The effect of composition on spinel crystals equilibrium in low-silica high-level waste glasses

The liquidus temperature (T_L) and the equilibrium mass fraction of spinel were measured in the regions of low-silica (less than 42 mass% SiO₂) high-level waste borosilicate glasses within the spinel primary phase field as functions of glass composition. The components that varied, one at a time, were Al₂O₃, B₂O₃, Cr₂O₃, Fe₂O₃, Li₂O, MnO, Na₂O, NiO, SiO₂, and ZrO₂. In the low-silica region, Cr₂O₃ increased the T_L substantially less, and Li₂O and Na₂O decreased the T_L significantly less than in the region with 42-56 mass% SiO₂. The temperature at which the equilibrium mass fraction of spinel was 1 mass% was 25-64 °C below the T_L.     

Redox freezing temperature and Bartenev equation in a 25Na2O-15B2O3-60SiO2 glass doped with chromium and manganese

In a melt with the base mol% composition 25Na₂O-15B₂O₃-60SiO₂, doped with chromium and manganese, a redox reaction takes place during cooling the melt. This reaction was studied using high temperature UV-vis spectroscopy. Above 600 °C, the reaction is in equilibrium and shifted during cooling to the Cr³⁺ and Mn³⁺ species. At temperatures between 500 and 600 °C, the kinetics of the redox reaction is decisive and the cooling rate plays an important part. At temperatures < 500 °C, the reaction is frozen in. The smaller the cooling rate, the smaller is the Cr⁶⁺ concentration and the lower is the fictive redox temperature.The kinetics of the reaction was described by a differential equation assuming Arrhenian behaviour. The equation was numerically solved and fictive temperatures were calculated. These temperatures depended on cooling rate similar to Bartenev equation. Activation energies calculated hereof were around 38 kJ?mol⁻¹ larger than those inserted into the kinetic equation. The experimentally determined activation energy is 565 kJ?mol⁻¹, a value much larger than the activation energies of diffusion of the polyvalent elements. The rate determining step in the case of the Cr³⁺/Cr⁶⁺/Mn²⁺/Mn³⁺ system is the electron transfer reaction, because a notable structural rearrangement is necessary during the course of the electron transfer reaction (Cr³⁺ and Cr⁶⁺ occur in octahedral and tetrahedral coordination, respectively). The latter leads to a large inner reorganisation energy and to an activation energy similar to that of the viscous flow. In the case of the redox reaction between copper and arsenic, the activation energy is much smaller (210 kJ?mol⁻¹), because here the coordination numbers do not change during the course of the redox reaction.     

Thermal properties and crystallization of iron phosphate glasses containing up to 25 wt% additions of Si-, Al-, Na- and U-oxides

The thermal properties (expansion, T_g and T_SOFT.) of glasses, having 56-66% P₂O₅, 14.8-34.2% Fe₂O₃ and 2-25 wt% additions of SiO₂, Al₂O₃, Na₂O and UO₂, were comparatively estimated from dilatometric measurements in similar conditions. The T_g reversibility was clearly verified by varying the heating rates between 1 and 5 °C min⁻¹. From linear equations fits of the various glass properties as functions of the six components it is suggested the iron, sodium and uranium oxides decrease the thermal expansion (for 50 < T ? 300 °C), T_g and T_SOFT. From DTA/XRD analysis of three glasses it was confirmed the crystallization tendency decreased with increasing the UO₂ level in the glasses. Leaching test data for two compositions containing Na₂O suggest addition of UO₂ increases the chemical durability of the related glass. The roles of UO₂, Na₂O and Fe-oxide species as structural components of the glass network are discussed.     

