Effects of modifier additions on the thermal properties, chemical durability,\ud oxidation state and structure of iron phosphate glasses

摘要

Modified iron phosphate glasses have been prepared with nominal molar compositions [(1�x)�(0.6P2O5–\ud0.4Fe2O3)]�xRySO4, where x = 0–0.5 in increments of 0.1 and R = Li, Na, K, Mg, Ca, Ba, or Pb and y = 1 or 2. In\udmost cases the vast majority or all of the sulfate volatalizes and quarternary P2O5–Fe2O3–FeO–RyOz\udglasses or partially crystalline materials are formed. Here we have characterized the structure, thermal\udproperties, chemical durability and redox state of these materials. Raman spectroscopy indicates that\udincreasing modifier oxide additions result in depolymerization of the phosphate network such that the\udaverage value of i, the number of bridging oxygens per –(PO4)– tetrahedron, and expressed as Qi,\uddecreases. Differences have been observed between the structural effects of different modifier types\udbut these are secondary to the amount of modifier added. Alkali additions have little effect on density;\udslightly increasing Tg and Td; increasing a and Tliq; and promoting bulk crystallization at temperatures\udof 600–700 �C. Additions of divalent cations increase density, a, Tg, Td, Tliq and promote bulk crystallization\udat temperatures of 700–800 �C. Overall the addition of divalent cations has a less deleterious effect\udon glass stability than alkali additions. 57Fe Mössbauer spectroscopy confirms that iron is present as Fe2+\udand Fe3+ ions which primarily occupy distorted octahedral sites. This is consistent with accepted structural\udmodels for iron phosphate glasses. The iron redox ratio, Fe2+/RFe, has a value of 0.13–0.29 for the\udglasses studied. The base glass exhibits a very low aqueous leach rate when measured by Product Consistency\udTest B, a standard durability test for nuclear waste glasses. The addition of high quantities of\udalkali oxide (30–40 mol% R2O) to the base glass increases leach rates, but only to levels comparable with\udthose measured for a commercial soda-lime-silica glass and for a surrogate nuclear waste-loaded borosilicate\udglass. Divalent cation additions decrease aqueous leach rates and large additions (30–50 mol%\udRO) provide exceptionally low leach rates that are 2–3 orders of magnitude lower than have been measured\udfor the surrogate waste-loaded borosilicate glass. The P2O5–Fe2O3–FeO–BaO glasses reported here\udshow particular promise as they are ultra-durable, thermally stable, low-melting glasses with a large\udglass-forming compositional range.