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Ag composites
Scripta Materialia 54 (2006) 1501–1504 www.actamat-journals.com
Maxwell–Wagner relaxation in CaCu3Ti4O12/Ag composites C.C. Wang a, Y.J. Yan b, L.W. Zhang
a,*
, M.Y. Cui a, G.L. Xie a, B.S. Cao
a
a
b
Physics Department, Laboratory of Advanced Materials, Tsinghua University, Beijing 100084, PR China Electron Microscopy Laboratory, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 7 August 2005; received in revised form 19 December 2005; accepted 25 December 2005 Available online 25 January 2006
Abstract CaCu3Ti4O12/Ag composites with different Ag weight fractions were prepared by sintering thoroughly mixed CaCu3Ti4O12 and Ag2O powders at 1050 C in air. The samples were found to consist of Ag-deficient insulating surface layers and Ag-rich more conductive inner parts. Dielectric measurements reveal that, apart from the distinct dielectric features for pure CaCu3Ti4O12, another dielectric relaxation appears near room temperature. This relaxation has been interpreted based on the Maxwell–Wagner model due to discontinuity of the charge concentration across the interface between the surface and inner layers. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: CaCu3Ti4O12; Dielectric properties; Ag addition; Maxwell–Wagner relaxation
1. Introduction The cubic perovskite-related CaCu3Ti4O12 (CCTO) has generated extensive interest due to its giant dielectric constant of 105 with weak temperature dependence in a broad temperature range from 100 K to nearly 600 K [1– 3]. The dielectric constant shows a 100-fold reduction around 100 K, below which the dielectric constant keeps an almost constant value of 100. Experiments of highresolution X-ray [2] and neutron powder diffraction [1] revealed that this remarkable dielectric behavior is not related to a ferroelectric transition or relaxor behavior. Some researchers suggested that this stunning dielectric behavior is intrinsic [1,3], while others claimed that this behavior arises from extrinsic effects such as spatial inhomogeneity [4], contact effect [5], as well as the internal barrier layers capacitor (IBLC) [6–9]. Based on the IBLC model, it is expected that the dielectric property of a CCTO ceramic sample would be strongly dependent on the conductivity of the grain boundaries, which can be tuned by annealing in reducing/oxidizing *
Corresponding author. Tel.: +86 10 62772762. E-mail address: [email protected] (L.W. Zhang).
atmospheres or by using suitable additions. Silver is widely used in high-temperature superconducting cuprates [10–12] and ferroelectric materials [13–15] to improve the boundary conditions. In high-temperature superconductors, addition of Ag is beneficial to grain enlargement and alignment, improving the ductility and superconductivity. While for ferroelectric materials, the incorporation of Ag has the advantages of lowering the sintering temperature and improving the dielectric properties. In this paper, CCTO/ Ag composites were prepared and the effects of the Ag addition were studied. Our results show that Ag is mainly located at the grain boundary in forms of clusters and distributed inhomogeneously across the thickness of the samples. A more conductive Ag-rich inner layer is sandwiched with the insulating Ag-deficient surface layers. A Maxwell– Wagner type relaxation was observed near room temperature due to the Ag-induced inhomogeneity. 2. Experimental procedures Single phase CCTO powder was synthesized by the solid-state reaction method. Details of the sintering processes were reported elsewhere [16]. The CCTO powder was thoroughly mixed with 5, 10 and 15 wt.% Ag2O
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.12.047
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C.C. Wang et al. / Scripta Materialia 54 (2006) 1501–1504
powder (denoted as CCTO/Ag5, CCTO/Ag10, and CCTO/ Ag15, respectively), pressed into disks and sintered at 1050 C for 16 h. X-ray diffraction (XRD) was performed on a Rigaku D/max-RB diffractometer using CuKa radiation. Electrical properties were measured by a standard four-point method. The distribution of Ag was detected using a scanning electron microscope (SEM, Model: JSM-6301F) and energy dispersive spectroscopy (EDS). Temperature dependent dielectric properties were measured using a QuadTech 1730 LCR Digibridge in a frequency range from 100 Hz to 100 kHz with a heating rate of 3 K/min. Silver paste was used as the electrodes. 3. Results and discussion XRD patterns indicate that the as-sintered disks are of a single phase with a cubic perovskite-related structure as shown in Fig. 1(a) for a CCTO/Ag15 sample. The calculated lattice parameter of a = 0.7396 nm is fairly consistent with that reported (0.7393 nm) by Subramanian [1]. No obvious Ag signal was detected. However, when the outermost surface layer of the sample was reduced by 0.2 mm, the XRD pattern from the exposed surface shows several reflections of Ag (Fig. 1(b)). No reaction phase between CCTO and Ag was observed in all the samples studied. This indicates that the outmost layer is Ag-deficient, while the inner part of the sample is Ag-rich. This inhomogeneous distribution of Ag was also observed by SEM-EDS line scans across the thickness of the as-sintered disks, which is typified by Fig. 2(a) for CCTO/Ag10. It is clearly seen that the concentration of Ag registers a peak with its peak position near the center of the sample. With increasing Ag fraction in the composites, the peak becomes broader. Ag loss in the surface layers might be due to the gasification of Ag during the sintering process. At the heating-up stage the starting Ag2O was decomposed into metallic Ag. The sintering temperature (1050 C) is higher than the melting point of Ag (960 C). Ag in the surface layers
(a)
(b)
Fig. 1. XRD patterns for the CCTO/Ag15 sample obtained from the surface layer (a) and inner part (b).
