CCTO Grains Research Articles

Constructing multiple heterogeneous shell on the CaCu3Ti4O12 core for better dielectric performances

A core-shell synthesis is proposed in this study to improve the dielectric loss and the resistivity of CaCu3Ti4O12 (CCTO), which exhibits a colossal permittivity, make it very interesting for capacitor applications. However, the grain boundaries of this ceramic material play a key role for the good dielectric properties. To control these boundaries can be performed by producing shells around the CCTO grains with different oxide mixtures. The design of core-shell structure is a possible approach to control the thickness and homogeneity of the shell, which will improve the properties of this material. In this work, as-prepared CCTO particles, synthetized by solid-state reaction, were successful coated with double or triple shell (SiO2, Al2O3, TiO2) using conventional sol–gel processing derivative from Stöber method. This coating with several binary oxides is made for the first time and has an innovative character in the improvement of properties for an application The CCTO coated with shell composed of SiO2 + Al2O3 + TiO2 mixture showed the best improvement with a colossal permittivity about 7.104, low loss tangent close to 0.05 and high resistivity equal to 109 Ω cm at 1 kHz. Indeed, a clear increasing of resistivity and decreasing of dielectric loss were observed for these samples compared with the resistivity of CCTO (≈105 Ω cm). Moreover, simulations of impedance curves give the capacitance and resistivity of the grain boundaries, which confirm by their linear behavior that the global properties of this material depend on the behavior of the grain boundaries in agreement with the Internal Bayer Layer Capacitors model.

Influence of CTO additives on microstructure and electrical properties of CCTO ceramics

CaCu3Ti4O12/CaTiO3(CCTO/CTO) composite ceramics were prepared through in-situ reaction and solid-state sintered at 1040°C for 10 h. The phase composition, microstructure, morphology, valence, and electrical properties of ceramics are systematically studied by X-ray diffraction(XRD), Field emission scanning electron microscopy(FESEM), Electron probe micro-analysis (EPMA), X-ray photoelectron spectroscopy(XPS), and LCR instruments. It can be found that the FESEM image shows that the addition of CTO inhibits the growth of CCTO grains, and there are nanocrystals inside the grains. The EPMA mapping demonstrated which nanocrystals are composed of CCTO and a small amount of CTO. Impedance spectroscopy observed two semi-circular arcs, which correspond to the effects of micron-level grains and nano-level nanocrystals, respectively. These two mechanisms significantly affect the colossal dielectric response of CCTO/CTO composite ceramics.

Preferentially oriented Fe-doped CaCu3Ti4O12 films with high dielectric constant and low dielectric loss deposited on LaAlO3 and NdGaO3 substrates

The colossal dielectric constant of calcium copper titanate (CCTO) thin films is generally accompanied by high dielectric losses which limit its potential for electronic miniaturization. Strategies for reducing the dielectric loss while keeping large dielectric constants are needed for various electrical applications of CCTO. This work aims to reduce the loss by means of doping preferentially oriented CCTO films with Fe3+ ions. Highly oriented undoped and Fe-doped calcium copper titanate (Fe-doped CCTO) thin films were fabricated by synthesizing CCTO on LaAlO3(100) and NdGaO3(100) substrates via a sol-gel spin casting method. The films with different Fe-doping concentrations (0, 1.6 and 2.6 wt%) were structurally and chemically characterized via energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. X-Ray Diffraction (XRD) patterns confirmed that our doped CCTO films had a cubic perovskite structure with well-defined preferential orientations of (h00) and (hh0) on LAO and NGO substrates, respectively. Preferentially oriented films having room temperature dielectric constants on the order of 1000 in conjunction with values of loss tangent smaller than 0.012 in the frequency range of 50 Hz-2 MHz were achieved. Temperature dependence of the dielectric constant and loss tangent revealed that Fe-doping decreased the dielectric loss via the lowering of the electrical conduction through CCTO grains.

Dielectric and non-ohmic analysis of Sr2+ influences on CaCu3Ti4O12-based ceramic composites

In this study, CaCu3Ti4O12 ceramic composites were prepared through solid-state reaction by adding Sr2+ and removing Cu2+. Application of the Rietveld method and structure refinement revealed the presence of pure CaCu3Ti4O12 (CCTO) phase for x = 0.00 and Sr0.75Ca0.25TiO3 (SCTO) phase for sample x = 3.00. The other samples presented a mixture of both phases. The sample microstructures showed that increasing the amount of SCTO phase led to the formation of cubic CCTO grains. The x = 0.15 sample presented giant dielectric permittivity results, 3.28 × 105, associated with increased grain size and greater grain boundary capacitance, thus indicating it could be a promising material for class III capacitors. On the other hand, greater presence of the SCTO phase displayed potential non-ohmic behavior, as seen with sample x = 2.70, which has a high nonlinear coefficient (α ∼ 30.0) and low current leakage, (IL ∼ 9.0 μA).

