The effect of calcining temperatures on the phase purity and electric properties of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics are prepared by the traditional solid-state reaction method under the same sintering conditions. The effect of calcining temperatures for the powders before sintering on the microstructure and electric properties of CCTO ceramics has been investigated. The XRD patterns for the powder calcined at 950 °C show that some measure of second phases (CaTiO₃, TiO₂ and CuO) can be found except a considerable amount of CCTO phase in them and the content of second phases decrease markedly as the calcining temperature is raised to 1000 °C. The XRD patterns for the powder calcined at 1050 °C indicate that the powder has been basically formed into a single CCTO phase except a small quantity of CaTiO₃ phase, which is attributed to CuO volatilizing in the calcining process. Furthermore, the XRD patterns for the CCTO pellets sintered at 1080 °C/10 h manifest that all the second phases have disappeared after the sintering process except that a very weak peak of CaTiO₃ can still be seen in the XRD pattern for the pellets made of the powder calcined at 1050 °C. The electric properties measurement demonstrates that the lower calcining temperature for the raw powder is helpful to increase the values of permittivity and the higher calcining temperature is helpful to improve the non-ohmic properties. The non-ohmic characteristic has a behavior reverse to that of the permittivity, which can be ascribed to the change in the height of Schottky barriers.     

Dielectric relaxation behaviors of pure and Pr6O11-doped CaCu3Ti4O12 ceramics in high temperature range

Pure and Pr₆O₁₁-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were prepared by conventional solid-state reaction method. The compositions and structures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of Pr-ion concentration on dielectric properties of CCTO were measured in the ranges of 60 Hz-3 MHz and 290-490 K. The third phase of Ca₂CuO₃ was observed from the XRD of CCTO ceramics. From SEM, the grain size was decreased obviously with high valence Pr-ion (mixing valence of Pr³⁺ and Pr⁴⁺) substituting Ca²⁺. The room temperature dielectric constant of Pr-doped CCTO ceramics, sintered at 1323 K, was an order of magnitude lower than the pure CCTO ceramics due to the grain size decreasing and Schottky potential increasing. The dielectric spectra of Pr-doped CCTO were flatter than that of pure CCTO. The loss tangent of Pr-doped CCTO ceramics was less than 0.20 in 2 × 10²-10⁵ Hz region below 440 K. The complex impedance spectra of pure and Pr-doped CCTOs were fitted by ZView. From low to high frequency, three semicircles were observed corresponding to three different conducting regions: electrode interface, grain boundary and grain. By fitting the resistors R and capacitors C, the activation energies of grain boundary and electrode contact were calculated. All doped CCTOs showed higher activation energies of grain boundary and electrode than those of pure CCTO ceramics, which were concordant with the decreasing of dielectric constant after Pr₆O₁₁ doping.     

Dielectric properties of soft chemical method derived CaCu3Ti4O12 thin films onto Pt/TiO2/Si(1 0 0) substrates

Calcium copper titanate, CaCu₃Ti₄O₁₂ (CCTO), thin film has been deposited by the soft chemical method on Pt/Ti/SiO₂/Si (1 0 0) substrates at 700 °C for 2 h. The peaks were indexed as cubic phase belonging to the Im−3 space group. The film exhibited a duplex microstructure consisting of large grains of 130 nm in length and regions of fine grains (less than 80 nm). The CCTO film capacitor showed a dielectric loss of 0.031 and a dielectric permittivity of 1020 at 1 MHz. The J-V behavior is completely symmetrical, regardless of whether the conduction is limited by interfacial barriers or by bulk-like mechanisms. Based on impedance analyses, the equivalent circuit of CCTO film consisting of a resistor connected in series with two resistor-capacitor (RC) elements.     

An aqueous-solution based low-temperature pathway to synthesize giant dielectric CaCu3Ti4O12—Highly porous ceramic matrix and submicron sized powder

A simple, cost-effective and environment-friendly pathway for preparing highly porous matrix of giant dielectric material CaCu₃Ti₄O₁₂ (CCTO) through combustion of a completely aqueous precursor solution is presented. The pathway yields phase-pure and impurity-less CCTO ceramic at an ultra-low temperature (700 °C) and is better than traditional solid-state reaction schemes which fail to produce pure phase at as high temperature as 1000 °C (Li, Schwartz, Phys. Rev. B 75, 012104). The porous ceramic matrix on grinding produced CCTO powder having particle size in submicron order with an average size 300 nm. On sintering at 1050 °C for 5 h the powder shows high dielectric constants (>10⁴ at all frequencies from 100 Hz to 100 kHz) and low loss (with 0.05 as the lowest value) which is suitable for device applications. The reaction pathway is expected to be extended to prepare other multifunctional complex perovskite materials.     

