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Dielectric properties of bismuth doped CaCu3Ti4O12 ceramics
Materials Science and Engineering B 177 (2012) 494–498
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Dielectric properties of bismuth doped CaCu3 Ti4 O12 ceramics L.F. Xu, P.B. Qi, S.S. Chen, R.L. Wang, C.P. Yang ∗ Faculty of Physics and Electronics, Hubei University, Wuhan 430062, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 16 December 2011 Accepted 6 February 2012 Available online 19 February 2012 Keywords: CCTO ceramics Dielectric properties Oxygen deficiency
a b s t r a c t Ca1−3x/2 Bix Cu3 Ti4 O12 (x = 0.0–0.3) ceramics were prepared by the conventional solid-state reaction method. X-ray powder diffraction analysis confirmed the formation of cubic CCTO phase except for subtle peaks of CuO. SEM micrographs suggested that the morphologies of doped CCTO ceramics had been sheet-like for high Bi-doping amount, and the dominant grain size decreasing could be seen for the small content of Bi-doping CCTO. Dielectric properties of pure and doped CCTO were investigated in a broad temperature range of 20–420 K. The results showed that bismuth doping could decrease the dielectric loss but suppress the dielectric temperature stability at the same time. Bi doped CCTO ceramics presented different relaxation properties. As to pure CCTO and BCCTO (x = 0.3) only one MW relaxation (Relaxation I) could be found, which moves to higher frequency with temperature increasing. However, two relaxation processes (Relaxation I and II) appear for BCCTO (x = 0.1–0.2). By means of complex impedance spectra analysis and Arrhenius fitting, we successfully separated the different conductive segments and explained the mechanisms of the two relaxation processes. Relaxation I appeared at low temperature could be attributed to the VO • doping energies inside CCTO grains which did not showed significant changing of activation energy after bismuth doping. For Relaxation II at higher temperature than Relaxation I, with activation energy obviously depending on the Bi-ion concentration, may be related with the VO • • point defects at the grain boundaries. © 2012 Elsevier B.V. All rights reserved.
1. Introduction CaCu3 Ti4 O12 (CCTO) is a perovskite-like compound with colossal dielectric constant (CDC) and almost temperature independent in the frequency range of dc to 106 Hz. It shows high potential for technological applications, such as memory devices based on capacitive components and microwave devices. Some researchers suggested that the origin of CDC is intrinsic [1], however, by the first principles calculation, the static dielectric constant of CCTO was only about 40 [2]. Therefore, it has been speculated that the dielectric response is due to barrier-layer capacitances associated with one or more of the following: grain boundaries, twin boundaries, dislocation networks, and Schottky barrier or interfacial polarization effects [3–8]. The later behaviors were generally attributed to Maxwell–Wagner (MW) mechanism, which is the characteristic of electrical microstructure inhomogeneous materials [9]. M. H. Cohen et al. [7] discussed six possible morphologies for inhomogeneity in the local dielectric response of CCTO by supposing the properties of the internal boundary regions to be opposite from those of the bulk. (If one conducts, the other is insulating, and vice versa.) It was found that large dielectric constant result either when the conducting regions approach a percolation threshold or when
∗ Corresponding author. Tel.: +86 27 88665447; fax: +86 27 88663390. E-mail address: [email protected] (C.P. Yang). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.02.001
they percolate but are blocked at the sample surface or electrode interface. On the other hand, oxygen vacancies and/or impurities in CCTO can lead to conductivity in either the bulk (grains) or grain boundaries. And the oxygen vacancies are very sensitive to sintering parameters, like temperature, heating rate, duration of heat-treatment and atmosphere [4–7]. Besides the sintering conditions, the microstructure and the dielectric properties of the CCTO are strongly influenced by doping with other elements [10–13]. Especially Luo et al. [14,15] represented the electric and dielectric properties of bismuth doped CCTO ceramics based on the IBLC model. The grain size and second phase were affected by Bi-doping apparently, and then changed the electric and dielectric properties significantly. The purpose of the present work is to further investigate the effects of Bi-ion concentration on the structure and dielectric properties of CCTO ceramics by broad temperature range (20–420 K) measurement. Moreover, we will separate the different conductivity segments of CCTO by both dielectric spectra and complex impedance spectra. By the traditional solid-state reaction method, different proportion of Bi3+ from Bi2 O3 was introduced into CCTO with the nominal composition Ca1−3x/2 Bix Cu3 Ti4 O12 (x = 0.0, 0.1, 0.2 and 0.3). Bi3+ has higher valence than Ca2+ , so we intended to introduce oxygen vacancies and impurities by superseding Ca site with Bi3+ and explore the influence of Bi-ion concentration on properties of grain and grain boundary, which may contribute to the dielectric responses of CCTO.
