Journal of Advanced Dielectrics

A lead-free relaxor ferroelectric, Sr${}_{0.15}$Ba${}_{0.85}$Bi₂Nb₂O₉, was synthesized via solid-state reaction and the temperature-dependence of the heat capacity was measured in a wide temperature range. The dielectric permittivity was also measured between 500Hz and 5MHz in the same temperature range. No anomaly has been detected in the heat capacity curve for the whole temperature range covered in the present experiments, while broad peaks have been observed in the dielectric permittivity with high frequency dispersion. A typical relaxor behavior has been observed from the dielectric analysis. The Debye’s temperature has showed a minimum value near the freezing temperature. The results are discussed considering the spin-glass model and the high frequency dispersion, which has been observed for the studied relaxor system.

Due to their excellent electrical insulation properties and processability, polymer materials are used in many electrical products. It is widely believed that space charge plays an important role for the electric field distribution, conduction, ageing, and electric breakdown of polymeric insulation. This paper reviews measurements and characteristics of space charge behavior which mainly determined by the pulsed electro-acoustic (PEA) measurement technique. Particular interests are the effects of the applied voltage, the electrodes, temperature, humidity, microstructure, additives, and filler materials on accumulation, distribution, transport, and the decay of space charge in polymeric materials. This review paper is to provide an overview on various space charge effects under different conditions, and also to summarize the information for polymeric materials with suppressed space charge and improved electrical behavior.

Progresses in thermoelectric (TE) materials will contribute to solving the world’s demands for energy and global climate protection. It also calls for higher ZT to achieve ideal commercial conversion efficiency. As an effective way, nanostructuring can reduce the thermal conductivity by the selective scattering of phonons or enhance Seebeck coefficient via modification of the density of the states, resulting in good ZT value. Meanwhile, TE properties of nanostructured materials should depend on the size and morphology of the microstructure features. This review emphasizes the developments in the TE bulk materials at the nanoscale in the past several years and summarizes the understanding in this active field.

The influence of single and double layered gold (Au) nanocrystals (NC), embedded in SiO₂ matrix, on the electrical characteristics of metal–oxide–semiconductor (MOS) structures is reported in this communication. The size and position of the NCs are varied and study is made using Sentaurus TCAD simulation tools. In a single NC-layered MOS structure, the role of NCs is more prominent when they are placed closer to SiO₂/Si$-$substrate interface than to SiO₂/Al–gate interface. In MOS structures with larger NC dots and double layered NCs, the charge storage capacity is increased due to charging of the dielectric in the presence of NCs. Higher breakdown voltage and smaller leakage current are also obtained in the case of dual NC-layered MOS device. A new phenomenon of smearing out of the capacitance–voltage curve is observed in the presence of dual NC layer indicating generation of interface traps. An internal electric field developed between these two charged NC layers is expected to generate such interface traps at the SiO₂/Si interface.

The temperature coefficient of permittivity ($\mathrm{\text{TC}}\epsilon$) of BaTiO₃–Bi(Me)O₃ solid solutions were investigated. It was determined that as the tolerance factor was decreased with the addition of Bi(Me)O₃, the $TC\epsilon$ shifted from large negative values to $\mathrm{\text{TC}}\epsilon$ values approaching zero. It is proposed that the different bonding nature of the dopant cation affects the magnitude and temperature stability of the permittivity. This study suggests that the relationship between tolerance factor and $\mathrm{\text{TC}}\epsilon$ can be used as a guide to design new dielectric compounds exhibiting temperature-stable high permittivity characteristics, which is similar to past research on perovskite and pyrochlore-based microwave dielectrics.

We have investigated dielectric properties of aromatic polythiourea (ArPTU, a polar polymer containing high dipolar moments with very low defect levels) thin films that were developed on Pt/SiO₂ substrate. The detected response is compared to the response of commercially available polymers, such as high density polyethylene (HDPE) and polypropylene (PP), which are at present used in foil capacitors. Stable values of the dielectric constant ${\epsilon }^{\prime }\approx 5$ (being twice higher than in HDPE and PP) over broad temperature and frequency ranges and dielectric losses as low as in commercial systems suggest ArPTU as a promising candidate for future use in a variety of applications.

