The synthesis and properties of CaCu3Ti4O12 thin films

The CaCu₃Ti₄O₁₂ (CCTO) thin films were synthesized via a metal‐organic solution containing stoichiometric amounts of the metal cations at 700 °C for 1 h. The stable metal‐organic solution was prepared by dissolving calcium nitrate, copper nitrate, and tetrabuty titanate in grain alcohol. The phases, microstructures, and electric properties of CCTO thin films were characterized by X‐ray diffraction, scanning electron microscopy, atomic force microscope, and electric measurements. The results show that the CCTO thin films have homogeneous microstructure, smooth surface, low leakage current, and high values of dielectric constant. The low leakage current can be attributed to the small surface roughness. The high value of dielectric constant can be attributed to the internal barrier layer capacitor mechanism and metal‐insulator‐semiconductor junction of CCTO thin films. Copyright © 2013 John Wiley & Sons, Ltd.     

Dielectric and impedance properties of Sr(Sm0.5Nb0.5)O3 ceramics

Perovskite types Sr(Sm_0.5Nb_0.5)O₃, (SSN) ceramics have been prepared through solid state reaction route. The scanning electron microscopy provides information on the quality of the samples and uniform grain distribution over the surface of the samples. The field dependence of the dielectric response was measured in a frequency range from 50 Hz to 1 MHz and in a temperature range from 60 °C to 420 °C indicates polydispersive nature of the materials. An analysis of the dielectric constant (?′) and tangent loss (tanδ) with frequency is performed assuming a distribution of relaxation times as confirmed by the scaling behavior of electric modulus spectra. The frequency dependence of the electric modulus peak is found to obey Arrhenius law with activation energy of ∼0.026 eV. The complex plane impedance plot shows the grain boundary contribution for higher value of dielectric constant in the law frequency region. The frequency dependence of electrical data is also analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behavior of imaginary part of electric modulus M″ suggests that the relaxation describes the same mechanism at various temperatures in SSN.     

Comparative study of dielectric,mechanical, and nuclear magnetic relaxations in linear polyethylene. II. Relaxation spectroscopy of the α, β, and γ loss bands with special emphasis on fine structure

Dielectric, mechanical, and NMR retardation (correlation) spectra for relaxations in linear polyethylene were calculated in normalized form and intercompared. For each of the two local-mode relaxations in the γ region, called γ₁ and γ₂, these spectra are found to be in excellent agreement. For the α region, the spectra for two mechanical processes, called α₁ and α₂, two NMR processes, called α′ and α, and one dielectric process α were calculated. Excellent agreement is found between the spectra for the dielectric α and NMR α′ processes and also spectra for the mechanical α₂ and NMR α processes, due to molecular motion in the interior of crystals. However, the spectrum for the mechanical α₁ process is different from that for the dielectric α and NMR α′ processes, though the activation energy for the first process is almost the same as for the other two. This behavior is interpreted on the assumption that the dielectric α and NMR α′ processes are caused by molecular motion in lamellar surface layers while the mechanical α₁ process is due to grain-boundary slip with viscous resistance of the surface layers in the boundaries. The shapes of the spectra, including the spectrum for the β process, are not affected by diluent.     

Contact Free Impedance Methodology for Investigating Enzymatic Reactions into Dielectric Polymer Microchip

A methodology for free contact microchannel impedance measurements through a dielectric microchip was developed for monitoring the kinetics of enzymatic reactions. For that purpose, we propose a procedure which consists of subtracting the impedance contribution of the dielectric polymer layer, which separates the two parallel microband electrodes embedded in it, from the global microchip impedance. This operation allows microchannel impedance enhancement for real time monitoring of impedance modulus changes without direct electrical contact. Application for determination of kinetic parameters of enzyme‐substrate reaction independently of optical or electrochemical properties of the substrates is demonstrated. Hydrolysis 4‐nitrophenylphosphate (pNPP) and 4‐aminophenylphosphate (pAPP), which are two substrates for Alkaline Phosphatase (ALP), are taken as examples. Moreover, signal amplification response of the impedance modulus is achieved by the use of superparamagnetic microbeads as enzyme supports. Plotting the maximum rate against the ALP concentration gives rise to straight lines with a slope that is the hydrolysis catalytic pseudo first‐order rate constant, k_cat. Sensitivity, selectivity and reproducibility of these measurements have been demonstrated comparatively with both substrates. k_cat values were 103 s^?1 and 52 s^?1 with pAPP and pNPP, respectively.     