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Table 1 XRD parameters and XRF results of pure and Bi-doped CCTO ceramics. Sample
Crystal system
Gain size (m)
˚ Lattice parameter (A)
Ca:Bi:Ti:Cu (molratio, XRF results)
x=0 x = 0.1 x = 0.2 x = 0.3
Cubic Cubic Cubic Cubic
27.4 3.7 4.3 4.8
7.3905 7.4201 7.4122 7.4005
1:2.99:4.09 0.85:0.08:3:4.29 0.69:0.17:3:4.30 0.54:0.25:3:4.28
Fig. 1. XRD patterns of the sintered CCTO ceramics.
2. Experimental CaCu3 Ti4 O12 (pure, as well as Bi3+ doped) pellets were prepared using traditional solid-state reaction and sintering process. Precursor powders of CaCO3 (99%), TiO2 (99%), and CuO (99%) were directly combined in the necessary stoichiometric ratios while doped samples employed Bi2 O3 (99%) as a powder (All raw materials are home-made). Thorough mixing was achieved by milling the powders in ethanol for no less than 4 h. The mixed powders were calcined at 1273 K (1000 ◦ C) for 10 h before being milled a second time. The dried powders were ground for 4 h in ethanol and then uniaxially cold pressed into pellets with 12 mm diameter and thickness about 1 mm under 14 MPa pressure. The brown pellets were then sintered in air using the schedule 10 K/min to 1373 K (1100 ◦ C), 12 h hold, and then furnace cooled to room temperature. The X-ray diffraction (XRD) patterns of the sintered samples were obtained in a range of 10–80◦ at room temperature, in a Japan MAXima X XRD-7000 system, with Cu K␣ radiation ( = 0.1506 nm) at 40 kV and 30 mA, with a step of 2◦ /min. The XRF measurements were performed utilizing an XRF-1800 (Shimadzu, Japan). The microstructures of the fractured surfaces were examined using a Scanning Electron Microscope (SEM s-4800, Japan). The dielectric properties and impedance spectrum were determined on samples with opposite sides painted with silver paste, using a WK6420 impedance analyzer with a Janis closed-cycle-refrigerator in the frequency and temperature range of 500 Hz to 3 MHz and 20–420 K respectively. 3. Results and discussion 3.1. Structure of Bi-doped CCTO ceramics The XRD (Fig. 1) confirms the presence of CCTO as a single phase for Bi2 O3 concentration up to 0.3. For pure CCTO peaks of CuO could be seen and fade away after bismuth doping. The segregation of
CuO could also be found in many other researches [15,16]. Moreover, peaks of Ca appear after bismuth doping and become stronger with doping amount increasing, and no bismuth-related phase is observed in the XRD patterns for Bi-doped samples. It could be inferred that the bismuth atoms have entered the lattice and almost taken place of some calcium except for minimal copper atom. The crystallite sizes (Lc ) of CCTO, obtained from the XRD patterns of representative samples, are presented in Table 1. These values show that the CCTO crystallite size would decrease after bismuth doping. And from XRF results we can see bismuth increasing and calcium decreasing with Bi-doping content increasing. Therefore, we can conclude that bismuth has replaced Ca-site. Fig. 2 shows the microstructure of the samples. The morphology of the undoped specimen consists of some huge grains, surrounded by small ones. An obvious decrease of the largest grain size is observed after Bi-doping which is consistent with the XRD calculation results. But the smaller grains did not show so apparent size decreasing as the bigger ones. BCCTO-2 and -3 are characterized by close packing sheet-like grains, surrounded by blurry grain boundary which may be caused by the overmuch liquid composed of Bi2 O3 during high temperature sintering. 