The purpose of this work is in development of the model that allows to investigate the conformations of macromolecules near the interface “dielectric-metal” depending on the conditions of formation of the polymer coating. In the modified model of “sticky tape”, one part of macromolecule is anchored to the metal surface while the other can be elongated due to effective mean (molecular) field of dipolar type formed by free ends of other chains. The dynamic Monte-Carlo method for Langmuir’s model is used for calculation of adhesion force taking into account the interaction energy of monomers with the metal surface. It is shown that conformation of polymer chain is defined by temperature conditions of its formation. The obtained results are confirmed by the data of production tests on polymer coatings in JSC “Severstal”.

The theory of direct magnetoelectric (ME) effect in a bilayer of ferromagnetic and piezoelectric is developed taking into account nonuniform distribution of electrical field on a piezoelectric layer thickness. The simultaneous solution of the motion equations for piezoelectric and ferromagnetic mediums allowed to numerically and analytically calculate the dependence of natural mechanical oscillation on structure parameters and to determine the dependence of ME effect coefficient on frequency of the variable magnetic field.

Ceramic samples of (Bi${}_{1-x}$Pb_x)NbO₄ ($x=0$, 0.025, 0.05, 0.10, 0.15, 0.20) with 0.75wt.% V₂O₅ addition sintered at 920${}^{\circ }$C, 940${}^{\circ }$C and 960${}^{\circ }$C are investigated. Pb is selected as a substitute for Bi${}^{3+}$ in BiNbO₄ ceramics as it exists in two stable valence states of $+$2 and $+$4 and the average valency matches to that of Bi${}^{3+}$. The average Shannon radius (for octahedral coordination) of Pb${}^{2+}$ (1.19Å) and Pb${}^{4+}$(0.775Å) cations is 0.9825Å, which is similar to that of Bi${}^{3+}$ ion (1.03Å). The dense ($>94$%) polycrystalline (Bi${}_{1-x}$Pb_x)NbO₄ samples fabricated mostly reveal orthorhombic (Pnna) phase ($\alpha$-BiNbO₄, Sp. Gp. 52) by powder XRD. Presence of satellite Pb₂Nb₂O₇ phase, the amount of which is increasing with increase in Pb content, is also noticed. The microwave dielectric constant (${\epsilon }_r^{\prime }$) values of the niobates are found to increase from 42 to 71, whereas the quality factor (Q_u.f) values are found to decrease from 5400 to 550 GHz with increasing substitution of Pb. The compositions so synthesized are important as hardly there are any microwave dielectric ceramics available with $45<{\epsilon }_r^{\prime }<75$.

Influence of BiFeO₃ (BF) on phase formation, unit cell parameters, microstructure, dielectric and ferroelectric properties of solid solutions close to the morphotropic phase boundary in the (Na${}_{0.5}$Bi${}_{0.5})$TiO₃–(K${}_{0.5}$Na${}_{0.5})$NbO₃ system additionally modified by the low-melting KCl additives has been studied. The formation of pure perovskite structure samples decrease in the unit cell parameters and increase in the $T_C$ value stimulated by the BF addition have been revealed. It was proved that modification of compositions by small amounts of the BF and KCl additives leads to improvement of dielectric parameters.

CaCu₃Ti${}_{4-x}$Y_xO${}_{12}$ (0$\le x\le$0.12) ceramics were fabricated with conventional solid-state reaction method. Phase structure and microstructure of prepared ceramics were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The impedance and modulus tests both suggested the existence of two different relaxation behavior, which were attributed to bulk and grain boundary response. In addition, the conductivity and dielectric permittivity showed a step-like behavior under 405K. Meanwhile, frequency independence of dc conduction became dominant when above 405K. In CCTO ceramic, rare earth element Y${}^{3+}$ ions as an acceptor were used to substitute Ti sites, decreasing the concentration of oxygen vacancy around grain-electrode and grain boundary. The reason to the reduction of dielectric behavior in low frequencies range was associated with the Y doping in CCTO ceramic.