Improved dielectric and nonlinear properties of CaCu3Ti4O12 ceramics with Cu-rich phase at grain boundary layers

The formation and compositions of grain boundary layers are very important factors to improve the electrical properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics. In present work, the dielectric and nonlinear properties of the CCTO ceramics are enhanced by controlling the Cu-rich phase degree at grain boundary layers. The dense CCTO ceramics were prepared successfully through mixing the nanometer and micrometer powders and using the cold isostatic pressing process. The average grain size of these CCTO ceramics is about 30.71(±11.76) ∼ 62.01(±32.16) μm, and their grain microstructures show the Cu-rich phases at grain boundary layers. The CCTO ceramics with the mass ratios of nanometer and micrometer powders 7:3 display a giant dielectric constant of 5.4 × 10⁴, low dielectric loss of 0.048 at 10³ Hz, enhanced nonlinear coefficients of 11.12, as well as the noteworthy breakdown field of 4466.17 V/cm. The complex impedance spectroscopy results indicate that the giant dielectric behavior is due to the electrically heterogeneous grain/grain boundary characteristics from internal barrier layer capacitance (IBLC) model. The lower dielectric loss and the higher breakdown field are attributed to the high resistance grain boundary layers with the Cu-rich phase. The improved nonlinear properties are related to the increased Schottky barrier height at grain boundary. This work may provide a potential way to design the CCTO ceramics with excellent electrical properties from the viewpoint of controlling the response of the Cu-rich phase grain boundary.     

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.     

Colossal permittivity of (Gd + Nb) co-doped TiO2 ceramics induced by interface effects and defect cluster

Material with high dielectric constant plays an important role in energy storage elements. (Gd + Nb) co-doped TiO₂ (GNTO) ceramics with giant dielectric permittivity (>10⁴), low dielectric loss, good temperature and frequency stability in broad range of 30–150 °C and 10²–10⁶ Hz have been systematically characterized. Especially, a low dielectric loss of 0.027 and a giant dielectric permittivity of 5.63 × 10⁴ at 1 kHz are attained for the composition with x = 0.01. Results of complex impedance spectroscopy, I–V curve and frequency dependent dielectric constant under DC bias indicate that internal barrier layer capacitance (IBLC) effect, electrode effect and electron-pinned defect-dipole (EPDD) effect contribute to the colossal permittivity (CP) property simultaneously.     

Good dielectric performance and broadband dielectric polarization in Ag,Nb co-doped TiO2

Due to the demand of miniaturization and integration for ceramic capacitors in electronic components market, TiO₂-based ceramics with colossal permittivity has become a research hotspot in recent years. In this work, we report that Ag⁺/Nb⁵⁺ co-doped (Ag_1/4Nb_3/4)_xTi_1−xO₂ (ANTOx) ceramics with colossal permittivity over a wide frequency and temperature range were successfully prepared by a traditional solid–state method. Notably, compositions of ANTO0.005 and ANTO0.01 respectively exhibit both low dielectric loss (0.040 and 0.050 at 1 kHz), high dielectric permittivity (9.2 × 10³ and 1.6 × 10⁴ at 1 kHz), and good thermal stability, which satisfy the requirements for the temperature range of application of X9R and X8R ceramic capacitors, respectively. The origin of the dielectric behavior was attributed to five dielectric relaxation phenomena, i.e., localized carriers' hopping, electron–pinned defect–dipoles, interfacial polarization, and oxygen vacancies ionization and diffusion, as suggested by dielectric temperature spectra and valence state analysis via XPS; wherein, electron-pinned defect–dipoles and internal barrier layer capacitance are believed to be the main causes for the giant dielectric permittivity in ANTOx ceramics.     

Origin of giant dielectric response in LiCuNb3O9 distorted perovskite ceramics

LiCuNb₃O₉ ceramics with the distorted cubic perovskite structure were prepared by a solid-state reaction method. The ceramic exhibited a very large value of permittivity (∼4.4 × 10⁴ at 100 kHz) at room temperature (∼300 K) and a low-temperature dielectric relaxation behaviour following the Arrhenius law. The origin of the giant dielectric response of the LiCuNb₃O₉ ceramics was correlated with the structure of the ceramics. The barrier layers in the grain boundaries and the mixed-valent structure of Cu⁺/Cu²⁺ were found to contribute to the giant permittivity of the ceramics and confirmed by X-ray spectroscopy and complex impedance spectroscopy analyses.     

