Evolution of the intergranular phase during sintering of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics have been processed by solid state reaction and sintered at 1100 °C for different times. A clear increase of the dielectric constant of the material up to values of 6 × 10⁴ has been observed with the sintering time. This increase is accompanied by a limited grain growth and intergranular Cu-oxide phase thickness reduction. The disappearance of the Cu-oxide phase is caused by the incorporation of Cu cations into the grains, contributing to the increase of the dielectric constant. Raman spectroscopy shows the decrease of TiO₆ octahedral rotational modes with the sintering time due to the incorporation of Cu cations into the CCTO grains. XANES measurements show that the Cu main oxidation state is Cu²⁺ and does not change with the sintering time. The fitting of the experimental dielectric constant to the Internal Barrier Layer Capacitance (IBLC) model reveals the change of the intergranular phase dielectric constant, caused by a compositional change due to the incorporation of Cu into the CCTO grains.     

Dielectric properties of Sm-doped CaCu3Ti4O12 ceramics

The effects of Sm substitution on structure, dielectric properties and conductivity of CaCu₃Ti₄O₁₂ ceramics were investigated. Ca_1?xSm_xCu₃Ti₄O₁₂ (x=0.0%, 0.5%, 1.0%) ceramics were synthesized by the solid-state reaction method. Single phase crystal of the ceramics with space group Im3 was obtained. With increasing Sm content, the dielectric loss of Ca_1?xSm_xCu₃Ti₄O₁₂ ceramics improved but the dielectric constant also decreased significantly, with both the low- and high-temperature dielectric relaxations suppressed.     

Dielectric properties of CaCu3Ti4O12 based multiphased ceramics

A “soft chemistry” method, the coprecipitation, has been used to synthesize the perovskite CaCu₃Ti₄O₁₂ (CCT). Three main types of materials were obtained for both powders and sintered ceramics: a monophased consisting of the pure CCT phase, a biphased (CCT + CaTiO₃), and a three-phased (CCT + CaTiO₃ + copper oxide (CuO or Cu₂O)). These ceramics, sintered at low temperature, 1050 °C, present original dielectric properties. The relative permittivity determined in the temperature range (−150 < T < 250 °C) is significantly higher than the one reported in the literature. Internal barrier layer capacitor is the probable mechanism to explain the particular behaviour. Moreover, the presence of a copper oxide phase beside the perovksite CCT plays an important role for enhancing the dielectric properties.     

Dielectric behaviour of Hf-doped CaCu3Ti4O12 ceramics obtained by conventional synthesis and reactive sintering

CaCu₃(Ti_4?xHf_x)O₁₂ ceramics (x = 0.04, 0.1 and 0.2) were prepared by conventional synthesis (CS) and through reactive sintering (RS), in which synthesis and sintering of the material take place in one single step. The microstructure and the dielectric properties of Hf-doped CCTO (CCTOHf) have been studied by XRD, FE-SEM, AFM, Raman and impedance spectroscopy (IS) in order to correlate the structure, microstructure and the electrical properties. Samples prepared by reactive sintering show slightly higher dielectric constant than those prepared by conventional synthesis in the same way than the pure CCTO. Dielectric constant and dielectric losses decrease slightly increasing Hf content. For CCTOHf ceramics with x > 0.04 for CS and x > 0.1 for RS, a secondary phase HfTiO₄ appears. As expected, the reactive sintering processing method allows a higher incorporation of Hf in the CCTO lattice than the conventional synthesis one.     

