| Popis: |
The nonlinear high temperature impedance of iron- phosphate glasses doped with niobium oxide was measured. Glass samples of the composition of 35P2O5-30Fe2O3-35Nb2O5 (in %mol) were prepared by the conventional melt quenching technique. Appropriate amounts of reagents ((NH4)2HPO4 (≥99.9%, POCH), Fe2O3 (≥99.9%, POCH) and Nb2O5 (≥99.9%, PLUKA AG) were thoroughly mixed in an agate mortar and put on electric stove in temperature of 573K in order to carry out the decomposition process of (NH4)2HPO4. Next powders were melted in porcelain crucibles at 1573 K for 1-2 h. The melts were poured on preheated (to 573 K) brass plate and pressed by another plate to obtain flat circular samples (dimensions: 1-2 mm thick, 20-30 mm in diameter). The obtained pellets were annealed at 673 K for 1 h and cooled with furnace. For the electrical measurements gold electrodes were evaporated at the preheated samples. Nonlinear Impedance measurements were carried out in the the temperature range from 363 K to 723 K, with the ac voltage of 1 Vrms with Concept 40 broadband dielectric spectrometer. The higher harmonic components (harmonic 0, harmonic1 and harmonic2) were measured up to frequency of 1000 Hz. Here the impedance for harmonic components was defined as the ratio of the voltage base wave to the n-th harmonic current component: Zn∗= U0∗/In∗, where Zn⁎ including the base wave generally depend on the sample voltage U1⁎ base wave amplitude. From Zn⁎ allother independent variables are calculated. The dependence of current density on the cosinusoidal electric field E(t)= E0cos(ωt) leads to the following expression |
