Microstructure and electrical conduction of iron-doped barium titanate glass-ceramics

Glasses with the mol % composition 20.1Na(2)O/23TiO(2)/23.1BaO/3Al(2)O(3)/7.6B(2)O(3)/17.4SiO(2)/5.8Fe(2)O(3) were synthesized. During heat treatment at 550 degrees C for 5 min to 7 h, cubic BaTiO3 crystallized. The microstructure of the glass-ceramics was studied by scanning electron microscopy. The average size of the crystalline particles varied from 190 to 790nm, depending on the crystallization time. Impedance measurements of selected glass-ceramics at temperatures from-183 to 97 degrees C using frequencies from 100Hz to 1MHz witness insulating proper-ties. The ac-conductivity at 100Hz increases with temperature from 10(-10) S/m at -183 degrees C to 10(-6) S/m at 97 degrees C. The ac-conductivity is established predominantly in the glass phase with Na+-ions as charge carriers and could be well fitted by the universal as well as by the modified Jonscher power law. However, for the intermediate temperature range, the modified Jonscher equation seems to better explain the physical mechanisms of conduction dispersion with frequency. In the whole studied temperature range, the power "s" in the dispersion equation slightly varies with the glass-ceramics? thermal history but shows a relatively large decrease with increasing temperature. The higher conductivity of the sample with the highest BaTiO3 volume fraction is associated with the lowest s-value and largest number of non-bridging oxygen ions in the structure.