In situ synthesis and characterization of polyaniline–CaCu3Ti4O12 nanocrystal composites

High dielectric constant (ca. 2.4 × 10⁶ at 1 kHz) nanocomposite of polyaniline (PANI)/CaCu₃Ti₄O₁₂ (CCTO) was synthesized using a simple procedure involving in situ polymerization of aniline in dil. HCl. The PANI and the composite were subjected to X-ray diffraction, Fourier transform infrared, thermo gravimetric, scanning electron microscopy and transmission electron microscopy analyses. The presence of the nanocrystallites of CCTO embedded in the nanofibers of PANI matrix was established by TEM. Frequency dependent characteristics of the dielectric constant, dielectric loss and AC conductivity were studied for the PANI and the composites. The dielectric constant increased as the CCTO content increased in PANI but decreased with increasing frequency (100 Hz–1 MHz) of measurement. The dielectric loss was two times less than the value obtained for pure PANI around 100 Hz. The AC conductivity increased slightly up to 2 kHz as the CCTO content increased in the PANI which was attributed to the polarization of the charge carriers.     

Microstructures and electrical responses of pure and chromium-doped CaCu3Ti4O12 ceramics

Pure and chromium-doped CCTO (CaCu₃Ti₄O₁₂) ceramics were prepared by a conventional solid-state reaction method, and the effects of chromium doping on the microstructures and electrical properties of these ceramics were investigated. Efficient crystalline phase formation accompanied by dopant-induced lattice constant expansion was confirmed through X-ray diffraction studies. Scanning electron microscopy (SEM) results show that doping effectively enhanced grain growth or densification, which should increase the complex permittivity. The dielectric constant reached a value as high as 20,000 (at 1 kHz) at a chromium-doping concentration of 3%. The electrical relaxation and dc conductivity of the pure and chromium-doped CCTO ceramics were measured in the 300-500 K temperature range, and the electrical data were analyzed in the framework of the dielectric as well as the electric modulus formalisms. The obtained activation energy associated with the electrical relaxation, determined from the electric modulus spectra, was 0.50-0.60 eV, which was very close to the value of the activation energy for dc conductivity (0.50 ± 0.05 eV). These results suggest that the movement of oxygen vacancies at the grain boundaries is responsible for both the conduction and relaxation processes. The short-range hopping of oxygen vacancies as “polarons” is similar to the reorientation of the dipole and leads to dielectric relaxation. The proposed explanation of the electric properties of pure and chromium-doped CCTO ceramics is supported by the data from the impedance spectrum.     

Dielectric properties of iron doped calcium copper titanate, CaCu2.9Fe0.1Ti4O12

CaCu_2.9Fe_0.1Ti₄O₁₂ (CCFTO) has been prepared by a novel semi-wet route and its dielectric properties have been studied in the temperature range 300-500 K. It is found that dielectric constant (?) decreases drastically in the frequency range 100 Hz to 1 MHz. Complex plane impedance and modulus analysis was done to understand this drastic decrease in ?. Oxidation state of various ions was studied using X-ray photoelectron spectroscopy (XPS). The decrease in the permittivity of CCFTO can be attributed to two factors: the suppression of the Ca/Cu disorder in CCFTO which is observed in CaCu₃Ti₄O₁₂ (CCTO) and the absence of the grain boundary internal barrier layer capacitance mechanism.     

High frequency response to the impedance complex properties of Nb-doped CaCu3Ti4O12 electroceramics

Impedance analyses was performed on undoped and Nb-doped CaCu₃Ti₄O₁₂ (CaCu₃Ti_4−xNb_xO_12+x/2; x = 0, 0.01, 0.03, 0.05, 0.1) to investigate their electrical properties. The pellet samples were prepared using the solid state reaction method. Silver electrode was deposited on both pellets’ surfaces for electrical measurement. The thermally etched samples showed tiny bumped domains within the grains. The existence of both domain and grain boundaries are believed to strongly influence the dielectric constant of CaCu₃Ti₄O₁₂ (CCTO). Undoped CCTO showed two arcs of impedance complex plane while Nb-doped samples have three arcs. Each arc represents the constituent elements of the CCTO. The highest frequency arc is evidence that CCTO consists of conductive domains which measure about 1 Ω and are insulated by two types of barriers, i.e. domain boundary and grain boundary.     

