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.     

Effect of neodymium oxide on the solubility of MoO3 in an aluminoborosilicate glass

High molybdenum and rare earth concentrations in soda-lime aluminoborosilicate glasses may lead to the crystallization of molybdenum-rich phases such as alkali and alkaline-earth molybdates (Na₂MoO₄, CaMoO₄) and also rare earth (RE)-rich phases such as apatite (Ca₂RE₈(SiO₄)₆O₂) during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy wasteforms. To understand the effect of neodymium addition (from 0 to 16 wt.% Nd₂O₃) on the phase separation and the crystallization tendency of molydbate phases and on the structure of a Mo-bearing nuclear glass belonging to the SiO₂-B₂O₃-Al₂O₃-Na₂O-CaO-MoO₃ system, crystallization and structural studies have been performed by X-ray diffraction, scanning electron microscopy, electron microprobe analysis, Raman and optical absorption spectroscopies. The results obtained show that the addition of an increasing amount of Nd₂O₃ induces a significant increase of the solubility of molybdenum in the glass, characterized by a decrease of the phase separation and of the crystallization tendency of molybdate phases. The increase of chemical disorder in the structure of Mo-bearing glasses when Nd₂O₃ is added - and more precisely in the depolymerized regions where Nd³⁺ cations and [MoO₄]²⁻ entities are located - could be at the origin of the evolution of the molybdenum solubility in the glass.     

Second-order optical non-linearity initiated in Li2O–Nb2O5–SiO2 and Li2O–ZnO–Nb2O5–SiO2 glasses by formation of polar and centrosymmetric nanostructures

Amorphous nanoheterogeneities of the size less than 100 Å have been formed in glasses of the Li₂O–Nb₂O₅–SiO₂ (LNS) and Li₂O–ZnO–Nb₂O₅–SiO₂ (LZNS) systems at the initial stage of phase separation and examined by transmission electron microscopy, small-angle X-ray and neutron scattering. Both LNS and LZNS nanoheterogeneous glasses exhibit second harmonic generation (SHG) even when they are characterized by fully amorphous X-ray diffraction (XRD) patterns. Chemical differentiation and ordering of glass structure during heat treatments at appropriate temperatures higher T_g lead to drastic increase of SHG efficiency of LNS glasses contrary to LZNS ones in the frame of amorphous state of samples. Following heat treatments of nanostructured glasses result in crystallization of ferroelectric LiNbO₃ and non-polar LiZnNbO₄ in the LNS and LZNS glasses, respectively. Taking into account similar polarizability of atoms in LNS and LZNS glasses, the origin of the principal difference in the second-order optical non-linearity of amorphous LNS and LZNS samples is proposed to connect predominantly with the internal structure of formed nanoheterogeneities and with their polarity. Most probably, amorphous nanoheterogeneities in glasses may be characterized with crystal-like structure of polar (LiNbO₃) phase initiating remarkable SHG efficiency or non-polar (LiZnNbO₄) phase, which do not initiate SHG activity. It gives an opportunity to vary SHG efficiency of glasses in a wide rage without remarkable change of their transparency by chemical differentiation process at the initial stage of phase separation when growth of nanoheterogeneities is ‘frozen’. At higher temperatures, LiNbO₃ crystals identified by XRD precipitate in LNS glasses initiating even more increase of SHG efficiency but visually observable transparency is impaired.     

Morphology‐controlled CaMoO4 nanorods via a facile microwave‐assisted EDTA chelating agent process

This work reports on the shape‐controlled synthesis of CaMoO₄ nanorods via a high‐efficient microwave irradiation‐assisted chelating agent method. The phase and microstructure of these materials were systematically characterized by X‐ray diffraction and field‐emission scanning electron microscopy. It is noteworthy that EDTA plays pivotal roles as the complexing and capping agent in the oriented growth of uniform CaMoO₄ nanorods. The synthetic process gives the guidance to understand the morphological evolution of CaMoO₄ microstructures in microwave system, and provides a facile and designed strategy to fabricate functional one dimensional metallic molybdates.     

