Leading role of grain boundaries in colossal permittivity of doped and undoped CCTO

The role of grain boundaries in the colossal permittivity ɛ of doped and undoped calcium copper titanate, CaCu₃Ti₄O₁₂ (CCTO), is illustrated by a first correlation – over four orders of magnitude – between ɛ and the capacity of grain boundaries, not that of grains, deduced from the analysis of impedance measurements. The DC resistance of the CCTO sample which is essential to make efficient capacitors for technological applications, as well as the loss factor tan(δ), are found to be correlated with the resistance of the grain boundaries rather than that of the grains. The correlation extends over almost seven orders of magnitude. These findings, consistent with the internal barrier layer capacitance (IBLC) model, indicate the leading role of grain boundaries in the origin of the capacitance of CCTO samples.     

Effects of strontium/lanthanum co-doping on the dielectric properties of CaCu3Ti4O12 prepared by reactive sintering

The extremely high dielectric constant of the cubic perovskite CaCu₃Ti₄O₁₂ (CCTO) has attracted increasing attention for a variety of capacitive elements in microelectronic device applications. In this research, the influence of Sr and La replacing Ca and Cu, respectively, to simultaneously controlling the intrinsic properties of grain boundaries in a co-doped CCTO ceramic has been investigated. The preparation was done using high purity compounds milled and mixed by mechano-synthesis and further consolidated by reactive sintering without calcination. Characterization by XRD confirmed the formation of single-phase CCTO ceramic and a residual amount CaTiO₃. The microstructure and composition analyzed by SEM/EDX showed a smaller grain size for the co-doped CCTO. Impedance measurements indicated the smallest dielectric loss for the co-doped ceramics compare to pure and single-doped CCTO, while reaching a higher dielectric permittivity than single-doped ceramics. The CCTO-SrLa sample also showed high thermal stability of the dielectric permittivity between 100 and 470?K, and the lowest loss between 200 and 300?K. This behavior was attributed to the lower bulk resistance exhibited by the co-doped sample.     

A new perovskite-related ceramic with colossal permittivity and low dielectric loss

We designed a new type of perovskite-related dielectric energy storage material Na_1/3Cd_1/3Bi_1/3Cu₃Ti₄O₁₂ with colossal permittivity via an ordinary solid-state method. Remarkably, the Na_1/3Cd_1/3Bi_1/3Cu₃Ti₄O₁₂ ceramic sintered at 1030 °C displays a decent dielectric performance of colossal permittivity ∼1.5 × 10⁴ and low dielectric loss ∼0.04 at 1 kHz. Electric heterogeneity structure in Na_1/3Cd_1/3Bi_1/3Cu₃Ti₄O₁₂ ceramics was clarified, which consists of insulating grain boundaries and semiconducting grains. Notably, internal barrier layer capacitor effect was adopted to explain the decent dielectric performance based on the analysis of dielectric response behavior and complex impedance. Three dielectric anomalies were evidenced in dielectric temperature spectra. Our finding in this work not only explored the dielectric response of the new type of giant dielectric ceramics Na_1/3Cd_1/3Bi_1/3Cu₃Ti₄O₁₂ but also provided candidate materials for high energy storage density capacitors.     

Investigation on the origin of the giant dielectric constant in CaCu3Ti4O12 ceramics through analyzing CaCu3Ti4O12-HfO2 composites

A full range of CaCu₃Ti₄O₁₂-HfO₂ (CCTO-HfO₂) composites were prepared by sintering mixtures of the two components at 1000 °C for 10 h. X-ray diffraction studies confirmed the two-phase nature of the composites. The evolution of the microstructure in the composites, in particular, the size distribution of CCTO grains, was examined by scanning electron microscopy. The studies showed that, as more HfO₂ was added, the abnormal grain growth of CCTO and coarsening of the microstructure were gradually suppressed. As a result, the average CCTO grain size was reduced from 50 to 1 μm. The measured dielectric constants agree well with the values calculated from Lichtenecker's logarithmic law, using only the dielectric constants of pure CCTO and HfO₂ as two end points. The agreement suggests to us that the dielectric constant of CCTO is dominated by domain boundaries within the grains rather than by grain boundaries between the grains.     