Fig. 2. (a) SEM-EDS line-scan profile of Ag across the thickness of CCTO/Ag10; (b) EDS mapping image of Ag taken from the inner part of CCTO/Ag10, the bar is 10 lm.
may find it easier to escape from the surface during the sintering process. As presented in Fig. 2(b) the SEM-EDS mapping image of silver taken from the polished surface of CCTO/Ag10 with Ag-deficit surface layer removed shows that the distribution of Ag is inhomogeneous as commonly reported in Ag-doped cuprates [10–12] and ferroelectric materials [13–15]. For CCTO/Ag10 and CCTO/Ag15 samples the Ag inclusions in the inner part are already percolated. As a result the resistivity of the inner part exhibits a metallic behavior, as typically shown in inset of Fig. 5(a) for a CCTO/Ag15 sample, with a room temperature resistivity q = 3.025 · 103 X cm, much smaller than that of q 107 X cm for pure CCTO [17]. Fig. 3 shows the variation in capacitance and loss tangent with temperature between 25 and 310 K for CCTO/ Ag5. Although the surface layers of both sides of the sample were removed (to a depth of 0.2 mm), the main features of the dielectric spectra are the same as those reported in the literature for pure CCTO, i.e. there exist two dielectric plateaus at higher and lower temperatures, between which the capacitance (dielectric constant) drops steeply, accompanied by a peak in the loss tangent. This result implies that the addition of 5 wt.% Ag2O has no significant influence on the dielectric behavior of the sample. A close inspection reveals that, besides the mentioned peak in the
C.C. Wang et al. / Scripta Materialia 54 (2006) 1501–1504
Fig. 3. Temperature dependence of the capacitance and loss tangent for CCTO/Ag5 at various frequencies.
loss tangent, another smaller peak appears around 250 K, and the corresponding capacitance exhibits a weak steplike increase. It suggests that a new relaxation process occurs. To clarify this, the loss tangent curves around 250 K were replotted in Fig. 4. It can be seen that the peak position strongly depends on the measuring frequency, indicating a thermally activated behavior. The inset of Fig. 4 shows the measuring frequency, f, versus the reciprocal of the peak position, Tp, defined as the temperature where d(tan d)/dT = 0. The obtained data fall perfectly on a straight line in the half-logarithmic presentation, implying that the dielectric behavior of the relaxation follows the Arrhenius law s ¼ s0 expðE=k B T Þ
ð1Þ
Fig. 4. Loss tangent as a function of temperature for CCTO/Ag5 in the temperature range from 200 to 370 K. The inset shows the plot of f versus 1/Tp. The symbols are the experimental points and the straight line is the result of a linear fit according to the Arrhenius law.
