Behaviour of ruthenium dioxide particles in borosilicate glasses and melts

Ruthenium-glass systems are formed during the vitrification of nuclear waste. They are also widely used in micro-electronics because of their unique electrical properties. However, the interaction of this element with the glass matrix remains poorly understood. This work focuses on a RuO₂ particles-nuclear alumino-borosilicate glass system in which the electrical conductivity is known to vary considerably with the RuO₂ content and to become electronic above about 0.5-0.7 vol.% RuO₂ [R. Pflieger, M. Malki, Y. Guari, J. Larionova, A. Grandjean, J. Am. Ceram. Soc., accepted for publication]. Some RuO₂ segregation was observed in SEM/TEM investigations but no continuous chain of RuO₂ particles could be seen. Electron relays between the particles are then necessary for a low-rate percolation, such as the nanoclusters suggested by Adachi et al. [K. Adachi, S. Iida, K. Hayashi, J. Mater. Res. 9 (7) (1994) 1866; K. Adachi, H. Kuno, J. Am. Ceram. Soc. 83 (10) (2000) 2441], which could consist in dissolved ruthenium. Indeed, several observations made here clearly indicate the presence of dissolved ruthenium in the glass matrix, like the modification of the glass density in presence of RuO₂ particles or the diffusion-limited growth of RuO₂ particles in the melt.     

X-ray absorption studies of chlorine valence and local environments in borosilicate waste glasses

Chlorine (Cl) is a constituent of certain types of nuclear wastes and its presence can affect the physical and chemical properties of silicate melts and glasses developed for the immobilization of such wastes. Cl K-edge X-ray absorption spectra (XAS) were collected and analyzed to characterize the unknown Cl environments in borosilicate waste glass formulations, ranging in Cl-content from 0.23 to 0.94 wt.%. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data for the glasses show trends dependent on calcium (Ca) content. Near-edge data for the Ca-rich glasses are most similar to the Cl XANES of CaCl₂, where Cl⁻ is coordinated to three Ca atoms, while the XANES for the Ca-poor glasses are more similar to the mineral davyne, where Cl is most commonly coordinated to two Ca in one site, as well as Cl and oxygen nearest-neighbors in other sites. With increasing Ca content in the glass, Cl XANES for the glasses approach that for CaCl₂, indicating more Ca nearest-neighbors around Cl. Reliable structural information obtained from the EXAFS data for the glasses is limited, however, to ClCl, ClO, and ClNa distances; ClCa contributions could not be fit to the glass data, due to the narrow k-space range available for analysis. Structural models that best fit the glass EXAFS data include ClCl, ClO, and ClNa correlations, where ClO and ClNa distances decrease by approximately 0.16 Å as glass Ca content increases. XAS for the glasses indicates Cl⁻ is found in multiple sites where most Cl-sites have Ca neighbors, with oxygen, and possibly, Na second-nearest neighbors. EXAFS analyses suggest that ClCl environments may also exist in the glasses in minor amounts. These results are generally consistent with earlier findings for silicate glasses, where Cl⁻ was associated with Ca²⁺ and Na⁺ in network modifier sites.     

X-ray absorption studies of bismuth valence and local environments in borosilicate waste glasses

X-ray absorption spectra (XAS) were collected and analyzed to characterize bismuth (Bi) environments in borosilicate glass formulations developed for the immobilization of high level nuclear wastes (HLW), from the bismuth phosphate process. Therefore, the structural role of Bi in these glasses is of interest; in addition in the present study, more particular interest in Bi originated from unusual foaming that was observed during melt cooling, where it was initially suspected that Bi³⁺ reduction to Bi⁰ may generate oxygen that caused the foaming. Observations from scanning electron microscopy of some HLW glass samples indicated a Bi-phosphate association. Bi L_III XAS of 13 Bi-containing waste glass formulations of various compositions were measured that exhibited varying degrees of melt foaming. The Bi XAS are similar for all glasses investigated, and indicate Bi³⁺O₃ nearest-neighbor environments with Bi–O distances near 2.13 Å. This environment is similar to the most localized Bi coordination characteristics in the crystalline Bi-silicates, eulytite (Bi₄Si₃O₁₂) and bismutoferrite (BiFe₂Si₂O₈OH). However, the Bi-environments in the glasses are distinctly different from the Bi-site in crystalline BiPO₄; therefore, XAS indicates no evidence of Bi-phosphate domains in the glasses measured. No XAS evidence was observed in any of the glasses investigated for Bi clustering, such as metallic Bi, or Bi–O–Bi bonding. Since the local Bi environments look similar for all glasses investigated, Bi XAS data and analyses show no association of the melt foaming problems with changes of Bi environments in the corresponding glass. The foaming may be dependent on chromate or phosphate behavior in the glass structure.     