Effects of LiF addition on microstructure and dielectric properties of CaCu3Ti4O12 ceramics

The effects of small amounts of lithium fluoride sintering aid on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. CCTO polycrystalline ceramics with 0.5 and 1.0 mol% LiF, and without additive were prepared by solid state synthesis. Good densification (>90% of the theoretical density) was obtained for all prepared materials. Specimens without the sintering aid and sintered at 1090 °C exhibit secondary phases as an outcome of the decomposition reaction. The mean grain size is controlled by the amount of LiF in specimens containing the additive. Impedance spectroscopy measurements on CaCu₃Ti₄O₁₂ ceramics evidence the electrically heterogeneous nature of this material consisting of semiconductor grains along with insulating grain boundaries. The activation energy for grain boundary conduction is lower for specimens prepared with the additive, and the electric permittivity reached 53,000 for 0.5 mol% LiF containing CCTO.     

Enhancement of giant dielectric response in Ga-doped CaCu3Ti4O12 ceramics

The influences of Ga³⁺ doping ions on the microstructure, dielectric and electrical properties of CaCu₃Ti₄O₁₂ ceramics were investigated systematically. Addition of Ga³⁺ ions can cause a great increase in the mean grain size of CaCu₃Ti₄O₁₂ ceramics. This is ascribed to the ability of Ga³⁺ doping to enhance grain boundary mobility. Doping CaCu₃Ti₄O₁₂ with 0.25 mol% of Ga³⁺ caused a large increase in its dielectric constant from 5439 to 31,331. The loss tangent decreased from 0.153 to 0.044. The giant dielectric response and dielectric relaxation behavior can be well described by the internal barrier layer capacitor model based on Maxwell?Wagner polarization at grain boundaries. The nonlinear coefficient, breakdown field, and electrostatic potential barrier at grain boundaries decreased with increasing Ga³⁺ content. Our results demonstrated the importance of ceramic microstructure and electrical responses of grain and grain boundaries in controlling the giant dielectric response and dielectric relaxation behavior of CaCu₃Ti₄O₁₂ ceramics.     

Effect of calcium stoichiometry on the dielectric response of CaCu3Ti4O12 ceramics

Ca_xCu₃Ti₄O₁₂ (x = 0.90, 0.97, 1.0, 1.1 and 1.15) polycrystalline powders with variation in calcium content were prepared via the oxalate precursor route. The structural, morphological and dielectric properties of the ceramics fabricated using these powders were studied using X-ray diffraction, scanning electron microscope along with energy dispersive X-ray analysis, transmission electron microscopy, electron spin resonance (ESR) spectroscopy and impedance analyzer. The X-ray diffraction patterns obtained for the x = 0.97, 1.0 and 1.1 powdered ceramics could be indexed to a body-centered cubic perovskite related structure associated with the space group Im3. The ESR studies confirmed the absence of oxygen vacancies in the ceramics that were prepared using the oxalate precursor route. The dielectric properties of these suggest that the calcium deficient sample (x = 0.97) has a reduced dielectric loss while retaining the high dielectric constant which is of significant industrial relevance.     

Low temperature preparation of CaCu3Ti4O12 ceramics with high permittivity and low dielectric loss

Low temperature preparation of CaCu₃Ti₄O₁₂ ceramics with large permittivity is of practical interest for cofired multilayer ceramic capacitors. Although CaCu₃Ti₄O₁₂ ceramics have been prepared at low temperatures as previously reported, they have rather low permittivity. This work demonstrates that CaCu₃Ti₄O₁₂ ceramics can not only be prepared at low temperatures, but they also have large permittivity. Herein, CaCu₃Ti₄O₁₂ ceramics were prepared by the solid state reaction method using B₂O₃ as the doping substance. It has been shown that B₂O₃ dopant can considerably lower the calcination and sintering temperatures to 870 °C and 920 °C, respectively. The relative permittivity of the low temperature prepared CaCu₃Ti_4−xB_xO₁₂ ceramics is about 5 times larger than the previously reported results in the literature. Furthermore, the dielectric loss of the CaCu₃Ti_4−xB_xO₁₂ ceramics is found to be as low as 0.03. This work provides a beneficial base for the future commercial applications of CaCu₃Ti₄O₁₂ ceramics with large permittivity for the cofired multilayer ceramic capacitors.     