Colossal Permittivity in Microwave‐Sintered Barium Titanate and Effect of Annealing on Dielectric Properties

Colossal permittivity (ε′ = 301,484 at room temperature and 1 kHz) of barium titanate was induced in ceramics synthesized using the microwave sintering method. Three different sintering processes (conventional, spark plasma, and microwave) were performed to better understand colossal permittivity in sintered barium titanate. The dielectric permittivity measurements revealed that the appearance of colossal permittivity has strong dependence on the sintering temperature and atmosphere, and less on the grain size of the sintered ceramics. However, the as‐sintered barium titanate samples produced by microwave sintering show high dielectric loss (tanδ > 1) consistent with oxygen reduction during the microwave sintering process and consequent accumulation of oxygen vacancies and associated charge carriers at the grain boundary. Since the highly conductive state of as‐sintered ceramics precludes their use in dielectric applications, thermal annealing at different conditions was performed to recover insulating characteristics. Microwave‐sintered barium titanate with post annealing process (950°C for 12 h in air) showed low dielectric loss (tanδ = 0.045) at room temperature and 1 kHz, while still showing a much higher permittivity (ε′ = 36,055) than conventionally sintered barium titanate (ε′ = 3500).     

High permittivity and low dielectric loss of the (Ca0.9Sr0.1)1-xLa2x/3Cu3Ti4O12 ceramics

In this work, we have systematically studied the effects of La³⁺/Sr²⁺ dopants on the crystal structure, microstructure, dielectric response and electrical properties of (Ca_0.9Sr_0.1)_1-xLa_2x/3Cu₃Ti₄O₁₂ (x = 0, 0.025, 0.05 and 0.075) ceramics. XRD results show that the lattice parameter increases with the increase in the La³⁺ content. SEM micrographs illustrate that a small amount added of La³⁺ can reduce the grain size of CCTO during sintering. With increasing La³⁺ content, the grains grow larger. Dielectric measurements indicated that all doped samples synthesized by the solid-state reaction exhibit giant dielectric constants ε'>10⁴ over a large frequency range (10 Hz to 1 MHz) and at any temperature below 600 K. In particular, the ceramic with x = 0.05 exhibits a colossal dielectric permittivity ～5.49 × 10⁴; which increases by about 50% compared to that of the undoped ceramic. In addition, the doped ceramic also presents a low dielectric loss ～ 0.08 at 20 °C and 0.6 kHz. The giant dielectric properties of these samples can be explained by the (IBLC) model.     

Suppressing resistance degradation in SrTiO3-based colossal permittivity capacitor material

At present, research on colossal permittivity materials is extensive but challenging to achieve simultaneous properties of colossal permittivity, low loss, and high resistivity. Resistance degradation also restricts industrial application of colossal permittivity materials. In this work, a new method has been proposed to improve resistivity of colossal permittivity ceramics by making metal ions diffuse on ceramic grain boundaries, thus inhibiting the diffusion of oxygen vacancies at grain boundaries. Sr_0.99La_0.01TiO₃(SLT10) ceramics were synthesized by traditional solid-state method, and then Bi₂O₃ (35%)-Al₂O₃ (10%)-MgO (20%)-CuO (25%)-SiO₂ (10%) mixed oxidant was selected to percolate into ceramics. The resistivity of SLT10 ceramics improved remarkably (from 2.1×10⁸ Ω cm to 1.23×10¹¹ Ω cm under DC 100 V) with a colossal permittivity (16695 @1 kHz) and a low dielectric loss (0.016 @1 kHz), as well as excellent frequency stability (20 Hz–2 MHz) and temperature stability (-170 °C to 375 °C). The source of high insulation resistivity of the SLT10 ceramic sample was discussed. Subsequent examination uncovered that grain in the SLT10 ceramics percolated with metal ions displayed semiconducting characteristics, wherein insulation grain boundaries significantly influenced the ceramic's resistivity and served as formidable potential barriers constraining long-range movement of charge carriers. Experimental analysis demonstrated that the resistance degradation behavior of the SLT10 ceramics was suppressed, the breakdown voltage was increased, and the service life was extended.     

