Ian L. Pegg

Systematic investigation of the phase behavior in binary fluid mixtures. I. Calculations based on the Redlich–Kwong equation of state

The original Redlich–Kwong equation, together with the usual quadratic mixing rules, has been used to calculate phase diagrams for binary fluid mixtures and to classify them according to the system of van Konynenburg and Scott. Global phase diagrams (maps) showing the extent of the various phase diagram classes in the space of the Redlich–Kwong parameters are presented. While for molecules of equal size the results are very similar to those known for the van der Waals equation, the maps become topologically different for molecules of unequal sizes; some complicated phase diagram classes, which otherwise cover small domains on the maps or cannot be found at all, become quite important.

Raman studies of sulfur in borosilicate waste glasses: sulfate environments

Abstract Raman spectroscopy has been used to characterize sulfur environments in a variety of borosilicate glass formulations developed for long-term radioactive waste storage. The spectra of these glasses all have S–O symmetrical stretch modes (ν1) near 1000 cm −1 from tetrahedral SO4 (sulfate) environments. The Raman data indicate that the sulfate environments are independent of the borosilicate network; in particular, isolated SO4 tetrahedra in the glass are surrounded by network modifying cations, such as Na. By changing the type of network modifying cations in the borosilicate glass, the ν1 peak shifts, in such a way that larger cation charge densities correlate to higher ν1 frequencies. The ν1 peak for Li, Ca, Na, K, and Cs borosilicate glasses is broadened and shifted to lower frequencies with respect to ν1 for Li, Ca, Na, K, and Cs sulfate crystals, respectively; this indicates that sulfate tetrahedra in borosilicate glass are more disordered and more weakly bonded to their surrounding environments than sulfate tetrahedra in the corresponding sulfate crystals.

X-ray absorption studies of manganese valence and local environment in borosilicate waste glasses

Abstract X-ray absorption data were collected and analyzed to characterize the manganese environments in borosilicate glass formulations to be used for immobilization of nuclear wastes. Mn can become a significant constituent in some radioactive wastes, because of the use of Mn-compounds in waste pretreatment processes. Sixteen borosilicate glasses were investigated, which were synthesized to simulate the Mn environments in the anticipated waste glasses, where MnO concentrations range from 0.4 to 13.6 wt%. The X-ray absorption near edge structure (XANES) for all glasses investigated indicate that most of the manganese within these samples is divalent. The extended X-ray absorption fine structure (EXAFS) analysis results for the glasses show average Mn–O distances near 2.07 A, coordination numbers between 4.3 and 5.2, and large first-shell Debye–Waller factors. The EXAFS findings indicate that Mn2+ in borosilicate glass is most likely within a distribution of environments that include 4- and 5-coordinated sites. EXAFS data and fitting results also show that the average manganese environments in these glasses are statistically invariant with respect to composition as well as to synthesis conditions.

Structural characterization of high-zirconia borosilicate glasses using Raman spectroscopy

Polarized Raman spectra were obtained for a collection of borosilicate glasses that have been developed as candidate compositions for the immobilization of wastes generated from the reprocessing of Zircaloy-clad spent nuclear fuel. Raman spectra were obtained for borosilicate glasses with zirconia compositions as high as 21 wt%, as well as for crystalline ZrO2 (baddeleyite) and crystalline ZrSiO4 (zircon). As zirconia content in the glass is increased, two trends in the spectra indicate that the partially polymerized silicate tetrahedral network becomes more depolymerized: one, the polarized mid frequency envelope near 450 cm ˇ1 , assigned to Si‐O‐Si symmetrical bend modes, decreases in area; and two, the higher frequency band assigned to Si‐O stretch in Q 2 units (silicate chains) increases in area, while band areas decrease for modes assigned to Si‐O stretch in more polymerized Q 3 and Q 4 units (silicate sheets and cages). These trends take place whether the glass composition is relatively simple or considerably more complex. As zirconia concentrations in the glass increase beyond 15 wt%, a series of sharp lines are observed in the spectra from baddeleyite crystals, and to a minor extent Zn‐Cr spinel phases, superimposed on broad features from the glass matrix. A low intensity, broad band near 1400 cm ˇ1 in the glass spectra is probably due to B‐O stretch modes within BO3 units. ” 2000 Elsevier Science B.V. All rights reserved.