Optimized dual-function varistor-capacitor ceramics of core-shell structured xBi2/3Cu3Ti4O12/(1-x)CaCu3Ti4O12 composites

xBi_2/3Cu₃Ti₄O₁₂/(1-x)CaCu₃Ti₄O₁₂ composites were prepared by traditional solid-state reaction method. Extremely high nonlinear coefficient of 25 and breakdown field of 18.92 kV·cm⁻¹ were obtained in small current range of 0.1−1 mA·cm^-2. In addition, reduced dielectric loss of 0.055 was achieved with high dielectric constant of 1369. Optimized nonlinear and dielectric properties were integrated to make the composites a promising dual-function varistor-capacitor candidate. Microstructure analysis discovered two areas with various Bi/Ca ratio, designated as Bi-H and Bi-L respectively. It was found that the maximum ratio of Bi-H/Bi-L heterogeneous interface corresponded to optimized nonlinear and dielectric performance, which was associated with elevated potential barrier height and huge grain boundary resistance. Combined with relaxation analysis, a core-shell structure was proposed to elaborate microstructure evolution in xBi_2/3Cu₃Ti₄O₁₂/(1-x)CaCu₃Ti₄O₁₂ composite. According to the core-shell model, variation of heterogeneous interface was illustrated on how to influence nonlinear properties, which was well fitted to experimental results.     

Origin of the temperature stability of dielectric constant in CaCu3Ti4O12

The temperature and dc bias stability of the dielectric constant and loss tangent of CaCu₃Ti₄O₁₂ samples sintered under different oxygen atmospheres are discussed. The results suggest that the metal-oxygen vacancy related defects not only provide the charge carriers for the conduction (defect doping) but also contribute to the huge permittivity in the way of defect dipoles repositioning under charge carrier hopping. The charge localization in a specific copper-oxygen vacancy defect complex is the reason of the huge and stable permittivity and low dielectric loss in the middle temperature range, 90 K-200 K (20 Hz), while the implementation of the large barrier layer height needs a contribution by the titanium oxygen vacancy related trap charges in the grain boundaries, which also lead to a second permittivity stable range in a higher temperature range 200 K–300 K.     

Dielectric properties of CaCu3Ti4O12 ceramics with Zn substitution for Cu

Abstract

CaCu_3–xZn_xTi₄O₁₂ (x is from 0 to 1·0) polycrystalline samples were fabricated via a two-step solid state reaction process. The lattice parameter of the monophasic CaCu₃Ti₄O₁₂ phase increased as Zn content increased. Scanning electron microscopy (SEM) images of the CCTO ceramic show bimodal grain size distribution and the grain size decrease largely with the appearance of Zn₂TiO₄ second phase. The dielectric permittivity of pure CCTO ceramic is ～1·5×10⁴ at f?=?100 Hz. The dielectric constant of the sample largely increased with Zn substitution in the frequency range f<10⁴ Hz. The highest dielectric constant was 6·2×10⁴ at f?=?100 Hz with Zn substitution of x?=?0·8. The improved dielectric properties are believed to be related to the presence of a thin grain boundary barrier layer. The resistivity of the grain boundary decreased largely with Zn substitution as evidenced from the impedance plots.     

Nonlinear I–V electrical behaviour of doped CaCu3Ti4O12 ceramics

In this work a comparative study of undoped CaCu₃Ti₄O₁₂ (CCTO) and doped with Fe³⁺(CCTOF) and Nb⁵⁺(CCTON) ceramics, was aimed to modify the electronic transport. XRD patterns, FE-SEM microstructural analysis, impedance spectroscopy and I–V response curves were afforded to correlate the microstructure with the nonlinear I–V behaviour. The appearance of nonlinear behaviour in doped CCTO samples has been correlated with the ceramic microstructure that consists in n-type semiconductor grains, surrounded by a grain boundary phase based on CuO. The presence of this secondary grain boundary phase is the responsible of the assisted liquid phase sintering in CCTO ceramics. Doped samples showed cleaner grain boundaries than CCTO and nonlinearity in the I–V response.     