Influence of CoO addition on phase separation and crystallization of glasses of the ZnO–Al2O3–SiO2–TiO2 system

The evolution of structure, phase composition and spectroscopic properties of CoO-doped (up to 5 mol%) titania-containing zinc aluminosilicate glasses with their heat-treatment has been studied using Raman scattering, small angle X-ray scattering, X-ray diffraction analysis and optical absorption spectra. Addition of cobalt oxide was observed to facilitate amorphous phase separation of the parent glass and gahnite, ZnAl₂O₄, crystallization. Cobalt oxide entered phases formed during low-temperature heat-treatments (720 °C), i.e., amorphous phase, enriched in ZnO, Al₂O₃ and TiO₂ and crystalline phase of gahnite. The absorption of these glass-ceramics was defined mainly by tetrahedral Co²⁺ ions located in gahnite nanocrystals. As the temperature was increased further, traces of anosovite solid solution appeared and then decomposed. Even after high-temperature heat-treatments, a certain portion of Co²⁺ ions remained in amorphous zinc aluminotitanate phase and in octahedral sites of inversed gahnite spinel. In glass-ceramics, the residual high silica amorphous phase contained a small quantity of [TiO₄] centers, which content was smaller in Co:ZAS samples as compared with non-doped glass-ceramics.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

Structural changes during the crystallization of the Bi4Ge3O12 glasses

Bismuth-germanate glass ceramics with the composition 40% Bi₂O₃-60% GeO₂ (in molar percents) were prepared through controlled crystallization of melt-quenched glass. The Raman and FTIR spectra recorded in the as-made glasses show broad bands at 240, 400, 780 cm^− 1 and 400, 745 cm^− 1 have been assigned Ge-O bonds which appear right after preparation. X-ray diffraction has shown that the as-made glasses are amorphous, but after annealing above the crystallization temperature at 558 °C, BGO nano-crystallites with a size of about 50 nm precipitate in the glass matrix. The Raman and FTIR spectra reveal sharp peaks associated to the “internal” and “external vibrations” of GeO₄ tetrahedral groups inside the BGO nano-crystallites. In the glass ceramic sample the transparency region is shifted at longer wavelengths compared to as-made glass, due to the Rayleigh scattering on the BGO nano-crystallites.     

Non-isothermal crystallization and nanostructuring in potassium niobium silicate glasses

Potassium niobium silicate (KNS) glasses the composition of which is characterized by the K₂O/Nb₂O₅ molar ratio ranging from 0.85 to 1.2 and SiO₂ 50-54 mol% were examined in order to clarify the influence of chemical composition on formation of transparent nanostructured state of glasses. Differential thermal analysis, X-ray diffraction and scanning electron microscopy were used to study the non-isothermal crystallization of the KNS glasses as well as their morphological features. It was found that all glasses devitrify in three steps forming unidentified phases at the first two ones while at higher temperature (1000-1100 °C) the crystallization of K₃Nb₃O₆Si₂O₇ takes place. For prolonged heat treatment time (more than 5 h) at high temperature (1050-1100 °C) the transformation of this phase into the KNbSi₂O₇ ferroelectric one occurs in some extent. Nanostructuring occurs at the first stage of the devitrification process. It results from two partially overlapped processes: amorphous phase separation and subsequent crystallization. It was shown that only for the glass with the K₂O/Nb₂O₅ molar ratio equal to 0.85 and SiO₂ 50 mol% it is possible to separate the above processes by isothermal heat treatments at 680 °C obtaining fully transparent nanostructured samples. These samples contain nanocrystals 10 times smaller than the amorphous inhomogeneities of the phase separated matrix in which are dispersed.     

Crystallization of amorphous silicates far from equilibrium part II: Experimental insight into the key role of decoupled cation mobilities

The crystallization of amorphous and porous Ca–Mg-silicates prepared by a sol–gel method is studied in the glass transition range during isothermal and continuous heating experiments. A starting material of diopside (CaMgSi₂O₆) composition was primarily studied because it is a reference system to study crystal nucleation and growth. The annealed products were characterized by X-ray diffraction and transmission electron microscopy. In the glass transition range, the crystallization is not congruent. It follows a systematic sequence in which the most Ca-rich silicates present in the phase diagram crystallize first. This trend does not obey equilibrium thermodynamics predictions. Instead, this sequence is the result of the decoupled mobilities of network-modifying and network-forming cations. The high surface/volume ratio of gels likely exacerbates this effect compared to compositionally comparable glasses. Altogether, the study shows that local dynamics controls kinetic phase transitions in the glass transition range.     