Investigation on the growth of CaCu3Ti4O12 thin film and the origins of its dielectric relaxations

Using the radio frequency magnetron sputtering, CaCu₃Ti₄O₁₂ (CCTO) thin films were deposited on platinized silicon substrates. The influence of annealing temperature on structures and morphologies of the thin films was investigated. The high annealing temperature increased the crystallinity of the films. Temperature dependence of the dielectric constant revealed an amazing different characteristic of the dielectric relaxation at ∼10 MHz, whose characteristic frequency abnormally increased with the decrease of the measuring temperature unlike the relaxations due to extrinsic origins. Meanwhile, the dielectric constant at high frequencies was close to the value derived from the first principle calculation. All these gave the evidences to ascribe this relaxation to the intrinsic mechanism.     

Effect of the addition of B2O3 and BaO-B2O3-SiO2 glasses on the microstructure and dielectric properties of giant dielectric constant material CaCu3Ti4O12

The effect of the addition of glassy phases on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics was investigated. Both single-component (B₂O₃) and multi-component (30 wt% BaO-60 wt% B₂O₃-10 wt% SiO₂ (BBS)) glass systems were chosen to study their effect on the density, microstructure and dielectric properties of CCTO. Addition of an optimum amount of B₂O₃ glass facilitated grain growth and an increase in dielectric constant. However, further increase in the B₂O₃ content resulted in its segregation at the grain boundaries associated with a reduction in the grain size. In contrast, BBS glass addition resulted in well-faceted grains and increase in the dielectric constant and decrease in the dielectric loss. An internal barrier layer capacitance (IBLC) model was invoked to correlate the dielectric constant with the grain size in these samples.     

Influences of Sr2+ Doping on Microstructure,Giant Dielectric Behavior,and Non-Ohmic Properties of CaCu3Ti4O12/CaTiO3 Ceramic Composites

The microstructure, dielectric response, and nonlinear current-voltage properties of Sr²⁺-doped CaCu₃Ti₄O₁₂/CaTiO₃ (CCTO/CTO) ceramic composites, which were prepared by a solid-state reaction method using a single step from the starting nominal composition of CCTO/CTO/xSrO, were investigated. The CCTO and CTO phases were detected in the X-ray diffraction patterns. The lattice parameter increased with increasing Sr²⁺ doping concentration. The phase compositions of CCTO and CTO were confirmed by energy-dispersive X-ray spectroscopy with elemental mapping in the sintered ceramics. It can be confirmed that most of the Sr²⁺ ions substituted into the CTO phase, while some minor portion substituted into the CCTO phase. Furthermore, small segregation of Cu-rich was observed along the grain boundaries. The dielectric permittivity of the CCTO/CTO composite slightly decreased by doping with Sr²⁺, while the loss tangent was greatly reduced. Furthermore, the dielectric properties in a high-temperature range of the Sr²⁺-doped CCTO/CTO ceramic composites can be improved. Interestingly, the nonlinear electrical properties of the Sr²⁺-doped CCTO/CTO ceramic composites were significantly enhanced. The improved dielectric and nonlinear electrical properties of the Sr²⁺-doped CCTO/CTO ceramic composites were explained by the enhancement of the electrical properties of the internal interfaces.     

Electrochemical impedance spectroscopic study of perfluorovinyl ether copolymer with tetrafluoroethylene in the swollen state

The temperature and frequency dependences of the dielectric impedance of the perfluorovinyl ether copolymer with tetrafluoroethylene have been studied. The impedance spectra of the films swollen in water have been measured in the frequency range of 10^?1–10⁷ Hz at 213–278 K. The model of equivalent electric chains has been suggested for analysis of the frequency dependence of dielectric impedance. The parameters of this model have been calculated. The correlation of these parameters with effects in a polymer matrix is described. Specifically, the pure ohmic conductivity is determined for the copolymer under study.     