Thermo-optic properties of B2O3 doped Li2O-Al2O3-SiO2 glass-ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li₂O-Al₂O₃-SiO₂ glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The dS/dT value of 2.6 mol% B₂O₃-doped glass-ceramic was 12.5  × 10⁻⁶/°C, which was 0.9 ×  10⁻⁶/°C smaller than that of B₂O₃-free glass-ceramic. On the other hand, reduction in dS/dT through B₂O₃ doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass-ceramic through B₂O₃ doping is caused by the reduction in ?. The reduction in ? from B₂O₃ doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B₂O₃-doped glass-ceramic crystallized under 196 MPa was 11.7 ×  10⁻⁶/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass.     

Structural investigation of sodium borate glasses and melts by Raman spectroscopy. II. Conversion between BO4 and BO2O units at high temperature

Raman scattering spectra were recorded for pure boron oxide and sodium borate glasses and their melts at the temperature ranging from room temperature to 1200 °C to investigate the structural changes occurring in the melts. The amounts of short-range order structures (SRO), BO₂O⁻ and BO₃, were estimated from the high frequency bands at 1100-1600 cm⁻¹. The ratio of 4-fold coordinated boron oxide BO₄, N₄, at high temperature could be derived for the glass melts as a function of Na₂O concentration. In Na₂O ?20 mol% region, N₄ showed a slight decrease while the remarkable decrease of N₄ was found in the region of Na₂O ?25 mol% with increasing temperature. The enthalpy of the equilibrium reaction,

     

Optimization of Cr8O21 targets for Pulsed Laser Deposition

This work describes the preparation of Cr₈O₂₁ pellets with optimised mass density to be used as targets for Pulsed Laser Deposition (PLD) of chromium dioxide thin films. Cr₈O₂₁ is synthesised by thermal decomposition of CrO₃, at 270 °C. An attempt to reduce the grain size of the Cr₈O₂₁ powder to the nanometer scale has been made in order to increase the density of the pressed and sintered pellets serving as targets. The morphology of those starting fine powders as well as of targets (before and after laser ablation) and the corresponding thin films were characterized and studied using X‐ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Raman spectroscopy. The composition of the films is a mixture of crystallised Cr₂O₃ and amorphous CrO₂. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chromium phase behavior in a multi-component borosilicate glass melt

This paper reports the phase behavior of a multi-component borosilicate glass melt with 0–3 mass% Cr₂O₃ at 800–1500 °C in equilibrium with air. Both upper and lower liquidus temperatures were observed. When the temperature was between the upper and lower liquidus temperatures, eskolaite (Cr₂O₃) formed in melts with >2 mass% Cr₂O₃. Below the lower liquidus temperature, a dispersed chromate phase appeared in the melt that eventually became macroscopically segregated. The chemical durability of the glasses was virtually unaffected by chromium concentration. The particular glass studied was prototypic for vitrifying high-Cr high-level radioactive wastes stored in underground tanks at the Hanford site. The results suggest a significant potential cost benefit for Hanford tank waste cleanup through increased waste solubility in glass.     

A low silica, barium borate glass-ceramic for use as seals in planar SOFCs

A low silica, barium borate glass-ceramic for use as seals in planar SOFCs containing 64 mol%BaO, 3 mol%Al₂O₃ and 3 mol%SiO₂ was studied. Coefficient of thermal expansion (CTE) between 275-550 °C, glass transition temperature (T_g), and dilatometric softening point (T_s) of the parent glass were 11.9 × 10⁻⁶ °C⁻¹, 552 °C, and 558 °C, respectively. Glass-ceramic was produced by devitrification heat treatment at 800 °C for 100 h. It was found that nucleation heat treatment, seeding by 3 wt%ZrO₂ as glass-composite and pulverization affected the amount, size and distribution of crystalline phases. SEM-EDS and XRD results revealed that crystalline phases presented in the devitrified glass-ceramic were barium aluminate (BaAl₂O₄), barium aluminosilicate (BaAl₂Si₂O₈) possibly with boron associated in its crystal structure, and barium zirconate (BaZrO₃). CTE of the devitrified glass-ceramic was in the range of (10.1-13.0) × 10⁻⁶ °C⁻¹. Good adhesion was obtained both in the cases of glass and devitrified glass-ceramic with YSZ and AISI430 stainless steel. Interfacial phenomena between these components were discussed.     

