The Positive Temperature Coefficient of Resistivity in BiFeO3 Films

Investigation of relaxor and diffuse dielectric phase transitions of Ba1-XBixTi0.8Fe0·2O3 materials

Different doping elements have been used to enhance the dielectric properties of BaTiO₃ ceramic. In this work, the effect of substitution of Ba by Bi in A site and Ti by Fe in B site on structural, dielectric and electrical properties of Ba_1-xBi_xTi_0.80Fe_0·20O₃ ceramics at (x = 0.00, 0.05, 0.10 and 0.15) was investigated by X-ray diffraction, Raman spectroscopy, Scanning Electron Microscopy (SEM), Mössbauer spectroscopy as well as dielectric measurements. The Rietveld refinement results revealed that the prepared compounds crystallize in both tetragonal (P4mm) and hexagonal (P6₃/mmc) phases for x = 0.00 and 0.05 while at x = 0.10 and 0.15, the hexagonal phase disappears and only the tetragonal phase is fitted. The Raman spectra confirmed the disappearance of hexagonal phase in benefit of tetragonal phase as the Bi³⁺ substitution increases. Based on Mössbauer analyses results, all the samples are in paramagnetic state at room temperature and the Fe is oxidized under Fe³⁺ without the presence of Fe²⁺ or Fe⁴⁺ ions. The dielectric measurements as function of temperature are studied and tree broad and relaxor phase transitions were detected: from rhombohedral to orthorhombic phase T_R-O and to tetragonal ferroelectric phase T_O-T then to cubic paraelectric phase T_m. These phase transitions were displaced to the lower temperature with increasing of Bi³⁺ substitution. The values of ε'_r increase gradually with increasing of Bi³⁺contents which confirmed the enhancement of dielectric properties of BaTi_0·80Fe_0·20O₃ by Bi substitution on Ba site. The diffuse phase transitions were described by fitting the modified Uchino relation. The Cole-Cole analyses showed that both the grain and grain boundaries resistivity values are higher for Bi³⁺ substituted samples which are responsible to the dielectric properties improvement.

Preparation of Discontinuous Cu/SiO2 Multilayers-AC Conduction and Determining the Measurement Uncertainty

This paper presents a test stand for testing alternating current electrical parameters of Cu-SiO₂ multilayer nanocomposite structures obtained by the dual-source non-reactive magnetron sputtering method (resistance, capacitance, phase shift angle, and dielectric loss angle tangent δ). In order to confirm the dielectric nature of the test structure, measurements in the temperature range from room temperature to 373 K were carried out. The alternating current frequencies in which the measurements were made ranged from 4 Hz to 7.92 MHz. To improve the implementation of measurement processes, a program was written to control the impedance meter in the MATLAB environment. Structural studies by SEM were conducted to determine the effect of annealing on multilayer nanocomposite structures. Based on the static analysis of the 4-point method of measurements, the standard uncertainty of type A was determined, and taking into account the manufacturer's recommendations regarding the technical specification, the measurement uncertainty of type B.

Investigation into Layer Number Effect on Breakdown Strength of Multi-Layer Polymer Films

The layer number effect on electric breakdown strength (E_BD) of multi-layer polymer films is investigated using 10-μm polypropylene (PP) films under a dc condition. The layer number, n, of the films during the test is as large as 120. It is observed that the relation between E_BD and n conforms to a minus power law, i.e., E_BD(n) = E_1′n^−a, where the power exponent, a, is 0.27, E_1′ is a constant. By reviewing the experimental data in references, it is found that the power law holds true for different types of polymers in different test conditions, but the value of a varies from 0.072 to 0.5. The variation of a is explained in perspective of the discontinuous structures within films and those between films. A small value of a means a good purity level of the film, which is due to the decrease of the size of the inter-layer defects. A large value of a means a poor purity level of the films, which is due to the increase of the amount of intra-layer defects. Both factors influence the value of a, leading to the variation of a.