Abnormal dielectric relaxations and giant permittivity in SrTiO3 ceramic prepared by plasma activated sintering

In this study, the dielectric properties of SrTiO₃ ceramics prepared by plasma-activated sintering (PAS) were investigated. One of the striking findings is that the material exhibits giant room temperature permittivity (k∼3.5 × 10⁴) and low dielectric loss (∼0.05) at 1 kHz, with the permittivity exceeding that of the conventionally prepared SrTiO₃（ST） ceramics (k∼300) by two orders of magnitude. The enhancement of the polarizability was caused by the high concentration of defect dipoles. In this paper, two dielectric relaxation modes of the PAS ceramics below 0°C have been mainly discussed. One dielectric relaxation mode showed higher activation energy than that of the dielectric peak in the same temperature range for the conventional SrTiO₃-based ceramics. This mode was sensitive to humidity, and the strength of this mode was associated with the oxygen vacancies concentration in the ceramics. The other mode exhibited abnormal slowing down of relaxation rate with increasing temperature, which is contrary to the typical dielectric relaxation behavior, and the anomaly persisted over a narrow temperature range. Both modes were observed at the same interface between the grain and grain boundaries.     

Enhanced dielectric performance of (Ag1/4Nb3/4)0.01Ti0.99O2 ceramic prepared by a wet-chemistry method

The (Ag_1/4Nb_3/4)_0.01Ti_0.99O₂ ceramic, with ultra-high permittivity and relatively low dielectric loss, was synthetized by a sol-seed method. Significant influences of Ti concentration on the phase structure, microstructure, and dielectric performances were observed. By optimizing the Ti concentration, high active powders and effectively controlled grain uniformity were obtained, which is great benefit for depressing the calcining temperature. The sample shows an ultra-high permittivity (ε_r ~43271 at 1 kHz) and relatively low dielectric loss (tanδ ~ 0.048 at 1 kHz) at room temperature, meanwhile, the temperature coefficient of ε_r keeps in ±15% within the temperature range from −78 to 130°C, fortunately for X7R capacitor. Based on the XPS results, the defect cluster structure was revealed and giant permittivity is closely associated with the electron pinned defect dipole.     

Colossal dielectric response and relaxation behavior in novel system of Zr4+ and Nb5+ co-substituted CaCu3Ti4O12 ceramics

In this work, we developed a novel system of isovalent Zr⁴⁺ and donor Nb⁵⁺ co-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics to enhance dielectric response. The influences of Zr⁴⁺ and Nb⁵⁺ co-substituting on the colossal dielectric response and relaxation behavior of the CCTO ceramics fabricated by a conventional solid-phase synthesis method were investigated methodically. Co-doping of Zr⁴⁺ and Nb⁵⁺ ions leads to a significant reduction in grain size for the CCTO ceramics sintered at 1060 °C for 10 h. XRD and Raman results of the CaCu₃Ti_3.8-xZr_xNb_0.2O₁₂ (CCTZNO) ceramics show a cubic perovskite structure with space group Im-3. The first principle calculation result exhibits a better thermodynamic stability of the CCTO structure co-doped with Zr⁴⁺ and Nb⁵⁺ ions than that of single-doped with Zr⁴⁺ or Nb⁵⁺ ion. Interestingly, the CCTZNO ceramics exhibit greatly improved dielectric constant (~10⁵) at a frequency range of 10²–10⁵ Hz and at a temperature range of 20–210 °C, indicating a giant dielectric response within broader frequency and temperature ranges. The dielectric properties of CCTZNO ceramics were analyzed from the viewpoints of defect-dipole effect and internal barrier layer capacitance (IBLC) model. Accordingly, the immensely enhanced dielectric response is primarily ascribed to the complex defect dipoles associated with oxygen vacancies by co-doping Zr⁴⁺ and Nb⁵⁺ ions into CCTO structure. In addition, the obvious dielectric relaxation behavior has been found in CCTZNO ceramics, and the relaxation process in middle frequency regions is attributed to the grain boundary response confirmed by complex impedance spectroscopy and electric modulus.     