CaCu_3Ti_4O_(12)(CCTO)陶瓷的研究进展

CaCu_3Ti_4O_(12)(也简称为CCTO)是一种非铅基、具有高介电常数以及高热稳定性的新型介电陶瓷。这种性质使得它无论在科研领域还是在实际应运中都将成为研究的热点。首先介绍了CCTO目前的几种制备工艺,并对这几种工艺作出简单对比分析。其次介绍了CCTO巨介电常数机理来源的几种机制,然后详尽介绍掺杂改性对CCTO特性的影响,最后对CCTO的研究现状做出总结。     

Effect of cesium and cerium substitution on the dielectric properties of CaCu3Ti4O12 ceramics

In this study, CaCu₃Ti₄O₁₂ (CCTO) ceramics were doped with cesium and cerium atoms to possibly improve the electrical properties of these widely used ceramics. In all cases, pure phase perovskites were produced where cesium doping enhanced the grain growth and cerium doping produced grain growth inhibition. The cesium doping showed an improvement in loss tangent performance, in contrast to the cerium doping which showed a negative result. A high dielectric constant >15,000 with a dielectric loss lower than 0.06 was observed for cesium 2.0 mol% doped at high frequencies. These results were related to the change in microstructure and the properties of grain boundary after doping.     

Role of oxygen on the phase stability and microstructure evolution of CaCu3Ti4O12 ceramics

Phase stability and microstructure evolution of polycrystalline CaCu₃Ti₄O₁₂ (CCTO) ceramics were studied by controlling the partial pressure of oxygen (from a poor to an oxygen rich atmosphere) during the sintering process at high temperatures. The samples were analyzed by X-ray powder diffraction, scanning electron microscopy and X-ray energy dispersive spectroscopy. Our results show that the oxygen partial pressure during the sintering process is an important parameter that controls the phase stability, non-stoichiometry, and decomposition process of the CCTO phase as well as the densification and grain growth mechanisms on these polycrystalline ceramics. These results provided us further insight into the important role of copper reduction and copper/oxygen diffusion on the crystalline structure and morphological characteristics of polycrystalline CCTO ceramics.     

Electrical properties of Co-doped CaCu3Ti4O12

Co-doped CaCu₃Ti₄O₁₂ samples were synthesized by solid-phase reaction. Electrical properties were studied by impedance spectroscopy in wide temperature (25–450 °C) and frequency (10 Hz–10 MHz) intervals. It was shown that the presence of the copper oxide interlayer significantly reduces the value of the dielectric constant. The amount of impurity copper in the CaCu₃Ti_4-хCo_хO_12-δ samples (x = 0.06; 0.12; 0.24) rise with an increase in the cobalt content. The samples are characterized by a granular microstructure, with an average grain size ranging from 2 to 10 μm. The impedance of the samples was simulated at a temperature of 25 °C and in the range of 100–450 °C. It was found that the samples are characterized by low- and high-frequency polarization. The conductivity activation energy varied from 0.94 to 0.87 eV depending on the cobalt content. The CaCu₃Ti_3.94Co_0.06O_12-δ sample are characterized by the best values of the dielectric permittivity and the dielectric loss tangent, ε = 400 and tanδ = 0.2 (at 1 MHz and room temperature), respectively.     