3.2. Dielectric properties of Bi-doped CCTO ceramics Fig. 3 shows the frequency spectra of the relative permittivity (ε ) of undoped and Bi-doped (x = 0.1–0.3) CCTO ceramics. All room temperature (RT = 296 K) dielectric constant of the doped samples (∼104 ) are an order of magnitude lower than the pure ceramics (near 105 ). According to a simple series IBLC model, the dielectric constant of CCTO ceramics can be simply expressed by ε ≈ dεg /t, where d is the grain size, t is the boundary thickness, and εg is the dielectric constant of grain boundary. The effective dielectric constant would be around 103 –104 [17,18]. As the doping concentration increasing, the accumulation of Bi-ion will be exacerbated and the grain boundary will be thickened; moreover, from XRD a little increasing of CuO at the grain boundary for Bisubstituting may also increase the thickness of the gain boundary, so the dielectric constant will decrease. Compared to BCCTO ceramics, dielectric spectra of pure CCTO shows more stable temperature characteristics of ∼105 at 140–420 K and 500–105 Hz (Fig. 3(a)). All doped and pure samples show a sharp decreasing in ε values as the frequency higher than 106 Hz. The drastic decrease may indicate the presence of semi-conductive grains [19]. Only one dielectric relaxation region (I) can be observed for pure CCTO and BCCTO-3 in the available frequency and temperature range, which moves to higher frequencies as temperature increasing. Whereas for BCCTO-1 and -2, another relaxation region (II) at lower frequencies than Relaxation I comes up as the temperature higher than 350 K (Figs. 3(b and c)) which maybe related with the electrode surface. Fig. 4 shows dielectric loss spectra of undoped and Bi-doped (x = 0.1–0.3) CCTO ceramics. For CCTO and BCCTO-3, only one loss peak (I) at 80–170 K appears which is corresponding to Relaxation I in Fig. 3(a and d) and moves to higher frequencies as temperature increasing. For BCCTO-1 and -2, another loss peak (II) could be seen above 350 K at lower frequencies than Peak I, which accords well with Relaxation II in Fig. 3(b and c).
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Fig. 2. Microstructure of the sintered CCTO ceramics: (a) CCTO; (b) BCCTO-1; (c) BCCTO-2 and (d) BCCTO-3.
In order to analyze these data, we used the cole–cole plot, that is, the graphic of Z vs. Z , as shown in Fig. 5. At RT (Fig. 5(b)) only a large arc (Semicircle II) appears for all samples with nonzero intercepts (inset in Fig. 5(b)). The arc and intercepts are assigned as the contribution of CCTO grains and grain boundaries, respectively [20]. At low temperature (LT), for instance, 140 K (Fig. 5(a)), the second semicircle (I) appears which accords well with the high frequency plateau region in the dielectric permittivity spectra (at LT in Fig. 3). The large arc of semicircle I represented the more resistive grain boundaries than gains, which could be contributed by the CuO phase and Bi-ion accumulation. The intercepts in the insert of Fig. 5(a) are close to zero so we conclude Semicircle I should be corresponded to the contribution of CCTO grains.
Note in particular that the inflection point of the two arcs (Semicircles I and II) and Peak I (Fig. 4) occur at the same frequency, implying that the dielectric relaxations of CCTO and BCCTO at low temperature are related with MW relaxation, which is caused by electrical microstructure inhomogeneity between CCTO grain and grain boundary. At high temperature (HT) of 380 K, the third Semicircle III (BCCTO-1 and -2) comes up. The frequency of the inflection point of Semicircle II and III in the cole–cole plots is the same as that of the relaxation peaks (Peak II) in the corresponding Fig. 4(b and c), which demonstrates that the occurrence of dielectric relaxations of BCCTO-1 and -2 at high temperatures are related with MW relaxation of grain boundary and electrode interface.
Fig. 3. Dielectric spectra of CCTO ceramics at selected temperatures (20–420 K).
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Fig. 4. Dielectric loss spectra of CCTO ceramics at selected temperatures (20–420 K).
