO3 , CuO, and TiO2 powders (analytical purity) were employed as the raw materials and milled for more than 10 h. The dried mixture powders were pre-sintered at 950 ◦ C for 5 h and then the pre-sintered powders were pressed to green pellets (12 mm in diameter) with polyvinyl alcohol binder. Finally, the pellets were sintered at 1100 ◦ C for 10 h in air. The phase compositions of these as-sintered samples were measured by the X-ray diffraction (XRD) equipment (Rigaku D-Max 3A, Cu K␣ radiation). The ceramic samples were polished and pasted by silver paste on both sides, and then treated at 600 ◦ C for 30 min to form the electrodes. The dielectric response was measured by using a HP 4192A gain phase analyzer at various frequencies at an oscillation voltage
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Y.-H. Lin et al. / Materials Science and Engineering B 177 (2012) 1773–1776
Mn, Cu and Ti K-edge XAS spectra have been performed using the IFEFFIT package [11].
3. Result and discussion
Fig. 1. Dielectric behaviors of various Mn-doped CCTO samples at room temperature.
of 0.1 V in the frequency range from 100 Hz to 10 MHz. These measurements were performed in the temperature range from 130 K to 500 K. Each measured temperature was kept constant with an accuracy of ±1 K. The X-ray absorption near edge structure (XANES) spectra were collected at 1W1B of Beijing synchrotron radiation facility, running with the electron energy as 2.5 GeV and electric current between 160 and 250 mA. The Mn, Ti and Cu K-edge X-ray absorption structure (XAS) spectra are collected in a fluorescence mode using Lytle detector filled with mixed Ar/N2 gas at room temperature. The normalization and background subtraction of all the
Fig. 1 shows the room temperature dielectric response of various CCMTO samples between 100 Hz and 5 MHz. The results indicate that the Mn doping has significant effects on the dielectric properties of CCTO at room temperature. Obviously, the dielectric constants of CCMTO-2 ceramic sample decrease to about 102 at 1 kHz, which is only about 1% high as the typical dielectric constant of pure CCTO ceramic. Why does the dielectric permittivity of these CCMTO ceramics show large differences before and after Mn doping? Our previous EDX results revealed that the Mn4+ ions can enter into both of the grain and grain boundary regions, which can lead to the various compositions of the grain/grain boundary and related electrical characteristics. These may inhibit the double Schottky potential barrier at the grain boundaries [12]. Thus the Schottky barriers at grain boundaries are declined consequently the value of IBLC becomes smaller, resulting in lower permittivity. As previously reported [13,14], during the heating process, Cu2+ tended to be unstable and reduced to Cu+ in the pure CCTO system. In order to maintain the charge balance, the ionic formula of CCTO can be expressed as Ca2+ (Cu1−3x 2+ Cu2x + Tix 4+ )3 Ti4 4+ O12 . During the cooling process, Cu+ can be oxidized to Cu2+ , therefore, electrons move into the Ti 3d band and finally form Ca2+ (Cu1−x 2+ Tix 4+ )3 (Ti6x 3+ Ti4−6x 4+ )O12 . As Mn doped in the CCTO system, this variation of valence state of Cu
Fig. 2. Decomposition analysis for the Cu K XANES of CCMTO samples.
Fig. 3. Decomposition analysis for the Mn K XANES of CCMTO samples.
Y.-H. Lin et al. / Materials Science and Engineering B 177 (2012) 1773–1776
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Fig. 4. Decomposition analysis for the Ti K XANES of CCMTO samples.
and Ti will also result in the changes of Mn4+ /Mn3+ in the grain and grain boundary. In order to further reveal the variation of valence state of Mn, Cu and Ti ions in these Mn-doped CCTO samples, we performed XANES spectra measurements for these three CCMTO ceramic samples. Fig. 2 shows the decomposition analysis for the Cu K XANES of CCMTO samples into these of CCTO, Cu2 O (Cu+ ), and CuO (Cu2+ ). It seems that the fitted curves well reproduce the experimental spectra. With increasing the Mn concentration, the ratio of Cu+ component to the total amount of the components increases obviously (from 23.3% to 58.7% as denoted in Fig. 2). Considering the Mn substitution of Ti sites in the nominal composition of CaCu3 Mnx Ti4−x O12 samples, the changes of Cu valence states will lead to the variations of Mn and Ti valence state for charge balance in the CCMTO system. Therefore, we also performed the Mn and Ti K XANES spectra of CCMTO as shown in Figs. 3 and 4. It can be seen that the Mn3+ component increases with increasing Mn concentration (from 13.9% to 20.0% as denoted in Fig. 3). Although we cannot obtain the good fitted curves of Ti K XANES spectra shown in Fig. 4, the results can also illustrate that the Ti4+ concentration changes greatly in these XANES spectra. These variations of the valence state of Cu, Mn and Ti will have an influence on the electrical behaviors of grain and grain boundary regions in the bulk polycrystalline ceramics. Normally, the temperature dependence of impedance spectroscopy (IS) combined with dielectric modulus M*–f plots can be desirable for analysis of these grain and grain boundary characters. M*-modulus is defined as following equation [15,16]: M ∗ (ω) =
ε (ω) + iε (ω) ε (ω)2 + ε (ω)2
= M (ω) + iM (ω)
typical M –f plots of CCMTO-1 at various measured temperatures. The activation energy of grain and grain boundary can be evaluated by the Arrhenius equation [18]:
ln = −
Ea 1 · + ln 0 kB T
where Ea , kB , and T are the activation energy, Boltzmann constant, and absolute temperature, respectively. Fig. 6 shows the fitted Arrhenius plots of grain (a) and grain boundary phase (b). The results show that the activation energy of grain for Mn-doped CCTO specimen was increased from 0.054 eV of CCTO to 0.256 eV of CCMTO-2. At the same time, the activation energy of grain boundary was also decreased from 0.724 eV of CCTO to 0.258 eV of CCMTO-2. In other words, the semiconduction of grain and the resistance of grain boundary decrease with the increase of the content of Mn. Therefore, the similar grain and boundary activation energies for CCMTO-2 sample make the invalidation of the internal boundary layer capacitance effect. Therefore, the dielectric constant of Mn-doped CCTO ceramics was dramatically decreased in this research.
