[en] Nucleation and growth of crystal in an oxide glass was studied in a Si B Al Zr Nd Ca Na O system. The nucleation and growth process were monitored by thermal analysis and isothermal experiments. The effect of the network modifier was studied. Therefore for a Ca rich sample the crystallization is homogeneous in the bulk showing a slow increase of crystallinity as temperature increases. On the other hand, a Na rich sample undergoes several crystallization processes in the bulk or from the surface, leading to bigger crystals. The activation energy of the viscous flow and the glass transition are of same magnitude when that of crystallization is a lot smaller. Early diffusion of element is done with a mechanism different than the configurational rearrangements of the liquid sate. The global density and small size of the crystals within the Ca rich matrix confirmed that it would be a profitable waste form for minor actinides. (authors)

[en] The ease with which a liquid adjusts to the shape of its container is a well-known consequence of the hallmark of the molten state, atomic mobility. Atomic mobility is the very reason why liquids flow, even though another salient feature evident through daily experience is that the viscosity increases when the temperature decreases. In fact, if crystallization does not occur, the viscosity eventually becomes so high that flow can no longer take place during the timescale of an experiment. The resulting material is a glass, i.e., a solid with the frozen-in disordered atomic arrangement of a liquid. Glasses have been produced for millennia, but the kinetic nature of the liquid-glass transition and its influence on the properties of glasses have long remained elusive. We will not specifically address these aspects, however, because they have already been extensively discussed in the geochemical literature from a relaxational (Dingweil and Webb, 1990) or thermochemical standpoint (Richet and Bottinga, 1983, 1986). In this review, we will focus on features of liquids that are directly related to atomic mobility, namely, the existence of those contributions to physical properties of liquids that have been termed configurational (Simon, 1931; Bernal, 1936)

[en] High-temperature XANES experiments at the Fe K-edge have been used to study the kinetics of iron oxidation in a supercooled melt of Fe-bearing pyroxene composition. These experiments, made just above the glass transition between 600 and 700 deg C, show that variations in relative abundances of ferric and ferrous iron can be determined in situ at such temperatures. The kinetics of iron oxidation do not vary much with temperature down to the glass transition. This suggests that rate-limiting factor in this process is not oxygen diffusion, which is coupled to relaxation of the silicate network, but diffusion of network modifying cations along with a counter flux of electrons. To give a firmer basis to redox determinations made from XANES spectroscopy, the redox state of a series of a samples was first determined from wet chemical, Moessbauer spectroscopy and electron microprobe analyses. (authors)

[en] New nuclear glass compositions, able to immobilize highly active liquid wastes arising from high burn-up UO₂ fuel reprocessing, are being studied. Investigations are being performed on rare earth rich glasses, known as durable matrices. After a preliminary study, a basic glass composition was selected (Glass A, wt. %): 51.0 SiO₂ - 8.5 B₂O₃ - 12.2 Na₂O - 4.3 Al₂O₃ - 4.8 CaO - 3.2 ZrO₂ - 16.0 Nd₂O₃. The aim of this study is to determine the local environment of the rare earth in this glass and its evolution according to neodymium. To achieve this objective, glasses were prepared from the baseline Glass A with variable neodymium oxide amounts (from 0 to 30 wt. % Nd₂O₃). By coupling characterization methods such as EXAFS (Extended X-Ray Absorption Fine Structure) spectroscopy at the neodymium LIII-edge, optical absorption spectroscopy, ¹¹B, ²⁷Al MAS-NMR and Raman spectroscopy, pieces of information the rare earth surroundings in the glass were obtained. (authors)

[en] Understanding of the stability and weathering of glasses used for storing fission products is hampered by a general lack of basic thermochemical information. Models have been setup to predict Gibbs free energies of dissolution of glasses, but ascertaining their accuracy is made difficult by the very lack of reliable experimental data with which model results should be compared. As enthalpies of formation can in principle be determined from usual solution calorimetry experiments, the lack of Gibbs-free energy data for glasses mainly stems from the fact that, as disordered substances, glasses do not obey the third principle and have indeed large configurational entropies. These entropies can be determined from thermochemical measurements only when there exist a congruently melting crystalline compound with the same composition. Using available data, we have calculated the Gibbs-free energies of formation of a series of silicate glasses for which such a calorimetric determination is possible. With these results, we assess the predictions of Paul's model (1977) for calculating Gibbs-free energies of dissolution. As the complex compositions of the borosilicate glasses used for nuclear waste storage prevent determining configurational entropies by calorimetric methods, we point out how these can be determined instead from viscosity measurements. We finally discuss the implications of this approach for modeling of water-glass interactions. (authors)

[en] Within the framework of structural studies of borosilicate glasses, a revision of the Yun, Bray and Dell (YBD) model is proposed here. The YBD model is slightly modified in order to take into account more recent experimental results, and in particular the formation of Na₂O. B₂O₃.2SiO₂ danburite-like units in borosilicate glasses. The following more general formula is proposed for R*, the threshold value of R (R = [Na₂O]/[B₂O₃]), beyond which glass depolymerisation begins upon addition of modifier to the glass: R* = 0.5 + K/2N where N is the average number of SiO₂ units included in the borosilicate groups (reedmergnerite and danburite-like), and K = [SiO₂]/[B₂O₃]. (authors)