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Fig. 5. The capacitance (main panel) and loss tangent (inset (b)) at 50 kHz as a function of temperature for CCTO/Ag10 and CCTO/Ag15 samples with the two surface layers removed. Inset (a) shows the temperature dependence of resistivity for the inner part of CCTO/Ag15.
where s is the relaxation time, s0, the pre-exponential factor, E, the activation energy, and kB, the Boltzmann constant. A linear fit yields the values of the parameters s0 = 7.96 · 1013 s and E = 0.48 eV, respectively. For comparison, Fig. 5 displays the results of dielectric measurements at 50 kHz for the insulating surface layerremoved samples of CCTO/Ag10 and CCTO/Ag15. The dielectric behaviors are quite different from that of CCTO/Ag5: the capacitance (shown in the main panel) decreases almost exponentially with increasing temperature, while tan d (inset (b)) increases near linearly with increasing temperature, especially in the temperature range T > 100 K. The linear relationship between tan d and T has already been observed in La2CuO4+y ceramics and was suggested to be related to the conductivity [18]. These results demonstrate that the Ag-rich inner part from CCTO/Ag10 and CCTO/Ag15 samples behaves more or less like a percolated conductor rather than CCTO itself. Interestingly, when CCTO/Ag10 and CCTO/Ag15 disks were polished from one side to remove the insulating surface layer of one of two sides, the dielectric behaviors of the one-side-polished samples exhibit the same features as that observed in CCTO/Ag5 (Fig. 3). Typical results obtained from CCTO/Ag15 were plotted in Fig. 6, from which not only the distinct dielectric behaviors from pure CCTO in the lower temperature range (T < 200 K) is observed, but also a higher temperature relaxation can be clearly seen. The inset of this figure provides the Arrhenius plot of the higher temperature relaxation. The parameters of s0 and E deduced from the linear fit were found to be 1.96 · 1013 s and 0.53 eV, respectively. We now turn our attention to the origin of the high-temperature relaxation. As already mentioned above, Ag distribution in the composites is inhomogeneous. Each sample consists of insulating surface layers and a more
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C.C. Wang et al. / Scripta Materialia 54 (2006) 1501–1504
was given. After systematic investigations, we found that in pure CCTO the high-temperature relaxation originates from the inhomogeneity in oxygen distribution [16], instead of the inhomogeneous distribution of Ag for CCTO/Ag composites reported here. But in both cases, the relaxation is of the Maxwell–Wagner type. 4. Conclusions
Fig. 6. Temperature dependence of the capacitance and loss tangent for a CCTO/Ag15 sample with one-side-surface layer removed at various frequencies. The inset is the Arrhenius plot of f versus 1/Tp. The symbols are the experimental points and the straight line is the result of a linear fit according to the Arrhenius law.
In summary, we have prepared CCTO/Ag ceramic composites by sintering the thoroughly mixed CCTO and Ag2O powders with different fractions at 1050 C. It was found that the Ag distribution is not homogeneous in each sample. The surface layers are Ag-deficient and insulating and the inner layer is Ag-rich and more conductive. With increasing Ag weight fraction, the surface layer becomes thinner. Two dielectric relaxations were observed, with the low temperature one resulting from pure CCTO and the high-temperature one originating from Maxwell– Wagner relaxation between the surface and inner layers. Acknowledgements
conductive inner part. Hence, it is strongly indicative of a Maxwell–Wagner type relaxation, associated with an inhomogeneous dielectric medium containing two or more layers of materials with different permittivity and conductivity. An external electric field, inducing an electric current along the material, produces a charge carrier concentration discontinuity across the interface between more and less conductive regions. The build-up of charge results in the Maxwell–Wagner type relaxation. For the CCTO/Ag10 and CCTO/Ag15 samples, the Ag content is so high that the Ag particles accumulated at the grain boundary in the inner part are percolated as observed in Fig. 2(b), leading to the metal-like resistance and capacitance behaviors of the inner parts (Fig. 5 and its insets). If only one of the two insulating surface layers is removed, as for the main feature of the response to the external alternating current electric field the sample is equivalent to a circuit consisting of an insulating pure CCTO-like surface layer in series with a metal-like inner part. Therefore, the pureCCTO-like