Effect of La-doping on the properties of CaCu₃Ti₄O₁₂ dielectric ceramics

Abstract

Nano-size Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ (χ = 0.00, 0.05, 0.10, 0.15 and 0.20) precursor powders were prepared via the sol–gel method and the citrate auto-ignition route and then processed into micro-crystal Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics under heat treatment. Characterization of the as-obtained ceramics with XRD and SEM showed an average grain sizes of ∼1–2 μm, indicating La³⁺ amount to have little impact on grain size. The room-temperature dielectric constant of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics sintered at 1000 °C was of the order of 10³–10⁴ despite the variation of χ values. Compared with CaCu₃Ti₄O₁₂, La³⁺-doped CaCu₃Ti₄O₁₂ showed a flatter dielectric constant curve related to frequency. It was found that the loss tangent of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics was less than 0.20 in ∼600–10⁵ Hz region, which rapidly decreased to a minimum value of 0.03 by La³⁺doping with χ = 0.05. Our measurement of the ceramics conductivities (σ) also indicated that the appropriate introduction of La³⁺ into CaCu₃Ti₄O₁₂ would distinctly result in its dielectric properties.

Introduction

The family of ACu₃Ti₄O₁₂ compounds has been intensively studied since 1967 [1]. In particular, the ceramic CaCu₃Ti₄O₁₂ (CCTO), due to its relatively high-dielectric constant (∼10⁴) and low-loss tangent (∼10⁻¹) in a broad frequency range, is gaining increasing scientific attention for its potential applications in microelectronic devices [1], [2], [3], [4], [5], [6]. Meanwhile, the family of A_2/3Cu₃Ti₄O₁₂, where A is a trivalent rare-earth ion, which has a similar structure of CCTO, has been reported frequently as well as the effect of chemical composition on the dielectric properties [1].

In recent years, it has been discovered that the dielectric constant of La_2/3Cu₃Ti₄O₁₂ ceramic is ∼418 at 100 kHz at room temperature [1], which is much lower than that of CaCu₃Ti₄O₁₂ (∼10⁴). On the other hand, the high dielectric constant of the classic ceramic BaTiO₃ at room temperature could be significantly enhanced up to >25,000 by La-dopant, with appropriate compositions, temperatures and a novel doping mechanism [7]. Although the CCTO and BaTiO₃ ceramics have similar structures and high-dielectric constant, they have distinct mechanisms for the dielectric properties, which make it both necessary and interesting to investigate CCTO ceramic by doping of La³⁺. Furthermore, it is known that the radius of the substituting element is an important parameter to determine the incorporation site [8]. The radius of ions as La³⁺ (1.15 Å) and Ca²⁺ (1.05 Å) are so close in value, which is another reason why we prefer La³⁺ to other ions as substituting Ca²⁺.

In this paper, we managed to partially substitute Ca²⁺ with La³⁺ in CCTO for the first time. The different proportion of La³⁺ from La(NO₃)₃·6H₂O was introduced into CCTO to synthesize Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ (χ = 0.00, 0.05, 0.10, 0.15 and 0.20) ceramics through a sol–gel method and citrate auto-ignition way. The advantages of sol–gel method lie in the fact that the powders are characterized by nanometer sizes, a low sintering temperature and extraordinary mechanical properties [9]. The effects of La³⁺ doping on the dielectric properties of CCTO ceramic were studied.