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Abstract
The effects of charge compensation on dielectric and electrical properties of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics (x = 0−0.05) prepared by a solid-state reaction method were studied based on the configuration of defect dipoles. A single phase of CaCu3Ti4O12 was observed in all ceramics with a slight change in lattice parameters. The mean grain size of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics was slightly smaller than that of the undoped ceramic. The dielectric loss tangent can be reduced by a factor of 13 (tanδ ~0.017), while the dielectric permittivity was higher than 104 over a wide frequency range. Impedance spectroscopy showed that the significant decrease in tanδ was attributed to the highly increased resistance of the grain boundary by two orders of magnitude. The DFT calculation showed that the preferential sites of Al and Nb/Ta were closed together in the Ti sites, forming self-charge compensation, and resulting in the enhanced potential barrier height at the grain boundary. Therefore, the improved dielectric properties of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics associated with the enhanced electrical properties of grain boundaries. In addition, the non-Ohmic properties were also improved. Characterization of the grain boundaries under a DC bias showed the reduction of potential barrier height at the grain boundary. The overall results indicated that the origin of the colossal dielectric properties was caused by the internal barrier layer capacitor structure, in which the Schottky barriers at the grain boundaries were formed.
Highlights
Introduction iationsColossal dielectric permittivity of ceramic oxides with very large dielectric constant (ε0 > 104 ) has been extensively studied for use in electronics applications, especially for capacitive-based devices such as ceramic capacitors [1,2,3,4,5,6,7,8,9,10,11]
Since the colossal dielectric permittivity of CaCu3 Ti4 O12 (CCTO) ceramics was reported by Subramanian, et al [12], many simple and complex oxides such as NiO, ZnO, TiO2, SnO2, BiFeO3, and CCTObased ceramics, have been investigated [1,2,3,13,14,15,16,17,18,19,20,21,22,23,24]
The tanδ values at 1 kHz of the CCTO, NbTaAl025, and NbTaAl05 rapid decrease in ε′ at 20 °C appeared in a frequency range of >106 Hz, corresponding to ceramics were 0.227, 0.042, and 0.017, respectively
Summary
Nb2 O5 (Sigma-Aldrich, 99.99%), Ta2 O5 (Aldrich, 99.99% purity), Al2 O3 (Sigma-Aldrich, 99.99% purity), CaCO3 (Sigma-Aldrich, 99.0% purity), CuO (Merck, 99.9% purity), and. Details of the preparation method were provided in our previous publication [2]. The CaCu3 Ti4-x (Ta0.25 Nb0.25 Al0.5 )x O12 samples with x = 0, 0.025, and 0.05 are referred to as the CCTO, NbTaAl025, and NbTaAl05 ceramics, respectively. Rietveld quantitative phase analysis was carried out using the X’Pert High Score Plus v3.0e software package by PANalytical. The parameters and coefficients optimized were the zero shift, scale factor, background (with function type: polynomial), profile half-width parameters (v, u, w), lattice parameters. Rietveld quantitative phase anal of 15 ysis was carried out using the X’Pert High Score Plus v3.0e software package by PANalytical. The parameters and coefficients optimized were the zero shift, scale factor, background (with function type: polynomial), profile half-width parameters (v, u, w), lattice parameters (a, b,site c),occupancies atomic site occupancies (Wyckoff), and preferred parameter.
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