Dielectric properties of CaCu3Ti4O12 improved by chromium/lanthanum co-doping

The dielectric properties of Cr + La co-doped CaCu₃Ti₄O₁₂ ceramics prepared by a solid-state reaction method were evaluated and compared to Cr-doped, La-doped, and parent CaCu₃Ti₄O₁₂ (CCTO). Their structure and grain size were evaluated by X-ray diffraction and scanning electron microscopy, respectively. No secondary phase was detected based on the XRD analysis. The results show that, the room temperature dielectric loss of the co-doped samples is reduced to 43% compared to CCTO and their dielectric permittivity is higher than the un-doped, Cr-doped, and La-doped samples at frequencies over 325 kHz, 30 kHz, and 12 Hz, respectively. Furthermore, the temperature stability of the co-doped sample is significantly more convenient than that of CCTO, and its dielectric loss is three times lower. The results also indicated that the co-doping method is effective in reducing the dielectric loss, still maintaining the high dielectric permittivity.     

Controllable low-temperature flash sintering and giant dielectric performance of (Zn,Ta) co-doped TiO2 ceramics

In this study, the challenge of high-temperature and long-time sintering of (Zn, Ta) co-doped TiO₂ ceramics is solved successfully using flash sintering technology. Joule heating and a high heating rate make the sample compact rapidly at low temperatures (1050 °C in 24 min). When the electric field was equal to 200 V/cm, high permittivity (ε' ～ 1.32 × 10⁴), low dielectric loss tangent (tan δ ～ 0.27), and nonlinear coefficient ($\alpha$ ～ 5.8) values were obtained. Flash sintered samples have more free electrons, resulting in a high dielectric constant. Further, the higher the electric field, the smaller the grain resistance of the sample; this condition is conducive to reducing dielectric loss. giant dielectric performance is explained by the combined action of the electron-pinned defect dipole theory and the internal barrier layer capacitance effect. Therefore, this study provides a promising prospect for the green preparation of co-doped TiO₂ giant dielectric ceramics.     

Colossal permittivity and impedance analysis of tantalum and samarium co-doped TiO2 ceramics

In this study, (Ta_0.5Sm_0.5)_xTi_1−xO₂ (x = 0, 0.02, 0.06, 0.15) ceramics (referred to as TSTO) were fabricated by a standard solid-state reaction. As revealed by the X-ray diffraction (XRD) spectra, the TSTOs exhibit a tetragonal rutile TiO₂ structure. All the TSTO ceramics display colossal permittivity (~ 10²–10⁵). Moreover, the optimal ceramic, (Ta_0.5Sm_0.5)_0.02Ti_0.98O₂, exhibits high performance over a wide temperature range from 20 °C to 160 °C. At 1 kHz, the dielectric constant and dielectric loss are 2.30 × 10⁴ and 0.11 at 20 °C; they are 3.85 × 10⁴ and 0.64 at 160 °C. Dielectric and impedance spectra analyses for the TSTO ceramics indicate that the CP behavior over a broad temperature range in (Ta+Sm) co-doped TiO₂ could be explained by the internal barrier layer capacitance (IBLC) model, which consists of semiconducting grains and insulating grain boundaries.     

Significantly enhanced breakdown field with high grain boundary resistance and dielectric response in 0.1Na0.5Bi0.5TiO3-0.9BaTiO3 doped CaCu3Ti4O12 ceramics

Electrical performances are strongly associated with the electrical heterogeneity of grains and grain boundaries for CaCu₃Ti₄O₁₂ (CCTO) ceramics. In this work, the dielectric ceramics of 0.1Na_0.5Bi_0.5TiO₃-0.9BaTiO₃ (NBT-BT) doped CCTO were fabricated by a conventional solid-state reaction method, and the ceramics were sintered at 1100 °C for 6 h. Relatively homogeneous microstructures are obtained, and the average grain sizes are characterized about 0.9∼1.5 μm. Impressively, a significantly enhanced breakdown field of 13.7 kV/cm and a noteworthy nonlinear coefficient of 19.4 as well as a lower dielectric loss of 0.04 at 1 kHz are achieved in the 0.94CCTO-0.06(NBT-BT) ceramics. It is found that the improved electrical properties are attributed to the increased grain boundary resistance of 3.7 × 10⁹ Ω and the Schottky barrier height of 0.7 eV. This is originated from the NBT-BT compound doping effect. This work demonstrates an effective approach to improve electrical properties of CCTO ceramics by NBT-BT doping.     