capacitance behavior was displayed. At the same time, due to the large difference in the conductivity between the surface layer and the inner part, the Maxwell–Wagner type relaxation was observed at 250 K (Fig. 6). On the other hand, in CCTO/Ag5 both the surface layers and inner part are insulating. The inhomogeneous distribution of Ag between the surface layers with less conductivity and the inner part with more conductivity causes the Maxwell–Wagner type relaxation at 250 K (Fig. 4), accompanying the dielectric relaxation exhibited by a pure CCTO compound (Fig. 3). Finally, it is worth pointing out that the high-temperature relaxation was also occasionally observed for pure CCTO in the literature [5]. Unfortunately, no explanation
We acknowledge the financial support from the National 973 Project of China, the National Natural Science Foundation of China (Grant No. 10474050), and the Tsinghua University Natural Science Foundation. This work was also supported in part by the China Postdoctoral Science Foundation. References [1] Subramanian MA, Li D, Duan N, Reisner BA, Sleight AW. J Solid State Chem 2000;151:323. [2] Ramirez AP, Subramanian MA, Gardel M, Blumberg G, Li D, Vogt T, et al. Solid State Commun 2000;115:217. [3] Homes CC, Vogt T, Shapiro SM, Wakimoto S, Ramirez AP. Science 2001;293:673. [4] Cohen MH, Neaton JB, He LX, Vanderbilt D. J Appl Phys 2003;94: 3299. [5] Lunkenheimer P, Fichtl R, Ebbinghaus SG, Loidl A. Phys Rev B 2004;70:172102. [6] Sinclair DC, Adams TB, Morrison FD, West AR. Appl Phys Lett 2002;80:2153. [7] Fang TT, Shiau HK. J Am Chem Soc 2004;87:2072. [8] Adams TB, Sinclair DC, West AR. Adv Mater 2002;14:1321. [9] Chung SY, Kim ID, Kang SJL. Nature Mater 2004;3:774. [10] Singh RK, Bhattacharga D, Tiwari P, Narayan J, Lee CB. Appl Phys Lett 1992;60:255. [11] Ganapathi L, Kumar A, Narayan J. J Appl Phys 1989;66:5935. [12] Apte PR, Pinto R, Chourey AG, Pai SP. J Appl Phys 1994;75:4258. [13] Hwang HJ, Watari K, Sando M, Toriyama M, Niihara K. J Am Ceram Soc 1997;80:791. [14] Wang C, Fang QF, Zhu ZG. Appl Phys Lett 2002;80:3578. [15] Chen CY, Tuan WH. J Am Ceram Soc 2000;83:2988. [16] Wang CC, Zhang LW. Appl Phys Lett, in press. [17] Capsoni D, Bini M, Massarotti V, Chiodelli G, Mozzatic MC, Azzoni CB. J Solid State Chem 2004;177:4494. [18] Wang CC, Cui YM, Xie GL, Chen CP, Zhang LW. Phys Rev B 2005; 72:064513.
Maxwell–Wagner relaxation in CaCu3Ti4O12/Ag composites C.C. Wang a, Y.J. Yan b, L.W. Zhang
a,*
, M.Y. Cui a, G.L. Xie a, B.S. Cao
a
a
b
Physics Department, Laboratory of Advanced Materials, Tsinghua University, Beijing 100084, PR China Electron Microscopy Laboratory, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 7 August 2005; received in revised form 19 December 2005; accepted 25 December 2005 Available online 25 January 2006
Abstract CaCu3Ti4O12/Ag composites with different Ag weight fractions were prepared by sintering thoroughly mixed CaCu3Ti4O12 and Ag2O powders at 1050 C in air. The samples were found to consist of Ag-deficient insulating surface layers and Ag-rich more conductive inner parts. Dielectric measurements reveal that, apart from the distinct dielectric features for pure CaCu3Ti4O12, another dielectric relaxation appears near room temperature. This relaxation has been interpreted based on the Maxwell–Wagner model due to discontinuity of the charge concentration across the interface between the surface and inner layers. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: CaCu3Ti4O12; Dielectric properties; Ag addition; Maxwell–Wagner relaxation
1. Introduction The cubic perovskite-related CaCu3Ti4O12 (CCTO) has generated extensive interest due to its giant dielectric constant of 105 with weak temperature dependence in a broad temperature range from 100 K to nearly 600 K [1– 3]. The dielectric constant shows a 100-fold reduction around 100 K, below which the dielectric constant keeps an almost constant value of 100. Experiments of highresolution X-ray [2] and neutron powder diffraction [1] revealed that this remarkable dielectric behavior is not related to a ferroelectric transition or relaxor behavior. Some researchers suggested that this stunning dielectric behavior is intrinsic [1,3], while others claimed that this behavior arises from extrinsic effects such as spatial inhomogeneity [4], contact effect [5], as well as the internal barrier layers capacitor (IBLC) [6–9]. Based on the IBLC model, it is expected that the dielectric property of a CCTO ceramic sample would be strongly dependent on the conductivity of the grain boundaries, which can be tuned by annealing in reducing/oxidizing *
Corresponding author. Tel.: +86 10 62772762. E-mail address: [email protected] (L.W. Zhang).