Enhanced dielectric properties and grain boundary potentials in sulfur-doped CaCu3Ti4O12 thin films

CaCu₃Ti₄O₁₂ (CCTO) has been reported to possess a colossal dielectric constant owing to the intrinsic interfacial polarization via charge accumulations across the grain boundary. Herein, we explore the effects of unusual anion-doping on the dielectric properties of sputter-deposited CCTO thin films using an example of sulfur-doping. A post-annealing process of the films was utilized in a flowing H₂S atmosphere for the sulfur-doping. The incorporation of sulfur into the perovskite structure was evidenced with the changes in chemical states, such as the reduced cations of Cu⁺ and Ti³⁺, the increased concentration of oxygen vacancies, and the formation of S-O bonds. The sulfurized CCTO thin films demonstrated an enhanced relative permittivity of ∼620 at 100 Hz, which is substantially better than that of the unsulfurized film. Direct measurement of the grain-boundary potential using Kelvin probe force microscopy suggests that the enhanced relative permittivity is associated with an increased Schottky barrier height.     

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.     

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.     

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.     

Direct and hybrid microwave solid state synthesis of CaCu3Ti4O12 ceramic: Microstructures and dielectric properties

CaCu₃Ti₄O₁₂ (CCTO) electroceramic possesses unusual giant dielectric permittivity up to ε?=?10⁴ at low frequency range and room temperature. CCTO dielectric properties strongly depend on its microstructure therefore it is essential to pay attention to the processing techniques which impact grain size and microstructure. In this work, direct and hybrid microwave solid state synthesis was specifically designed and used for the synthesis of CCTO. The microwave process was also compared to the conventional process which involves usual infrared heating. The structural (XRD) and microstructural (SEM) characterizations indicate that microwave synthesis is particularly efficient to get rapidly pure CCTO powder. The fully automated 915?MHz single-mode microwave cavity used for hybrid synthesis allows a perfect control of the temperature distribution and heating rate. Therefore hybrid microwave synthesis leads to a fine, mono-disperse and practically pure CCTO powder in the range of 300 – 500?nm. The advantages of the hybrid microwave heating method are discussed and compared to the conventional and direct microwave heating processes. From the powders synthesized by the different routes, dense compacts were sintered in air at 1050?°C in a conventional furnace. Microstructural characterizations reveal abnormal grain growth during sintering which levels dielectric properties. All exhibit a giant dielectric constant ε?>?10³ at room temperature which decreases drastically to ε?=?90 at 10?K. Those properties are discussed according to the well-established Internal Barrier Layer Capacitor (IBLC) model.     

Dielectric properties of conventional and microwave sintered Lanthanum doped CaCu3Ti4O12 ceramics for high-frequency applications

The colossal dielectric response of La-doped CaCu₃Ti₄O₁₂ ceramics has been probed at room temperature for a frequency of 1Hz–20 MHz. In this work, the La-doped (CaCu₃Ti₄O₁₂)_x samples for x = 0.1, 0.2, and 0.3 have been sintered at 1100 °C using two different heating modes. SEM and EDS analysis investigated the microstructural chrysalis, grain size distribution, and the inhibitions of Cu-rich phase segregation into grain boundaries by the effect of La³⁺. The presence of main cubic single-phase of CCTO and the diminutive Bragg peak shift due to ion size effect of La³⁺ and Ca²⁺ have been identified by XRD for both conventional (CS) and microwave sintered (MWS) samples. XPS study revealed the effect of La³⁺ on the binding energies of Cu and Ti in CCTO. The dielectric properties namely dielectric constant (?), tan δ, and dielectric relaxation peaks were measured using BDS in which CS and MWS La-doped samples demonstrated (?) ～ >10⁴ and ～ >10³ along with low tan δ for x ≥ 0.1 at medium and high frequency (10⁴–10⁷Hz) than pure CCTO.     

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.     

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.     

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.     

Ionic conduction,colossal permittivity and dielectric relaxation behavior of solid electrolyte Li3xLa2/3-xTiO3 ceramics

Perovskite-type solid electrolyte lanthanum lithium titanate (LLTO), exhibiting high intrinsic ionic conductivity, has been attracting interests because of its potential use in all solid-state lithium-ion batteries. In this work, we prepared LLTO ceramics by solid state reaction method and studied their conductivity and dielectric properties systematically. It is found that the bulk conductivity of LLTO is several orders of magnitude higher than the grain boundary conductivity. In addition, colossal permittivity was observed in LLTO ceramics in wide frequency/temperature ranges. Two non-Debye type relaxation peaks were observed in the imaginary part of permittivity, resulting from Li⁺ ions motion and accumulation near interfaces of grains/grain boundaries/electrodes. It is suggested that colossal permittivity may originate from the lithium ion dipoles inside the samples and the interfacial polarization of lithium ion accumulation near the grain boundaries. These results clarify the relations among colossal permittivity, relaxation behavior and ionic conduction in solid ion conductor ceramics.     