Elemental analysis of edible grains by micro-PIXE: Common buckwheat case study

The aim of this study was the adaptation of the micro-PIXE method for analysis of nutritionally relevant heavy elements in different tissues of the grain of common buckwheat (Fagopyrum esculentum), as a representative nutritionally interesting grain food source. At 57% of the buckwheat grain biomass, the endosperm was a modest nutrient source when compared to the cotyledons, at 17% of the biomass. These latter contained high concentrations of trace elements, representing 91% of the total grain Zn, 87% for P, 70% for S, 62% for Mg, 60% for K, 54% for Cu, 53% for Mn and 35% for Fe. The husk provided storage for 85% of the total Ca, 84% for Al, 83% for Si, 76% for Cl, 69% for Ti and 46% for Fe. Knowledge on these preferential elemental constitutions of the different grain tissues makes the possibility of designing target products with nutritionally optimal constitution more feasible. These data represent a basis for a more targeted approach to nutritional improvement of grains intended for human consumption.     

Heavy ion Rutherford backscattering spectrometry (HIRBS) study of barium diffusion in borosilicate glass

Diffusion of barium in borosilicate glasses containing sodium and calcium has been studied using heavy ion Rutherford backscattering spectrometry (HIRBS) using 30 MeV ¹⁹F beam. A thin layer (25 μg/cm²) of BaCl₂ was vacuum evaporated on the polished surface of the glass samples which were then annealed at different temperatures (573-723 K) and subsequently subjected to HIRBS measurements. It was observed that introduction of 10 mole% of calcium in place of sodium results in increase in the activation energy of diffusion of barium by about 30%. This indicates that alkaline earth ions retard the diffusion process in borosilicate glass. The results corroborate the data obtained on diffusion of sodium by radiotracer method. The results have been explained in terms of the increasing rigidity of the glass network with introduction of alkaline earth ions.     

First-principles study on dissolution and diffusion properties of hydrogen in molybdenum

Employing a first-principles method, we have investigated dissolution and diffusion properties of hydrogen (H) in molybdenum (Mo), one of potential candidates for plasma facing materials in a nuclear fusion Tokamak. We show that single H atom is energetically favorable sitting at the tetrahedral interstitial site (TIS) instead of octahedral interstitial site and diagonal interstitial site. This can be confirmed by the electron localization function result. Bader charge analysis suggests that the bonding between H and surrounding Mo is mainly ionic mixed with slight covalent component. Double H atoms tend to be paired up at the two neighboring TIS’s along the 〈1 1 0〉 direction with the distance of ∼0.221 nm and the binding energy of 0.03 eV. This suggests a weak attractive interaction between H atoms, with the implication that self-trapping of H and thus formation of the H₂ molecules are quite difficult in an intrinsic Mo environment. We demonstrate that the diffusion barrier of H that jumps between the TIS’s is 0.16 eV, and the dissolved concentration of H in the intrinsic Mo is 2.6 × 10⁻⁸ at a typical temperature of 600 K. The diffusion coefficients of H, D, and T are different due to the different masses, which are calculated to be 1.3 × 10⁻⁷ m²/s, 9.2 × 10⁻⁸ m²/s, and 7.5 × 10⁻⁸ m²/s at 600 K.     

