Journal of Advanced Dielectrics

CoCr₂O₄ nanoceramics are prepared by sol–gel auto combustion method. Synchrotron X-ray diffraction analysis affirms the single-phase pristine cubic structure with space group $Fd3m$. Debye–Scherrer method estimates the crystallite size of main intense peak to be $\sim 27.15$nm. Prominent bands obtained in infrared spectra at 448 and 599cm${}^{-1}$ are due to metal–oxygen stretching bond present at tetrahedral and octahedral sites. Dielectric parameters decrease as frequency increases from $10^3$ to $10^6$Hz that can be interpreted by Maxwell–Wagner-type interfacial polarization. Complex impedance spectra (Nyquist plot) reveal arc like behavior, which is mainly due to intergrain (grain boundary) resistance that also exhibits conducting nature of the nanoceramics. Weak ferroelectricity is mainly associated with the partial reversal of the polarization. Leakage current behavior follows the Ohmic and Child square law. Electron conduction process was interpreted by space-charge limited current (SCLC) mechanism. Leakage current behavior observed in cobalt chromite nanoceramics is mainly attributed to the oxygen vacancies.

Two sets of experimental data of piezo-catalytic degradations are adapted to fit with three-parameter Mittag–Leffler functions. One set is degradation on methyl orange (MO) using catalysis of nano-sized barium titanates, and another set is on Rhodamine-B (RhB) using molybdenum disulfide nano-flowers as catalysts. The fitting results suggest that these piezo-catalytic degradations can be described either as Davidson–Cole type response function or Havriliak–Negami type of dielectric relaxation. This work confirms completely the universality in chemical reaction kinetics, which can be described by fractional differential equations with Mittag–Leffler functions as their solutions.

This paper investigates a system of 0.93${\text{Bi}}_{0.5}$${\text{Na}}_{0.5}$TiO₃–0.06BaTiO₃–0.01${\text{K}}_{0.5}$${\text{Na}}_{0.5}$NbO₃–$x$CuO (BNT–BT–KNN–$x$CuO, $x=0-0.04$mol.%) ceramics, which were fabricated by the conventional solid-state process through the granulation of vacuum freeze drier. The results show that the CuO doping made a significant enhancement on the piezoelectric properties of the BNT–BT–KNN ceramics. With the doping of CuO, the transition temperature between ferroelectric phase and ergodic relaxor state is reduced to near room temperature, resulting in pinched $P$–$E$ loops and “sprout” shape $S$–$E$ curves. For the composition with $x=0.01$, a high unipolar strain of 0.39% under 5kV/mm contributes a large $d_{33}^{\ast }\sim 780$pm/V at room temperature, which is competitive with the other BNT-based ceramics.

The characteristics of a two-layer structure on the basis of the layers of varistor ceramics and polymeric PPTC nanocomposite being in thermal contact for the purpose of using it as a limiter of constant voltage and long-term varying electrical overvoltages are analyzed. A theoretical model of such a structure has been developed, and its main electrical characteristics are simulated. It is shown that the provision of the required output voltage limitation is performed by selecting the classification voltage of the varistor layer. The maximum current of the varistor layer required for heating the structure is determined by the intensity of heat transfer to the environment. It has established a satisfactory agreement between the theoretical and experimental electrical characteristics for the structure based on the layers used in commercial varistors and PPTC fuses.

Low frequency Raman spectra of (${\text{Pb}}_{0.97}$${\text{La}}_{0.02})$(${\text{Zr}}_{0.864}$${\text{Sn}}_{0.04}$${\text{Ti}}_{0.096})$O₃ ceramic samples have been studied near cubic to antiferroelectric phase transition at about 200^∘C. A set of low frequency soft modes were observed restoring below the transition point, in addition to the known one above 100cm${}^{-1}$. These modes show strong damping anomalies at the transition point that supposes considerable intermode interactions via dampings.

Research on synthesis, characterization and determination of processing — structure — property relationships of commercially important ferroelectric thin films has been performed. The sol–gel type solution deposition technique was applied to produce good quality thin films of ${\text{Ba}}_{0.6}$${\text{Sr}}_{0.4}$TiO₃ (BST60/40) chemical composition on the stainless steel substrates. The thin films were characterized in terms of their microstructure, crystal structure, phase composition, piezoelectric and dielectric properties. It was found that the BST60/40 thin film adopted the cubic structure at room temperature with an elementary cell parameter $a=3.971(8)$ Å. Morphology of the thin film surface was studied with Atomic Force Microscopy (AFM). Average roughness of the thin films surface was found ($\mathrm{\text{Sa}}=0.055\mu$m). Piezoresponse Force Microscopy (PFM) was applied for the thin film characterization. Active piezoelectric regions were found in BST60/40 thin film. Therefore, dielectric response measured at room temperature was studied in assumption of piezoelectric electric equivalent circuit.

Detailed structural and dielectric properties of Lanthanum-doped barium titanate ${\text{Ba}}_{1-x}$La_x${\text{Ti}}_{(1-x∕4)}$O₃ ceramic powders BLTx (where $x=0.00$; 0.10; 0.20; 0.30 and 0.40)/BT, BLT10, BLT20, BLT30 and BLT40, synthesized by the sol gel process, calcined at 900^∘C for 3h and sintered at 1250^∘C for 6h, have been investigated. The phase formation and crystal structure of the samples were checked by X-ray diffraction (XRD) and Raman spectroscopy. The samples crystallize in the pure perovskite structure that transforms from tetragonal to pseudocubic under doping with La; results that have been confirmed by Rietveld Refinement technique. The estimated average crystallite size of the samples was about 23nm. Dielectric parameters (dielectric permittivity and losses) were determined in the temperature range room temperature (RT) — 280^∘C and in the frequency range 500Hz–2MHz. La doping gives rise to a strong decrease of the ferro-to-paraelectric transition temperature, and the frequency dependence of the permittivity shows that the samples with $x=0.00$ and $x=0.10$ reach their resonance frequency. The frequency dependence of impedance and electric modulus properties were studied over a wide frequency range from 1kHz to 2MHz at various temperatures to confirm the contributions from grains and grain-boundaries. The complex impedance analysis data have been presented in the Nyquist plot which is used to identify the corresponding equivalent circuit and fundamental circuit parameters; it was found that the grain boundaries resistance is dominant at room temperature. The frequency dependence of the parameters permittivity, losses and AC conductivity reveals that the relaxation process is of the Maxwell–Wagner type of interfacial polarization.

Surface-modified BaTiO₃ of 230nm average grain size were synthesized by coating BaTiO₃ particles with 3wt.% Al₂O₃ and 1wt.% SiO₂. Ceramic films of different thicknesses were prepared via tape casting and laminating processes followed by two-steps sintering method. After sintering, the average grain size of Surface-modified BaTiO₃ ceramic slightly increased to 275nm, and great enhancement of AC breakdown strength (BDS) from 184kV/cm to 665kV/cm was obtained as the thickness of ceramic films decreased from 63$\mu$m to 12$\mu$m, resulting in improvement of discharge energy density from 1.14J/cm³to 4.06J/cm³. Because of their low-cost, easily fabrication, lead-free, improved AC BDS and discharge energy density, surface-modified BaTiO₃ ceramic films appeared promising for applications in multilayer energy storage capacitors.