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.     

Decrease in the dielectric loss of CaCu3Ti4O12 at high frequency by Ru doping

CaCu_3−xRu_xTi₄O₁₂ (x=0, 0.03, 0.05 and 0.07) electronic ceramics were fabricated using a conventional solid-state reaction method. The microstructure, grain sizes and dielectric properties as well as the impedance behaviours of the ceramics were carefully investigated. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) results indicate that ruthenium (Ru) dopant inhibits the growth of grains during the sintering process by promoting the formation of high melting point oxides of Ca and Ti. The study on the frequency dependence of dielectric properties suggests that Ru doping shifts the dielectric loss peak of CCTO to a much lower frequency, thereby reducing the dielectric loss of CCTO at high frequency (f>1.0 MHz) accordingly. When doped with proper amount of Ru, the high frequency dielectric loss of CCTO is reduced to a very low value (tanδ<0.05). Our study conclusively suggests that Ru-doped CCTO, with sufficiently low dielectric loss and decent permittivity, presents potential applications at high frequency.     

Optimize the dielectric properties of CaCu3Ti4O12 ceramics by adjusting the conductivities of grains and grain boundaries

Reduction of dielectric loss for CCTO ceramics is a prerequisite for their applications. Considering internal barrier layer capacitance effect, improving the capacitance and grain boundary resistance is an effective way to reduce dielectric loss. Therefore, more conductive Ti³⁺ and Cu⁺ ions were introduced to grains by adding carbon to ceramic bodies, improving the permittivity of CCTO ceramics. Annealing was performed to increase the grain boundary resistance. The dielectric loss of the CCTO ceramics thus prepared, which maintain a giant permittivity, is significantly reduced. Specifically, the CCTO ceramic with carbon addition, which was sintered at 1080 °C for 8 h and air annealed at 950 °C for 2 h, exhibits a giant permittivity of about 2.50(5)×10⁴ and a low dielectric loss of less than 0.050(2) from below 20 Hz to 50 kHz at room temperature. Meanwhile, its dielectric loss at 1–10 kHz is less than 0.050(2) from below room temperature to about 100 °C.     

Influence of micro- and nanoparticles of zirconium oxides on the dielectric properties of CaCu3Ti4O12

This work presents the results of Zr oxide doping of a CaCu₃Ti₄O₁₂ (CCTO) ceramic prepared by a solid-state reaction. Different stoichiometries (ZrO and ZrO₂) and grain sizes (micro- and nanoparticles) were added as dopants at concentrations of 0.5 and 1.0 wt%. Zr-doping controls the grain size growth, leading to a reduction of the grain size as observed by scanning electron microscopy. For both dopant concentrations, all of the samples exhibited lower dielectric loss and a smaller dielectric constant than those of undoped CCTO. The sample doped with 0.5% of the non-stoichiometric ZrO exhibits a dielectric constant over 3200 and a dissipation factor of 0.02 at 1 kHz. The impedance spectroscopy analysis confirms that the decrease of dielectric loss is mainly due to an increase in resistivity at grain boundaries, which is attributed to the suppression of oxygen-loss promoted by dopants.     

High-resolution FIB-TEM-STEM structural characterization of grain boundaries in the high dielectric constant perovskite CaCu3Ti4O12

In this work, the grain boundaries composition of the polycrystalline CaCu₃Ti₄O₁₂ (CCTO) was investigated. A Focused Ion Beam (FIB)/lift-out technique was used to prepare site-specific thin samples of the grain boundaries interface of CCTO ceramics. Scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectrometry (EDXS) and Electron Energy Loss Spectroscopy (EELS) systems were used to characterize the composition and nanostructure of the grain and grain boundaries region. It is known that during conventional sintering, discontinuous grain growth occurs and a Cu-rich phase appears at grain boundaries. This Cu-rich phase may affect the final dielectric properties of CCTO but its structure and chemical composition remained unknown. For the first time, this high-resolution FIB-TEM-STEM study of CCTO interfacial region highlights the composition of the phases segregated at grain boundaries namely CuO, Cu₂O and the metastable phase Cu₃TiO_4.     

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.