Structural role of molybdenum in nuclear glasses: an EXAFS study

The Mo environment has been investigated in inactive nuclear glasses using extended X-ray absorption spectroscopy (XAS). Mo is present in a tetrahedron coordinated to oxygen in the form of molybdate groups [MoO₄]²⁻ (d(Mo-O)=1.78 Å). This surrounding is not affected by the presence of noble metal phases in the nuclear glass. Relying on the XAS results, on the bond-valence model and on molecular dynamics simulations of a simplified borosilicate model glass, we show that these groups are not directly linked to the borosilicate network but rather located within alkali and alkaline-earth rich domains in the glass. This specific location in the glass network is a way to understand the low solubility of Mo in glasses melted under oxidizing conditions. It also explains the possible phase separation of a yellow phase enriched in alkali molybdates in molten nuclear glasses or the nucleation of calcium molybdates during thermal aging of these glasses. Boron coordination changes in the molten and the glassy states may explain the difference in the composition of the crystalline molybdates, as they exert a direct influence on the activity of alkalis in borosilicate glasses and melts.     

An in-situ study of the dissolution of γ-zirconium hydride in zirconium

The dissolution of γ-zirconium hydride has been examined in single crystal specimens of zirconium using a hot stage in a high voltage electron microscope. It was found that a significant proportion of the dislocations generated by the hydride needles during growth were not annihilated when dissolution occurred on heating to well above the solvus temperature. This is contrary to earlier work where similar experiments were carried out with thinner specimens at conventional accelerating voltages. Electron irradiation completely prevented annihilation of the hydride dislocations during dissolution. The results are discussed in relation to (a) repeated nucleation at the same sites during thermal cycling, (b) external shape changes (growth), (c) strain steps observed during thermal cycling under creep conditions, (d) positron annihilation experiments, and (e) the terminal solid solubility of hydrogen in zirconium.     

EXAFS study on Ag-doped silicate glasses irradiated with low-mass ions

We present an EXAFS study on Ag-doped soda-lime glass waveguides obtained by ion-exchange and treated by low-mass ion-irradiation to promote the formation of metallic clusters. For differently prepared samples we have determined the Ag local order before and after the treatment. In the as-exchanged glasses Ag is coordinated with O at a distance significantly shorter than the Na---O bond length in the unexchanged glass. In irradiated samples we found the formation of metal aggregates. Structural parameters of clusters in high-temperature irradiated sample are compatible with a compressed fcc structure while in room-temperature irradiated specimen a more complicated structure comes out.     

Studies on immobilization of thorium in barium borosilicate glass

The barium borosilicate glass (BBS) matrix has shown considerable solubility of ThO₂ at 1000 °C. As seen by X-ray diffractometry (XRD) and Electron probe micro analysis (EPMA) up to 15.86 wt% of ThO₂ could be dissolved in this matrix. The homogeneity of thoria loaded glass was convincingly ascertained by EPMA. Attempts to load more than 16 wt% ThO₂ led to the phase separation of crystalline phases identified as major phase of ThO₂ and minor percentage of ThSiO₄ phase with altogether different morphologies, as seen by XRD. Interestingly, the back scattered images of thorite crystals point towards the presence of chemical zoning. The results being reported in this paper are of interest especially with respect to immobilization of other actinides in borosilicate glass matrix.     

Plutonium solubility and self-irradiation effects in borosilicate glass

This article presents the limitations for the immobilization of plutonium in borosilicate glasses. A first one is related to the solubility of this element in glass. The effects of the temperature and redox conditions during glass processing were studied. Glass specimens containing plutonium and plutonium surrogates are fabricated. The results show that trivalent elements (La, Gd, Nd, etc.) exhibit greater solubility than tetravalent elements (Pu, Th, Hf). Fabricating the plutonium-doped glass samples under reductive conditions reduced the Pu to trivalent oxidation state and increased its solubility to 4 wt% PuO₂ without reaching the solubility limit. A structural approach based on the results of EXAFS and NMR spectroscopy suggests that the structural role of the trivalent and tetravalent elements corresponds to that of intermediate network modifiers and intermediate network formers, respectively.