New insights into understanding the defect structures and relationship of frequency dependences of dielectric permittivity and ac conductivity of CaCu3Ti4O12

In view of the inconclusive defect structure of CaCu3Ti4O12 (CCTO) ceramics, a defect model has been proposed in this investigation underlying the results of the previous works and updated knowledge in the literature papers concerning the band structure, charge transfer between Cu and Ti ions, and processing of CCTO. This model is capable of explaining why CCTO grain interior becomes semiconductive and grain and domain boundaries are insulating, and the grain-boundary resistance would dramatically drop due to the effect of sintering temperature/time and dopants. The ac conductivity of undoped and CaSiO3-doped CCTO associated with electrical charge carrier motion was investigated as a function of frequency at different temperatures. In order to identify the physical entity, the frequency dependence of ac conductivity at different temperatures is further characterized by the peak frequencies of the frequency dependence of the imaginary part of impedance (Z″) and electrical modulus (M″). The related physical entity has been specified by the activation energy and s value of power-law equation. It is suggested that each region cannot be specified by a unique physical entity; instead, it is a transition between relaxations of the related physical entity as the frequency increases. The electron hopping inside percolation clusters containing local polarized clusters of the related physical entity with the main contribution to the ac conductivity may determine the corresponding dielectric permittivity of each region.

Influence of MgTiO3-Doping on Microstructure and Dielectric Properties of CaCu3Ti4O12 Ceramics

The application of CaCu3Ti4O12 (CCTO) is limited due to its high low-frequency dielectric loss. An internal barrier layer capacitance effect seems to be the most widely accepted explanation for the giant-e′ phenomenon of CCTO ceramics. According to the effect, wrapping the semiconducting grains of CCTO ceramics in insulating materials is an effective way to reduce the low-frequency dielectric loss. CCTO/MgTiO3 composite ceramics were prepared, and their crystalline structures, microstructures, dielectrical properties and complex impedance were systematically investigated. The results show that MgTiO3 has been wrapped around the CCTO grains in our CCTO-based ceramics. Compared with pure CCTO ceramics, the low-frequency dielectric loss was not reduced by MgTiO3-doping, because the resistance of MgTiO3 phase boundaries in the CCTO/MgTiO3 composite ceramics as prepared is lower than the grain boundaries resistance of pure CCTO ceramics. After annealing in oxygen, the low-frequency permittivity and dielectric loss of CCTO/MgTiO3 composite ceramics significantly decreased, which should be attributed to the increase of the resistance of insulating MgTiO3 phase boundaries and CCTO grains.

Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach

Grain boundaries of CaCu3Ti4O12 (CCTO) materials have been shown to play leading role in colossal permittivity. Core-shell design is an attractive approach to make colossal dielectric capacitors by controlling the grain boundaries. Core-shell grains of CCTO surrounded by Al2O3 shell were synthesized by ultrasonic sol-gel reaction from alumina alkoxide precursor. The influence of alumina shell by comparison with bare CCTO grains was studied. Particularly, microstructure, dielectric and electric effects on sintered ceramics are reported. The average grain size and the density are increased compared to undoped CCTO leading to an improvement of permittivity from 58,000 to 81,000 at 1 kHz. Furthermore a decrease of dielectric loss is found in a frequency range of 102–103 Hz. Moreover, the activation energy of grain boundaries is increased from 0.55 to 0.73 eV and the electrical properties such as breakdown voltage, non-linear coefficient and resistivity are improved with the aim of making industrial capacitors.