Nd:YAG laser irradiation effects on electrical properties of polycrystalline Li0.5Fe2.5O4

The polycrystalline spinel structured Li_0.5Fe_2.5O₄ ferrite have been prepared by conventional double sintering ceramic method. The samples were palletized and irradiated by Nd:YAG laser with different laser fluencies and characterized by infrared spectroscopy and DC electrical resistivity in order to obtain phase, crystal structure and conduction mechanism in pristine and irradiated samples. The infrared spectroscopy is employed to study the local symmetry and conduction mechanism in crystalline solids before and after irradiation. The DC electrical resistivity measured by two-probe technique from room temperature to beyond Curie temperature with steps of 10 K increases after laser irradiation. Variation of dielectric properties like dielectric constant and dielectric loss tangent is also measured as a function of temperature. A significant reduction in the values of dielectric constant and dielectric loss tangent has been observed with the increase of laser dose.     

Microstructure, dielectric and ferroelectric properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (NBTB) and 0.05BiFeO3-0.95NBTB ceramics: Effect of sintering atmosphere

0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ (NBTB) and 0.05BiFeO₃-0.95NBTB (BF-doped NBTB) lead-free ceramics were prepared by solid state reaction method. The ceramics were sintered at 1180 °C for 2 h in O₂ and N₂. All ceramics exhibited a single phase of perovskite structure. Relative amount of tetragonal phase was related to the sintering atmospheres. Both grain size and shape were influenced by the sintering atmospheres. Sintering the ceramics in N₂ weakened their dielectric anomalies corresponding to the transition from ferroelectric phase to the so-called “intermediate phase”. When the NBTB and BF-doped NBTB ceramics were sintered in N₂, their maximum dielectric constant and the degree of diffuseness of the transition from the “intermediate phase” to paraelectric phase increased, but their Curie temperatures decreased. The difference in dielectric properties of the ceramics sintered in different atmospheres was closely related to the difference in oxygen vacancy concentration. The correlation between ferroelectric properties and sintering atmospheres is associated with a competing effect among oxygen vacancy concentration, A-site vacancy concentration and percentage of tetragonal phase.     

Electric and dielectric behavior of CaCu3Ti4O12-based thin films obtained by soft chemical method

CaCu₃Ti₄O₁₂ (stoichiometric) and Ca_1.1Cu_2.9Ti₄O₁₂ (non-stoichiometric) thin films have been prepared by the soft chemical method on Pt/Ti/SiO₂/Si substrates, and their electrical and dielectric properties have been compared as a function of the annealing temperature. The crystalline structure and the surface morphology of the films were markedly affected by the annealing temperature and excess calcium. The films show frequency-independent dielectric properties at room temperature which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 570-nm-thick CCTO thin films annealed at 600 °C at 10 kHz was found to be 124. The best non-ohmic behavior (α = 12.6) presented by the film with excess calcium annealed at 500 °C. Resistive hysteresis on the I-V curves was observed which indicates these films can be used in resistance random access memory (ReRAM).     

Preparation and Dielectric Properties of CaCu3Ti4O12 Ceramics with Different Additives

Li₂CO₃, MgCO₃, BaCO₃, and Bi₂O₃ dopants were introduced into CaCu₃Ti₄O₁₂ (CCTO) ceramics in order to improve the dielectric properties. The CCTO ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure, and dielectric behavior were carefully investigated. The pure structure without any impurity phases can be confirmed by the x-ray diffraction patterns. Scanning Electron Microscopy (SEM) analysis illuminated that the grains of Ca_0.90Li_0.20Cu₃Ti₄O₁₂ ceramics were greater than that of pure CCTO. It was important for the properties of the CCTO ceramics to study the additives in complex impedance spectroscopy. It was found that the Ca_0.90Li_0.20Cu₃Ti₄O₁₂ ceramics had the higher permittivity (>45000), the lower dielectric loss (<0.025) than those of CCTO at 1 kHz at room temperature and good temperature stability from ?30 to 75 °C.     

