engleski

Ionic conductivity in Nb2O3-K2O-P2O5 glasses

Niobium phosphate glasses show potentials in a variety of applications, from immobilisation of nuclear wastes to optical lenses and electrodes. Addition of alkali ions makes them ionic conductors with wide applications in electrical devices. The aim of the study was to investigate structural and electrical properties of the glasses of ternary system xNb2O5-(100-x)[45K2O-55P2O5], (10ŁxŁ50, mol%). Glasses were prepared by microwave heating mixtures of Nb2O5, KOH and NH4H2PO4 in microwave oven with nominal power of 1100 W during 30 min. The melt was quenched in air by pouring it into a steel mould to form rectangular bars which were annealed for 3 h at 773 K. The relationships between structural changes and electrical properties have been investigated by Raman spectroscopy, impedance spectroscopy (IS) and thermally stimulated polarisation current (TSPC) method. Impedance was measured in the frequency from 0.01 Hz to 3 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. The TSPC was measured in temperature range from 293 to 480 K with heating rate of 5 K min-1. The polarisation was performed by a dc bias of 100 V and the resulting current was measured in series with the sample using a electrometer. The Raman spectra of glasses show that with increasing Nb2O5 content tetrahedral phosphate groups (PO43-) are replaced by octahedral (NbO67-) groups and mixed structures are formed by O-P-O-Nb-O-Nb-O-P chains. Despite the fact that Nb2O5 is not classified as a network forming oxide, for a large Nb2O5 concentration a niobate network is created. The dc conductivity, sdc, calculated from TSPC measurements increases with increasing K2O from 23 to 45 mol%. On the other hand, very low conductivity (~ 10-12 W m-1 at room temperature) of glasses with 23 and 27 mol% of K2O is related to glass structure. It seems that mobility of potassium ions is lower in the glasses with octahedral coordinated niobium groups than in that with predominantly tetrahedral coordinate phosphate groups. The dielectric properties such as e˘(w), e˘˘(w) and sac and their variation with frequency and temperature indicate an increase in relaxation intensity with increase in the concentration of K2O.

phosphate glasses; Raman spectroscopy; ionic conductivity

nije evidentirano