Abnormal capacitance–voltage behaviors of bismuth-doped CaCu3Ti4O12 ceramics

Ca[Formula: see text]Bi[Formula: see text]Cu3Ti4O[Formula: see text] ([Formula: see text] = 0.0, 0.1, 0.2 and 0.3; BCCTO) ceramics were prepared by traditional solid-state sintering method. All samples had pure cubic perovskite-like structure. A drastic grain size reduction was observed with bismuth doping. Dielectric spectra showed two obvious relaxation steps corresponding to two series of peaks in the imaginary part of electric modulus spectra and dielectric loss spectra too. Activation energy fitting by electric modulus spectra reflected different conducting segments in BCCTO ceramics of grains and oxygen vacancies below room temperature. Normalized capacitance dependent of extra bias showed different voltage–capacitance coefficients at special frequencies that suggested multirelaxation mechanisms related with grain and oxygen vacancy. A positive capacitance curvature implies dipolar relaxations in CCTO grains. Whereas a negative curvature suggests oxygen-related relaxations at the interface.

The effect of cobalt-doping on microstructure and dielectric properties of CaCu3Ti4O12 ceramics

Ceramics of CaCu3-xCoxTi4O12 (CCCTO, x = 0, 0.2, 0.4 and 0.6), with cubic lattice structure, were fabricated using a semi-wet method. The dielectric properties of the prepared samples were studied at 1 kHz frequency in the temperature range of 300–600 K, as well as in the frequency range of 50 Hz–200 kHz at room temperature. The results showed that the dielectric constants of the as-prepared samples in the frequency range of 50 Hz–200 kHz, at room temperature, are reasonably high (1.7 × 104 ≤ ε ≤ 6 × 104). It was also found out that the dielectric loss (tanδ) of the prepared samples decreased by increasing the cobalt concentration at low and middle frequencies. The dielectric loss decreased from 0.87 for an undoped sample CaCu3Ti4O12 (CCTO), to 0.15 for a doped sample (CCCTO) with x = 0.6, at 50 Hz frequency. However, the dielectric constant (ε) values for the samples doped with cobalt were found to be higher than that of for the undoped sample. Moreover, the XRD patterns as well as the SEM images and EDX results revealed that the Cu-rich phase (CuO) decreased at the CCTO grain boundaries, while at the same time Co2TiO4 and CaTiO3 phases with good dielectric insulating behavior having been formed at the boundaries. This causes the increase of the resistance at the grain boundaries leading to lower leakage current which in turn results in a decrease of the dielectric loss. These results were confirmed by using the impedance spectroscopy analysis. Our results indicate that the cobalt doped CCTO ceramics can be suggested as good candidates to be used in fabricating microelectronic and memory devices with high dielectric constant and low dielectric loss.

Development of performance‐improved global positioning system (GPS) patch antenna based on sol‐gel‐synthesized dual‐phase (Bi4Ti3O12)X – (CaCu3Ti4O12)1–X composites

As a first attempt, a series of composite materials of general formula (Bi4Ti3O12)X – (CaCu3Ti4O12)1–X (BTOX – CCTO1–X) (X = 0–1.0) is synthesized by a sol‐gel combustion method. The phase changes, including their formation and evolution during the gradual substitution of BTO into CCTO are analyzed from the X‐ray diffraction pattern. The Jana2006 refinement plotting method proves that formation of an ideal dual‐phase composite system with separate orthorhombic and cubic phases occurs. A surface morphological study revealed that the BTOX – CCTO1–X composites are composed of small CCTO grains embedded in large BTO grains and both coexisted with a bimodal distribution. The possible vibrational modes of interactions between the constituent phases are determined from Raman spectra. Our inexpensive, combustion method for BTO0.2 – CCTO0.8 gives a high dielectric constant (ε′ = 3232) at 100 Hz at room temperature. The nature behind these data is revealed that the BTO‐CCTO composite can be applicable for the fabrication of miniaturized global positioning system (GPS) patch antennas. The fabricated GPS patch antenna shows optimum results of small size (2.37 mm × 1.18 mm), return loss (‐14.419) and wide bandwidth (350 MHz) to be operated at 1.57 GHz.

Phase Investigation and Multi Objective Optimization of Dielectric Behaviour of Bismuth Titanate–Calcium Copper Titanate Nanocomposites Using Grey Relational Grade Analysis

First time, bismuth titanate–calcium copper titanate (BTOX–CCTO1−X) composite materials were produced through sol–gel combustion process. X-ray diffraction analysis revealed that the obtained products were phase pure with dual phase composite system. Atomic percentage of each element present in the composite, observed by energy dispersive spectroscopy, was consistent with that in the raw materials as per stoichiometry. Field emission scanning electron microscopy showed evidence that the large CCTO grains embedded in small BTO grains and both co-existed with a bimodal distribution. The composite materials showed excellent dielectric behaviour that made them promising candidates for the high dielectric embedded capacitors. An efficient grey relational grade approach was introduced for improving the condition of major controlled parameters like frequency, operating temperature and molar composition and optimizing these parameters to obtain maximum dielectric constant and minimum dielectric loss simultaneously.

