Synthesis of powellite-rich glasses for high level waste immobilization

Increasing amounts of MoO₃ were added to SiO₂-B₂O₃-Na₂O-CaO-Al₂O₃ glasses in order to trap molybdenum as powellite (nominally CaMoO₄). Different heat treatments were performed to study their influences on powellite crystallization by X-ray diffraction and EPR. The glass compositions studied in this work lead to glass-ceramics rich in CaMoO₄, up to [MoO₃] = 5 mol% no poorly durable Na₂MoO₄ phase was identified by XRD. Trivalent actinides surrogates (Gd³⁺) were observed to incorporate into CaMoO₄ crystals.     

Glass formation in the MoO3–Nd2O3–La2O3–B2O3 system

In MoO₃–Nd₂O₃–B₂O₃ and MoO₃–Nd₂O₃–La₂O₃–B₂O₃ systems, glasses were obtained in the region between 20 and 30 mol% Ln₂O₃. A liquid-phase separation region was observed near the MoO₃–B₂O₃ side up to 20 mol% Ln₂O₃ (La, Nd). The amorphous phases were characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), UV–VIS and infrared spectroscopy (IR). According to DTA data B₂O₃-rich glasses are stable up to 630 °C while glasses rich in MoO₃ are stable up to 430 °C. The glasses are transparent in the visible region. Structural models for the glasses network were suggested on the basis of IR spectral investigations. It was established that BO₃ (1380 cm⁻¹), BO₄ (1100–950 cm⁻¹) and MoO₄ (860 cm⁻¹) groups build up the glass network. MoO₆ units (band at 880 cm⁻¹) together with BO₃ units participate in the formation of the glass network with a high MoO₃ content (80–90 mol%).     

Effect of BaO on the crystallization kinetics of glasses along the Diopside-Ca-Tschermak join

We report on the effect of BaO on the crystallization kinetics of glasses in the diopside (CaMgSi₂O₆)-Ca-Tschermak (CaAl₂SiO₆) system. Partial substitution (i.e. 5%, 10% and 20%) of Ba²⁺ for Ca²⁺ was attempted in composition CaMg_0.8Al_0.4Si_1.8O₆, in three different glasses while partial substitution of B³⁺ for Al³⁺ was made in the fourth glass. Structural investigations on the glasses have been made by density measurements, molar volume and Infra-red spectroscopy (FTIR). Non-isothermal crystallization kinetic studies have been employed to study the mechanism of crystallization in all the four glasses. The Avrami parameter for the glass powders is ∼2, indicating the existence of intermediate mechanism of crystallization. Crystallization sequence in the glasses has been followed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and FTIR. Augite crystallized out being the dominant phase in all the glass-ceramics, while different polymorphs of BaAl₂Si₂O8 were present as secondary or minor phases.     

Effect of boron oxide addition on the Nd environment in a Nd-rich soda-lime aluminoborosilicate glass

The environment of Nd³⁺ ions has been studied using optical absorption spectroscopy and EXAFS at the Nd L₃-edge, in a series of soda lime aluminoborosilicate glasses with increasing B₂O₃ content. The proportion of BO₄ units has been determined by ¹¹B MAS NMR in an equivalent glass series with La³⁺ ions replacing the majority of Nd³⁺ ions, and complementary information has been obtained by measuring the Nd³⁺ decay fluorescence times in these latter glasses. In these glasses with low Al₂O₃ content, the R′ ratio, with R′ = [Na₂O^exc] / [B₂O₃] and [Na₂O^exc] = [Na₂O] − [Al₂O₃] − [ZrO₂], plays a key role in controlling the structural organization and crystallization resistance, in a similar way as the R ratio in the Dell and Bray model of sodium borosilicate glasses. At R′ > 0.5, the Nd³⁺ ions are located in a mixed silicate-borate environment and, by slow cooling of the melt, they tend to crystallize within a silicate apatite phase close to the Ca₂Nd₈(SiO₄)₆O₂ composition. At R′ < 0.5, the structural results are compatible with Nd³⁺ ions located in a borate-type environment (not excluding Si neighbors), and, by slow cooling of the melt, they segregate with Ca²⁺ ions within a Si-depleted separated borosilicate phase.     