Planar waveguides by ion exchange in Er-doped tellurite glass

This work reports the preparation of planar waveguides by Ag⁺ → Na⁺ ion exchange in Er³⁺-doped tellurite glass with a composition of 75TeO₂-2GeO₂-10Na₂O-12ZnO-1Er₂O₃ (mol%). The metric, of T_x − T_g, indicates that the glass has good thermal stability. Measurments of refractive index, absorption spectrum, luminescence and lifetime were made. The glass was chemically stable during the ion exchange process. Monomode and multimode planar waveguides in the tellurite glasses have been prepared. We determined the depth of the guides, effective diffusion coefficient and the activation energy. The depths of the waveguides could be controlled by varying ion exchange temperatures and times (250-280 °C, and 3-12 h were used).     

LEED-AES-Untersuchungen der Oxydation von austenitischen Cr-Ni-Stählen an der (100)-Ebene

Oxidation of austenitic stainless steel has been studied on the (100)-face. Temperature region examinated reaches from room temperature to 1000°C. Oxidation begins with formation of chromium oxide (Cr₂O₃). After this iron oxide (Fe₃O₄) covering the chromium oxide arises gradually. At 500°C Fe₃O₄ is destroyed, and a layer of chromium oxide increases. At 700°C the LEED-pattern was observed to represante Cr₂O₃(111), and at 750°C you can prove FeO(111) on that. At 1000°C the oxide layer is destroyed. All the oxides grow in form of islands.     

Thermal stability, spectra and laser properties of Yb: lead-zinc-telluride oxide glasses

A series of new glasses of 70TeO₂-(20 − x) ZnO-xPbO − 5La₂O₃-2.5K₂O-2.5Na₂O (mol%) doped with Yb³⁺ is presented. Thermal stability, spectra and laser properties of Yb³⁺ ions have been measured. It found that 70TeO₂-15PbO-5ZnO-5La₂O₃-2.5K₂O-2.5Na₂O composition glass had fine stability ((T_x−T_g)>190 °C), high-stimulated emission cross-section of 1.25 pm² for the ²F_5/2 → ²F_7/2 transition and existed measured fluorescence lifetime of 0.94 ms and the broad fluorescence effective linewidth of 72 nm. Evaluated from the good potential laser parameters, this system glass is excellent for short pulse generation in diode pumped lasers, short pulse generation tunable lasers, high-peak power and high-average power lasers.     

A square-wave voltammetric study on the diffusivities of polyvalent elements in CaO/BaO/Al2O3/SiO2 glass melts

Diffusion coefficients of various polyvalent ions (Sn²⁺, As³⁺, As⁵⁺, Sb³⁺, Sb⁵⁺, Cr³⁺, Ti⁴⁺, V⁴⁺, V⁵⁺ and Fe³⁺) were measured in melts with the basic compositions of 10CaO·10 BaO·10Al₂O₃·70SiO₂ and 10CaO·10BaO·15Al₂O₃·65SiO₂ by means of square-wave voltammetry. At temperatures in the range of 1300-1600 °C, linear correlations between logD and 1/T were observed. At 1400 °C, the diffusion coefficients obtained are compared with those obtained from other glass melt compositions.     