(1)
The peak frequency of M –M cole–cole plots are accompanied with dielectric relaxation frequency. Moreover, according to Eq. (1), the real and imaginary part of M* is expressed as:
M (ω) = −ωC0 Z = C0
M (ω) = ωC0 Z = ωC0
Rgb ω 1 + (ω)2 Rg +
Rgb 1 + (ω)2
(2)
(3)
Therefore, local resistivity of the grain was calculated by using C = 1/2M max , R = −1/ωmax C [15,17]. Then, the resistivity of the grain boundary can be extracted from impedance spectra. Fig. 5 shows a
(4)
Fig. 5. M –f plots of CCMTO-1 sample at various temperatures.
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Y.-H. Lin et al. / Materials Science and Engineering B 177 (2012) 1773–1776
Fig. 6. The fitted activation energies of (a) grain and (b) grain boundary of Mn-doped CCTO samples.
4. Conclusion In summary, we prepared Mn-doped CCTO ceramics and found that the Mn doping shows remarkable influences on the dielectric behaviors. With the Mn doping increasing, the dielectric constant drops from ∼104 to ∼102 . The XANES spectra reveal that valence states of Ti, Mn and Cu in the grain and grain boundary changes greatly, which lead to the difference of the activation energy of grain and grain boundary disappearing with increasing the Mn doping concentration, and thus show this abnormal dielectric behavior. Acknowledgments This work was supported by the Ministry of Science and Technology of China through a 973-Project under Grant No. 2009CB623303, NSF of China (50972068, 50921061 and 51025205), and NSF of Beijing (2092016) and Scientific Research Project of Tsinghua University No. 2009THZ08063. We also thank M. Kobayashi for the analysis of XANES spectra. References [1] A.P. Ramirez, M.A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, S.M. Shapiro, Solid State Communications 115 (2000) 217–220.
[2] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Science 293 (2001) 673–676. [3] L. Wu, Y. Zhu, S. Park, S. Shapiro, G. Shirane, J. Tafto, Physical Review B 71 (2005) 014118. [4] R.K. Grubbs, E.L. Venturini, P.G. Clem, J.J. Richardson, B.A. Tuttle, G.A. Samara, Physical Review B 72 (2005) 104111. [5] D.C. Sinclair, T.B. Adams, F.D. Morrison, A.R. West, Applied Physics Letters 80 (2002) 2153. [6] T.B. Adams, D.C. Sinclair, A.R. West, Physical Review B 73 (2006) 094124. [7] Y.-H. Lin, J. Cai, M. Li, C.-W. Nan, J. He, Applied Physics Letters 88 (2006) 172902. [8] S.-Y. Chung, I.-D. Kim, S.-J.L. Kang, Nature Materials 3 (2004) 774–778. [9] M.A. Ramirez, P.R. Bueno, J.A. Varela, E. Longo, Applied Physics Letters 89 (2006) 212102. [10] Y.-H. Lin, J. Cai, M. Li, C.-W. Nan, J. He, Applied Physics 103 (2008) 074111. [11] B. Ravel, M. Newville, Journal of Synchrotron Radiation 2 (2005) 537–541. [12] J. Cai, Y.-H. Lin, B. Cheng, C.-W. Nan, J. He, Y. Wu, X.M. Chen, Applied Physics Letters 91 (2007) 252905. [13] J. Li, M.A. Subramanian, H.D. Rosenfeld, C.Y. Jones, B.H. Toby, A.W. Sleight, Chemistry of Materials 25 (2004) 5223–5255. [14] T. Fang, L. Mei, Journal of the American Ceramic Society 90 (2007) 638–640. [15] M. Li, A. Feteira, D.C. Sinclair, A.R. West, Applied Physics Letters 88 (2006) 232903. [16] G.M. Tsangaris, G.C. Psarras, N. Koulumbi, Journal of Materials Science 33 (1998) 2027–2037. [17] M. Guo, T. Wu, T. Liu, S. Wang, X. Zhao, Journal of Applied Physics 99 (2006) 124113. [18] D.L. Sun, A.Y. Wu, S.T. Yin, Journal of the American Ceramic Society 91 (2008) 169–173.