Structural analysis of some sodium and alumina rich high-level nuclear waste glasses

Sodium- and aluminum-rich high-level nuclear waste glasses are prone to nepheline (NaAlSiO₄) crystallization. Since nepheline removes three moles of glass-forming oxides (Al₂O₃ and SiO₂) per mole of Na₂O, the formation of this phase can result in severe deterioration of the chemical durability in a given glass. The present study aims to investigate the relationships between the molecular-level structure and the crystallization behavior of sodium alumino-borosilicate-based simulated high-level nuclear waste glasses with infrared spectroscopy (FTIR) and X-ray diffraction, respectively. The molecular structure of most of the investigated glasses comprise a mixture of Q² and Q³ (Si) units while aluminum and boron are predominantly present in tetrahedral and trigonal coordination, respectively. The increasing boron content has been shown to suppress the nepheline formation in the glasses. The structural influence of various glass components on nepheline crystallization is discussed.     

Structural study of PbO-MoO3-P2O5 glasses by Raman and NMR spectroscopy

Glasses in the ternary system PbO-MoO₃-P₂O₅ were prepared in three compositional series (100 − x)[0.5PbO-0.5P₂O₅]-xMoO₃ (A), 50PbO-yMoO₃-(50 − y)P₂O₅ (B) and (50 − z)PbO-xMoO₃-50P₂O₅ (C) and their structure was studied by Raman and ³¹P NMR spectroscopies. In the compositional series (100 − x)[0.5PbO-0.5P₂O₅]-xMoO₃ homogeneous glasses were prepared in the concentration region of 0-70 mol% MoO₃. Their glass transition temperature increases with increasing MoO₃ content having a maximum at x = 50 mol% MoO₃. ³¹P MAS NMR spectra reveal that in the glass series (A) the incorporation of MoO₃ results in the shortening of phosphate chains and gradual transformation Q² units into Q² and Q⁰ units, prevailing in glasses with a high MoO₃ content. Octahedral structural units MoO₆ dominate in most glass compositions and they are present also in the structure of Pb(MoO₂)₂(PO₄)₂ compound corresponding to the glass composition 50Pb(PO₃)₂-50MoO₃. The analysis of Raman spectra of glasses of the (B) series with a high MoO₃ content showed the transformation of octahedral MoO₆ units into tetrahedral MoO₄ units.     

Glass formation and structure of glasses in the ZnO―Bi2O3―WO3―MoO3 system

The glass formation region in the ternary ZnO―Bi₂O₃―WO₃ system is determined by melt quenching technique (cooling rates 10¹-10² K/s). New original glasses are obtained in a narrow concentration range with high WO₃ content (60-75 mol%). Homogeneous glasses of the composition (100 − x)[0.2ZnO·0.3Bi₂O₃·0.5WO₃]xMoO₃, were obtained between 20 and 60 mol% MoO₃. Characterization of the amorphous samples was made by x-ray diffraction (XRD), differential thermal analysis (DTA) and infrared spectroscopy (IR). The thermal stability of glasses decreases with the increasing of MoO₃ content. The glass transition temperature, T_g, varies between 340-480 °C, while the crystallization temperature, T_x, varies between 388-531 °C. The tungstate glasses possess higher crystallization temperature (T_x over 500 °C) in comparison with the other vanadate and molybdate non-traditional glasses. The glass network is realized by transformation of three-dimensional structure of WO₃ into a layered one, consisting mainly of WO₆ units. We supposed that the network of quaternary glasses is built up by MoO₄, MoO₆ and WO₆. At low concentration ZnO and Bi₂O₃ facilitate the disorder in the supercooled melts, while at high concentration stimulate crystallization processes. These oxides belong to the intermediate ones.     

Laser-induced crystallization of β′-RE2(MoO4)3 ferroelectrics (RE: Sm, Gd, Dy) in glasses and their surface morphologies

The glasses with the compositions of 21.25RE₂O₃-63.75MoO₃-15B₂O₃ (RE: Sm, Gd, Dy) were prepared and the formation of β′-RE₂(MoO₄)₃ ferroelectrics was confirmed in the crystallized glasses obtained through a conventional crystallization in an electric furnace. The features of the glass structure and crystallization behavior were clarified from measurements of Raman scattering spectra. Continuous-wave Nd:YAG laser with a wavelength of 1064 nm (laser power: 0.6-0.9 W, laser scanning speed: S = 1-16 μm/s) was irradiated to 10.625Sm₂O₃-10.625Gd₂O₃ (or Dy₂O₃)-63.75MoO₃-15B₂O₃ glasses, and the structural modification was induced at the glass surface. At the scanning speed of S = 10 μm/s, crystal lines consisting of β′-Gd_2−xSm_x(MoO₄)₃ or β′-Dy_2−xSm_x(MoO₄)₃ crystals were patterned on the glass surface. It was found that those crystal lines have the surface morphology with periodic bumps. At S = 1 μm/s, it was found that crystal lines consist of the mixture of paraelectric α-Gd_2−xSm_x(MoO₄)₃ and ferroelectric β′-Gd_2−xSm_x(MoO₄)₃ crystals, indicating the phase transformation from the β′ phase to the α phase during laser irradiation. Homogeneous crystal lines with β′-RE₂(MoO₄)₃ ferroelectrics have not been written in this study, but further research is continuing.     