atmospheres or by using suitable additions. Silver is widely used in high-temperature superconducting cuprates [10–12] and ferroelectric materials [13–15] to improve the boundary conditions. In high-temperature superconductors, addition of Ag is beneficial to grain enlargement and alignment, improving the ductility and superconductivity. While for ferroelectric materials, the incorporation of Ag has the advantages of lowering the sintering temperature and improving the dielectric properties. In this paper, CCTO/ Ag composites were prepared and the effects of the Ag addition were studied. Our results show that Ag is mainly located at the grain boundary in forms of clusters and distributed inhomogeneously across the thickness of the samples. A more conductive Ag-rich inner layer is sandwiched with the insulating Ag-deficient surface layers. A Maxwell– Wagner type relaxation was observed near room temperature due to the Ag-induced inhomogeneity. 2. Experimental procedures Single phase CCTO powder was synthesized by the solid-state reaction method. Details of the sintering processes were reported elsewhere [16]. The CCTO powder was thoroughly mixed with 5, 10 and 15 wt.% Ag2O
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.12.047
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C.C. Wang et al. / Scripta Materialia 54 (2006) 1501–1504
powder (denoted as CCTO/Ag5, CCTO/Ag10, and CCTO/ Ag15, respectively), pressed into disks and sintered at 1050 C for 16 h. X-ray diffraction (XRD) was performed on a Rigaku D/max-RB diffractometer using CuKa radiation. Electrical properties were measured by a standard four-point method. The distribution of Ag was detected using a scanning electron microscope (SEM, Model: JSM-6301F) and energy dispersive spectroscopy (EDS). Temperature dependent dielectric properties were measured using a QuadTech 1730 LCR Digibridge in a frequency range from 100 Hz to 100 kHz with a heating rate of 3 K/min. Silver paste was used as the electrodes. 3. Results and discussion XRD patterns indicate that the as-sintered disks are of a single phase with a cubic perovskite-related structure as shown in Fig. 1(a) for a CCTO/Ag15 sample. The calculated lattice parameter of a = 0.7396 nm is fairly consistent with that reported (0.7393 nm) by Subramanian [1]. No obvious Ag signal was detected. However, when the outermost surface layer of the sample was reduced by 0.2 mm, the XRD pattern from the exposed surface shows several reflections of Ag (Fig. 1(b)). No reaction phase between CCTO and Ag was observed in all the samples studied. This indicates that the outmost layer is Ag-deficient, while the inner part of the sample is Ag-rich. This inhomogeneous distribution of Ag was also observed by SEM-EDS line scans across the thickness of the as-sintered disks, which is typified by Fig. 2(a) for CCTO/Ag10. It is clearly seen that the concentration of Ag registers a peak with its peak position near the center of the sample. With increasing Ag fraction in the composites, the peak becomes broader. Ag loss in the surface layers might be due to the gasification of Ag during the sintering process. At the heating-up stage the starting Ag2O was decomposed into metallic Ag. The sintering temperature (1050 C) is higher than the melting point of Ag (960 C). Ag in the surface layers
(a)
(b)
Fig. 1. XRD patterns for the CCTO/Ag15 sample obtained from the surface layer (a) and inner part (b).
Fig. 2. (a) SEM-EDS line-scan profile of Ag across the thickness of CCTO/Ag10; (b) EDS mapping image of Ag taken from the inner part of CCTO/Ag10, the bar is 10 lm.
may find it easier to escape from the surface during the sintering process. As presented in Fig. 2(b) the SEM-EDS mapping image of silver taken from the polished surface of CCTO/Ag10 with Ag-deficit surface layer removed shows that the distribution of Ag is inhomogeneous as commonly reported in Ag-doped cuprates [10–12] and ferroelectric materials [13–15]. For CCTO/Ag10 and CCTO/Ag15 samples the Ag inclusions in the inner part are already percolated. As a result the resistivity of the inner part exhibits a metallic behavior, as typically shown in inset of Fig. 5(a) for a CCTO/Ag15 sample, with a room temperature resistivity q = 3.025 · 103 X cm, much smaller than that of q 107 X cm for pure CCTO [17]. Fig. 3 shows the variation in capacitance and loss tangent with temperature between 25 and 310 K for CCTO/ Ag5. Although the surface layers of both sides of the sample were removed (to a depth of 0.2 mm), the main features of the dielectric spectra are the same as those reported in the literature for pure CCTO, i.e. there exist two dielectric plateaus at higher and lower temperatures, between which the capacitance (dielectric constant) drops steeply, accompanied by a peak in the loss tangent. This result implies that the addition of 5 wt.% Ag2O has no significant influence on the dielectric behavior of the sample. A close inspection reveals that, besides the mentioned peak in the
C.C. Wang et al. / Scripta Materialia 54 (2006) 1501–1504
Fig. 3. Temperature dependence of the capacitance and loss tangent for CCTO/Ag5 at various frequencies.
