Search Results (4)

CuMn_1−xMg_xO₂ (x = 0–0.06) polycrystalline samples were prepared using the hydrothermal method at T = 100 °C for 24 h in Teflon-line stainless steel autoclaves. The samples were crystallized, forming crednerite structures (C2/m space group), and the Mg²⁺ substitution onto the Mn³⁺ site induced small changes in the unit cell parameters and volume. Based on complex impedance measurements made between 20 Hz and 2 MHz, at different concentrations of Mg ions (x), the electrical conductivity (σ), the electric modulus (M), and the complex dielectric permittivity (ε) were determined. The conductivity spectrum, σ(f, x), follows the Jonscher universal law and enables the determination of the static conductivity (σ_DC) of the samples. The results showed that, when increasing the concentration x from 0 to 6%, σ_DC varied from 15.36 × 10⁻⁵ S/m to 16.42 × 10⁻⁵ S/m, with a minimum of 4.85 × 10⁻⁵ S/m found at a concentration of x = 4%. Using variable range hopping (VRH) and correlated barrier hopping (CBH) theoretical models, the electrical mechanism in the samples was explained. The band gap energy (W_m), charge carrier mobility (μ), number density (N_C) of effective charge carriers, and hopping frequency (ω_h) were evaluated at different concentrations (x) of substitution with Mg. In addition, using measurements of the temperature dependence of σ_DC(T) between 300 and 400 K, the thermal activation energy (E_A) of the samples was evaluated. Additionally, the dielectric behavior of the samples was explained by the interfacial relaxation process. This knowledge of the electrical properties of the CuMn_1−xMgxO₂ (x = 0–0.06) polycrystalline crednerite is of interest for their use in photocatalytic, electronic, or other applications. Full article

The complex dielectric permittivity, ε (f, H) = ε′ (f, H) − i ε″ (f, H), in the microwave frequency range f, of (0.1–3) GHz and polarizing field values H, in the range of (0–135) kA/m, was measured for a kerosene-based ferrofluid with magnetite particles. A relaxation process attributed to interfacial type relaxation was highlighted, determining for the first time in the microwave field, the activation energy of the dielectric relaxation process in the presence of the magnetic field, E_A(H), in relation to the activation energy in zero field, E_A(H = 0). Based on the complex permittivity measurements and the Claussius–Mossotti equation, the dependencies on frequency (f), and magnetic field (H), of the polarizability (α) and electrical conductivity (σ), were determined. From the dependence of α(f,H), the electric dipolar moment, p, of the particles in the ferrofluid, was determined. The conductivity spectrum, σ(f,H), was found to be in agreement with Jonscher’s universal law and the electrical conduction mechanism in the ferrofluid was explained using both Mott’s VRH (variable range hopping) model and CBH (correlated barrier hopping) model. Based on these models and conductivity measurements, the hopping distance, R_h, of the charge carriers and the maximum barrier height, W_m, for the investigated ferrofluid was determined for the first time in the microwave field. Knowledge of these electrical properties of the ferrofluid in the microwave field is useful for explaining the mechanisms of polarization and control of electrical conductivity with an external magnetic field, in order to use ferrofluids in various technological applications in microwave field. Full article

Two ceramic samples of sodium tantalate (NaTaO₃), doped with metal ions of copper (Cu; sample S1) or aluminum (Al; sample S2), were obtained by the sol-gel method. Complex impedance measurements in the frequency range (200 Hz–2 MHz) and at temperatures between 30 °C and 90 °C allowed identification of a transition temperature from semiconductor-type behavior to conductor-type behavior for each sample (52 °C for sample S1 and 54 °C for sample S2). In the temperature range with semiconductor behavior, the activation energy of each sample was determined. Based on the Mott’s variable-range hopping (VRH) model, the density of localized states at the Fermi level, N(E_F), the hopping distance (R) and the hopping energy (W) were determined, for the first time, on NaTaO₃ samples doped with Cu or Al metal ions. The increase in N(E_F) of sample S2 compared to N(E_F) of sample S1 was explained by the decrease in the hopping distance of charge carriers in sample S2 compared to that in sample S1. Additionally, using the correlated barrier hopping (CBH) model, the energy band gap (W_m) and the hopping (crossover) frequency (ω_h) at various temperatures were determined. Knowledge of these electrical properties is very important for explaining the electrical conduction mechanisms in metal ion-doped compounds, with perovskite structure being of interest for the use of these materials in the conversion of thermoelectric energy, photocatalytic applications, electronics or other applications. Full article

Two samples of Zn_xMn_1−xFe₂O₄ (x = 0, sample A; and x = 0.4, sample B) were synthesized by the hydrothermal method. From complex impedance measurements in the range 100 Hz–2 MHz and for temperatures T between 30 and 130 °C, the barrier energy between localized states ΔE_relax was determined for the first time in these samples. For sample B, a single value of ΔE_relax was highlighted (0.221 eV), whilst, for sample A, two values were obtained (0.012 eV and 0.283 eV, below 85 °C and above 85 °C, respectively), associated with two zones of different conductivities. Using the Mott’s VRH model and the CBH model, we determined for the first time both the bandgap energy barrier (W_m) and the hopping (crossover) frequency (ω_h), at various temperatures. The results show that, for sample A, W_m has a maximum equal to 0.72 eV at a temperature between 70 and 80 °C, whilst, for sample B, W_m has a minimum equal to 0.28 eV at a temperature of 60 °C, the results being in good agreement with the temperature dependence of the static conductivity σ_DC(T) of the samples. By evaluating σ_DC and eliminating the conduction losses, we identified, using a novel approach, a dielectric relaxation phenomenon in the samples, characterized by the activation energy E_A,rel. At various temperatures, we determined E_A,rel, which ranged from 0.195 eV to 0.77 eV. These results are important, as understanding these electrical properties is crucial to various applications, especially in technologies where temperature variation is significant. Full article