Structural features of hafnium iron phosphate glasses

Structural features and properties of a series of hafnium iron phosphate glasses have been investigated by Mössbauer spectroscopy and X-ray diffraction. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The isomer shift values obtained from the Mössbauer fits show that both Fe(II) and Fe(III) ions are in octahedral or distorted octahedral coordination. The crystalline HfP₂O₇ phase was detected in all the samples by powder X-ray diffraction but this did not degrade the chemical durability of the glasses as the dissolution rates of the glasses are comparable to that of base iron phosphate glass.     

Amorphous nanostructuring in potassium niobium silicate glasses by SANS and SHG: a new mechanism for second-order optical non-linearity of glasses

Potassium niobium silicate (KNS) glasses xK₂OxNb₂O₅(1−2x)SiO₂ with x=0.167; 0.182; 0.200; 0.220 and 0.250 have been subjected to prolonged heat treatments in a wide temperature range above T_g. As a result, glasses exhibiting liquid-type phase separation phenomena have been isolated. Moreover for each glass composition, the temperature zones have been determined to produce transparent, opalescent or opaque materials which have been studied by X-ray diffraction (XRD), small-angle neutron scattering (SANS) and second harmonic generation (SHG) techniques. SANS data unambiguously point at nanostructuring of KNS glasses in the scale of 5-20 nm under appropriate heat treatments near T_g. In contrast to initial KNS glasses, nanostructured glasses exhibit SHG activity. For earliest stages of phase separation SHG-active glasses are characterized by fully amorphous XRD patterns. Further development of phase separation in glasses with increasing of their opalescence leads to diminishing SHG, and subsequently partial crystallization takes place giving opaque materials. Since relative maximum of SHG efficiency corresponds to non-crystalline nanostructured glasses, such new transparent second-order non-linear media may be of both scientific and practical interest. With regard to non-crystalline structure of nano-inhomogeneities, SHG mechanism in the glasses is supposed to be due to a combination of third-order non-linearity with a spatial modulation of linear polarizability.     

The effect of PbF2 content on the microstructure and upconversion luminescence of Er-doped SiO2-PbF2-PbO glass ceramics

The Er³⁺ doped transparent oxyfluoride glass ceramics were obtained by appropriate heat treatment of the precursor glasses with composition (mol%) 50SiO₂-xPbF₂-(50 − x)PbO-0.5ErF₃. The microstructure and optical properties of the glasses and glass ceramics were determined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), absorption spectra and luminescence spectra. The intensity of upconversion luminescence significantly increased in glass ceramics compared to that in precursor glass. The emission bands centered around 660 nm (⁴F_9/2 → ⁴I_15/2) and 410 nm (²H_9/2 → ⁴I_15/2) were simultaneously observed in glass ceramics but cannot be seen in the corresponding precursor glass. The influence of different PbF₂ content on the microstructure and upconversion luminescence of the samples was analyzed in detail. The results indicated that with the increase of PbF₂ content, the Ω₂ was almost the same and the ratios of red to green upconversion luminescence decreased in glass ceramics.     

Structural characterization of phosphate glasses doped with silver

The present paper reports the influence of silver oxide addition on the local structure of 2P₂O₅ · CaO · 0.05ZnO glass matrix. The glass samples were investigated through several methods: X-ray powder diffraction (XRD), scanning electron microscopy (SEM), infrared absorption (FT-IR) and Raman scattering. The X-ray diffraction patterns confirm the vitreous character of these samples over the explored compositional range and the SEM pictures confirm this information. The phosphate structural units of the network former are assessed from FT-IR and Raman spectra as ultra-, meta-, pyro- and orthophosphate units. A slight structural depolymerization process of these phosphate-based glasses was evidenced for higher silver oxide content. In vitro behavior of the bulk glass sample with the highest silver oxide content was tested by immersion in simulated body fluid (SBF). X-ray diffraction and SEM measurements made on the SBF treated sample revealed growth of a crystalline phase on the surface sample.     