loss tangent, another smaller peak appears around 250 K, and the corresponding capacitance exhibits a weak steplike increase. It suggests that a new relaxation process occurs. To clarify this, the loss tangent curves around 250 K were replotted in Fig. 4. It can be seen that the peak position strongly depends on the measuring frequency, indicating a thermally activated behavior. The inset of Fig. 4 shows the measuring frequency, f, versus the reciprocal of the peak position, Tp, defined as the temperature where d(tan d)/dT = 0. The obtained data fall perfectly on a straight line in the half-logarithmic presentation, implying that the dielectric behavior of the relaxation follows the Arrhenius law s ¼ s0 expðE=k B T Þ
ð1Þ
Fig. 4. Loss tangent as a function of temperature for CCTO/Ag5 in the temperature range from 200 to 370 K. The inset shows the plot of f versus 1/Tp. The symbols are the experimental points and the straight line is the result of a linear fit according to the Arrhenius law.
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Fig. 5. The capacitance (main panel) and loss tangent (inset (b)) at 50 kHz as a function of temperature for CCTO/Ag10 and CCTO/Ag15 samples with the two surface layers removed. Inset (a) shows the temperature dependence of resistivity for the inner part of CCTO/Ag15.
where s is the relaxation time, s0, the pre-exponential factor, E, the activation energy, and kB, the Boltzmann constant. A linear fit yields the values of the parameters s0 = 7.96 · 1013 s and E = 0.48 eV, respectively. For comparison, Fig. 5 displays the results of dielectric measurements at 50 kHz for the insulating surface layerremoved samples of CCTO/Ag10 and CCTO/Ag15. The dielectric behaviors are quite different from that of CCTO/Ag5: the capacitance (shown in the main panel) decreases almost exponentially with increasing temperature, while tan d (inset (b)) increases near linearly with increasing temperature, especially in the temperature range T > 100 K. The linear relationship between tan d and T has already been observed in La2CuO4+y ceramics and was suggested to be related to the conductivity [18]. These results demonstrate that the Ag-rich inner part from CCTO/Ag10 and CCTO/Ag15 samples behaves more or less like a percolated conductor rather than CCTO itself. Interestingly, when CCTO/Ag10 and CCTO/Ag15 disks were polished from one side to remove the insulating surface layer of one of two sides, the dielectric behaviors of the one-side-polished samples exhibit the same features as that observed in CCTO/Ag5 (Fig. 3). Typical results obtained from CCTO/Ag15 were plotted in Fig. 6, from which not only the distinct dielectric behaviors from pure CCTO in the lower temperature range (T < 200 K) is observed, but also a higher temperature relaxation can be clearly seen. The inset of this figure provides the Arrhenius plot of the higher temperature relaxation. The parameters of s0 and E deduced from the linear fit were found to be 1.96 · 1013 s and 0.53 eV, respectively. We now turn our attention to the origin of the high-temperature relaxation. As already mentioned above, Ag distribution in the composites is inhomogeneous. Each sample consists of insulating surface layers and a more
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C.C. Wang et al. / Scripta Materialia 54 (2006) 1501–1504
was given. After systematic investigations, we found that in pure CCTO the high-temperature relaxation originates from the inhomogeneity in oxygen distribution [16], instead of the inhomogeneous distribution of Ag for CCTO/Ag composites reported here. But in both cases, the relaxation is of the Maxwell–Wagner type. 4. Conclusions
Fig. 6. Temperature dependence of the capacitance and loss tangent for a CCTO/Ag15 sample with one-side-surface layer removed at various frequencies. The inset is the Arrhenius plot of f versus 1/Tp. The symbols are the experimental points and the straight line is the result of a linear fit according to the Arrhenius law.