Fig. 5. Cole–cole curves of CCTO ceramics at three typical temperatures and the Arrhenius plots of ln vs. 1/T: (a) 170 K (LT); (b) 296 K (RT); (c) 380 K (HT); (d) the Arrhenius plots of BCCTO (x = 0–0.3) at different temperatures.
To further establish the possible relaxation mechanisms, the activation energy of dielectric relaxation was obtained from the temperature dependence of dielectric loss response in Fig. 4. Debye-like relaxations with Arrhenius kinetics can be described as observed by others [21]: = 0 exp
E a kB T
,
(1)
where 0 is the prefactor, Ea is the activation energy for the relaxation, kB is Boltzmann constant, T is the absolute temperature, and the relaxation time = RC. Then, ln vs. 1000/T are plotted in
Fig. 5(d). In the figure, T is the temperature of the dielectric loss peak at a given . It is seen that BCCTO-1 and -2 exhibit two Arrhenius segments at HT and LT, respectively, whereas CCTO and BCCTO-3 only show one relaxation region at LT. It can be noted that the activation energy values of 70 and 80 meV at LT are compared with the VO • doping energy level region (10–70 eV) [22]. Activation energies of 832 meV and 528 meV at HT for BCCTO-1 and -2 are close to 700 meV from the conduction activated energy of the electrons of VO •• [23]. Furthermore, according to cole–cole plots (Figs. 5(a–c)) there are three semicircles corresponding to three different conductive segments: grain, grain boundary and electrode surface. As
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a result, we conclude that the LT relaxations are attributed to the VO • doping energy inside CCTO grains, and the HT relaxations are related to the conduction activated energy of VO •• from the point defects of the grain boundaries. 4. Conclusion In this work, we have investigated the influences of Bi2 O3 doping on the dielectric response of CaCu3 Ti4 O12 ceramics. Heavy Bi3+ doped CCTO shows different morphology. Only one MW relaxation process was observed for pure CCTO and 0.3 Bi-doped CCTO ceramics in the available temperature range of 20–420 K, but for 0.1 and 0.2 Bi-doped CCTO ceramics, except for the high frequency relaxation, another MW relaxation was observed. By the complex impedance spectra measurement we found that these two MW relaxation regions were related with the competition of three different conductive segments (grain, grain boundary and electrode surface). Furthermore, from the activation energies fitting, the low temperature relaxation should be originated from the VO • doping energy inside CCTO grains, and the middle frequency dielectric relaxation of 0.1 and 0.2 Bi-doped CCTO at high temperature than 350 K should be attributed to the VO •• energy that related to the point defects in the grain boundary.
References [1] A.P. Ramirez, M.A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, S.M. Shapiro, Solid State Commun. 115 (2000) 217. [2] L.X. He, J.B. Neaton, M.H. Cohen, Phys. Rev. B 65 (2002) 214112. [3] J.Y. Li, X.T. Zhao, S.T. Li, M.A. Alim, J. Appl. Phys. 108 (2010) 104104. [4] L. He, J.B. Neaton, D. Vanderbilt, M.H. Cohen, Phys. Rev. B 67 (2003) 012103. [5] M.A.L. Cordeiro, F.L. Souza, E.R. Leite, A.J.C. Lanfredi, Appl. Phys. Lett. 93 (2008) 182912. [6] T.T. Fang, H.Y. Chung, J. Appl. Phys. 109 (2011) 014102. [7] M.H. Cohen, J.B. Neaton, L. He, D. Vanderbilt, J. Appl. Phys. 94 (2003) 3299. [8] M. Li, Z.J. Shen, M. Nygren, A. Feteira, D.C. Sinclair, A.R. West, J. Appl. Phys. 106 (2009) 104106. [9] A.V. Hippel, Dielectrics and Waves, Wiley, New York, 1952. [10] G. Chiodelli, V. Massarotti, D. Capsoni, M. Bini, C.B. Azzoni, M.C. Mozzati, P. Lupotto, Solid State Commun. 