Giant electrical conductivity enhancement in BaO-V2O5-Bi2O3 glass by nanocrystallization

The effects of the annealing of 20BaO-30V(2)O(5)-50Bi(2)O(3) glass on the structural and electrical properties were studied by scanning electron micrographs (SEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC) density (d) and dc conductivity (sigma). The XRD and SEM observations have shown that the sample under study undergoes structural changes: from amorphous at the beginning, to partly crystalline after nanocrystallization at crystallization temperature (T-c) for I h and to colossal crystallization after the annealing at the same temperature for 24 h. The average size of these grains after nanocrystallization at T-c for I h was estimated to be about 25-35 nm. However, the glass heat treated at T-c = 580 degrees C for 24 h the microstructure changes considerably. The nanomaterials obtained by nanocrystallization at T-c for I h exhibit giant improvement of electrical conductivity up to four order of magnitude and better thermal stability than the as-received glass. The major role in the conductivity enhancement of this nanomaterial is played by the developed interfacial regions "conduction tissue" between crystalline and amorphous phases, in which the concentration of V4+-V5+ pairs responsible for electron hopping is higher than inside the glassy matrix. The annealing at T-c for 24 h leads to decrease of the electronic conductivity. This phenomena lead to disappearance of the above mentioned "conduction tissue" for electrons and substantial reduction of electronic conductivity. The high temperature (above theta/2) dependence of conductivity could be qualitatively explained by the small polaron hopping (SPH) model. The physical parameters obtained from the best fits of this model are found reasonable and consistent with the glass compositions. (C) 2009 Elsevier B.V. All rights reserved.