engleski

Effect of controlled crystallization on the ionic and electronic conductivity in phosphate glasses

Glass-ceramics are advanced functional materials that contain crystalline phase(s) embedded in a glass matrix. Their electrical properties can be modified by tuning the nature and amount of phase(s) that crystallize in the glass upon the heat-treatment. This study shows how crystallization influences the mobility of electrons(polarons) and sodium ions in binary WO3- P2O5 and Na2O-WO3-P2O5 glasses, respectively. The parent 60WO3-40P2O5 and 40Na2O-50WO3-10P2O5 (in mol%) glasses were prepared by conventional melt-quenching technique and were subsequently heat-treated at temperatures above their glass transition temperature and at their crystallization temperature (700 °C, 800 °C and 935 °C for 60WO3-40P2O5 glass and 400 °C and 490 °C for 40Na2O-50WO3-10P2O5 glass) for various times (from 1 to 24 hours). The electrical properties of prepared glass-ceramics were investigated by impedance spectroscopy in a wide frequency (0.01 Hz – 1 MHz) and temperature (-30 °C – 250 °C) range while structural properties were evaluated by X-ray powder diffraction (XRPD) and SEM-EDS analysis. In the case of 60WO3-40P2O5 glass, heat-treatments from 700 °C to 935 °C induced evolution of structure from amorphous to the structures with dominant W2O3(PO4)2 and minor WO3 crystalline phases. The electrical conductivity of amorphous sample prepared at 700 °C decreased in comparison to the parent glass due to the rearrangement of the glass network upon heating which accompanies breaking of the bonds in the tungsten clusters which consequently impedes transport of polarons Nevertheless, with further increase in heat-treatment time and temperature a significant rise in conductivity was observed as a consequence of crystallization of conductive W2O3(PO4)2 and WO3 phases. The highest conductivity was obtained for glass-ceramic prepared at 935 °C for 24h where rapid electron transport occurs through WO3 crystallites. On the other hand, 40Na2O-50WO3-10P2O5 sample heat- treated at 400 °C remained amorphous with similar ionic conductivity as the parent glass, whereas treatment at 490 °C resulted in partially crystallized sample with slightly lower conductivity. Here, the glass-ceramic was found to contain Na2W2O7 crystallites embedded in amorphous glassy matrix. In the crystalline Na2W2O7 phase, sodium ions move between interstitial positions of corner-sharing WO6 octahedral and WO4 tetrahedral due to which their mobility is slower and hence overall ionic conductivity of the glass-ceramic lower. Therefore, in contrast to the electronically conductive binary tungsten- phosphate glass in which the crystallization produces a significant increase in conductivity, the crystallization processes in sodium tungsten- phosphate glass have a negative effect on the mobility of sodium ions causing a decrease in ionic conductivity in glass-ceramics.

glass-ceramics ; electrical properties

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