engleski

Impedance spectroscopy of PbO-Fe2O3-P2O5 glasses

Introduction Lead-free iron phosphate glasses melted in air are electronically conducting glasses where the polaronic conduction is due to electron hopping from low to high iron valence state [1]. In these glasses, the charge carrier concentration is related to the total concentration of iron ions and to the ratio of Fe(II) ions to the total quantity of iron (Fe(II)/Fetot). The aim of this study was to investigate structural and electrical properties of (43.3-x)PbO-(13.7+x)Fe2O3-43P2O5, (0 x 30) glasses. The contribution of iron ions to the total electrical conductivity and the effect of glass crystallization on the origin of electrical conductivity have been examined. Experiments The glasses were prepared from the appropriate mixtures of reagent grade NH4H2PO4, Fe2O3 and PbO and melted between 1373 K and 1473 K for 2 h in air in high purity alumina crucibles. The melt was quenched in air by pouring it into a 1 x 1 x 5 cm3 steel mold. The samples were transferred to a furnace and annealed at 723 K for 3 h. The structural changes in glasses were investigated by Raman, while, electrical properties have been measured by impedance spectroscopy in the frequency range from 0.01 Hz to 4 MHz and temperature range from 303 to 473 K. The impedance spectra were analyzed numerically by means of electrical equivalent circuit modelling, using complex non-linear least square procedure. Results and Discussion The Raman spectra of the glasses show predominantly pyrophosphate structure and high tendency for crystallization with increasing Fe2O3 content  33.7 mol%. The changes in the glass network and glass stability against crystallization are followed by characteristic features of impedance spectra. The impedance spectra of glass samples indicate single conduction mechanism attributed to electron hopping from Fe(II) to Fe(III) ions. Consequently, electronic conductivity increases with increasing Fe2O3 content and Fe(II)/Fetot ratio in glasses. On the other hand, in crystallized compositions, electrical properties are strongly affected by microstructure. The impedance spectra contain features related to microstructure: interior of the grains (crystalline phase) and grain boundaries (glassy phase). Equivalent circuit modelling is used to separate grains from grain boundaries contribution to the electrical conductivity. With increasing Fe2O3 content in crystallized samples, electrical conductivity associated with grains increases to a constant value, whereas, for a grain boundaries decreases. This is related to enhanced tendency for crystallization, which diminishes the glassy phase in compositions with high Fe2O3 content. Conclusions The impedance spectra of glass samples indicate a single conduction mechanism attributed to the electron hopping from Fe(II) to Fe(III) ions. The impedance spectra of partially crystallized samples contain features related to grains and grain boundaries and equivalent circuit modeling successfully resolves conductivity of individual phase. It was shown that electrical conductivity in this case depends strongly on microstructure. References: [1]L. Murawski et al., J. Non-Cryst. Solids 32 (1979), 91.

iron phosphate glasses; impedance; electrical conductivity

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