engleski

Controlled crystallization of tungsten phosphate glass - a pathway towards highly conductive glass- ceramics

In the last decades, various families of oxide glasses have been investigated as potential components in solid-state electrical devices. However, despite numerous advantages which glassy materials offer, the limiting factor for their application is usually low electrical conductivity. A versatile approach for improving electrical performance of glasses is thermally induced crystallization which modifies properties by tuning the nature and amount of crystalline phase(s) as well as (micro)structure. The goal of this study was to correlate electrical and structural changes which occur upon heat-treatment of WO3-P2O5 glass at different conditions and to prepare highly electronically conductive glass- ceramics. The parent 60WO3-40P2O5 (in mol%) glass was prepared by conventional melt-quenching technique and heat-treated at 700 °C, 800 °C and 935 °C for various times (from 1 to 24 hours). The electrical properties of prepared glass-ceramics were investigated by impedance spectroscopy in a wide frequency and temperature range while structural properties were evaluated by X-ray powder diffraction (XRPD) and SEM-EDS analysis. Upon heat-treatment at 700 °C, the material retains fully amorphous structure, however with two types types of grains ; the large ones with the composition of the parent glass and the small ones which act as precursors for crystallization of W2O3(PO4)2. At higher heat-treatment temperatures, the precursor grains evolve in large prismatic crystallites of W2O3(PO4)2 which become a dominant phase in all glass-ceramics. Along with W2O3(PO4)2 crystallites, the glass-ceramics prepared at 800 and 935 °C contain WO3 nanocrystallites which increase in amount with heat-treatment temperature and time and reach. The electrical conductivity of prepared glass-ceramics is electronic in nature and shows a strong dependence on the evolution of crystalline phases. The samples prepared at 700 °C exhibit lower electrical conductivity than the parent bulk glass due to the glass network rearrangement and breakage of the tungsten clusters that form easy pathways for polaronic conduction. However, a faster electronic transport can be seen in the samples prepared at higher temperatures owning to the formation W2O3(PO4)2 and WO3 crystalline phases. Moreover, highly conductive glass-ceramics were obtained at 935 °C where the increase of semiconducting WO3 nanocrystallites enhances the conduction of the glass-ceramics to the order of ~10–4 (ohms-cm)–1 at 30 °C.

glass-ceramics ; electrical properties

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