Microwave synthesis and sintering characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

CaCu₃Ti₄O₁₂ (CCTO) was synthesized and sintered by microwave processing at 2·45 GHz, 1·1 kW. The optimum calcination temperature using microwave heating was determined to be 950°C for 20 min to obtain cubic CCTO powders. The microwave processed powders were sintered to 94% density at 1000°C/60 min. The microstructural studies carried out on these ceramics revealed the grain size to be in the range 1–7 μm. The dielectric constants for the microwave sintered (1000°C/60 min) ceramics were found to vary from 11000–7700 in the 100 Hz–00 kHz frequency range. Interestingly the dielectric loss had lower values than those sintered by conventional sintering routes and decreases with increase in frequency.     

Chemical solution deposition of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin film

CaCu₃Ti₄O₁₂ (CCTO) thin film was successfully deposited on boron doped silica substrate by chemical solution deposition and rapid thermal processing. The phase and microstructure of the deposited films were studied as a function of sintering temperature, employing X-ray diffractometry and scanning electron microscopy. Dielectric properties of the films were measured at room temperature using impedance spectroscopy. Polycrystalline pure phase CCTO thin films with (220) preferential orientation was obtained at a sintering temperature of 750°C. There was a bimodal size distribution of grains. The dielectric constant and loss factor at 1 kHz obtained for a film sintered at 750°C was k ∼ 2000 and tan δ ∼ 0.05.     

Rietveld analysis of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films obtained by RF-sputtering

Calcium copper titanate, CaCu₃Ti₄O₁₂, CCTO, thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO₂/Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The crystalline structure and the surface morphology of the films were markedly affected by the growth conditions. Rietveld analysis reveal a CCTO film with 100 % pure perovskite belonging to a space group Im3 and pseudo-cubic structure. The XPS spectroscopy reveal that the in a reducing N₂ atmosphere a lower Cu/Ca and Ti/Ca ratio were detected, while the O₂ treatment led to an excess of Cu, due to Cu segregation of the surface forming copper oxide crystals. The film present frequency -independent dielectric properties in the temperature range evaluated, which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 600-nm-thick CCTO films annealed at 600 °C at 1 kHz was found to be 70. The leakage current of the MFS capacitor structure was governed by the Schottky barrier conduction mechanism and the leakage current density was lower than 10^?7 A/cm² at a 1.0 V. The current–voltage measurements on MFS capacitors established good switching characteristics.     

Effects of dc bias on dielectric relaxations in CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

Effects of dc bias on dielectric relaxations in CaCu₃Ti₄O₁₂ ceramics were investigated via an improved dielectric spectroscopy. A new low-frequency dielectric relaxation, which was assigned to space charge polarization, was found shifting towards higher frequency with increasing bias voltage in the improved spectra. It was suggested that the Schottky barrier at grain boundary was lowered under dc bias resulting in higher possibility for carriers to migrate. Therefore, the relaxation time was decreased, which was in accordance with rightward shift of this relaxation under increased dc bias. In addition, dependence of the widely reported high-frequency relaxation (>?10⁵ Hz) and middle-frequency relaxation (10³–10⁵ Hz) on bias voltage was also discussed. Permittivity contributed by either high-frequency or middle-frequency relaxation presented inverse dependence on dc bias. Discrepancy on barrier parameters was obtained assuming both of them physically correlated with the barrier at grain boundary.     

Electric properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>(BIT)–CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> (CCTO) composite substrates for high dielectric constant devices

A study of the effect of the presence of BIT (Bi₄Ti₃O₁₂) in the dielectric and optical properties of the CaCu₃Ti₄O₁₂ (CCTO) is presented. The samples were prepared by the solid state procedure. Mechanical alloying followed by the solid state procedure has been used successfully to produce powders of CaCu₃Ti₄O₁₂ (CCTO) and BIT (Bi₄Ti₃O₁₂) to be used in the composites. We also look at the effect of the grain size of the BIT and CCTO in the final properties of the composite. The samples were studied using X-Ray diffraction, scanning electron microscopy (SEM), Raman and infrared spectroscopy. We also did a study of the dielectric function K and dielectric loss of the samples. The role played by the grain size of CCTO and BIT in the dielectric constant and structural properties of the substrates are discussed. For frequencies below 10 MHz the K value presented by the CCTO100 sample is always higher than the K value presented by the BIT100 sample. At 100 Hz the value of K 1900 for the CCTO100 sample and 288 for the BIT100 sample. However for the composite sample one has an unexpected result. The dielectric constant is higher for all the frequencies under study. At 100 Hz the value of the K is around 10.000 for the BIT10 sample. Which is more than one order bigger compared to the CCTO100 value for the same frequency. Therefore, these measurements confirm the potential use of such materials for small high dielectric planar devices. These composites are also attractive for capacitor applications and certainly for microelectronics, microwave devices (cell mobile phones for example), where the miniaturization of the devices is crucial.     