132 (2004) 241. [11] D. Capsoni, M. Bini, V. Massarotti, G. Chiodelli, M.C. Mozzati, C.B. Azzoni, J. Solid State Chem. 177 (2004) 4494. [12] S. Kwon, C.C. Huang, E.A. Patterson, D.P. Cann, E.F. Alberta, W.S. Hackenberger, Mater. Lett. 62 (2008) 633. [13] B.S. Prakash, K.B.R. Varma, J. Mater. Sci.: Mater. Electron. 17 (11) (2006) 899. [14] F. Luo, J. He, J. Hu, Y.-H. Lin, Adv. Mater. Res. 105–106 (2010) 274. [15] F. Luo, J. He, J. Hu, Y.-H. Lin, J. Appl. Phys. 105 (2009) 076104. [16] B.S. Prakash, K.B.R. Varma, J. Mater. Sci. 42 (2007) 7467. [17] Y.H. Lin, J.N. Cai, M. Li, C.W. Nan, J.L. He, J. Appl. Phys. 103 (2008) 074111. [18] T.-T. Fang, L.-T. Mei, H.-F. Ho, Acta Mater. 54 (2006) 2867. [19] T.B. Adams, D.C. Sinclair, A.R. West, Adv. Mater. 14 (2002) 1321. [20] L. Fang, M.R. Shen, F.G. Zheng, Z.Y. Li, J. Yang, J. Appl. Phys. 104 (2008) 064110. [21] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoto, A.P. Ramirez, Science 293 (2001) 673. [22] F.D. Morrison, D.C. Sinclair, A.R. West, J. Am. Ceram. Soc. 84 (2001) 531. [23] L. Ni, X.M. Chen, Appl. Phys. Lett. 91 (2007) 122905.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Dielectric properties of bismuth doped CaCu3 Ti4 O12 ceramics L.F. Xu, P.B. Qi, S.S. Chen, R.L. Wang, C.P. Yang ∗ Faculty of Physics and Electronics, Hubei University, Wuhan 430062, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 July 2011 Received in revised form 16 December 2011 Accepted 6 February 2012 Available online 19 February 2012 Keywords: CCTO ceramics Dielectric properties Oxygen deficiency
a b s t r a c t Ca1−3x/2 Bix Cu3 Ti4 O12 (x = 0.0–0.3) ceramics were prepared by the conventional solid-state reaction method. X-ray powder diffraction analysis confirmed the formation of cubic CCTO phase except for subtle peaks of CuO. SEM micrographs suggested that the morphologies of doped CCTO ceramics had been sheet-like for high Bi-doping amount, and the dominant grain size decreasing could be seen for the small content of Bi-doping CCTO. Dielectric properties of pure and doped CCTO were investigated in a broad temperature range of 20–420 K. The results showed that bismuth doping could decrease the dielectric loss but suppress the dielectric temperature stability at the same time. Bi doped CCTO ceramics presented different relaxation properties. As to pure CCTO and BCCTO (x = 0.3) only one MW relaxation (Relaxation I) could be found, which moves to higher frequency with temperature increasing. However, two relaxation processes (Relaxation I and II) appear for BCCTO (x = 0.1–0.2). By means of complex impedance spectra analysis and Arrhenius fitting, we successfully separated the different conductive segments and explained the mechanisms of the two relaxation processes. Relaxation I appeared at low temperature could be attributed to the VO • doping energies inside CCTO grains which did not showed significant changing of activation energy after bismuth doping. For Relaxation II at higher temperature than Relaxation I, with activation energy obviously depending on the Bi-ion concentration, may be related with the VO • • point defects at the grain boundaries. © 2012 Elsevier B.V. All rights reserved.