[en] This study is focused on the behavior of ternary SiO₂-Na₂O-B₂O₃ borosilicate glasses at temperatures between 298 and 1800 K. Unpolarized Raman spectra were measured up to high temperature. SiO₂-Na₂O-B₂O₃ glass samples were prepared with different values of the ratio R [Na₂O]/[B₂O₃], while the ratio K = [SiO₂]/[B₂O₃] was kept constant and equal 2.12. Spectra were measured at room temperature in samples with 0.43 ≤ R ≤ 1.68, and the effect of the modifier content was clearly observed in these glasses, only in partial agreement with previous literature results. In particular, the formation in the glass of sodium-danburite units Na₂O-B₂O₃-2SiO₂ was postulated. This feature led to a new assessment of R^*, the critical value of R above which every new alkali atom added to the system breaks a Fo-O-Fo (Fo=glass former) bridge causing depolymerization of the glass. A revised formula is proposed to obtain the value of R^* as a function of K. Raman spectra measured at high temperature yielded important information about the temperature-dependent evolution of the borosilicate system. In particular, borate and borosilicate units including tetra-coordinated boron seem to be unstable at high temperature, where the formation of metaborate chains or rings is fostered. Above 1500 degrees C, evaporation of borate compounds is clearly observed, stemming from the small sample size. (authors)

[en] The presence of sulfur in radioactive waste to be incorporated in borosilicate glasses entails difficulties mainly due to the relatively low solubility of sulfates in the vitreous phase. In this work a study is presented on the effects of the ratio R = [Na₂O]/[B₂O₃], the type of sulfate added and the addition Of V₂O₅ on the incorporation of sulfates in borosilicate glasses. Glass samples were prepared at the laboratory scale (up to 50-100 g) by melting oxide and sulfate powders under air in Pt/Au crucibles. XRF and ICP/AES chemical analysis, SEM/EDS, microprobe WDS and Raman spectroscopy were employed to characterize the fabricated samples. The main experimental results confirm that the incorporation of sulfates in borosilicate glasses is favored by the network depolymerization, which evolves with the ratio R. The addition Of V₂O₅ seems to accelerate the kinetics of sulfur incorporation in the glass and, probably, increase the sulfate solubility by modifying the borate network and fostering the formation of voids of shape and size compatible with the sulfur coordination polyhedron in the glassy network. The kinetics of X₂SO₄ incorporation in the glass seems to be slower when X = Cs. (authors)

[en] The structure of iron-bearing sodium borosilicate glasses with up to 10 mol% FeO has been investigated in the range 0.15 ≤ Fe³⁺/SFe ≤ 0.95. According to Moessbauer spectroscopy, Fe³⁺ and Fe²⁺ are mainly in tetrahedral and octahedral coordination, respectively, although other coordination states exist for both cations. From XANES experiments, we conclude that increasing Fe content and varying redox states have only a minor effect on the relative proportions of BO₃ and BO₄ units. In Raman spectra, a decrease of the proportion of BO₄ species present in danburite-like units (Na₂O.B₂O₃.2SiO₂) is found upon increasing iron content and oxidizing state. Whereas the insensitivity of the overall boron speciation to iron content and redox state points to weak interactions between boron and iron, the changes affecting BO₄ species do indicate a more subtle interplay between Fe³⁺ and the other tetrahedrally coordinated cations (Si,B) because of the competition between tetrahedral Fe³⁺ and B³⁺ for charge compensation by Na⁺. (authors)

[en] Sulphur is present in several kinds of nuclear waste destined to confinement in a glass matrix. These species are hardly incorporated in borosilicate glasses, due to the poor miscibility of molten sulphates and glass melt at high temperature. This point constitutes one of the main technological difficulties in the current fabrication of nuclear glasses. Several studies carried out in the last two decades indicate that the liquid state miscibility of sulphates and glass melt, determining the waste incorporation in the final glass, is dependent on the composition of the glass and on the temperature and duration of its preparation. In particular, the ratios K = [SiO₂] / [B₂O₃] and R = [Na₂O] / [B₂O₃] (where [.] indicates the mol %), as well as the presence of V₂O₅ in the glass matrix play an important role in the determination of both kinetics and thermodynamics of sulphates incorporation in the vitreous matrix. In this work a study on the physico-chemical effects of the ratio R, the melt reactivity, the type of sulphate added, and the presence of V₂O₅ on the solubility of sulphates in borosilico-vanadate glasses is presented. Glass samples were prepared at the laboratory scale (up to 50-100 g) by melting oxide and sulphate powders in Pt/Au or Pt/Rh crucibles at a fixed viscosity level of 100 Po in air. Several glass specimens were powdered and re-melted at 1200 deg. C in the presence of sulphates in order to simulate conditions as close as possible to those of an industrial melter. XRF and ICP AES chemical analysis, SEM EDS and Raman spectroscopy were employed to characterise the fabricated samples. Raman spectra, in particular, reveal the structural modifications that condition the sulphate incorporation in the glass. A critical review of the obtained results is compared with the literature data, to yield a more systematic description of the mentioned factors effects on sulphate behaviour in the borosilico-vanadate / system. (author)