Effects of Nd-substitution on microstructures and dielectric characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

Ca_1−3x/2Nd _x Cu₃Ti₄O₁₂ (x = 0, 0.1, 0.2) ceramics were prepared by a solid state reaction process, and single-phased structures were obtained for all the compositions. The dielectric characteristics of pure and Nd-substituted CaCu₃Ti₄O₁₂ ceramics were investigated together with the microstructures. The mixed-valent structures of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ in the present ceramics were confirmed by X-ray photoelectron analysis. The dielectric relaxation in the low temperature range was examined in detail and the variation of dielectric constant and dielectric loss was attributed to the modification mixed-valent structures.     

Influence of Zr doping on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

Pure and Zr-substituted CaCu₃(Ti_1−x Zr _x )₄O₁₂ (x = 0, 0.01, 0.02, 0.03) ceramics were prepared by the Pechini method. X-ray powder diffraction analysis indicated the formation of single-phase compound, and all the diffraction peaks were completely indexed by the body-centered cubic perovskite-related structure. The effects of Zr⁴⁺ ion substituting partially Ti⁴⁺ ion on the dielectric properties were investigated in frequency range between 100 Hz and 1 GHz. The low frequency (f ≤ 10⁵ Hz) dielectric constant decreases with Zr substitution and the high frequency (f ≥ 10⁷ Hz) dielectric constant is unchanged. Interestingly, a low-frequency relaxation was observed at room temperature through Zr substitution. The observed dielectric properties in Zr-substituted samples were discussed using the internal barrier layer capacitor model. A corresponding equivalent circuit was adopted to explain the dielectric dispersion. The characteristic frequency of low-frequency relaxation rises due to the decrease of the resistivity of grain boundary with Zr substitution, which is likely responsible for the large low-frequency response at room temperature.     

Effect of high pressures and temperatures on the structure and properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

We have prepared ceramic CaCu₃Ti₄O₁₂ samples by solid-state reaction and investigated the effect of high-pressure/high-temperature processing (p = 8.0 GPa, t = 1100°C) on the structure and electrical properties of CaCu₃Ti₄O₁₂.     

Influence of different dopant sites by lanthanum on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The effects of La³⁺ doped in calcium copper titanate (CCTO) at Ca²⁺ site and Cu²⁺ site were examined. The doped compositions, La_0.1Ca_0.85Cu₃Ti₄O₁₂ (LCCTO) ceramics and CaLa_0.1Cu_2.85Ti₄O₁₂ (CLCTO) ceramics were prepared by the solid-state method. The microstructure, dielectric properties, complex impedance and nonlinear I–V characteristics were studied. And it was found that La³⁺ doped at Ca²⁺ site achieved lower sintering temperatures than that doped at Cu²⁺ site in CCTO ceramics. The dielectric loss (tan δ) of LCCTO ceramics was about 0.05 at 40 kHz when the sample was sintered at 1080 °C. Dielectric constant (ε′) of LCCTO ceramics was about 3.2 × 10⁴ when the sample was sintered at 1100 °C, which was larger than CLCTO ceramics examined under the same process condition with sintering temperatures vary. The impedance analysis revealed that LCCTO ceramics had an influence of resistance of grain boundaries, which was stronger than that of CLCTO ceramics. Meanwhile, both LCCTO ceramics and CLCTO ceramics had a nonlinear-Ohmic property.     

