Increasing Electrical Resistivity of P-Type BiFeO3 Ceramics by Hydrogen Peroxide-Assisted Hydrothermal Synthesis.

Bismuth ferrite (BiFeO3, BFO) is still widely investigated both because of the great diversity of its possible applications and from the perspective of intrinsic defect engineering in the perovskite structure. Defect control in BiFeO3 semiconductors could provide a key technology for overcoming undesirable limitations, namely, a strong leakage current, which is attributed to the presence of oxygen vacancies (VO) and Bi vacancies (VBi). Our study proposes a hydrothermal method for the reduction of the concentration of VBi during the ceramic synthesis of BiFeO3.Using hydrogen peroxide (H2O2) as part of the medium, p-type BiFeO3 ceramics characterized by their low conductivity were obtained. Hydrogen peroxide acted as the electron donor in the perovskite structure, controlling VBi in the BiFeO3 semiconductor, which caused the dielectric constant and loss to decrease along with the electrical resistivity. The reduction of Bi vacancies highlighted by a FT-IR and Mott-Schottky analysis has an expected contribution to the dielectric characteristic. A decrease in the dielectric constant (with approximately 40%) and loss (3 times) and an increase of the electrical resistivity (by 3 times) was achieved by the hydrogen peroxide-assisted hydrothermal synthesized BFO ceramics, as compared with the hydrothermal synthesized BFOs.

The Effect of Bi2O3 and Fe2O3 Impurity Phases in BiFeO3 Perovskite Materials on Some Electrical Properties in the Low-Frequency Field.

Pure bismuth ferrite (BFO) and BFO with impurity phases (Bi2O3 or Fe2O3) were synthesized by the hydrothermal method. Complex dielectric permittivity (ε) and electrical conductivity (σ) were determined by complex impedance measurements at different frequencies (200 Hz-2 MHz) and temperatures (25-290) °C. The conductivity spectrum of samples, σ(f), complies with Jonscher's universal law and the presence of impurity phases leads to a decrease in the static conductivity (σDC); this result is correlated with the increased thermal activation energy of the conduction in impure samples compared to the pure BFO sample. The conduction mechanism in BFO and the effect of impurity phases on σ and ε were analyzed considering the variable range hopping model (VRH). Based on the VRH model, the hopping length (Rh), hopping energy (Wh) and the density of states at the Fermi level (N(EF)) were determined for the first time, for these samples. In addition, from ε(T) dependence, a transition in the electronic structure of samples from a semiconductor-like to a conductor-like behavior was highlighted around 465-490 K for all samples. The results obtained are useful to explain the conduction mechanisms from samples of BFO type, offering the possibility to develop a great variety of electrical devices with novel functions.

Electric and Dielectric Properties in Low-Frequency Fields of Composites Consisting of Silicone Rubber and Al Particles for Flexible Electronic Devices.

Understanding the electrical conduction and dielectric polarization properties of elastomer-based composites is important for the design of flexible and elastic electronic devices and circuits. Five samples were manufactured by mixing silicone rubber (RTV-530) with Al particles in different volume fractions, x equal to 0%, 0.5%, 1%, 2.5% and 5.1%. Using the complex impedance measurements, the electric modulus, M, the electrical conductivity, σ, and the dielectric permittivity, ε, over the frequency range 100 Hz-200 kHz were analyzed. The electrical conductivity spectrum, σ(f), follows the Jonscher universal law and the DC conductivity of the samples, σDC, increases from 2.637·10-8 S/m to 5.725·10-8 S/m, with increasing x from, 0 to 5.1%. The conduction process was analyzed in terms of Mott's variable-range-hopping (VRH) model. The hopping distance of the charge carriers, Rh decreases with increasing x, from 7.30 nm (for x = 0) to 5.92 nm (for x = 5.1%). The frequency dependence of permittivity, ε(f) = ε'(f) - iε″(f), reveals a relaxation process with the maximum of ε″(f) shifting from 301 Hz to 385 Hz and values of ε'(f) increasing with the increase of x.

Experimental and theoretical investigations on thermal conductivity of a ferrofluid under the influence of magnetic field.

The effect of the strength and orientation of magnetic field with respect to the temperature gradient on the effective thermal conductivity [Formula: see text], in a kerosene-based ferrofluid with magnetite particles is reported. A new theoretical model to explain the experimental dependence [Formula: see text], obtained for both the parallel and perpendicular orientation of the magnetic field, relative to the temperature gradient is proposed, based on the Sillars equation (which is applied for the first time to a ferrofluid in this purpose). For computing [Formula: see text], we have considered that the particle agglomerations, arranged in field-induced microstructures, have ellipsoid forms and the ratio a/b between the major axis and the minor axis of the ellipsoid increases with increasing the magnetic field strength. Using the proposed theoretical model, we established for the first time a semi-empirical relationship between the ratio, a/b and the magnetic field, H, both for parallel and perpendicular H relative to the temperature gradient, determining then the dependence on H of [Formula: see text]. The theoretical results are in agreement with the experimental measurements. The reported results are of great practical importance and show that ferrofluids may be useful for incorporation in magnetic tuneable heat transfer devices or for other potential thermal applications.