A novel method to produce niobium phosphate glasses by microwave heating

Niobium phosphate glasses were produced by microwave heating mixtures of Nb2O5, (NH4)(2)HPO4, KOH, PbO, and BaCO3. Not all tried compositions were melted during the microwave irradiation. Absorption of microwaves was checked for individual compounds to determine the best chemical form to be used. The presence of KOH is indispensable in the mixture to allow microwave coupling. Glasses were also prepared by melting the same mixtures in an electric furnace. Glasses produced by microwave heating shows concentration of Al lower than the ones for glasses melted in electrical furnaces because only the precursors are directly heated by the microwave energy, reducing the diffusion of elements from the crucible to the liquid. Glasses with dissolution rates in aqueous solution at 90degreesC of 8.6 x 10(-10) g . cm(-2) . min(-1) were produced. The chemical durability of glasses increases as a function of the amount of PbO. The amount of water molecules, OH- radicals, and CO2 bonded to the glass structure decreases as the amount of Nb2O5 increases. (C) 2004 Kluwer Academic Publishers.