Study of structural and dielectric behaviour on carbon nano tube dispersed {xPVF: (1 − x)CH3COONH4} electrolyte

In the present work, improvement in ion transport property of polyvinyl formal (PVF)-based nanocomposite polymer electrolytes has been studied upon dispersal of multiwall carbon nanotube (MWCNT) filler. Nanocomposite polymer electrolyte (NCPE) films of xPVF: (1 ? x)CH₃COONH₄ (ammonium acetate) were prepared by solution cast technique. The formation of nanocomposite has been ascertained by X-ray diffraction (XRD) pattern, which also shows that doping of salt increases amorphousness through polymer salt complexation. Changes in surface morphology have been observed in optical microscopy and Scanning Electron Microscopic (SEM) images. Variation of dielectric constant, dielectric loss, tangent loss and modulus spectra with the change in frequency and temperature were studied with the aid of impedance spectroscopy.     

From Synthesis to Applications: Copper Calcium Titanate (CCTO) and its Magnetic and Photocatalytic Properties

Investigations focusing on electrical energy storage capacitors especially the dielectric ceramic capacitors for high energy storage density are attracting more and more attention in the recent years. Ceramic capacitors possess a faster charge-discharge rate and improved mechanical and thermal properties compared with other energy storage devices such as batteries. The challenge is to obtain ceramic capacitors with outstanding mechanical, thermal and storage properties over large temperature and frequencies ranges. ABO₃ as a type of perovskites showed a strong piezoelectric, dielectric, pyroelectric, and electro-optic properties useful as energy storage and environmental devices. CaCu₃Ti₄O₁₂ (CCTO) perovskite with cubic lattice (Im3 symmetry) was discovered to have a colossal dielectric constant (10⁴) that is stable over a wide range of frequencies (10 Hz–1 MHz) and temperature independence (100–300 K). The origin of this high dielectric constant is not fully established, specially because it is the same for single crystal and thin films. In this review, the history of CCTO will be introduced. The synthesis and the sintering approaches, the dopant elements used as well as the applications of CCTO will be reported. In addition to dielectrical properties useful to energy storage devices; CCTO could serve as photocatalytic materials with a very good performance in visible light.     

Capacitance scaling of grain boundaries with colossal permittivity of CaCu3Ti4O12-based materials

Samples of copper-deficient CaCu₃Ti₄O₁₂ (CCTO) compared to the nominal composition, all synthesized via organic gel-assisted citrate process, show huge change of grain boundaries capacitance as deduced from a fit of an RC element model to the impedance spectroscopic data. The grain boundary capacitance is found to scale with the permittivity measured at 1 kHz weighted by the size of the grains. This result is found consistent with the internal barrier layer capacitance (IBLC) model.     

Role of doping and CuO segregation in improving the giant permittivity of CaCu3Ti4O12

The dopant role on the electric and dielectric properties of the perovskite-type CaCu₃Ti₄O₁₂ (CCTO) compound is evidenced. Impedance spectroscopy measurements show that the relevant permittivity value attributed to sintered CCTO is due to grain boundary (g.b.) effects. The g.b. permittivity value of the pure CCTO can be increased of 1-2 orders of magnitude by cation substitution on Ti site and/or segregation of CuO phase, while the bulk permittivity keeps values 90<ε_r<180. Bulk and g.b. conductivity contributions are discussed: electrons are responsible for the charge transport and a mean bulk activation energy of 0.07 eV is obtained at room temperature for all the examined samples. The g.b. activation energy ranges between 0.54 and 0.76 eV. Defect models related to the transport properties are proposed, supported by electron paramagnetic resonance measurements.     

Effect of annealing on electrical responses of electrode and surface layer in giant-permittivity CuO ceramic

The effect of heat treatments on the electrical responses of the electrode and surface layer in a giant-permittivity CuO ceramic is investigated. It is found that the giant low-frequency relative permittivity of the CuO ceramic can be tuned by annealing in Ar and O₂—it can be reduced by annealing in Ar, and then it can be enhanced up to the initial value by annealing in O₂. The results indicate to the effect of oxygen vacancy concentration on the giant dielectric properties of the CuO ceramic. Interestingly, three sets of dielectric relaxations are observed in the O₂–annealed sample, which can be assigned as the effects of outmost surface layer, electrode, and grain boundary. Our results reveal that the giant low-frequency dielectric response in the CuO ceramic is associated with both of the interfacial polarization at the sample–electrode interface resulted from a non-Ohmic electrode contact and the outmost surface layer-inner part interface.     