Effect of Glass Addition on the Properties of CaCu<sub>3</sub>Ti<sub>4</sub>O<sub>12</sub> Ceramics

The effect of glass addition on the properties of CaCu3Ti4O12 (CCTO) ceramics was studied. CCTO ceramics with BaO-SrO-Nb2O5-B2O3-SiO2 (BSNBS) glass additive were prepared using solid state reaction method. The raw materials of CCTO and BSNBS were wet mixed separately for 24 hours, dried overnight and subsequently the BSNBS powders was melted at 1450°C for 2 hours while CCTO powders was calcined at 900°C for 12 hours. BSNBS glass then was grinded to form a fine powders. Different weight percentages of BSNBS powder were added into CCTO and the powders were wet mixed at 24 hours. The mixtures were dried, compacted at 250 MPa and then sintered at 1050°C for 10 hours. X-Ray Diffractometer (XRD) analysis showed the formation of CCTO phase for all sintered samples while secondary phase of CuO phase was identified for sintered BSNBS 1.0 sample. Observation on Scanning Electron Microscopy (SEM) micrographs showed abnormal grain growth was seen in CCTO sample and grain size of the samples become finer with increasing the concentration glass addition. CCTO sample obtained the highest dielectric constant (8670) while the lowest dielectric loss (0.38) was recorded by BSNBS 0.10 sample, measured at 1 MHz.

Dielectric and Nonohmic Properties of CaCu3Ti4O12/SrTiO3 Ceramics

In this work, (1 − x)CaCu3Ti4O12-xSrTiO3 [(1 − x)CCTO/xST, x = 0% to 2%] ceramic samples were prepared by the solid-state reaction method. The dielectric and electrical properties of CaCu3Ti4O12 (CCTO) and CaCu3Ti4O12/SrTiO3 (CCTO/ST) ceramics were investigated. The results show that a small amount of Sr2+ can enter the lattice of CCTO. The mean grain size of the ceramic samples increased greatly for x = 0.5% and then decreased for x = 0.75% to 2%. ST addition and Sr2+ preferential occupancy in CCTO grains should be responsible for the change of the microstructure. Interestingly, the dielectric constant (e) of the 0.5% ST-added sample increased significantly while the dielectric loss (tan δ) remained low. With further increase of the ST content, the e and tan δ values of the CCTO ceramics decreased monotonically while the nonlinear current–voltage properties were significantly enhanced. The change in the potential barrier height is thought to be the main cause for the opposite behaviors in the dielectric properties and nonohmic characteristics.

Improved dielectric properties of BaTiO3-added CaCu3Ti4O12 polycrystalline ceramics

The effects of the BaTiO3 (BTO) additive on the electrical properties of CaCu3Ti4O12 (CCTO) polycrystalline ceramics were systematically investigated. Various amounts of BTO powder up to 15 mol. % were added to CCTO powder. Each batch was ball-milled, pressed into pellets, and finally sintered at 1060°C for 12 h in air. Compared with pure CCTO sample (ɛ r ∼ 52,000 and tan δ ∼ 0.38 at 100 kHz), BTO-added CCTO samples commonly showed significantly reduced dielectric losses although their dielectric constants were decreased approximately by one order of magnitude (for instance, ɛ r ∼ 4,075 and tan δ ∼ 0.02 at 100 kHz for 5 mol. % BTO-added CCTO sample). In addition, the breakdown voltages of BTO-added CCTO samples were much higher than that of pure CCTO sample, and thus the leakage currents were greatly reduced at the applied voltage above ∼ 10 V. A large reduction in the dielectric losses and leakage currents is attributed to the secondary phases segregated at the CCTO grain boundary which are composed of CaTiO3, Ba4Ti12O27, and unreacted BTO.

