Enhanced giant dielectric response in Mg-substituted CaCu3Ti4O12 ceramics

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Abstract

Effects of Mg-substitution on the giant dielectric response in CaCu3Ti4O12 ceramics were investigated and discussed. A significantly enhanced giant dielectric response was obtained in Ca(Cu1−xMgx)3Ti4O12 ceramics, and the dielectric constant step increased with increasing Mg-substitution content. The results of X-ray photoelectron analysis confirmed the obvious increase of Ti3+/Ti4+ ratio, and the enhanced giant dielectric response should originate from the corresponding modification of the mixed-valent structure. The present results have great scientific significance in deepening the understanding on the origin of giant dielectric response and modulating the giant dielectric constant step in CaCu3Ti4O12.

Introduction

CaCu3Ti4O12 with cubic structure has intrigued much scientific attention because of its giant dielectric constant over a wide range of temperature and frequency, which make it potentially useful for important applications in microelectronics and memory devices, as the dielectric constant of a material ultimately decides the degree of miniaturization [1], [2]. The controversy surrounding the origin for the giant dielectric constant has not yet been put to rest. Much work has inclined to the explanation of barrier layer capacitance effects based on the Maxwell–Wagner relaxation mode, and the (1) electrode/ceramic interfaces, (2) grain boundaries, and (3) some inter grain inhomogeneities [3], [4], [5], [6], [7], [8]. On the other hand, more intrinsic mechanisms such as the structural frustration associated with the displacement of Ti ions [9] or nanoscale disorder at Ca/Cu site [10] et al. were proposed as the origin of the giant dielectric constant. Whether we call it an extrinsic or an intrinsic mechanism, the primary giant dielectric response in CaCu3Ti4O12 ceramics and single crystals should be dominated by some micro-scale inhomogeneities or polar micro-domains associated with defect chemistry in CaCu3Ti4O12, and it may help us to understand the structural origin [11], [12], [13], [14]. Different oxidation states of Ti and Cu were observed by x-ray photoelectron spectroscopy. Electron movement between Ti3+/Ti4+ and Cu+/Cu2+ and their correlation with oxygen vacancies had great influence on the dielectric properties of CaCu3Ti4O12 [15].

To deepen the understanding on the origin and modification of the giant dielectric response in CaCu3Ti4O12 ceramics, it is important to determine the effects of substitution of the invariable equivalent element for the variable-valent element of Cu or Ti, where the modification of the giant dielectric response can be expected by tailoring the mixed-valent structure of Ti3+/Ti4+ and Cu+/Cu2+. In the present work, the effects of Mg-substitution for Cu on the giant dielectric response in CaCu3Ti4O12 ceramics are investigated, and the giant dielectric response is greatly enhanced by such a substitution.

Section snippets

Experimental

Ca(Cu1−xMgx)3Ti4O12 (x=0, 0.05, 0.1) ceramics were prepared by a solid-state reaction process using the starting materials of CaCO3 (99.99%), CuO (99%), TiO2 (99.5%), and MgO (99.9%). The weighed raw materials were mixed by ball milling with zirconia media in ethanol for 24 h, and the mixtures were heated at 1273 K in air for 3 h after drying. The calcined powders with 8 wt% PVA (polyvinyl alcohol) were pressed into disks with 12 mm in diameter and 2 mm high under a pressure of about 98 MPa and

Results and discussions

All the diffraction peaks in X-ray diffraction patterns for Ca(CuxMg1−x)3Ti4O12 (x=0,0.05,0.1) ceramics (Fig. 1) are perfectly indexed based on the body-centered cubic structure with space group Im3. No second phase is observed up to x=0.1 and the lattice constants increase with Mg concentration (a=7.392Å, 7.396Å, and 7.398 Å for x=0, 0.05, and 0.1, respectively). The increased lattice constant should not be attributed to the ionic radius between Cu and Mg ions at square planar coordination

Conclusions

A significantly enhanced giant dielectric constant is obtained in Ca(Cu1−xMgx)3Ti4O12 ceramics, and the dielectric constant step increases with an increase of the Mg-substitution content. The results of X-ray photoelectron analysis confirm the obvious increase of the Ti3+/Ti4+ ratio, and the enhanced giant dielectric response should primarily originate from the corresponding modification of the mixed-valent structure. The present results have great scientific significance in deepening the

Acknowledgement

The present work was supported by National Science Foundation of China under Grant Nos. 50832005 and 50672083.

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