yu Li

Raman spectroscopic study of gadolinium(III) in sodium-aluminoborosilicate glasses

Abstract A Raman spectroscopy study was performed on a series of sodium-aluminoborosilicate glasses with Gd2O3 from 0 mass% up to its solubility (47 mass% or 13.58 mol%). Experimentally measured spectra were fitted with a Gaussian function for each individual band without any restriction on the band position, width, and intensity. The evolution of the spectroscopic bands appears to result from partitioning of the rare earth cation, as its dissolution mechanism, in these borosilicate glasses. Specifically, the evolution of the Raman bands was correlated well with Gd cations partitioning in the borate-rich environment at low Gd2O3 concentration, Gd2O3/[1/3B2O3] 1. Raman bands near 1420 and 710 cm −1 suggest the presence of a local Gd-metaborate environment, which appeared to occur at all Gd2O3 concentrations. The bands near 300 and 910 cm −1 further suggest the formation of Gd–O–Gd clusters in the silicate-rich environment at high Gd2O3 concentrations.

Crystallization of gadolinium- and lanthanum-containing phases from sodium alumino-borosilicate glasses

Abstract Lanthanide-containing glasses, commonly used for optical and laser applications, are also important in the vitrification of actinide-bearing radioactive wastes. In previous studies, we measured the glass forming regions of La2O3 and Gd2O3 in some sodium alumino-borosilicate glasses. Above their highest concentrations in these glasses, lanthanide silicate crystals with an apatite structure were found. In this paper, we characterize these crystals using powder X-ray diffraction (XRD), electron microscopy, energy dispersive spectroscopy (EDS), and selective area diffraction (SAD) to evaluate baseline glass composition effect and mixed La/Gd effect on the structure and chemistry of these crystals. When different lanthanide elements (lanthanum and gadolinium) co-exist in the glasses, complete lanthanide silicate solid solution is observed. Small amounts of boron can enter the gadolinium silicate structure if aluminum is present in the melt. The boron is probably substituting for the silicon in the crystal lattice. This substitution will cause a decrease in the unit cell parameters a0 and c0. A small amount of Na can also enter the crystal lattice, causing a decrease in the cell parameter a0, but an increase in c0. These results may help us to develop better understanding on the solution mechanism of lanthanide oxides in these glasses.

Energetics of dissolution of Gd2O3 and HfO2 in sodium alumino-borosilicate glasses

Abstract In addition to their industrial and consumer-oriented applications, sodium alumino-borosilicate glasses are leading candidates for encapsulation of reprocessed commercial and defense-related nuclear waste. Quantification of the thermochemical and physical properties of these glasses is necessary for a complete understanding of processes occurring in these systems. In this study, 16 glass samples were studied including the base glass (20Na 2 O–15B 2 O 3 –5Al 2 O 3 –60SiO 2 ), six Gd-doped glasses (0.45, 0.92, 1.72, 3.25, 4.74, 7.67, 10.53 mol% Gd 2 O 3 ) and eight Hf-doped glasses (0.15, 0.38, 0.77, 1.57, 3.27, 5.09, 7.06, 11.53 mol % HfO 2 ). Drop solution calorimetry using lead borate solvent at 976 K indicates that these glasses are energetically stable with respect to their binary crystalline oxides. There is a sharp change in the enthalpy of formation between glasses with more than 1.6 mol % Gd 2 O 3 or HfO 2 addition and glasses of lower Gd 2 O 3 or HfO 2 content. At higher doping levels, the apparent partial molar enthalpy of solution of HfO 2 in the glass is close to zero, consistent with the formation of nanometer-sized heterogeneities. A possible explanation is that glasses with more than 1.6 mol% added oxides consist of regions of Gd 2 O 3 or HfO 2 -rich glass perhaps containing regions of medium-range order dominated by Gd 2 O 3 or HfO 2 and/or their nanocrystals and regions of Gd 2 O 3 or HfO 2 poor glass. Glass transition temperatures from DSC experiments indicate similar trends with a change in slope near 1.6 mol % Gd 2 O 3 or HfO 2 . These data are consistent with structural and spectroscopic studies that suggest the onset of clustering and related changes in structure in this composition range.

