Abstract
The giant dielectric behavior of CaCu3Ti4O12 (CCTO) has been widely investigated owing to its potential applications in electronics; however, the loss tangent (tanδ) of this material is too large for many applications. A partial substitution of CCTO ceramics with either Al3+ or Ta5+ ions generally results in poorer nonlinear properties and an associated increase in tanδ (to ~0.29–1.15). However, first-principles calculations showed that self-charge compensation occurs between these two dopant ions when co-doped into Ti4+ sites, which can improve the electrical properties of the grain boundary (GB). Surprisingly, in this study, a greatly enhanced breakdown electric field (~200–6588 V/cm) and nonlinear coefficient (~4.8–15.2) with a significantly reduced tanδ (~0.010–0.036) were obtained by simultaneous partial substitution of CCTO with acceptor-donor (Al3+, Ta5+) dopants to produce (Al3+, Ta5+)-CCTO ceramics. The reduced tanδ and improved nonlinear properties were attributed to the synergistic effects of the co-dopants in the doped CCTO structure. The significant reduction in the mean grain size of the (Al3+, Ta5+)-CCTO ceramics compared to pure CCTO was mainly because of the Ta5+ ions. Accordingly, the increased GB density due to the reduced grain size and the larger Schottky barrier height (Φb) at the GBs of the co-doped CCTO ceramics were the main reasons for the greatly increased GB resistance, improved nonlinear properties, and reduced tanδ values compared to pure and single-doped CCTO. In addition, high dielectric constant values (ε′ ≈ (0.52–2.7) × 104) were obtained. A fine-grained microstructure with highly insulating GBs was obtained by Ta5+ doping, while co-doping with Ta5+ and Al3+ resulted in a high Φb. The obtained results are expected to provide useful guidelines for developing new giant dielectric ceramics with excellent dielectric properties.
Highlights
The giant dielectric properties (GDPs) of dielectric materials with a high dielectric constant ( ) have been extensively studied for use in electronics applications, such as capacitive devices used in high-energy storage devices [1,2,3,4,5,6,7,8,9,10,11,12,13]
The grain growth in polycrystalline ceramics is associated with grain boundary (GB) mobility (Mb), which is dependent on the diffusion of ions, atoms, and/or charged species across the GB [40]
The strong frequency dependence of the low-frequency for the Al025 ceramic (Fig. 4(a)) was considered to originate from the dielectric response of the sample electrode interface [5]. This result is consistent with the appearance of a low-frequency dielectric relaxation peak in the tan curve (102–103 Hz) (inset of Fig. 4(a)). These results clearly indicate the additional dielectric response from the sample–electrode interface, which was dominant when the Rgb of the Al025 ceramic significantly decreased from 5.08 104 to 5.96 103 ·cm at 110 °C [45,46]
Summary
The giant dielectric properties (GDPs) of dielectric materials with a high dielectric constant ( ) have been extensively studied for use in electronics applications, such as capacitive devices used in high-energy storage devices [1,2,3,4,5,6,7,8,9,10,11,12,13]. High values of 103 105 in the low-frequency range without detectable phase transitions have been reported for a wide range of functional electroceramics, such as doped TiO2 [4,17], doped SnO2 [18], doped NiO [19], CaCu3Ti4O12 (CCTO) and its related structures [3,5,20,21,22,23], and La2−xSrxNiO4 [24]. These ceramic oxides can be used in electronic devices, such as capacitors, sensors, and varistors. For use in capacitor applications, a low frequency- and temperature-dependence of (at 1 kHz) is important [4,5]
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