Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield,
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DOI:
10.7502/j.issn.1674-3962.2016.07.03
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Abstract:
MoO3 is one of the challenging oxides in nuclear waste vitrification in the UK. It has a poor solubility in the conventionally used nuclear waste glasses and its excess presence may cause the formation of “yellow phase” which is highly detrimental to vitrification process. This work investigates the compositional dependence of MoO3 solubility in borosilicate glasses with varying alkaline earth species as well as the effects of MoO3 addition on glass structure and properties. Among all alkaline earths Ca is the best in terms of MoO3 solubility, with 2.84 mol% in calcium borosilicate glass (CBS) without causing any visible phase separation. Magnesium borosilicate glass (MBS) has the lowest MoO3 solubility (≤1 mol%). XRD results indicate that, while the visibly homogeneous glasses all remain amorphous nature, the visibly heterogeneous glasses contain tiny molybdate crystals. According to SEM results, the separated particles forming within glass matrices are mostly spherical, submicron in diameter and randomly dispersed; the size of these particles are dependent on the extent to which MoO3 addition is excessive. Compositional analysis for separated phases in CBS glass with excess MoO3 suggests that the phases are rich in Mo and Ca, while TEM results prove the crystallinity of separated particles whose electron diffraction patterns are in accordance with those of CaMoO4. Two Raman bands are observed due to addition of MoO3 in glass; the bands are assigned to bending and stretching vibrations of MoO42– and their intensities increase with MoO3 addition. The positions of these bands shift with alkaline earth species in glass, indicating that the local environment of MoO42– is closely associated with alkaline earth cations in glass network. MoO3 addition to glass also results in decreased glass transition temperature Tg, which suggests a depolymerized network due to MoO42– incorporation. Glass density is increased with the increase amount of MoO3 in glass as long as the glass remains homogeneous.