Modulus spectroscopy of CaCu3Ti4O12 ceramics: clues to the internal barrier layer capacitance mechanism

To date, all existing literature on the so-called ‘high permittivity’ perovskite oxide CaCu3Ti4O12 (CCTO) in the form of ceramics, single crystals and thin films show the grains (bulk) to exhibit semiconductivity with room temperature, RT, resistivity of ∼10–100 Ω cm. Here we show that CCTO grains can be highly resistive with RT resistivity >1 GΩ cm when CCTO ceramics are processed at lower temperature (700 °C). With increasing processing temperature, the semiconducting CCTO phase commonly reported in the literature emerges from grain cores and grows at the expense of the insulating phase. For sintering temperatures of ∼1000–1100 °C, the grains are dominated by the semiconducting phase and the insulating phase exists only as a thin layer grain shell/grain boundary region. This electrical microstructure results in the formation of the so-called Internal Barrier Layer Capacitance (IBLC) or Maxwell–Wagner mechanism that produces the commonly reported high effective permittivity at radio frequencies in dense ceramics. The relationship between Cu loss at elevated processing temperatures and the transformation of the grain resistivity from an insulating to semiconducting state with increasing processing temperature is also discussed.

Dielectric properties of bismuth doped CaCu3Ti4O12 ceramics

Abstract Ca 1−3 x /2 Bi x Cu 3 Ti 4 O 12 ( x = 0.0–0.3) ceramics were prepared by the conventional solid-state reaction method. X-ray powder diffraction analysis confirmed the formation of cubic CCTO phase except for subtle peaks of CuO. SEM micrographs suggested that the morphologies of doped CCTO ceramics had been sheet-like for high Bi-doping amount, and the dominant grain size decreasing could be seen for the small content of Bi-doping CCTO. Dielectric properties of pure and doped CCTO were investigated in a broad temperature range of 20–420 K. The results showed that bismuth doping could decrease the dielectric loss but suppress the dielectric temperature stability at the same time. Bi doped CCTO ceramics presented different relaxation properties. As to pure CCTO and BCCTO ( x = 0.3) only one MW relaxation (Relaxation I) could be found, which moves to higher frequency with temperature increasing. However, two relaxation processes (Relaxation I and II) appear for BCCTO ( x = 0.1–0.2). By means of complex impedance spectra analysis and Arrhenius fitting, we successfully separated the different conductive segments and explained the mechanisms of the two relaxation processes. Relaxation I appeared at low temperature could be attributed to the V O doping energies inside CCTO grains which did not showed significant changing of activation energy after bismuth doping. For Relaxation II at higher temperature than Relaxation I, with activation energy obviously depending on the Bi-ion concentration, may be related with the V O point defects at the grain boundaries.

Relationship between microstructural evolution and electric properties of B2O3–CaCu3Ti4O12 composite ceramics

Calcium copper titanate (CaCu3Ti4O12, CCTO) powder was mixed with boric oxide (B2O3) glassy phase up to 8% by weight to prepare the composite ceramics. The effects of B2O3 incorporation on the microstructures, electric and dielectric properties of CCTO ceramics have been systematically investigated. Adding B2O3 results in both the decrease of the CCTO grain size. These microstructural changes conjointly affect the dielectric constant. In addition, further B2O3 increase leads to its aggregation at grain boundaries. Meanwhile, the B2O3 addition reduces the nonlinear coefficient and influences the breakdown electric field.

Dielectric Constant Enhancement in Calcium Copper Titanate Ceramics

The structural, morphological, and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics without and with 10% HfO2 by weight were investigated in this work. The main effect of adding HfO2 on the microstructure was the segregation of HfO2 subgrains within CCTO grains, which consequently causes the inhomogeneous electric structure of the HfO2-modified sample. The HfO2-modified sample presented the much larger dielectric constant than the pure CCTO one, which resulted from HfO2 subgrains’ effect as another new origin of the dielectric constant.

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

Abstract A full range of CaCu 3 Ti 4 O 12 –HfO 2 (CCTO–HfO 2 ) 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 HfO 2 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 HfO 2 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.

Nanoscale Investigation of Nb-Doped CaCu<sub>3</sub>Ti<sub>4</sub>O<sub>12</sub> Grains

CaCu3Ti4O12 (CCTO) is a promising material for microelectronic and microwave device applications due to its unique properties that posses high dielectric constant in the wide temperature range. In this work, atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses were applied for nanoscale imaging of Nb-doped CCTO grains. The Nb-doped CCTO pellets (CaCu3Ti4-xNbxO12+x/2; x = 0, 0.01, 0.03, 0.05, 0.1) were prepared via solid state reaction method and thermally etched at 940°C for an hour. From AFM and SEM images found that tiny bumped as well as terrace type domains are distributed within a grain. The domain size is ranging from 20 to 180 nm measured by AFM. The existence of domains on grain will produce grain boundary and domain boundary resistance inside CCTO. Both domain and grain resistance are believed to strongly influence the electrical properties of CCTO.