The influence of the segregation of Cu-rich phase on the microstructural and impedance characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The influence of sintering conditions on the microstructural features and impedance characteristics of the giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) was investigated. The microstructure and impedance characteristics were found to be strongly dependent on the sintering conditions. Sintering of the CCTO ceramics at elevated temperatures (>1100 °C) for prolonged durations resulted in the segregation of Cu-rich phase, mostly confined to the surface, which was in concomitance with the appearance of the additional semicircle at the low frequency end in Impedance (Z*) plots. The absence of this additional semicircle in the Cu-deficient CCTO ceramics and the appearance of the same in Cu-rich CCTO ceramics that were deliberately fabricated corroborated the above observations. Also, La_2/3Cu₃Ti₄O₁₂ (LCTO), a low dielectric constant member of CCTO family, which consisted of small grains without the segregation of Cu-rich phase at the grain boundary, did not reveal the presence of additional semicircle in the Z*plots.     

Electrical conductivity of nanostructured fluorite-like Sc<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>

Single-crystal and polycrystalline samples of Sc₄Ti₃O₁₂ have been shown to contain nanodomains (10–50 nm) with different degrees of ordering, coherent with the fluorite-like matrix. The oxygen-ion conductivity of this compound has been determined in the range 300–1000°C in air using impedance spectroscopy. The nanostructured single-crystal and polycrystalline samples are close in the activation energy for bulk conduction at both low and high temperatures: ?1.26 and 1.29 eV in the range 300–775°C, ?1.98 and 2.07 eV in the range 775–1000°C.     

Electrochemical properties of Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>/C and Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>/C/Ag nanomaterials

We have prepared and characterized lithium titanate-based anode materials, Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C/Ag, using polyvinylidene fluoride as a carbon source. The formation of such materials has been shown to be accompanied by fluorination of the lithium titanate surface and the formation of a highly conductive carbon coating. The highest electrochemical capacity (175 mAh/g at a current density of 20 mA/g) is offered by the Li₄Ti₅O₁₂-based anode materials prepared using 5% polyvinylidene fluoride. The addition of silver nanoparticles ensures a further increase in electrical conductivity and better cycling stability of the materials at high current densities.     

Very high thermal stability with excellent dielectric,and non-ohmics properties of Mg-doped CaCu<Subscript>3</Subscript>Ti<Subscript>4.2</Subscript>O<Subscript>12</Subscript> ceramics

In this work, the nominal CaCu_3?xMg_xTi_4.2O₁₂ (0.00, 0.05 and 0.10) ceramics were prepared by sintering pellets of their precursor powders obtained by a polymer pyrolysis solution method at 1100 °C for different sintering time of 8 and 12 h. Very low loss tangent (tanδ)?<?0.009–0.014 and giant dielectric constant (ε′) ～?1.1?×?10⁴–1.8?×?10⁴ with excellent temperature coefficient (Δε′) less than ±?15% in a temperature range of ??60 to 210 °C were achieved. These excellent performances suggested a potent application of the ceramics for high temperature X8R and X9R capacitors. It was found that tanδ values decreased with increasing Mg²⁺ dopants due to the increase of grain boundary resistance (R_gb) caused by the very high density of grain, resulting from the substitution of small ionic radius Mg²⁺ dopants in the structure. In addition, CaCu_3?xMg_xTi_4.2O₁₂ ceramics displayed non-linear characteristics with the significant enhancements of a non-linear coefficient (α) and a breakdown field (E_b) due to Mg²⁺doping. The high values of ε′ (14012), α (13.64) and E_b (5977.02 V/cm) with very low tanδ value (0.009) were obtained in a CaCu_2.90Mg_0.10Ti_4.2O₁₂ ceramic sintered at 1100 °C for 8 h.     

Annealing behavior of TiO<Subscript>2</Subscript>-sheathed Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires

TiO₂-sheathed Ga₂O₃ one-dimensional (1D) nanostructures were synthesized by thermal evaporation of GaN powders and then sputter-deposition of TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis results indicate that the Ga₂O₃ cores are of a single crystal nature with a monoclinic structure while the TiO₂ shells are amorphous. Photoluminescence (PL) emission is slightly decreased in intensity by TiO₂ coating, but it is significantly increased by thermal annealing in an oxygen atmosphere. The emission peak is also shifted from ~500 to ~550 nm by oxygen annealing. The increase in the green emission is due to the increase in the concentration of the Ga vacancies in the cores by the inflow of oxygen during oxygen annealing. On the other hand, annealing in a nitrogen atmosphere leads to a red shift of the emission to ~700 nm originating from nitrogen doping.     