Reaction-sintering method for ultra-low loss (Mg0.95Co0.05)TiO3 ceramics

Microwave dielectric properties and microstructures of (Mg_0.95Co_0.05)TiO₃ ceramics prepared by a new sintering method (reaction-sintering method) were investigated. A pure phase of (Mg_0.95Co_0.05)TiO₃ was obtained by the new method and excellent dielectric properties were observed due to uniformities of the microstructure and the phase. In contrast, the secondary phase (Mg_0.95Co_0.05)Ti₂O₅ was observed in samples prepared by conventional sintering method. In order to study the influence of secondary phase on the microwave dielectric properties quantitatively, the weight fraction of (Mg_0.95Co_0.05)Ti₂O₅ was calculated on the basis of Rietveld refinement. The pore-free?_r values of specimens prepared by two different methods indicated that porosity plays an important role in the ?_r values of (Mg_0.95Co_0.05)TiO₃ ceramics. Specimens sintered by reaction-sintering method at 1350 °C for 4 h possess excellent dielectric properties with an ?_r of 16.3, a Q × f value of 244,500 GHz, and a τ_f value of −53.5 ppm/°C.     

Low temperature processing of (Zr0.8Sn0.2)TiO4 ceramics with improved Q factor

Polycrystalline (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics have been synthesized by solid-sate reaction method. The effect of B₂O₃, ZnO-B₂O₃ or 5ZnO-2B₂O₃ glass addition (0.2-1.0 wt.%) on microwave dielectric properties of ZST ceramics are investigated. The increase in average grain size via growth of large grains and dissolution of small grains is explained by Ostwald ripening phenomena. The highest Q × f_o values are found to be 61,500, 48,500 and 51,900 GHz for the ZST dielectric resonators added with B₂O₃, ZnO-B₂O₃ and 5ZnO-2B₂O₃ respectively. The effect of liquid phase sintering on microstructure and microwave dielectric properties of ZST ceramics is discussed.     

Sinterability and dielectric properties of Ba0.55Sr0.4Ca0.05TiO3-CaTiSiO5-Mg2TiO4 composite ceramics

The composite ceramics of Ba_0.55Sr_0.4Ca_0.05TiO₃-CaTiSiO₅-Mg₂TiO₄ (BSCT-CTS-MT) were prepared by the conventional solid-state route. The sintering performance, phase structures, morphologies, and dielectric properties of the composite ceramics were investigated. The BSCT-CTS-MT ceramics were sintered at 1100 °C and possessed dense microstructure. The dielectric constant was tailored from 1196 to 141 as the amount of Mg₂TiO₄ increased from 0 to 50 wt%. The dielectric constant and dielectric loss of 40 wt% Ba_0.55Sr_0.4Ca_0.05TiO₃-10 wt% CaTiSiO₅-50 wt% Mg₂TiO₄ was 141 and 0.0020, respectively, and the tunability was 8.64% under a DC electric field of 8.0 kV/cm. The Curie peaks were broadened and depressed after the addition of CaTiSiO₅. The optimistic dielectric properties made it a promising candidate for the application of tunable capacitors and phase shifters.     

Influence of the mechanochemical conditions on the processing of Bi4SrTi4O15 ceramics from submicronic powdered precursors

Ferroelectric Bi₄SrTi₄O₁₅ ceramic samples have been processed and characterized. The ceramic precursors were obtained by the mechanochemical method with different activation conditions. Initial reagents have been activated in different types of mills, vibrating and planetary, for different milling times. SrCO₃ or SrO were used as initial reagents. Bi₄SrTi₄O₁₅ ceramics have been processed at several sintering temperatures, 1050 °C being the optimum temperature, for obtaining dense Aurivillius single phase ceramics. Also, dielectric properties as a function of temperature and frequency are reported for the Bi₄SrTi₄O₁₅ ceramics. Poling experiments were carried out, and piezoelectric coefficients are given.     

Effects of Kaolinite addition on the densification and dielectric properties of BaTiO3 ceramics

Commercial Kaolinite was employed as sintering aid to reduce the sintering temperature of BaTiO₃ ceramics. The effects of Kaolinite content and sintering temperature on the densification, microstructure and dielectric properties of BaTiO₃ ceramics have been investigated. The density characterization results show that the addition of Kaolinite significantly lowered the sintering temperature of BaTiO₃ ceramics to about 1200 °C. XRD results show BaTiO₃ ceramics with a low amount of Kaolinite exhibited perovskite structure, but 10.0 wt% Kaolinite additions resulted in the formation of a secondary phase, BaAl₂Si₂O₈. BaO-TiO₂-Al₂O₃-SiO₂ glass phase was formed and improved the average breakdown strength of ceramics, which was supported by SEM-EDX results. The Kaolinite content had shown a strong influence on the dielectric constant and the diffuse transition. BaTiO₃ ceramic with 4.0 wt% Kaolinite addition possessed well temperature stability of dielectric constant.     