Physical properties of lead iron phosphate glasses containing Cr2O3

The influence of Cr₂O₃ on glass forming characteristics and physical properties of PbO-Fe₂O₃-P₂O₅ glasses has been investigated by Raman and Mössbauer spectroscopies, X-ray diffraction analysis (XRD), Differential Thermal Analysis (DTA), Scanning Electron Microscopy (SEM) and impedance spectroscopy. Glasses of the general composition xCr₂O₃-(28.3-x)PbO-28.7Fe₂O₃-43.0P₂O₅, 0 ≤ × ≤ 10, (mol%) were prepared by conventional melt-quenching technique. The compositions containing up to 4 mol% Cr₂O₃ formed fully amorphous samples and their Raman spectra show systematic increase in the fraction of orthophosphate Q⁰ units with increasing Cr₂O₃ content and O/P ratio.On the other hand, compositions containing 8 and 10 mol% Cr₂O₃ partially crystallized during cooling and annealing to Fe₇(PO₄)₆, Fe₂Pb₃(PO₄)₄ and Cr₂Pb₃(PO₄)₄. A high tendency for crystallization of these melts is related to the high O/P (> 4) and Fe²⁺/Fe_tot (≈ 0.60) ratios.Electrical conductivity of xCr₂O₃-(28.3-x)PbO-28.7Fe₂O₃-43.0P₂O₅, 0 ≤ × ≤ 10, (mol%) compositions is independent of Cr₂O₃ and controlled entirely by the polaron transfer between Fe²⁺ and Fe³⁺ ions.     

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.     

Thermodynamics of the Cu/Cu-redox equilibrium in soda-silicate and soda-lime-silicate melts

The thermodynamics of the redox equilibrium of Cu⁺/Cu²⁺ were determined by square-wave voltammetry in glass melts with the base mol% compositions x Na₂O · (100 − x) SiO₂ (x = 15, 20, 26 and 33) and (26 − x) Na₂O · x CaO · 74 SiO₂ (x = 0, 5, 10 and 15) doped with 1 mol% CuO in the temperature range from 850 to 1150 °C. All recorded voltammograms showed two maxima attributed to the reductions of Cu²⁺ to Cu⁺ and Cu⁺ to metallic copper. Both peaks are shifted to smaller potentials with decreasing temperature. With increasing melt basicity, the [Cu⁺]/[Cu²⁺]-ratio first increases, and remains constant for optical basicities >0.56. The effect of composition on the redox equilibrium is explained by the incorporation of both Cu⁺ and Cu²⁺ in octahedral coordination into the melt structure.     

Structural properties of Cr2O3–Fe2O3–P2O5 glasses,Part I

The structural properties of xCr₂O₃–(40 − x)Fe₂O₃–60P₂O₅, 0 ⩽ x ⩽ 10 (mol%) glasses have been investigated by Raman and Mössbauer spectroscopies, X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The Raman spectra show that the addition of up to 5.3 mol% Cr₂O₃ does not produce any changes in the glass structure, which consists predominantly of pyrophosphate, Q¹, units. This is in accordance with O/P ≈ 3.5 for these glasses. The increase in glass density and T_g that occurs with increasing Cr₂O₃ suggests the strengthening of glass network. The Mössbauer spectra indicate that the Fe²⁺/Fe_tot ratio increases from 0.13 to 0.28 with increasing Cr₂O₃ content up to 5.3 mol%, which can be related to an increase in the melting temperature from 1423 to 1473 K. After annealing, the 10Cr₂O₃–30Fe₂O₃–60P₂O₅ (mol%) sample was partially crystallized and contained crystalline β-CrPO₄ and Fe₃(P₂O₇)₂. The SEM and AFM micrographs of the partially crystallized sample revealed randomly distributed crystals embedded in a homogeneous glass matrix. EDS analysis indicated that the glass matrix was rich in Fe₂O₃ (39.6 mol%) and P₂O₅ (54.9 mol%), but contained only 5.5 mol% of Cr₂O₃. These results suggest that the maximum solubility of chromium in these iron phosphate melts is 5.5 mol% Cr₂O₃.     