New evidence for tetrahedral triclusters in aluminosilicate glasses

The formation of thermodynamically stable 3/2-mullite (3 Al₂OAl₃·2 SiO₂) was investigated by scanning electron microscopy using reaction couples consisting of 2/1-mullite (2 Al₂O₃·1 SiO₂) plus SiO₂ glass, or Na₂O-SiO₂ glass, respectively. The mullite substrates were partially dissolved, thus leading to Al incorporation in the siliceous phases. In both reaction couples thin layers of stoichiometric 3/2-mullite form on the 2/1-mullite substrates. However, the major mullitization steps are different: The 2/1-mullite/SiO₂ reaction couple gives rise to 3/2-mullite crystallization within the bulk of the glass, whereas epitactic growth of c-axis orientated 3/2-mullite needles on the 2/1-mullite substrate was observed in the presence of Na₂O-SiO₂ glass. The differences in mullite nucleation were attributed to the existence or non-existence of tetrahedral triclusters in the as-reacted non-crystalline Al₂O₃-SiO₂ and Na₂O-Al₂O₃-SiO₂ phases, respectively. Triclusters of (Si,Al)O₄-tetrahedra in the Al₂O₃-SiO₂ glass may act as nuclei for 3/2-mullite crystallization in the bulk of the glass since these structural units also occur in mullite. In Na₂O-Al₂O₃-SiO₂ glasses triclusters are absent, and epitactical 3/2-mullite formation on the mullite substrate becomes more favorable energetically.     

Transparent phosphosilicate glasses containing crystals formed during cooling of melts

Effect of P₂O₅–SiO₂ substitution on spontaneous crystallization of SiO₂–Al₂O₃–P₂O₅–Na₂O–MgO melts during cooling was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and rotation viscometry. Results show that addition of P₂O₅ leads to amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases. It is due to the tendency of Mg²⁺ to form [MgO₄] linking with [SiO₄]. Molar substitution of P₂O₅ for SiO₂ enhances the network polymerization of silicate-rich phase in the melts, and thereby the spontaneous crystallization of cubic Na₂MgSiO₄ is also enhanced during cooling of the melts. In addition, the sizes of the local crystalline and separated glassy domains are smaller than the wavelength of the visible light, and this leads to the transparency of the obtained glasses containing crystals.     

Characterization of iron phosphate glasses prepared by microwave heating

Phosphate glasses have been prepared by melting batch materials in electric furnaces, induction furnaces, and in microwave ovens. In the present work mixtures of (NH₄)₂HPO₄ and Fe₃O₄ or Fe₂O₃ were exposed to microwave energy, heated to 1200 °C, and cast to produce iron phosphate glasses. Glasses were also produced in electric furnaces for comparison. The material was analyzed by X-ray diffraction, Mössbauer spectroscopy, and differential thermal analysis. For magnetite-based glasses produced in an electric furnace, the Fe²⁺/(Fe²⁺ + Fe³⁺) ratio is compatible with the value in the batch material. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio is higher for glasses produced in a microwave oven. Glasses with nominal composition 55Fe₃O₄–45P₂O₅ (mol%) produced in an electric furnace present an arranged magnetic phase with hyperfine field that could be associated to hematite (estimated to be 21%). All the glasses submitted to heat treatments for crystallization present the following crystalline phases: FePO₄, Fe₃(PO₄)₂, Fe(PO₃)₃, Fe(PO₃)₂ and Fe₇(PO₄)₆. The amount of these phases depends on the glass composition, and glass preparation procedure. Microwave heating allows to reach melting temperatures at high heating rates, making the procedure easy and economical, but care should be taken concerning the final Fe²⁺/(Fe²⁺ + Fe³⁺) ratio.     

