Influence of temperature on dielectric properties of Fe-doped CaCu3Ti4O12 ceramics

doi:10.1016/j.physb.2009.08.093

Physica B: Condensed Matter

Volume 405, Issue 1, 1 January 2010, Pages 386-389

https://doi.org/10.1016/j.physb.2009.08.093 Get rights and content

Abstract

CaCu₃Ti₄₋_xFe_xO₁₂ (0≤x≤0.2) ceramics were fabricated by a solid-state reaction method, and the dielectric properties were studied. Dielectric measurement performed at room temperature shows that with Fe content increasing from 0 to 0.2, the dielectric constant (ε′) decreases from about 5×10⁴ to 50 in the frequency range from 1 kHz to 1 MHz, while ε′ of all Fe-doped samples at high temperature re-increased to a high level of 10⁵ below 10 kHz, characterized by dielectric temperature spectrum. Interestingly, two dielectric relaxations named as I and II are observed with the increasing of temperature. In order to catch the intrinsic relaxation mechanism, the IBLC model and modulus approach are adopted to clarify these fantastic dielectric properties.

Introduction

In recent years, CaCu₃Ti₄O₁₂ (CCTO) ceramics have attracted considerable interests due to their high dielectric constant (>10⁴) and good stability until 1 MHz [1], [2], [3]. The dielectric constant abruptly drops to about 100 below 100 K, accompanying neither any phase transitions nor detectable changes of long-range crystallographic structure. In order to understand this unique response, a variety of characteristics of CCTO have been studied [4], [5], [6]. It is now widely accepted that the high dielectric constant at room temperature is associated with the internal barrier layer capacitance (IBLC) effect [7], [8], or electrode/sample contact effects [9], but not with an intrinsic effect. However, two dielectric relaxations are observed in dielectric property spectra measurement, which indicates that there are still some unknown factors contributing to the giant dielectric constant in CCTO. Prakash et al. argued that the additional dielectric relaxation originated from the surface layers due to the inhomogeneous of oxygen on the surfaces and the interior of the sample [10]. Li et al. suggested that the two dielectric relaxations may be ascribed to two Maxwell–Wagner relaxations, which occurred at the interfacial layer of grain boundaries and domain boundaries [11]. Furthermore, two dielectric relaxations are also observed in single-crystalline CCTO [12]. It is well known that there are no grain boundaries in single-crystalline CCTO.

Obviously, there are many open questions concerning the origin of the dielectric properties of CCTO. Grubbs et al. reported that Fe iron doping could produce remarkably large changes in dielectric response of Fe-doped CCTO ceramics [13], and they devoted to the low-temperature properties. In this work, we substitute Ti⁴⁺ with Fe³⁺ and investigate the dielectric properties of CCTO ceramics in high temperature where we found new additional relaxation responses.

Section snippets

Experimental

CaCu₃Ti₄₋_xFe_xO₁₂ (CCTFO, x=0, 0.01, 0.04, 0.12, and 0.2) ceramics were prepared by a solid-state reaction method. High purity CaCO₃ (99.99%), CuO (99.9%), TiO₂ (99.99%), and Fe₂O₃ (99.9%) powders were mixed and ball-milled in ethanol for 10 h. Powders were dried and calcined at 900 °C for 10 h in air, then ball-milled for another 6 h. The calcined powders were blended with approximately 5 wt% polyvinyl alcohol (PVA) to press into pellets by uniaxial pressing, and all specimens were sintered at 1100

Results and discussion

All samples could be indexed to cubic perovskite structure according to the standard card ICDD (05–0566), and no secondary phase was found. The detailed analysis was included in our previous research [14].

The SEM surface images of CCTFO ceramics (x=0, 0.01, 0.04, and 0.2) sintered at 1100 °C for 12 h are shown in Fig. 1(a–d), respectively. It can be seen that average grain size increases with increasing Fe doping content. Fig. 1(a) shows a discontinuous grain growth for the pure sample. When x

Conclusions

In conclusion, the influence of temperature on the dielectric properties in CCTFO ceramics has been investigated. With the increase of Fe doping content, dielectric constant decreases rapidly from around 5×10⁴ to 50 at room temperature. We prove that colossal dielectric constant is related to the electrical properties of grains and grain boundaries for CCTFO ceramics in the frequency range from 1 kHz to 1 MHz. When frequency is lower than 1 kHz, the high permittivity comes from the contributions

Acknowledgments

This work is partially supported by the National Basic Research Program of China (973) under Grant No. 2007CB310407 and the International S&T Cooperation Program of China under Grant No. 2006DFA53410 and 2007DFR10250.