1. Introduction CaCu3 Ti4 O12 (CCTO) is a perovskite-like compound with colossal dielectric constant (CDC) and almost temperature independent in the frequency range of dc to 106 Hz. It shows high potential for technological applications, such as memory devices based on capacitive components and microwave devices. Some researchers suggested that the origin of CDC is intrinsic [1], however, by the first principles calculation, the static dielectric constant of CCTO was only about 40 [2]. Therefore, it has been speculated that the dielectric response is due to barrier-layer capacitances associated with one or more of the following: grain boundaries, twin boundaries, dislocation networks, and Schottky barrier or interfacial polarization effects [3–8]. The later behaviors were generally attributed to Maxwell–Wagner (MW) mechanism, which is the characteristic of electrical microstructure inhomogeneous materials [9]. M. H. Cohen et al. [7] discussed six possible morphologies for inhomogeneity in the local dielectric response of CCTO by supposing the properties of the internal boundary regions to be opposite from those of the bulk. (If one conducts, the other is insulating, and vice versa.) It was found that large dielectric constant result either when the conducting regions approach a percolation threshold or when
∗ Corresponding author. Tel.: +86 27 88665447; fax: +86 27 88663390. E-mail address: [email protected] (C.P. Yang). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.02.001
they percolate but are blocked at the sample surface or electrode interface. On the other hand, oxygen vacancies and/or impurities in CCTO can lead to conductivity in either the bulk (grains) or grain boundaries. And the oxygen vacancies are very sensitive to sintering parameters, like temperature, heating rate, duration of heat-treatment and atmosphere [4–7]. Besides the sintering conditions, the microstructure and the dielectric properties of the CCTO are strongly influenced by doping with other elements [10–13]. Especially Luo et al. [14,15] represented the electric and dielectric properties of bismuth doped CCTO ceramics based on the IBLC model. The grain size and second phase were affected by Bi-doping apparently, and then changed the electric and dielectric properties significantly. The purpose of the present work is to further investigate the effects of Bi-ion concentration on the structure and dielectric properties of CCTO ceramics by broad temperature range (20–420 K) measurement. Moreover, we will separate the different conductivity segments of CCTO by both dielectric spectra and complex impedance spectra. By the traditional solid-state reaction method, different proportion of Bi3+ from Bi2 O3 was introduced into CCTO with the nominal composition Ca1−3x/2 Bix Cu3 Ti4 O12 (x = 0.0, 0.1, 0.2 and 0.3). Bi3+ has higher valence than Ca2+ , so we intended to introduce oxygen vacancies and impurities by superseding Ca site with Bi3+ and explore the influence of Bi-ion concentration on properties of grain and grain boundary, which may contribute to the dielectric responses of CCTO.
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Table 1 XRD parameters and XRF results of pure and Bi-doped CCTO ceramics. Sample
Crystal system
Gain size (m)
˚ Lattice parameter (A)
Ca:Bi:Ti:Cu (molratio, XRF results)
x=0 x = 0.1 x = 0.2 x = 0.3
Cubic Cubic Cubic Cubic
27.4 3.7 4.3 4.8
7.3905 7.4201 7.4122 7.4005
1:2.99:4.09 0.85:0.08:3:4.29 0.69:0.17:3:4.30 0.54:0.25:3:4.28
Fig. 1. XRD patterns of the sintered CCTO ceramics.
2. Experimental CaCu3 Ti4 O12 (pure, as well as Bi3+ doped) pellets were prepared using traditional solid-state reaction and sintering process. Precursor powders of CaCO3 (99%), TiO2 (99%), and CuO (99%) were directly combined in the necessary stoichiometric ratios while doped samples employed Bi2 O3 (99%) as a powder (All raw materials are home-made). Thorough mixing was achieved by milling the powders in ethanol for no less than 4 h. The mixed powders were calcined at 1273 K (1000 ◦ C) for 10 h before being milled a second time. The dried powders were ground for 4 h in ethanol and then uniaxially cold pressed into pellets with 12 mm diameter and thickness about 1 mm under 14 MPa pressure. The brown pellets were then sintered in air using the schedule 10 K/min to 1373 K (1100 ◦ C), 12 h hold, and then furnace cooled to room temperature. The X-ray diffraction (XRD) patterns of the sintered samples were obtained in a range of 10–80◦ at room temperature, in a Japan MAXima X XRD-7000 system, with Cu K␣ radiation ( = 0.1506 nm) at 40 kV and 30 mA, with a step of 2◦ /min. The XRF measurements were performed utilizing an XRF-1800 (Shimadzu, Japan). The microstructures of the fractured surfaces were examined using a Scanning Electron Microscope (SEM s-4800, Japan). The dielectric properties and impedance spectrum were determined on samples with opposite sides painted with silver paste, using a WK6420 impedance analyzer with a Janis closed-cycle-refrigerator in the frequency and temperature range of 500 Hz to 3 MHz and 20–420 K respectively. 