Gadolinium solubility in peraluminous borosilicate glasses

This paper discusses the results of a study with 18 peraluminous (Na/Al<1) borosilicate glasses to understand the effect of glass composition on Gd solubility. Above Gd solubility limit, phase separation occurs in some of the glasses; in the others, sodium gadolinium silicate crystallizes. For the samples in which phase separation occurs, Gd solubility is determined by the [AlO1.5]–[NaO0.5]–0.2[BO1.5] values in the melt. Increasing [AlO1.5]–[NaO0.5]–0.2[BO1.5] increases Gd solubility. For the samples in which sodium gadolinium silicate crystallizes, Gd solubility is determined by the concentration of Na. Increasing Na decreases Gd solubility. When Al concentration in the baseline glass is high, a minimum amount of Gd is needed to form a clear glass. Otherwise, mullite crystallizes. The minimum concentration of Gd is determined by the [AlO1.5]–[NaO0.5]–[BO1.5] value in the melt. The higher this value, the more Gd is needed to form a clear glass. In general, the solution behavior of Gd in peraluminous borosilicate melts is similar to that in peralkaline borosilicate melts.

Gadolinium Solubility in Peralkaline Borosilicate Glasses

Lanthanide-containing glasses, commonly used for optical and laser applications, are also important in the vitrification of actinide-bearing radioactive wastes. In order to study the effect of glass composition on the solubility of gadolinium, 25 peralkaline (Na/Al >1) borosilicate glasses were studied. Above Gd solubility, liquid-liquid phase separation was found in the glasses with Na/B less than 0.5; and in the glasses with Na/B more than 0.5, crystallization was found. For the samples from which liquid-liquid phase separation was observed, Gd solubility was mostly decided by the concentration of excess Na (e.g. Na-Al). Increasing excess Na will increase Gd solubility. For the samples from which crystallization was observed, Gd solubility was decided by the concentration of B, Si, and Al. Increasing B and Si will increase Gd solubility, but increasing Al will decrease Gd solubility. The solution mechanism of Gd in peralkaline borosilicate glasses is also discussed.

Gadolinium Borosilicate Glass-Bonded Gd-Silicate Apatite: A Glass-Ceramic Nuclear Waste Form for Actinides

A Gd-rich crystalline phase precipitated in a sodium gadolinium alumino-borosilicate glass during synthesis. The glass has a chemical composition of 45.39-31.13 wt% Gd2O3, 28.80-34.04 wt% SiO2, 10.75-14.02 wt% Na2O, 4.30-5.89 wt% Al2O3, and 10.75-14.91 wt% B2O3. Backscattered electron images revealed that the crystals are hexagonal, elongated, acicular, prismatic, skeletal or dendritic, tens of mm in size, some reaching 200 mm in length. Electron microprobe analysis confirmed that the crystals are chemically homogeneous and have a formula of NaGd9(SiO4)6O2 with minor B substitution for Si. The X-ray diffraction pattern of this phase is similar to that of lithium gadolinium silicate apatite. Thus, this hexagonal phase is a rare earth silicate with the apatite structure. We suggest that this Gd-silicate apatite in a Gd-borosilicate glass is a potential glass-ceramic nuclear waste form for actinide disposition. Am, Cm and other actinides can easily occupy the Gd-sites. The potential advantages of this glass-ceramic waste form include: (1) both the glass and apatite can be used to immobilize actinides, (2) silicate apatite is thermodynamically more stable than the glass, (3) borosilicate glass-bonded Gd-silicate apatite is easily fabricated, and (4) the Gd is an effective neutron absorber.

Extended electron energy loss fine structure simulation of the local boron environment in sodium aluminoborosilicate glasses containing gadolinium

Abstract Electron energy loss boron K-edge spectra of gadolinium-bearing sodium aluminoborosilicate glasses were acquired. Extended electron energy loss fine structure studies were performed in both extended and near edge region. Using Gd-metaborate, GdB3O6, as a model, we fitted the experimental B K-edge spectra of the glasses containing 30 and 47 mass% Gd2O3 in r-space with a non-linear least-square fitting method. Based on the results obtained from the current study, the majority of borate units appear to react with Gd cations and the local environment of Gd cations resembles that of the Gd-metaborate structure. Therefore, a distorted Gd-metaborate-like local atomic structure is proposed, in which each Gd cation is stabilized by one tetragonal BO4 unit and two trigonal BO3 units. The similarity of the chemical environment between the two high Gd-bearing glasses implies that the local Gd-metal borate environment, 1BO4:Gd:2BO3, is stable. When the Gd to B ratio in the glasses exceeded 1 to 3 ratio, according to the model, excess Gd cations should form bonds with silicate units. The modeling results are further supported by the energy loss near edge spectra data and previously results obtained from Raman and laser fluorescence deduced phonon side band studies.

