Thermochemistry of glass forming Y-substituted Sr-analogues of titanite (SrTiSiO 5 )
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Strontium titanium silicates are possible oxide forms for immobilization of short lived fission products in radioactive waste. Through beta decay, strontium decays to yttrium, and then to zirconium. Therefore, not only the stability of Sr-loaded waste forms, but also that of a potential decay product series with charge-balance in a naturally occurring mineral or a ceramic is of fundamental importance. Strontium titanosilicate (SrTiSiO5) is the Sr-analogue of titanite (CaTiSiO5). To incorporate the reaction 3Sr2+ = 2Y3+ + vacancy in the titanite composition, Y-substituted Sr-analogues of titanite, (Sr1–xY2/3x) TiSiO5 (x = 0, 0.25, 0.5, 0.75) were prepared by high temperature synthesis and were found to form glass upon cooling. The Y-end-member (Y2/3TiSiO5, x = 1) crystallized to a mixture of Y2Ti2O7, TiO2, and SiO2 upon quenching in air. The enthalpies of formation of Y-substituted Sr-titanite glasses were obtained from drop solution calorimetry in a molten lead borate (2PbOB2O3) solvent at 702 C. The enthalpies of formation from constituent oxides are exothermic but become less so with increasing Y content. The thermodynamic stability of the Y-substituted Sr-analogue of crystalline titanite may become marginal with increasing yttrium content. I. INTRODUCTION
Nuclear waste from reactors contains varying quantities of radioactive nuclides with relatively short half lives of 30 yr, namely 137Cs and 90Sr, in addition to the actinides. Development of Cs/Sr-loaded waste forms is of current interest because of the high radiation levels and emission of b and g–radiation associated with the decay of Cs and Sr and because of proposed schemes to separate the short lived isotopes and actinides into two different waste streams.1–3 Titanosilicates (TS, also referred to as ST, silicotitanates) as waste forms for Cs and Sr are attractive due to their successful function as selective ion exchangers and their chemical, mechanical, and structural stability comparable to that of borosilicates or aluminosilicates.4–9 Processing schemes to utilize Cs/Sr-loaded titanosilicate waste forms resemble those already well-developed to produce Synroc.10 Most studies related to the Cs/Sr-loaded titanosilicate waste forms have focused on their structure and selectivity.11–15 What is still lacking in this area is a fundamental understanding of the thermodynamics of the titanosilicate waste forms. Moreover, the relations among crystalline and amorphous or glassy states for waste forms are important due to the possibility of producing material instability through amorphization from radiation damage and during vitrification.16–19 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0413
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J. Mater. Res., Vol. 24, No. 11, Nov 2009
Strontium titanosilicate (SrTiSiO5) is the Sr-analogue of titanite (CaTiSiO5). Titanite (also called sphene) has been developed as a material for immobilizing highlevel nuclear fuel recycling wastes because it is a common accessory and leach-resistant mineral.20–22 It
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