Universality of ac conduction and phase transitions in some thermal energy storage compounds / Wafia Mohamed Gamal Eldein Saber ; Supervised Mohamed M. Abdelkader , Aboud I. Aboud

Thesis (Ph.D.) - Cairo University - Faculty of Science - Department of Physics

In the present thesis, six thermal energy storage compounds which belong to organic-inorganic hybrid layered halide perovskites of the alkylammonium series (n-CnH₂n+₁NH₃)₂MX₄ where n=12, 14 and 16, M=Cd and Zn and X=Cl, are chosen and are synthesized in a pure polycrystalline form. For each compound (hybrid), the ac conductivity vac ({uF077},T) and the complex dielectric permittivity f*(f*({uF077},T) =f'({uF077},T) - if"({uF077},T)) as function of temperature 100K{u2264}T{u2264}400K and some selected frequencies in the range (1kHz-100kHz) were accurately determined. The differential scanning calorimetry (DSC) (heating and cooling cycles) and the differential thermal analysis (DTA) thermograms were performed at a heating rate of 5⁰/min. From the combination of the measured thermal and electrical parameters mentioned above, it is obvious that each hybrid of the Cd series undergoes two solid-solid reversible structural phase transitions at temperatures ,denoted by T ( major) and T( minor), where one of them precedes the other, in addition to some intermediate temperatures (IT) which located in between are reported. For n=12 hybrid, (IT) is absent. For C₁₂Cd, the transition temperatures are detected at T{u2248}343.13 K (major) and T{u2248}330.32 K (minor) whereas for C₁₄Cd, the transition temperatures are observed at T{u2248}351.50 K (major) and T{u2248}344.66 K (minor) in addition to an intermediate transition at Tint{u2248}346.81K. For C₁₆Cd, the transition temperatures are detected at T{u2248}350.70K (major) and T{u2248}360.70K (minor) and an intermediate transition is found at Tint{u2248}353.75K.Thus, the data suggested that no general rule for the order of minor and /or major transitions i.e. Tmajor may and may not precedes tminor

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