Microstructures and dielectric relaxation behaviors of pure and tellurium doped CaCu3Ti4O12 ceramics prepared via vibro-milling method

In this work, we have reported microstructures and the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics doped with different proportions of TeO₂ dopant (mol%, x=0, 0.5%, 1.0%, 2.0%). The pure and tellurium doping CCTO ceramics were prepared by a conventional solid-state reaction method and the effects of TeO₂ doping on the electrical properties and microstructures of these ceramics were investigated. XRD analysis confirmed the formation of single-phase material in samples. Scanning electron microscopy (SEM) is used in the micro structural studies of the specimens, which showed that TeO₂ doping can reduce the mean grain size and increasing size of an abnormal grain growth. Lattice parameter increases slightly with tellurium doping in CCTO, the dielectric constant reached a value as high as 18,000 (at 1 kHz) at a tellurium-doping concentration of 2.0 mol% and showed temperature stability at high frequency. The loss tangent of Te-doped CCTO ceramics was less than 0.05 at 1 kHz region below 105 °C. The loss tangent properties could be interpreted by the internal barrier layer capacitor model and the impedance measurement data.     

Colossal permittivity in Ta-doped SrTiO3 ceramics induced by interface effects and defect structure: An experimental and theoretical study

The lack of systematic research on the phase structure, defect structure, and polarization mechanism hinders the full comprehension of the colossal permittivity (CP) behavior for SrTiO₃-based ceramics. For this purpose, Ta-doped SrTiO₃-based ceramics were synthesized in an N₂ atmosphere with a traditional method. When the appropriate amount of Ta was doped, colossal permittivity (ԑ_r ∼ 62505), low dielectric loss (tanδ ∼ 0.07), as well as excellent temperature stability (−70 °C–180 °C, ΔC/C_25°C ≤ ±15%) were obtained in the Sr_0.996Ta_0.004TiO₃ ceramic. The relationship between Ta doping, polarization mechanism, and dielectric performance was systematically researched according to experimental analysis and theoretical calculations. The first-principle calculations indicate that the Ta⁵⁺ ion prefers to replace the Sr-site. The defect dipoles and oxygen vacancies formed by heterogeneous-ion doping play an active role in regulating the dielectric performance of ceramics. In addition, the interface barrier layer capacitance (IBLC) effect associated with semi-conductive grains and insulating grain boundaries is the primary origin of colossal permittivity for Sr_1-xTa_xTiO₃ ceramics. The polarization mechanism and defect structure proposed in the study can be extended to the research of SrTiO₃ CP ceramics. The results have a good development prospect in colossal permittivity (CP) materials.     

Giant permittivity and low dielectric loss of SrTiO3 ceramics sintered in nitrogen atmosphere

The dielectric properties of SrTiO₃ ceramics sintered in nitrogen (N₂) exhibit a weak temperature- and frequency-dependent giant permittivity (>10⁴) as well as a very low dielectric loss (mostly < 0.02) over a broad temperature range from −100 to 200 °C. Based on the results of ac conductivity and structural analysis, the giant permittivity and low dielectric loss were due to the fully ionized oxygen vacancies and giant defect-dipoles. When further sintering these ceramics in air, the materials exhibit a large temperature- and frequency-dependent high dielectric loss, which were due to the ionization and motion of oxygen vacancies.     

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.     

Effects of sintering temperature on the internal barrier layer capacitor (IBLC) structure in CaCu3Ti4O12 (CCTO) ceramics

The formation of the internal barrier layer capacitor (IBLC) structure in CaCu₃Ti₄O₁₂ (CCTO) ceramics was found to be facilitated by the ceramic heat treatment. Electrically insulating grain boundary (GB) and semi-conducting grain interior areas were characterized by impedance spectroscopy to monitor the evolution of the IBLC structure with increasing sintering temperature T_S (975–1100 °C). The intrinsic bulk and GB permittivity increased by factors of ≈2 and 300, respectively and the bulk resistivity decreased by a factor of ≈10³. These trends were accompanied by increased Cu segregation from the CCTO ceramics as detected by scanning electron microscopy and quantitative energy dispersive analysis of X-rays. The chemical changes due to possible Cu-loss in CCTO ceramics with increasing T_S are small and beyond the detection limits of X-ray absorption spectroscopy near Cu and Ti K-edges and Raman Spectroscopy.