Bismuth niobate-based ceramic materials are of interest for embedded elements such as capacitors, resonators, and filters because they exhibit high relative dielectric permittivities and tend to be processible at temperatures in the 1000 C to 1200 C range. As part of ongoing studies of phase equilibria and structure-property relations in this class of electroceramics, the Bi2O3-Al2O3-Nb2O5 system has been determined. In contrast to published reports, we did not observe the formation of pyrochlore, despite detailed syntheses which included controlled heatings up to and above the solidus. Except for solid solutions of Al2O3 in binary Bi2O3-Nb2O5 phases (i.e. fluorite-type Bi3NbO7 and Bi5Nb3O15), no distinct ternary compounds were found in the Bi-Al-Nb-O system. Al2O3 was found to substitute for Fe2O3 in the Bi-Fe-Nb-O pyrochlore up to 30 mol%, resulting in a slight decrease in the cubic unit cell volume. The presence of Al3+ modified the dielectric properties of the Bi-Fe-Nb-O pyrochlore by increasing the relative permittivity from 125 to approximately 135 (1 MHz, 225 K), and by lowering the temperature of onset of conductive effects (from 350 K to 225 K).
Citation: Journal of the American Ceramic Society
Pub Type: Journals
Bi-Al-Nb-O, Bi2O3-Al203-Nb2O5, dielectric relaxation, phase diagram, pyrochlore