engleski

Effect of induced crystallization on electrical transport in polaronic phosphate glass-ceramics

The emergence of nanocrystals embedded in glass leads to the enhancement of properties thus opening new fields of the application and research of glass- ceramics. Polaronic iron phosphate glass of the composition 40Fe2O3-60P2O5 (mol%) was crystallized at various temperatures for various times (1, 6, 24 and 48 h). The structural changes and evolution of crystalline phases formed in glass- ceramics were studied by X-ray diffraction, Raman and Mossbauer spectroscopy, whereas microstucture was evaluated by Scanning Electron Microscopy (SEM). Depending on crystallization temperature and time three crystalline phases are found to be present in crystallized samples. Impedance spectroscopy (IS) was used for studying the electrical transport over vide temperature and frequency range. Different electrical processes present in the present glass-ceramics are identified and their contribution to overall conductivity were determined using various formalisms for presenting IS results. Two steps can be separated during crystallization processes. In the early stage of crystallization at about first crystallization temperature, TC1, the electrical conductivity decreases as a results of formation of randomly dispersed Fe3(P2O7)2 grains and a decrease of Fe2+-Fe3+ pairs concentration in glassy phase. The temperature at which the minimum in electrical conductivity appeared strongly depends on the amount of crystallinity. Above 15% of crystallinity the electrical conductivity increases as the grains of Fe3(P2O7)2 are better defined. At second crystallization temperature, TC2, the formation of second and third crystalline phase causes further increase in the electrical conductivity. Thus, it was found that the various electrical processes are present in these prepared glass-ceramics. The observed processes are resolved by the detail analysis combining various formalisms for determining IS experimental data. On the other hand, although the total electrical conductivity is higher above TC2, the grain boundary contribution is low due to a large grain boundary resistance. It seems that the imperfect crystalline grain contacts in these polycrystalline samples have an effect on the grain boundary transport.

Glass-ceramics ; Electrical transport ; Structure ; Crystallization

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