Control of the thermal properties of slow bioresorbable glasses by boron addition

This work studied the properties of glasses with the molar composition 63.8SiO₂-(11.6-x)Na₂O-(0.7 + x)B₂O₃-19.2CaO-3MgO-1.5Al₂O₃-0.2P₂O_5, in which x = 0, 1, 2, 3. These glasses are of interest for the development of slowly dissolving fibers to be incorporated in composites for medical applications. The thermal properties were recorded using hot stage microscopy, differential thermal analysis, and heat treatments in the range of 800°-1000 °C. The glass crystallization behavior was determined based on calculated values of the activation energy of crystallization and the Johnson-Mehl-Avrami exponent. The structural units in the glass network were identified using infrared spectroscopy. Finally, in vitro dissolution was tested in SBF solution.The addition of B₂O₃ increased the glass transition temperature and reduced the working temperature. When heat treated at 900 °C, the glass with the smallest amount of B₂O₃ formed two crystalline phases: magnesium silicate MgSiO₃ and wollastonite CaSiO₃. In the other compositions, only CaSiO₃ was observed after heat treatment at 950 °C. All the glasses crystallized preferentially from the surface. Changes in the liquidus and crystallization temperatures were related to changes in the glass structure. The formation of [BO₃] units led to glasses with improved resistance to crystallization and decreased liquidus temperature. In the glasses with 2.7 and 3.7 mol% B₂O₃, [BO₃] units were transformed into [BO₄] units. The formation of [BO₄] led to an increase in fragility and a decrease in resistance to crystallization. All the glasses dissolved slowly in simulated body fluid.     

Glass formation and structure of glasses in B2O3―Bi2O3―MoO3 system

The purpose of this paper is to study the glass formation tendency in the ternary system B₂O₃―Bi₂O₃―MoO₃ and to define the main structural units building the amorphous network. A wide glass formation area was determined which is situated near the Bi₂O₃―B₂O₃ side. A liquid phase separation region was observed near the MoO₃―B₂O₃ side for compositions containing below 25 mol% Bi₂O₃ and their microheterogeneous structure was observed by SEM. The phase formation was characterized by X-ray diffraction (XRD). By DTA was established the glass transition temperature (T_g) in the range of 380-420 °C and crystallization temperature (T_x) vary between 420 and 540 °C. The main building units forming the amorphous network are BO₃ (1270 and 1200 cm^− 1), BO₄ (930-880, 1050-1040 cm^− 1), MoO₄ (840-760 cm^− 1) and BiO₆ (470 cm^− 1). It was proved that Bi₂O₃ favors the BO₃ → BO₄ transformations while MoO₃ preserves BO₃ units in the amorphous network.     

Effect of Al2O3 on phase separation of SiO2-Nd2O3 glasses

The immiscibility boundary and the critical point of SiO₂-Nd₂O₃ system glass were determined as a function of Al₂O₃ addition. The critical temperature of the immiscibility boundary was observed to decrease with the addition of Al₂O₃. Using the regular solution model, the observed decrease of the immiscibility boundary was directly related to the decrease of the concentration fluctuation of Nd₂O₃ in SiO₂. It is concluded that the Al₂O₃ addition to Nd₂O₃ containing silica glass is beneficial in decreasing the concentration quenching effect, deterioration of the optical efficiency due to clustering of rare earth element, because Al₂O₃ addition diminishes the concentration fluctuation of Nd₂O₃ in silica glass.     

Synthesis and physical properties of glasses in the Sb2O3-PbCl2-MoO3 system

Lead chloroantimonite glasses form stable binary glasses that may accommodate numerous oxides or halides as a third component. Molybdenum trioxide is a glass progenitor leading to molybdate glasses. Ternary glasses have been synthesized and studied in the Sb₂O₃-PbCl₂-MoO₃ system. Compositional limits of glass formation are reported and two series of glass samples have been prepared corresponding to the general formulas: (90 − x)Sb₂O₃-xPbCl₂-10MoO₃ and (90 − x)Sb₂O₃-xMoO₃-10PbCl₂. Glass transition temperature is close to 290 °C at high Sb₂O₃ content and decreases as antimony oxide is substituted by MoO₃ or PbCl₂. Position, width and intensity of crystallization peak suggest that devitrification rate is small in some composition ranges. The evolution of density, thermal expansion, refractive index and microhardness has been studied as a function of composition parameter x. Deviations from linearity are observed. They suggest structural changes in the case of the MoO₃/Sb₂O₃ substitution while it appears that molar volume increases linearly versus lead content in the other series of glasses. Refractive index is close to 2.04. Optical transmission ranges from 550 nm in the visible spectrum to 5.5 μm in the infrared. It is limited by extrinsic absorption bands arising from hydroxyls and silicon impurities. Young's, bulk and shear moduli have been measured for the two series of samples.     