Impedance studies on the 5-V cathode material,LiCoPO<Subscript>4</Subscript>

Olivine-structured LiCoPO₄ is synthesized by a Pechini-type polymer precursor method. The structure and the morphology of the compounds are studied by the Rietveld-refined X-ray diffraction, scanning electron microscopy, Brunauer, Emmett, and Teller surface area technique, infrared spectroscopy, and Raman spectroscopy techniques, respectively. The ionic conductivity (σ ionic), dielectric, and electric modulus properties of LiCoPO₄ are investigated on sintered pellets by impedance spectroscopy in the temperature range, 27–50 °C. The σ (ionic) values at 27 and 50 °C are 8.8 × 10⁻⁸ and 49 × 10⁻⁸ S cm⁻¹, respectively with an energy of activation (E _a) = 0.43 eV. The electric modulus studies suggest the presence of non-Debye type of relaxation. Preliminary charge–discharge cycling data are presented.     

Dielectric,impedance and excitation performance of Aurivillius oxides: Bi4Pb2Zr2TiFeNbO18

The Aurivillius phase-based composition bearing chemical formula Bi₄Pb₂Zr₂TiFeNbO₁₈ (termed as BPZTFNO) was synthesized by a solid-state method. The structural analysis as obtained from the X-ray pattern has depicted an orthorhombic phase. The various size and morphology of grains are distributed throughout the natural surface as revealed from microstructure analysis. The dielectric and electrical study of the BPZTFNO is taken at a various sweep of frequency (1 kHz–1 MHz) and temperatures (25–400 °C). The ferroelectric-paraelectric phase transition is well illustrated from the temperature-dependent dielectric constant. The complex impedance analysis suggests the association of grain effect. The relaxation of non-Debye nature is shown in the impedance study. The formed sample is subjected to an electric field and further its excitation performance is examined suggesting its possible usage in devices.     

Temperature dependence of electron spin resonance in CaCu3Ti4O12 substituted with transition metal elements

The temperature dependence of the electron spin resonance (ESR) spectrum of copper in CaCu₃Ti₄O₁₂ (CCTO) polycrystalline samples doped with transition metal elements Mn, Fe, Ni is reported. The frequency dependence of the dielectric constant of the sample is also reported at room temperature. While the dielectric constant of undoped CCTO samples reaches ～10,000, it is found one hundred times lower in samples doped with only 0.5 or 1% of Mn or Fe. Copper is confirmed to give a g = 2.14 signal at room temperature for substituted and unsubstituted samples. Below the antiferromagnetic transition that occurs near 25 K, the signal is found shifted down to low fields for all samples. However the downshift is 10–20 times more important in Mn and Fe-doped samples compared to undoped or Ni-doped CCTO. ESR results in an undoped CCTO thin film grown by pulse laser deposition are also reported. While in the low-temperature antiferromagnetic phase the spectrum is multi-line and magnetic-field-orientation-dependent, it reduces to a narrow single line, independent of the orientation of the magnetic field, in the upper paramagnetic phase, similar to the undoped polycrystalline sample. All doped samples display much broader response in the upper phase. The results are discussed within the framework of the relationship between the high effective dielectric constant and the electrical conductivity of CCTO bulk.     

Structural,thermal and electrical properties of poly(methyl methacrylate)/CaCu3Ti4O12 composite sheets fabricated via melt mixing

Poly(methyl methacrylate) (PMMA) and CaCu₃Ti₄O₁₂ (CCTO) composites were fabricated via melt mixing followed by hot pressing technique. These were characterized using X-ray diffraction, thermo gravimetric, thermo mechanical, differential scanning calorimetry, fourier transform infrared (FTIR) and Impedance analyser for their structural, thermal and dielectric properties. Composites were found to have better thermal stability than that of pure PMMA. However, there was no significant difference in the glass transition (T _g ) temperature between the polymer and the composite. The appearance of additional vibrational frequencies in the range 400–600 cm^?1 in FTIR spectra indicated a possible interaction between PMMA and CCTO. The composite, with 38 vol% of CCTO (in PMMA), exhibited remarkably low dielectric loss at high frequencies and the low-frequency relaxation is attributed to the interfacial polarization/MWS effect. The origin of AC conductivity particularly in the high-frequency region was attributed to the electronic polarization.     