Relationship among the phase equilibria,microstructures, and dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics via different sintering time

The structure–property relationship of the CaCu₃Ti₄O₁₂ ceramics processed via conventional solid-state method was studied in terms of the different processing conditions. X-ray diffraction patterns of the tenorite CuO and cuprite Cu₂O secondary phases found on the unpolished and polished surfaces of CaCu₃Ti₄O₁₂ were explained by the reduction/reoxidation reaction as a function of sintering time. Based on the microstructures, grain growth of CaCu₃Ti₄O₁₂ continued from 0.5 to 4 h sintering while the further growth was limited to the small-sized grains after 8 h sintering. Also, WDS data indicated the Cu-deficient and Ti-excessive stoichiometry of CaCu₃Ti₄O₁₂ on both outer and inner regions regardless of sintering time. The change of dielectric constant and tan δ were shortly discussed with regard to the secondary phases and the microstructures of the different sintering hours.     

Structural investigation and giant dielectric response of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramic by Nd/Zr co-doping for energy storage applications

High performance dielectric materials are highly required for practical application for energy storage technologies. In this work, high-k pristine and modified calcium copper titanate having nominal formula Ca_0.95Nd_0.05Cu₃Ti_4?xZr_xO₁₂ (x?=?0.01, 0.03 & 0.10) were synthesized and characterized for structural and dielectric properties. Single phase formation of the synthesized compositions was confirmed by X-ray diffraction patterns and further analysed using Rietveld refinement technique. Phase purity of the synthesized ceramics was further confirmed by Energy-dispersive X-ray Spectroscopy (EDX) analysis. SEM images demonstrated that grain size of the modified CCTO ceramics was controlled by Zr⁴⁺ ions due to solute drag effect. Impedance spectroscopy was employed to understand the grain, grain boundaries and electrode contribution to the dielectric response. Nyquist plots were fitted with a 2R-CPE model which confirms the non-ideality of the system. Substitution of specific concentration of Nd and Zr improved the dielectric properties of high dielectric permittivity (ε′ ~?16,902) and minimal tanδ (≤?0.10) over a wide frequency range. The giant ε′ of the investigated system was attributed to internal barrier layer capacitance (IBLC) effect and reduced tanδ accredited to enhanced grain boundaries resistance due to substitution of Zr⁴⁺ ions at Ti⁴⁺ site.     

Dielectric properties of iron doped calcium copper titanate,CaCu<Subscript>3</Subscript>Ti<Subscript>3.9</Subscript>Fe<Subscript>0.1</Subscript>O<Subscript>12</Subscript> ceramic

Dielectric properties of iron doped CaCu₃Ti₄O₁₂ (CCTO), viz. CaCu₃Ti_3.9Fe_0.1O₁₂ (CCTFO) prepared by a novel semi-wet route have been investigated. X-ray diffraction of powder sintered at 900 °C show formation of single phase solid solution. Energy dispersive X-ray spectroscopy (EDX) confirmed the presence of CuO rich phase at grain boundaries of CCTFO. Nature of dielectric relaxation observed above room temperature is studied using complex plane impedance analysis and modulus spectroscopy. It has been found that out of the two relaxations reported earlier above room temperature, one occurring at lower temperature is due to grainboundaries interfacial polarization.     

Formation and structural phase transitions of mesoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> xerogels under hydrothermal conditions

We have studied the effect of the hydrothermal synthesis temperature on Al₂O₃ structure formation and examined the role of the phase composition of the precursor gel and surfactant in the formation of the pore structure of Al₂O₃. A technique has been proposed for the synthesis of TiO₂/Al₂O₃ binary xerogels, and the effect of TiO₂ content on the pore structure parameters and adsorption properties of TiO₂/Al₂O₃ has been investigated.     

Formation of solid solutions in the TiO<Subscript>2</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The phase formation and reaction kinetics in the TiO₂-Cr₂O₃ system have been studied by x-ray diffraction and electron microscopy. The Cr₂O₃ solubility in TiO₂ has been accurately determined, and the rate parameters of the formation of solid solutions in this system have been evaluated. The results demonstrate that Cr₂O₃ dissolves in rutile and not in anatase. Cr₂O₃ markedly reduces the temperature of the anatase-rutile phase transition.