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The colossal dielectric constant of calcium copper titanate (CCTO) thin films is generally accompanied by high dielectric losses which limit its potential for electronic miniaturization. Strategies for reducing the dielectric loss while keeping large dielectric constants are needed for various electrical applications of CCTO. This work aims to reduce the loss by means of doping preferentially oriented CCTO films with Fe³⁺ ions. Highly oriented undoped and Fe-doped calcium copper titanate (Fe-doped CCTO) thin films were fabricated by synthesizing CCTO on LaAlO₃(1 0 0) and NdGaO₃(1 0 0) substrates via a sol-gel spin casting method. The films with different Fe-doping concentrations (0, 1.6 and 2.6 wt%) were structurally and chemically characterized via energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. X-Ray Diffraction (XRD) patterns confirmed that our doped CCTO films had a cubic perovskite structure with well-defined preferential orientations of (h00) and (hh0) on LAO and NGO substrates, respectively. Preferentially oriented films having room temperature dielectric constants on the order of 1000 in conjunction with values of loss tangent smaller than 0.012 in the frequency range of 50 Hz-2 MHz were achieved. Temperature dependence of the dielectric constant and loss tangent revealed that Fe-doping decreased the dielectric loss via the lowering of the electrical conduction through CCTO grains.
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Most studies [25,27,29,32] showed a decrease in the dielectric loss and permittivity of calcium copper titanate as a result of doping, which was explained by the selective effect of dopants on the formation of barrier layers at grain boundaries and by a change in the grain resistance of ceramics due to doping. In particular, a possible substitution of copper (II) and titanium (IV) ions [34,41–45] in calcium copper titanate with iron (III) ions was reported. It was shown that the replacement of titanium [41,44–48] or copper ions [34,42,43,46,48] with iron led to a sharp decrease in the dielectric constant of calcium copper titanate [47,49,50].
The methods of magnetic dilution, thermal analysis, ESR (Electron Spin Resonance), Mössbauer and NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectroscopy were used to study iron-doped solid solutions of calcium copper titanate CaCu₃Ti_4-4xFe_4xO_12-δ (0.005 ≤ x ≤ 0.08, 0.2) and CaCu_3-3xFe_3xTi₄O_12+δ (0.005 ≤ x ≤ 0.06) synthesized by the standard ceramic method by replacing copper or titanium ions with iron. According to X-ray diffraction data, the samples of the both series contained no iron-based impurity phases. A characteristic feature of the Mössbauer spectra of the samples of both series was the doublet part in the form of an asymmetric triplet with an intensity ratio of 1:3:1. The analysis of the Mössbauer spectra of the samples of the both series showed the presence of three doublets in them with different quadrupole splitting and isomeric shift values of 0.3–0.4 mm/s, which is typical for Fe³⁺ ions in an octahedral coordination with varying degrees of distortion. According to the Mössbauer data and ESR spectroscopy, in the both series of solid solutions, the substitution (Fe→Ti or Fe→Cu) occurred mainly in the octahedral positions of titanium by means of heterovalent substitution Fe³⁺→Ti⁴⁺. In the ESR spectra, the increase in the iron content in the samples of the both series led to broadening of the Lorentz absorption line of Cu(II) ions with g ≈ 2.15 and to decrease in the g-factor. NEXAFS and XPS revealed that the doping of CaCu₃Ti₄O₁₂ (CCTO) with iron ions had no significant effect on the electronic state and polyhedral environment of Ca, Cu and Ti ions in CCTO, regardless of the degree and nature of the doping. The charge state of iron ions in the solid solutions corresponded to Fe(III), iron ions predominantly occupy the positions of titanium. According to the magnetic dilution data, the antiferromagnetic exchange appeared between the nearest iron (III) ions in the solid solutions of calcium copper titanate. The clusters of iron (III) ions interacted with Cu(II) ions through the formation of ferromagnetic exchange. There were no significant differences in the magnetic behavior of the iron-doped solid solutions of both series. According to the thermogravimetric data, there was no mass loss of the samples below the melting temperature (∼1033 °C). The DSC (Differential Scanning Calorimetry) curves of the both series of the solid solutions revealed the exothermic effect at 233–300 °C and a number of endothermic processes at temperatures above 946 °C, near the melting temperature of the samples.
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Influence of temperature on dielectric properties of Fe-doped CaCu3Ti4O12 ceramics

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

J. Phys. Chem. Solids

Solid State Commun.

Science

Phys. Rev. B

Physica B

J. Amer. Ceram. Soc.

Appl. Phys. Lett.

Physica B

J. Phys. D Appl. Phys.

Influence of temperature on dielectric properties of Fe-doped CaCu₃Ti₄O₁₂ ceramics