Role of aluminum ions on the dielectric and conducting properties of multiferroic Tb1−xAlxMnO3: Study at high temperatures

Dielectric and conducting properties of Tb_1−xAl_xMnO₃ (x = 0, 0.05) synthesized by the solid–state reaction method have been investigated. The Al ion has the same valence as substituted Tb but is nonmagnetic and its small size gives rise to microstructural strain which affects the multiferroic properties of the parent compound. Samples were characterized by means of complex impedance spectroscopy (CIS) in the frequency range from 40 Hz to 5 MHz, at temperatures above room temperature. The conductivity curves for the two samples are well fitted by the Jonscher law σ(ω) = σ_dc + Aωⁿ. Results of the fitting procedure indicate that for the studied samples, the hopping motion involves localized hopping without the species leaving the neighbors. Frequency dependence of the dielectric constant (ε″) and tangent loss (tan δ) display a dispersive behavior at low frequencies that can be explained on the basis of the Maxwell–Wagner model and Koop's theory. The relaxation dynamics of charge carriers has been studied by means of the electric modulus formalism. In turn, the variation of the imaginary part of the complex impedance, Z″, shows a peak at a relaxation angular frequency (ω_r) related to the relaxation time (τ) by τ = 1/ω_r. The complex impedance spectra (Nyquist plots) show well-defined semicircles which are strongly dependent on the Al-doping level and temperature. The complex impedance data have been modeled using electrical equivalent circuits.     

Determination of diffusion coefficient of lithium in substituted LiMn1.95Cr0.05O4 spinel using impedance technique

Electrodiffusion properties of chromium-substituted lithium-manganese spinel Li _x Mn_1.95Cr_0.05O₄ intended for application as a cathodic material for lithium-ion batteries is studied. The studies are carried out at 25°C using the electrochemical impedance spectroscopy technique in alkyl-carbonate electrolyte. In the analysis of impedance spectra, the apparatus of electric equivalent circuits was employed to determine surface layer resistances, double electric layer capacitance, differential intercalation capacity, chemical diffusion coefficient D of lithium, and other electrode characteristics. The issues of substantiating the choice of electric equivalent circuits and correct interpretation of their elements are discussed; dependences of the calculated model parameters on the electrode potential (lithium concentration in the electrode) are analyzed. The chemical diffusion coefficient of Li⁺ in Li _x Mn_1.95Cr_0.05O₄ found on the basis of the impedance spectra is in the range of 10^?9 to 10^?12 cm²/s under electrode potential variation in the range of 3.5–4.5 V (vs. Li/Li⁺) with a pronounced minimum of D in the middle of this range. Repeated cycling of the electrode is accompanied by a gradual increase in resistance of the solid-electrolyte interphase (SEI).     

Dielectric relaxations in PEEK by combined dynamic dielectric spectroscopy and thermally stimulated current

The molecular dynamics of a quenched poly(ether ether ketone) (PEEK) was studied over a broad frequency range from 10^?3 to 10⁶ Hz by combining dynamic dielectric spectroscopy (DDS) and thermo-stimulated current (TSC) analysis. The dielectric relaxation losses ε′′_KK has been determined from the real part ε′_T(ω) thanks to Kramers–Kronig transform. In this way, conduction and relaxation processes can be analyzed independently. Two secondary dipolar relaxations, the γ and the β modes, corresponding to non-cooperative localized molecular mobility have been pointed out. The main α relaxation appeared close to the glass transition temperature as determined by DSC; it has been attributed to the delocalized cooperative mobility of the free amorphous phase. The relaxation times of dielectric relaxations determined with TSC at low frequency converge with relaxation times extracted from DDS at high frequency. This correlation emphasized continuity of mobility kinetics between vitreous and liquid state. The dielectric spectroscopy exhibits the α_c relaxation, near 443 K, which has been associated with the rigid amorphous phase confined by crystallites. This present experiment demonstrates coherence of the dynamics of the PEEK heterogeneous amorphous phase between glassy and liquid state and significantly improve the knowledge of molecular/dynamic structure relationships.