The second factor is the effect of actinide decay on the long-term behavior of the glass. Borosilicate glass samples were doped with different curium contents (0.05, 0.5, 1.5 and 4.1 wt% of CmO₂). The macroscopic properties (density, microhardness and initial dissolution rate) of the glasses were characterized up to 4 × 10¹⁸  g⁻¹. No significant effect on the initial alteration rate was detected. The glass swelled slightly, saturating at about 0.5% after receiving a dose of about 2 × 10¹⁸  g⁻¹.

Further studies are ongoing to confirm the satisfactory long-term behavior of the borosilicate glass matrix at higher doses, and to determine the solubility limit of plutonium in reducing conditions.     

Synchrotron diffraction study of dissolution and precipitation kinetics of hydrides in Zircaloy-4

This paper presents observations of in situ heating and cooling cycles using synchrotron X-ray diffraction carried out on Zircaloy-4 plates charged to different levels of hydrogen content. The diffraction peak intensities were used to derive the weight fraction of hydrogen in solid solution and the solvi curves upon dissolution and precipitation. The heating and cooling rate did not seem to have a major influence on the solvi. Fast-cooling/dwell experiments showed that the precipitation kinetics are very fast. Finally the evolutions of the lattice strains from individual grain families were studied during dissolution and precipitation. The aim was to understand the role of interphase misfit stresses on the mechanisms of hydride formation and hysteresis between dissolution and precipitation. The results emphasise that changes in dissolved H concentration are responsible for a non-negligible part of the lattice distortion in the matrix, and that the diffraction strains must therefore be analysed with care.     

Immobilization of sulfate-containing wastes in borosilicate glass using fluoride additives

Moscow Scientific and Industrial Association “Radon.” Translated from Atomnaya énergiya, Vol. 76, No. 3, pp. 234–237, March, 1994.     

Structure of cesium loaded iron phosphate glasses: An infrared and Raman spectroscopy study

The structure of cesium loaded iron phosphate glasses (IPG) was investigated using infrared and Raman spectroscopy. The spectra of the cesium doped samples revealed a structural modification of the parent glass owing to the incorporation of cesium. The structural changes could be correlated with the variation observed in the glass transition temperature of these glasses. Increased Cs-mediated cationic cross linking appears to be the reason for the initial rise in glass transition temperature up to 21 mol% Cs₂O in IPG; while, breakdown of the phosphate network with increasing cesium content, brings down the glass transition temperature.     

Analysis of molecular oxygen formation in irradiated glasses: a Raman depth profile study

Depth profiles experiments have been performed by Raman spectroscopy on three alkali (Na, Li, K) borosilicate glasses irradiated with 1.8 MeV electrons at 1 and 3 GGy. These experiments show that molecular oxygen produced under β irradiation is concentrated near the glass surface according to a depth depending on the irradiation dose. Moreover, we observed that the polymerisation increase is the same in the entire volume sample. The average Si–O–Si angle decrease under irradiation is also homogeneous in the whole irradiated glass volume. From all results, we demonstrate that oxygen migrates up to the glass surface during irradiation without strong interaction with the glass network. Migration of oxygen and probably alkalis takes place through percolation channels with a possible departure of oxygen in some cases.     

新一代元素俘获谱测井仪(ECS)及其应用

元素俘获谱测井仪(ECS)是斯伦贝谢公司研发的新一代地球化学元素测井技术,近年来已在我国投入商用.它的显著特点是不但可以确定粘土矿物含量,而且可以确定粘土矿物类型.本工作在介绍该仪器结构、测井原理以及氧化物闭合模型求解矿物的原理的基础上,通过一定的闭合模型,探讨定量求取矿物类型和含量,以及碳酸盐岩、蒸发岩、砂岩和泥质等各种矿物成分的方法,总结了它在岩石骨架属性、渗透率估算、煤层识别、沉积相带划分、油藏工程压裂设计等方面的应用.