The effect of composition on UV-vis-NIR spectra of iron doped glasses in the systems Na2O/MgO/SiO2 and Na2O/MgO/Al2O3/SiO2

Glasses with the compositions xNa₂O · 10MgO · (90 − x)SiO₂, 10Na₂O · xMgO · (90 − x)SiO₂, 5Na₂O · 15MgO · xAl₂O₃ · (80 − x)SiO₂, xNa₂O · 10MgO · 10Al₂O₃ · (80 − x)SiO₂, 10Na₂O · 10MgO · xAl₂O₃ · (80 − x)SiO₂, 10Na₂O · 5MgO · 10Al₂O₃ · (80 − x)SiO₂ were melted and studied using UV-vis-NIR spectroscopy in the wavenumber range from 5000 to 30 000 cm⁻¹. At [Al₂O₃] > [Na₂O], the UV-cut off is strongly shifted to smaller wavenumbers and the NIR peak at around 10 000 cm⁻¹ attributed to Fe²⁺ in sixfold coordination gets narrower. Furthermore, the intensity of the NIR peak at 5500 cm⁻¹ increases. This is explained by the incorporation of iron in the respective glass structures.     

Dielectric analysis of tungsten-phosphoniobate 20A2O-30WO3 -10Nb2O5 -40P2O5 (A = Li, Na) glass-ceramics

New phosphate glasses of the quaternary system A₂O-Nb₂O₅-WO₃-P₂O₅, where X = Li and Na were prepared by the melt-quenching method. The introduction of WO₃ in the glass composition was based on the proposal of analysing the effect of the diminishing of the molar amount of the alkaline oxide and thus decreasing the molar ratio between network modifiers and network formers (M/F).In the present work we present the preparation of 20A₂O-30WO₃-10Nb₂O₅-40P₂O₅ (A = Li, Na) transparent glasses. These glasses were heat-treated in air, at 550 °C and 650 °C for 4 h. The structure of the obtained samples was studied by X-ray powder diffraction (XRD) and Raman spectroscopy and the morphology by scanning electron microscopy (SEM). The dc (σ_dc), ac (σ_ac) conductivity and dielectric spectroscopy measurements were performed in the function of the temperature and were related with the structural changes of the glass structures.     

The effect of alumina on the Sn/Sn redox equilibrium and the incorporation of tin in Na2O/Al2O3/SiO2 melts

Glasses with the basic compositions 10Na₂O · 10CaO · xAl₂O₃ · (80 − x)SiO₂ (x=0, 5, 15, 25) and 16Na₂O · 10CaO · xAl₂O₃ · (74 − x)SiO₂ (x=0, 5, 10, 15, 20) doped with 0.25-0.5 mol% SnO₂ were studied using square-wave-voltammetry at temperatures in the range from 1000 to 1600 °C. The voltammograms exhibit a maximum which increases linearly with increasing temperature. With increasing alumina concentration and decreasing Na₂O concentration the peak potentials get more negative. Mössbauer spectra showed two signals attributed to Sn²⁺ and Sn⁴⁺. Increasing alumina concentrations did not affect the isomer shift of Sn²⁺; however, they led to increasing quadrupole splitting, while in the case of Sn⁴⁺ both isomer shift and quadrupole splitting increased. A structural model is proposed which explains the effect of the composition on both the peak potentials and the Mössbauer parameters.     

Properties and structure of binary tin phosphate glasses

The properties and structures of binary xSnO*(100 − x)P₂O₅ (50 ≤ x ≤ 70 mol%) glasses were evaluated. The glass transition temperatures (T_g), determined by differential thermal analysis (DTA), range from 246 to 264 °C, for glasses prepared under identical conditions. The refractive index (n_D) increases from 1.701 to 1.833 as x increases from 50 to 70, and the Abbe number (ν_D) decreases from 29.1 to 20.4 over the same range. Infrared spectroscopy was used to estimate water contents in the glasses, which decreased with an increase in SnO content, from about 1570 ppm OH for x = 50 to about 50 ppm OH for x = 70, for glasses quenched from melts held at 1000 °C for 15 min. Residual water affects thermal properties, like T_g, and variations in water contents due to differences in melt processing explain the wide variety of glass properties reported in the literature. Raman spectroscopy indicates that progressively shorter phosphate chains are present in the structures of the binary Sn-phosphate glasses with increasing SnO contents.