Effect of melting temperature and time on iron valence and crystallizationof iron phosphate glasses

The effect of melting temperature and time on iron valence, dissolution rat e (DR) in deionized water, and crystallization of iron phosphate glasses wa s investigated using a 40Fe(2)O(3)-60P(2)O(5), mol%, batch composition. The concentration of Fe2+ ions in these glasses increased from 17% to 57% as m elting temperature increased from 1150 degrees C to 1450 degrees C, but rem ained nearly constant at about 20% for melting times longer than 1 h at 120 0 degrees C. Measurements by differential thermal analysis (DTA) combined w ith X-rap diffraction (XRD) and thermogravimetric analysis (TGA) showed tha t these glasses crystallized to Fe-3(P2O7)(2) and Fe-4(P2O7)(3) when heated in nitrogen between 600 degrees C and 820 degrees C, but with continued he ating in air at 820 degrees C the Fe-3(P2O7)(2) changed to Fe(PO4), which p roduced a weight gain in the sample associated with the oxidation of Fe2+ t o Fe3+ ions. The DR (in deionized water) of these glasses was generally ver y low (similar to 10(-9) g cm(-2) min(-1)) and nearly independent of the re lative concentration of Fe2+ or Fe3+ ions, but decreased with total iron co ntent. (C) 1999 Elsevier Science B.V. All rights reserved.