3. Results and discussion 3.1. Structure of Bi-doped CCTO ceramics The XRD (Fig. 1) confirms the presence of CCTO as a single phase for Bi2 O3 concentration up to 0.3. For pure CCTO peaks of CuO could be seen and fade away after bismuth doping. The segregation of
CuO could also be found in many other researches [15,16]. Moreover, peaks of Ca appear after bismuth doping and become stronger with doping amount increasing, and no bismuth-related phase is observed in the XRD patterns for Bi-doped samples. It could be inferred that the bismuth atoms have entered the lattice and almost taken place of some calcium except for minimal copper atom. The crystallite sizes (Lc ) of CCTO, obtained from the XRD patterns of representative samples, are presented in Table 1. These values show that the CCTO crystallite size would decrease after bismuth doping. And from XRF results we can see bismuth increasing and calcium decreasing with Bi-doping content increasing. Therefore, we can conclude that bismuth has replaced Ca-site. Fig. 2 shows the microstructure of the samples. The morphology of the undoped specimen consists of some huge grains, surrounded by small ones. An obvious decrease of the largest grain size is observed after Bi-doping which is consistent with the XRD calculation results. But the smaller grains did not show so apparent size decreasing as the bigger ones. BCCTO-2 and -3 are characterized by close packing sheet-like grains, surrounded by blurry grain boundary which may be caused by the overmuch liquid composed of Bi2 O3 during high temperature sintering. 3.2. Dielectric properties of Bi-doped CCTO ceramics Fig. 3 shows the frequency spectra of the relative permittivity (ε ) of undoped and Bi-doped (x = 0.1–0.3) CCTO ceramics. All room temperature (RT = 296 K) dielectric constant of the doped samples (∼104 ) are an order of magnitude lower than the pure ceramics (near 105 ). According to a simple series IBLC model, the dielectric constant of CCTO ceramics can be simply expressed by ε ≈ dεg /t, where d is the grain size, t is the boundary thickness, and εg is the dielectric constant of grain boundary. The effective dielectric constant would be around 103 –104 [17,18]. As the doping concentration increasing, the accumulation of Bi-ion will be exacerbated and the grain boundary will be thickened; moreover, from XRD a little increasing of CuO at the grain boundary for Bisubstituting may also increase the thickness of the gain boundary, so the dielectric constant will decrease. Compared to BCCTO ceramics, dielectric spectra of pure CCTO shows more stable temperature characteristics of ∼105 at 140–420 K and 500–105 Hz (Fig. 3(a)). All doped and pure samples show a sharp decreasing in ε values as the frequency higher than 106 Hz. The drastic decrease may indicate the presence of semi-conductive grains [19]. Only one dielectric relaxation region (I) can be observed for pure CCTO and BCCTO-3 in the available frequency and temperature range, which moves to higher frequencies as temperature increasing. Whereas for BCCTO-1 and -2, another relaxation region (II) at lower frequencies than Relaxation I comes up as the temperature higher than 350 K (Figs. 3(b and c)) which maybe related with the electrode surface. Fig. 4 shows dielectric loss spectra of undoped and Bi-doped (x = 0.1–0.3) CCTO ceramics. For CCTO and BCCTO-3, only one loss peak (I) at 80–170 K appears which is corresponding to Relaxation I in Fig. 3(a and d) and moves to higher frequencies as temperature increasing. For BCCTO-1 and -2, another loss peak (II) could be seen above 350 K at lower frequencies than Peak I, which accords well with Relaxation II in Fig. 3(b and c).
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Fig. 2. Microstructure of the sintered CCTO ceramics: (a) CCTO; (b) BCCTO-1; (c) BCCTO-2 and (d) BCCTO-3.