In summary, we have prepared CCTO/Ag ceramic composites by sintering the thoroughly mixed CCTO and Ag2O powders with different fractions at 1050 C. It was found that the Ag distribution is not homogeneous in each sample. The surface layers are Ag-deficient and insulating and the inner layer is Ag-rich and more conductive. With increasing Ag weight fraction, the surface layer becomes thinner. Two dielectric relaxations were observed, with the low temperature one resulting from pure CCTO and the high-temperature one originating from Maxwell– Wagner relaxation between the surface and inner layers. Acknowledgements
conductive inner part. Hence, it is strongly indicative of a Maxwell–Wagner type relaxation, associated with an inhomogeneous dielectric medium containing two or more layers of materials with different permittivity and conductivity. An external electric field, inducing an electric current along the material, produces a charge carrier concentration discontinuity across the interface between more and less conductive regions. The build-up of charge results in the Maxwell–Wagner type relaxation. For the CCTO/Ag10 and CCTO/Ag15 samples, the Ag content is so high that the Ag particles accumulated at the grain boundary in the inner part are percolated as observed in Fig. 2(b), leading to the metal-like resistance and capacitance behaviors of the inner parts (Fig. 5 and its insets). If only one of the two insulating surface layers is removed, as for the main feature of the response to the external alternating current electric field the sample is equivalent to a circuit consisting of an insulating pure CCTO-like surface layer in series with a metal-like inner part. Therefore, the pureCCTO-like capacitance behavior was displayed. At the same time, due to the large difference in the conductivity between the surface layer and the inner part, the Maxwell–Wagner type relaxation was observed at 250 K (Fig. 6). On the other hand, in CCTO/Ag5 both the surface layers and inner part are insulating. The inhomogeneous distribution of Ag between the surface layers with less conductivity and the inner part with more conductivity causes the Maxwell–Wagner type relaxation at 250 K (Fig. 4), accompanying the dielectric relaxation exhibited by a pure CCTO compound (Fig. 3). Finally, it is worth pointing out that the high-temperature relaxation was also occasionally observed for pure CCTO in the literature [5]. Unfortunately, no explanation
We acknowledge the financial support from the National 973 Project of China, the National Natural Science Foundation of China (Grant No. 10474050), and the Tsinghua University Natural Science Foundation. This work was also supported in part by the China Postdoctoral Science Foundation. References [1] Subramanian MA, Li D, Duan N, Reisner BA, Sleight AW. J Solid State Chem 2000;151:323. [2] Ramirez AP, Subramanian MA, Gardel M, Blumberg G, Li D, Vogt T, et al. Solid State Commun 2000;115:217. [3] Homes CC, Vogt T, Shapiro SM, Wakimoto S, Ramirez AP. Science 2001;293:673. [4] Cohen MH, Neaton JB, He LX, Vanderbilt D. J Appl Phys 2003;94: 3299. [5] Lunkenheimer P, Fichtl R, Ebbinghaus SG, Loidl A. Phys Rev B 2004;70:172102. [6] Sinclair DC, Adams TB, Morrison FD, West AR. Appl Phys Lett 2002;80:2153. [7] Fang TT, Shiau HK. J Am Chem Soc 2004;87:2072. [8] Adams TB, Sinclair DC, West AR. Adv Mater 2002;14:1321. [9] Chung SY, Kim ID, Kang SJL. Nature Mater 2004;3:774. [10] Singh RK, Bhattacharga D, Tiwari P, Narayan J, Lee CB. Appl Phys Lett 1992;60:255. [11] Ganapathi L, Kumar A, Narayan J. J Appl Phys 1989;66:5935. [12] Apte PR, Pinto R, Chourey AG, Pai SP. J Appl Phys 1994;75:4258. [13] Hwang HJ, Watari K, Sando M, Toriyama M, Niihara K. J Am Ceram Soc 1997;80:791. [14] Wang C, Fang QF, Zhu ZG. Appl Phys Lett 2002;80:3578. [15] Chen CY, Tuan WH. J Am Ceram Soc 2000;83:2988. [16] Wang CC, Zhang LW. Appl Phys Lett, in press. [17] Capsoni D, Bini M, Massarotti V, Chiodelli G, Mozzatic MC, Azzoni CB. J Solid State Chem 2004;177:4494. [18] Wang CC, Cui YM, Xie GL, Chen CP, Zhang LW. Phys Rev B 2005; 72:064513.