The Effects of Na 2 O, Al 2 O 3 , and 3203 ON HfO 2 Solubility in Borosilicate Glass

A single borosilicate glass composition has previously been shown to dissolve 10 and 25 mass% PuO 2 under oxidizing and reducing conditions, respectively. A simplified version of this glass has been thoroughly investigated to determine the effect of increasing the alkali:aluminum ratio on the HfO 2 solubility in borosilicate glasses. We are investigating HfO 2 solubility because specific Pu wastes are being considered for disposal in glass, and Hf(IV) serves as a structural surrogate for Pu(IV) and as a neutron absorber in glass. Three series of base glasses were produced using the same initial composition, but varying the oxides B 2 O 3 , Al 2 O 3 , or Na 2 O one at a time. In a fourth series of the same initial composition, both Na 2 O and A1 2 O 3 were varied. Hafnia was added to these glasses and the mixture equilibrated for 2 hours: 1 hour at 1450°C after 1 hour at 1560°C. A wide range of HfO 2 additions were made to the base glasses, and the solubility of HfO 2 determined to within ±1 mass%. The highest solubility determined was 14 mol% (35 mass%) HfO2 in a low-Al glass. We conclude that increasing Na 2 O/Al 2 O 3 increases the HfO 2 solubility, and increasing the B 2 O 3 content apparently has little effect on HfO 2 solubility in the borosilicate glasses studied.

Gadolinium Solubility Limits in Sodium-Aluminoborosilicate Glasses

Gadolinium and lanthanum solubility limits in a sodium-alumino-borosilicate glass system were studied. As melting temperature increased from 1,400 C to 1,450 C, 1,500 C and 1,550 C, the solubility of gadolinium in the baseline glass 15B{sub 2}O{sub 3}-5Al{sub 2}O{sub 3}-20Na{sub 2}O-60SiO{sub 2} (in molar composition) increased from 10.1 to 11.3, 12.2 and 13.1 (in mole percent of Gd{sub 2}O{sub 3}). The enthalpy change of Gd{sub 2}O{sub 3} dissolution in this baseline glass is about 43.6 kJ/mol. Boron effect on lanthanum solubility was studied using the following baseline glasses: xB{sub 2}O{sub 3}-20Na{sub 2}O-5Al{sub 2}O{sub 3}-60SiO{sub 2}, where x equals to 5, 10, 15, and 20, respectively. It was found that lanthanum solubility limit increased from 8.4 to 10.3, 12.5 and 14.9 (in mole percent of La{sub 2}O{sub 3}) as B{sub 2}O{sub 3} increased from 5.1 to 9.5, 13.1 and 16.2 mol%. Gd{sub 2}O{sub 3} and La{sub 2}O{sub 3} have similar solubility limits. Solubility limits of mixtures containing different ratios of Gd{sub 2}O{sub 3} in the baseline glass 15B{sub 2}O{sub 3}-20Na{sub 2}O-5Al{sub 2}O{sub 3}-60SiO{sub 2} were found insensitive to the ratio of La/Gd. As far as gadolinium is concerned, its solubility limit will decrease when other lanthanides are introduced.

Gadolinium and Hafnium Alumino-Borosilicate Glasses: Gd and Hf Solubilities

The solubilities of Hf and Gd in sodium alumino-borosilicate glasses based on the target compositions were examined and confirmed by electron microprobe analysis. The measured compositions of essentially crystal-free glasses are generally homogeneous and close to the target compositions. Therefore, the solubilities of Gd and Hf in sodium alumino-borosilicate glasses based on the target glass compositions are valid. However, for glasses containing precipitates (crystals grown from the melt) and undissolved HfO2 with overgrowths, the chemical compositions are often heterogeneous and may be significantly different from the target compositions. Precipitated crystalline phases include a rare earth silicate with the apatite structure (NaGd9(Si5.25B)O26) in a gadolinium sodium alumino-borosilicate glass and a HfO2 phase in hafnium sodium alumino-borosilicate glasses.