Structural characterization of Ni-based refractory glassy metals

X-ray scattering patterns of several Ni-based refractory alloy glasses indicated short range atomic order to at least four nearest neighbor shells. Comparisons between diffraction results from a synchrotron source vs. a standard laboratory source showed the necessity for low divergence and high intensity incident radiation in order to distinguish a low concentration of crystallites as small as ≈15 nm, which were present within an amorphous matrix. The divergence of both sources was examined by comparing the diffraction patterns of the same LaB₆ standard sample and using this as a reference standard to measure relative grain sizes. The crystallites in these glasses comprised between 0.0% and 7.5% by volume, and were relatively consistent amongst samples of the same composition. These crystalline peaks had potential matches with several metallic elements, compounds and oxides. It is seen that a very small composition range exists for near-perfect bulk metallic glass formers (as defined by a ≈0% crystallinity). Scattering electron microscopy was also performed to understand aspects of individual second phase particles such as size and distribution. Departure of radial distribution results from those predicted by hard sphere models indicated intermediate range order.     

The origin of nanostructuring in potassium niobiosilicate glasses by Raman and FTIR spectroscopy

The structure of potassium niobium silicate glasses in a wide compositional range has been studied by means of Raman and FTIR spectroscopy. The glasses spectra were compared to those of the KNbSi₂O₇ and K₃Nb₃O₆Si₂O₇ polycrystalline samples, obtained by crystallization of glasses of the same composition. It was found that the structure of such glasses is formed by SiO₄ tetrahedra and distorted NbO₆ octahedra. The amount of highly distorted (edge-sharing, non-bridging oxygens) octahedra results essentially unchanged from the glass composition. By contrast, the fraction of octahedra with a lower distortion degree (corner-sharing, bridging oxygens) increases with the Nb₂O₅ content. Raman and FTIR investigations indicate that during long heat treatments at temperatures near T_g, in the 23K₂O · 27Nb₂O₅ · 50SiO₂ glass, a structural change occurs regarding the amorphous matrix with a decrease of the niobium octahedra distortion. This can be related to a segregation process producing niobium rich regions nanometric in size. In the first heat treatment (2 h) the glass remains amorphous while for more prolonged heat treatments, nanocrystals of an unidentified phase are formed. In the same time the changes of the amorphous matrix hinder further crystallization.     

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.     

From amorphous phase separations to nanostructured materials in sol–gel derived ZrO2:Eu3+/SiO2 and ZnO/SiO2 composites

ZrO₂:Eu³⁺–SiO₂ and ZnO–SiO₂ composites have been synthesized by a sol–gel method by using a specific gelation and drying procedure. In the two cases we were able to produce large and transparent monolithic samples. Microstructural properties of these materials were investigated by thermo-differential and thermo-gravimetric analysis, X-ray diffraction, transmission electron microscopy and small angle X-ray scattering. The existence of a miscibility gap in both systems results in the formation of nanocomposites where crystallized zirconia or amorphous zinc oxide nanoparticles are dispersed in a silica glass matrix. These two kinds of nanocomposites are potential high efficiency luminescent materials because the nanoparticles size is easily controlled by the annealing conditions.     

Photoconductivity of oxychalcogenide glasses in the system As2Se3CdO

The photoconductivity of oxychalcogenide glasses in the system As₂Se₃CdO was investigated. When 2–3 mol % CdO was added, the photoresponse peak of the parent glass As₂Se₃ in the vicinity of 740 nm became broader and a little weaker. The addition of more than about 4 mol % CdO brought about a sharp and strong peak at 720 nm and a broad peak at 860 nm. X-ray diffraction and electron microscopic observations revealed the presence of crystalline CdSe in the glass matrix, indicating that the reaction As₂Se₃ + 3CdO → As₂O₃ + 3CdSe took place in the melting process of these glasses. Tempeature and light intensity dependences of the photocurrent lead to the conclusion that the above spectral photoresponse is greatly affected by the presence of the dispersed crystalline CdSe.     

The influence of strontium substitution in fluorapatite glasses and glass-ceramics

Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO₂3Al₂O₃1.5P₂O₅3CaO2CaF₂, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca₅(PO₄)₃F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The ²⁹Si, ²⁷Al and ³¹P NMR spectra for the glasses with different strontium contents were identical. The ¹⁹F spectra indicated the presence of F-Ca(n) and Al-F-Ca(n) species in the calcium glasses and in the strontium glasses F-Sr(n) and Al-F-Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium-strontium apatite and the fully strontium substituted glass to strontium fluorapatite.