Effect of addition of Ba(W0.5Cu0.5)O3 in Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3-Pb(Zr0.52Ti0.48)O3 ceramics on the sintering temperature, electrical properties and phase transition

In this work, we report on the Pb(Mg_1/3Nb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr_0.52Ti_0.48)O₃ (PMN-PZN-PZT) ceramics with Ba(W_0.5Cu_0.5)O₃ as the sintering aid that was manufactured in order to develop the low-temperature sintering materials for piezoelectric device applications. The phase transition, microstructure, dielectric, piezoelectric properties, and the temperature stability of the ceramics were investigated. The results showed that the addition of Ba(W_0.5Cu_0.5)O₃ significantly improved the sintering temperature of PMN-PZN-PZT ceramics and could lower the sintering temperature from 1005 to 920 °C. Besides, the obtained Ba(W_0.5Cu_0.5)O₃-doped ceramics sintered at 920 °C have optimized electrical properties, which are listed as follows: (K_p = 0.63, Q_m = 1415 and d₃₃ = 351 pC/N), and high depolarization temperature above 320 °C. These results indicated that this material was a promising candidate for high-power multilayer piezoelectric device applications.     

Overlapping large polaron tunneling conductivity and giant dielectric constant in Ni0.5Zn0.5Fe1.5Cr0.5O4 nanoparticles (NPs)

We report an orderly study on the structural and dielectric properties of Ni_0.5Zn_0.5Cr_0.5Fe_1.5O₄ nanoparticles (NPs) synthesized by a polyethylene glycol (PEG)-assisted hydrothermal technique. XRD, FT-IR, FE-SEM and EDX measurements were implemented for the structural, morphological and compositional investigations of the product. Dielectric spectroscopy was used for the dielectric property investigation of the sample. Average particle size of the nanoparticles was estimated using Debye-Scherrer's equation as 34 nm. Electrical properties of the sample have been investigated in the range of 1 Hz to 3 MHz (233-412 K). It is observed that the sample has a giant dielectric constant approaching to 10⁶ within the examined temperature range. It is also determined that the sample exhibits a dispersive phase transition around 305 K at which this giant value of dielectric constant has been obtained. This transition has been characterized by Diffuse Phase Transition. Temperature and frequency dependence of dielectric loss function has been attributed to surface charges for the short-time relaxations and to hopping electrons for the long-time relaxations. At low frequencies, dielectric loss function has been supported by the modified Cole-Cole equation. Frequency and temperature dependent conductivity behavior of the sample has been explained by Overlapping Large Polaron Tunneling model.     

Microwave-assisted combustion synthesis in a mechanically activated Al-TiO2-H3BO3 system

The Al₂O₃-TiB₂ in-situ composite has been fabricated by different techniques. In this work, the mechanical activation process has been used to aid microwave-assisted combustion synthesis (MACS) to produce the Al₂O₃-TiB₂ in-situ composite. For this purpose, the thermite mixture of Al, TiO₂ and boric acid (H₃BO₃) powders was used as the raw materials, and was mechanically activated at different milling speeds. The results of X-ray phase analysis of the mechanically activated samples after combustion synthesis showed that the Al₂O₃-TiB₂ in-situ composite has been successfully fabricated by thermal explosion mode of combustion synthesis in microwave, while no combustion synthesis occurred for the unmilled sample. Also, it was found that by increasing the milling speed from 250 to 400 rpm, the purity of the final products has been increased; while further milling speed up to 550 rpm reduced the purity of the final products. The effects of milling speed were also studied by means of differential scanning calorimetry (DSC) measurements. It was shown that by increasing the energy level of the reactants via milling speed, the ignition temperature and the intensity of exothermic peaks in the DSC curves have been changed. Finally, in order to have a good understanding about the in-situ formation of such ceramic composites, a reaction mechanism was proposed based on the experimental results. The synthesized composite exhibited high microhardness value of about 1950 Hv in dense parts.