Improved dielectric,nonlinear and magnetic properties of cobalt-doped CaCu3Ti4O12 ceramics

The dielectric properties and voltage–current nonlinearity of the pure and various cobalt-doped CaCu₃Ti₄O₁₂ (Co-doped CCTO) prepared by solid-state reactions were investigated. The improved dielectric properties in the Co-doped CCTO, with a dielectric constant $\text{ε'}$ ≈7.4?×?10⁴ and dielectric loss $\text{tan}\text{δ}$?≈?0.034, were observed in the sample with a Co doping of 5% (CCTO05) at room temperature and 1?kHz. The related multi-relaxations, RII (?20 to 40?°C) and RIII (100–150?°C), were demonstrated to be a Debye-like relaxation and a Maxwell–Wagner relaxation related to oxygen vacancies. The low dielectric loss of CCTO05 was associated with the high grain boundary resistance and the increase in cation vacancies. The improved nonlinear electrical properties (CCTO05, with nonlinear coefficients $\text{α}$?≈?5.22 and breakdown electric field ${\text{E}}_{\text{b}}$?≈?300.46?V/cm) and the ferromagnetism in Co-doped CCTO were also discussed.     

Microstructural evolution and dielectric properties of SiO2-doped CaCu3Ti4O12 ceramics

The abnormal grain growth (AGG) behavior of undoped and SiO₂-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. With the addition of 2 wt.% SiO₂, the AGG-triggering temperature decreased from 1100 to 1060 °C, and the temperature for obtaining a uniform and coarse microstructure decreased from 1140 to 1100 °C. The lowering of the AGG temperature by SiO₂ addition was attributed to the formation of a CuO-SiO₂-rich intergranular phase at lower temperature. The apparent dielectric permittivity of coarse SiO₂-doped CCTO ceramics was ∼10 times higher than that of fine SiO₂-doped CCTO ceramics at the frequency of 10³–10⁵ Hz. The doping of SiO₂ to CCTO ceramics provides an efficient route of improving the dielectric properties via grain coarsening. The correlation between the microstructure and apparent permittivity suggests the presence of a barrier layer near the grain boundary.     

Probing the effects of oxygen-related defects on the optical and luminescence properties in CaCu3Ti4O12 ceramics

The formation of oxygen-related lattice defects and their influence on optical and electronic properties in CaCu₃Ti₄O₁₂ (CCTO) ceramics were studied by controlling the oxygen partial pressure during the sintering process at high temperatures. The samples were analyzed using complementary bulk spectroscopic techniques such as photoluminescence spectroscopy, ultraviolet-visible spectroscopy, Fourier-transform infrared spectroscopy, and electron paramagnetic resonance. Our comprehensive study shows that the oxygen content during sintering process exerts considerable control over the type and concentration of oxygen-related defects in the crystalline CCTO structure. Our results also suggest that the formation of different types of oxygen-related defects in the structure modifies electronic energy levels and thus directly affects the electronic and optical properties of the CCTO phase. These results provided further insight into the important role of oxygen in the crystalline defect arrangement in polycrystalline CCTO ceramics.     

Deficient or excessive CuO-TiO2 phase influence on dielectric properties of CaCu3Ti4O12 ceramics

The influence of the CuO–TiO₂ phase (CT) on dielectric properties of the CCTO ceramic was investigated. CaCu_XTi_YO₁₂ (CC_XT_YO) powders were prepared based on the coprecipitated method, where 2.70 ≤ x ≤ 3.30 and 3.25 ≤ y ≤ 4.75. XRD patterns confirmed the presence of CCTO and also the secondary phases as CuO, TiO₂, and CaTiO₃ for each sample and aided in its quantification. Scanning Electron Microscopy (SEM) shows secondary phases evolution in the grain boundaries, and its influence on size and morphology of the grains. Impedance spectroscopy measurements showed that the ceramics with lower amount of CuO and TiO₂ phases (CT/deficient ceramics) exhibited the highest ε′ values (2.1 × 10⁴ at 1 kHz for CC_2.9T_3.75O ceramic). Also, CT/deficient ceramics showed lower tanδ values (0.090 at 1 kHz for CC_2.9T_3.75O ceramic) than ceramics prepared with excessive CuO–TiO₂ phase (0.241 at 1 kHz for CC_3.1T_4.25O ceramics). The deficiency of CuO and TiO₂ phases associated with high percentage of CCTO and CaTiO₃ phases resulted in ceramics with the higher ε′ values.