Post annealing effects and in situ monitoring of the phase transitions in CdBr2 powders رسالة ماجستير

Abstract

In this thesis, we focused on studying the in situ heating and post annealing effects, on the structural, optical and electrical properties of CdBr2 powders. The powders were subjected to an in situ heating processes in the temperature range of 20-200 °C. For these samples, before heating, multistructural phases existed. The most dominant structural phase was the triclinic. The heating cycle resulted in a relaxation of the crystal structural parameters and changed the dominant phase from triclinic to hexagonal. The structure transfer is accompanied with changes in crystallite size, strain, defect density and increased the stacking faults percentage. On the other hand, when CdBr2 powders were heat treated at elevate temperatures of 200 °C, 300 °C and 400 °C for one hour in rough vacuum media. The structural properties were different. Namely, the phase weight of the triclinic structure decreased from 68.3% to 53.9%, 46.0% and reached 13.7% as the annealing temperature increased from 25 °C, to 200 °C, 300 °C and reaches 400 °C, respectively. The most dominant structural phase for the samples annealed at 400 °C was monoclinic. The annealing cycle altered the lattice parameters, slightly enhanced the crystallite size and the microstrain. It is also decreased the defect density without altering the stacking fault percentages. The optical investigations on the as grown and heat treated samples have shown that the transparency of the powders in the ultraviolet range increases with increasing molar concentration and increasing vi annealing temperature. The energy band gap decreased from 5.11 eV to 5.10 eV as the annealing temperature increased from 25 °C to 400 °C. The electrical measurements have shown that the annealing process increases the electrical resistance of the identical samples. While the as grown samples displayed two resonance- antiresonance peaks in the radio and microwave ranges of spectrum, the samples annealed at 200 °C and 400 °C displayed one peak in the microwave range. The higher the annealing temperature, the larger the resonance frequency. The capacitance spectra display negative values near the resonance peaks. Consistently, the conductance spectra show negative conductance effect indicating the dominant of the tunneling process in the annealed samples. Measurements of the reflection coefficient magnitude for as grown and annealed powders in the microwave range have shown the applicability of the samples as band pass filters operating above 1.5 GHz.