In order to analyze these data, we used the cole–cole plot, that is, the graphic of Z vs. Z , as shown in Fig. 5. At RT (Fig. 5(b)) only a large arc (Semicircle II) appears for all samples with nonzero intercepts (inset in Fig. 5(b)). The arc and intercepts are assigned as the contribution of CCTO grains and grain boundaries, respectively [20]. At low temperature (LT), for instance, 140 K (Fig. 5(a)), the second semicircle (I) appears which accords well with the high frequency plateau region in the dielectric permittivity spectra (at LT in Fig. 3). The large arc of semicircle I represented the more resistive grain boundaries than gains, which could be contributed by the CuO phase and Bi-ion accumulation. The intercepts in the insert of Fig. 5(a) are close to zero so we conclude Semicircle I should be corresponded to the contribution of CCTO grains.
Note in particular that the inflection point of the two arcs (Semicircles I and II) and Peak I (Fig. 4) occur at the same frequency, implying that the dielectric relaxations of CCTO and BCCTO at low temperature are related with MW relaxation, which is caused by electrical microstructure inhomogeneity between CCTO grain and grain boundary. At high temperature (HT) of 380 K, the third Semicircle III (BCCTO-1 and -2) comes up. The frequency of the inflection point of Semicircle II and III in the cole–cole plots is the same as that of the relaxation peaks (Peak II) in the corresponding Fig. 4(b and c), which demonstrates that the occurrence of dielectric relaxations of BCCTO-1 and -2 at high temperatures are related with MW relaxation of grain boundary and electrode interface.
Fig. 3. Dielectric spectra of CCTO ceramics at selected temperatures (20–420 K).
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Fig. 4. Dielectric loss spectra of CCTO ceramics at selected temperatures (20–420 K).
Fig. 5. Cole–cole curves of CCTO ceramics at three typical temperatures and the Arrhenius plots of ln vs. 1/T: (a) 170 K (LT); (b) 296 K (RT); (c) 380 K (HT); (d) the Arrhenius plots of BCCTO (x = 0–0.3) at different temperatures.
To further establish the possible relaxation mechanisms, the activation energy of dielectric relaxation was obtained from the temperature dependence of dielectric loss response in Fig. 4. Debye-like relaxations with Arrhenius kinetics can be described as observed by others [21]: = 0 exp
E a kB T
,
(1)
where 0 is the prefactor, Ea is the activation energy for the relaxation, kB is Boltzmann constant, T is the absolute temperature, and the relaxation time = RC. Then, ln vs. 1000/T are plotted in
Fig. 5(d). In the figure, T is the temperature of the dielectric loss peak at a given . It is seen that BCCTO-1 and -2 exhibit two Arrhenius segments at HT and LT, respectively, whereas CCTO and BCCTO-3 only show one relaxation region at LT. It can be noted that the activation energy values of 70 and 80 meV at LT are compared with the VO • doping energy level region (10–70 eV) [22]. Activation energies of 832 meV and 528 meV at HT for BCCTO-1 and -2 are close to 700 meV from the conduction activated energy of the electrons of VO •• [23]. Furthermore, according to cole–cole plots (Figs. 5(a–c)) there are three semicircles corresponding to three different conductive segments: grain, grain boundary and electrode surface. As
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a result, we conclude that the LT relaxations are attributed to the VO • doping energy inside CCTO grains, and the HT relaxations are related to the conduction activated energy of VO •• from the point defects of the grain boundaries. 4. Conclusion In this work, we have investigated the influences of Bi2 O3 doping on the dielectric response of CaCu3 Ti4 O12 ceramics. Heavy Bi3+ doped CCTO shows different morphology. Only one MW relaxation process was observed for pure CCTO and 0.3 Bi-doped CCTO ceramics in the available temperature range of 20–420 K, but for 0.1 and 0.2 Bi-doped CCTO ceramics, except for the high frequency relaxation, another MW relaxation was observed. By the complex impedance spectra measurement we found that these two MW relaxation regions were related with the competition of three different conductive segments (grain, grain boundary and electrode surface). Furthermore, from the activation energies fitting, the low temperature relaxation should be originated from the VO • doping energy inside CCTO grains, and the middle frequency dielectric relaxation of 0.1 and 0.2 Bi-doped CCTO at high temperature than 350 K should be attributed to the VO •• energy that related to the point defects in the grain boundary.
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