Structure and properties of MoO3-containing zinc borophosphate glasses

ZnO–B₂O₃–P₂O₅ glasses doped with MoO₃ were investigated in the series (100?x)[0.5ZnO–0.1B₂O₃–0.4P₂O₅]–xMoO₃, where bulk glasses were obtained by slow cooling in air within the compositional region of 0 ? x ? 60 mol% MoO₃. The incorporation of MoO₃ into the parent zinc borophosphate glass results in a weakening of bond strength in the structural network, which induces a decrease in chemical durability and glass transition temperature. Raman spectra reflect the incorporation of molybdate groups into the glass network of the studied glasses by the presence of the polarized vibrational band at ≈976 cm^?1 ascribed to the MO_x symmetric stretching vibrations and the depolarized band at ≈878 cm^?1 ascribed to the Mo–O–Mo stretching vibration. The incorporation of molybdate units into the glass network results in the depolymerization of phosphate chains and the formation of P–O–Mo bonds, as reflected in Raman and ³¹P NMR spectra. According to the ¹¹B MAS NMR spectra, tetrahedral B(OP)_4?x(OMo)_x units are formed in the glasses, whereas only a small amount of BO₄ units is converted to BO₃ units in the MoO₃-rich glasses.     

Optical properties of zinc molybdenum phosphate glasses

Zinc molybdenum phosphate (ZnO–MoO₃–P₂O₅) glasses of different compositions were prepared and synthesized and some of their properties were studied. The optical band gaps have been deduced from spectral dependence curves. The infrared absorption spectra of these investigated glasses were found analogous to SrO–Fe₂O₃–P₂O₅. Annealing at different temperatures does not show a prominent change in absorption band positions. We correlate our findings with a possible structural explanation of these glasses.     

Effects of yttrium codoping on fluorescence lifetimes of Er3+ ions in SiO2–Al2O3 sol–gel glasses

In order to find a new glass host and optimize erbium doping for IR glass optical amplifiers in photonic applications, a study on the optimization of the emission of erbium ions in the SiO₂–Al₂O₃ glass by codoping with Y₂O₃ is performed. It is first attempted to make a new sol–gel glass host based on SiO₂, Al₂O₃, and Y₂O₃ doped with Er³⁺ ions of the composition (1−x)SiO₂–xAl₂O₃–yY₂O₃:0.65Er₂O₃ (in mol%), x varies from 0 to 65, and y from 0 to 4. The optimal proportion in mol% of SiO₂ and Al₂O₃ for the Er³⁺ emission (at a fixed optimal concentration of 0.65) was 65 – 35. The effect of Y₂O₃ content on photoluminescence, decay curve profiles and lifetime of the ⁴I_13/2 level of Er³⁺ in SiO₂–Al₂O₃ glass is observed. The largest quantum efficiency and the higher emission intensity are observed in the sample with 65Al₂O₃ and 4Y₂O₃. The emission intensity at 1530 nm is two times higher than in glasses without Y₂O₃. A shift of 3 nm to shorter wavelengths is observed. The emission spectral profiles are flatter and broader for the glasses containing Al and Y (bandwidth of 59.5 nm). The decay curves show strong difference profiles for the different samples. The increase of the lifetime value τ (about ms) of the ⁴I_13/2 level of Er³⁺ in the SiO₂–Al₂O₃ with the Y₂O₃ is discussed.     

Structural analysis of Na2OGa2O3SiO2 glasses by the X-ray diffraction method

The structure of Na₂OGa₂O₃SiO₂ glasses of four different compositions containing up to 45 mol. % Ga₂O₃ has been determined by the X-ray diffraction method. The radial distribution function D(r) obtained indicates that Ga³⁺ ions are in fourfold coordination and GaO₄ tetrahedra are formed in these glasses. The reduced intensity function S·i(S) and radial distribution function D(r) which were calculated based on a structural model for A-25, 0.25(Ga₂O₃)·0.75(Na₂O·2SiO₂)/3 glass agreed well with the observed ones.     

EPR study of crystalline and vitreous forms of the Li2OAl2O3SiO2 system

The epr spectra of V⁴⁺ and radiation centres have been studied in β-eucryptite (LiAlSiO₄), β-, γ-spodumene (LiAlSi₂O₆) and in glasses prepared by the fusion of the single crystals. It is shown that the electronic structures of the vitreous state in the Li₂OAl₂O₃SiO₂ system and that of the crystalline forms differ considerably. The change of the electronic structure on crystallization is not direct, but is realized through the intermediate state whose electronic structure differs from that of glasses and crystals.