X-ray absorption studies of vanadium valence and local environment in borosilicate waste glasses using vanadium sulfide, silicate, and oxide standards

X-ray absorption spectroscopic data were collected and analyzed to characterize vanadium in borosilicate glasses used for immobilization of sulfur-containing nuclear wastes. Data are presented for borosilicate glasses, some with and some without sulfur, that have V2O5 concentrations as high as 12 wt%, and for the sulfides: sulvanite and patronite, the silicates: cavansite, hadaraite, and roscoelite, and the oxide: vanadinite. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data for the glasses have no sulfur dependent features, but do show changes that parallel various redox conditions for the corresponding melts. EXAFS data for the glasses indicate V–O distances near 1.70 A that are considerably shorter than typical V–S distances found in the sulfides. Both XANES and EXAFS indicate that most or all vanadium in these glasses is in the form of V5+O4 tetrahedra; glasses synthesized under reducing conditions can have penta-coordinated V4+ populations up to approximately 20–25% of all vanadium present.

X-ray absorption studies of the local environment of Zr in high-zirconia borosilicate glasses

Zirconium X-ray absorption spectroscopic (XAS) data were collected and analyzed for a series of zirconia borosilicate glasses developed as potential formulations for immobilization of nuclear fuel reprocessing wastes. For the first time, XAS are presented for silicate glasses with zirconia concentrations as high as 21 wt% and for zektzerite (LiNaZrSi6O15), a crystalline standard that is chemically similar to the glasses. Two other glass series were investigated where the CaO and ZnO contents were varied individually, while the Zr content was held constant. The analysis results indicate that Zr4+ is predominantly in octahedral sites in the glasses. Trends in the data also show that the Zr environments in the glasses are sensitive to changes in ZrO2 concentration, but are less sensitive to changes in Zn or Ca content. With addition of zirconia, the Zr site population shifts from mostly six-coordinated with minor amounts of seven-coordinated Zr, to having a larger fraction of seven-coordinated Zr sites, while maintaining a majority of octahedral Zr. The seven-coordinated Zr atoms probably act as nucleation sites in the glass; as the zirconia content increases to 15 wt% and higher, the numbers of these sites become large enough to promote ZrO2 baddeleyite crystallization. There is no evidence of Zr–Zr pair correlations, similar to those observed in baddeleyite, in any of the crystal-free glasses.

Large low field magnetoresistance in La0.67Sr0.33MnO3 nanowire devices

Large low field magnetoresistance (LFMR) of about 28% is observed in La0.67Sr0.33MnO3 nanowires with 80 nm in diameter at T=300 K. A gradual decrease in the LFMR has been found with increase in wire diameter. The LFMR drops to zero for wires above 280 nm in diameter. The nanowires are grown by means of electrospinning process and exhibit distorted orthorhombic crystal structure. The large LFMR is considered as a grain boundary effect as observed in several perovskite systems. The large LFMR observed in these manganites with reduced dimensions may be useful for room temperature device applications.

The critical lines of the van der waals equation for binary mixtures around the van Laar point

Abstract In this paper it is pointed out that the differences in critical behavior of the various classes as observed in p-T diagrams are better understood in terms of density-concentration (or equivalent) coordinates. The position of the line of instabilities, which determines the transition from unstable to metastable or stable points on the critical line, is helpful in determining the class of a phase diagram, since its position is shown to be relatively insensitive to changes in the energy parameters. The critical lines and instability lines in p-T coordinates are either tangent or almost to each other, and moreover show several cusps, which makes it very difficult to determine the intersection points which are responsible for the change in character of the critical line. The procedure is illustrated by means of the algebraic equations for the critical lines near the van Laar point.

A glass dissolution model for the effects of S/V on leachate pH☆

Abstract The observed relationship between the ratio of the glass surface area to the volume of the leachant ( S / V ) and the leachate pH is discussed in terms of a simple model of the glass dissolution process. Data from leach tests on several nuclear waste glass compositions at different S / V ratios show that the leachate pH increases with time and stabilizes at a nearly constant value beyond about 28 days. This stabilized pH increases systematically with the S / V ratio of the test. The model developed here reproduces the essential features of the data and suggests that a single parameter describing the intrinsic rate of alkali diffusion and ion exchange from the glass is sufficient to represent the major glass composition dependence. Interestingly, the results are essentially independent of the rate constant for matrix dissolution. This study suggests that the diffusion-ion exchange process is central in determining the solution pH and its dependence on S / V . Under static or low-flow-rate conditions, the rate of the matrix hydrolysis reaction slows to a long-term value that is determined by the (diffusional) rate of alkali release.

Correlation between composition effects on glass durability and the structural role of the constituent oxides

In this article the effects on durability of composition variations in West Valley Nuclear Services Company preliminary waste glass composition WV205 are discussed. MCC-3 results at times from 7 to 180 days are presented for 50 glass compositions. The results are suggestive of a large plateau region where durability is good and weakly dependent on composition, adjoining a region in which durability is a much steeper function of composition. The same effect is observed when the redox state of the iron, which comprises --12wt% of the glass, is varied. The general trends are discussed in terms of the structural roles of the components. The effects of the alkalies and alkaline earths correlate quite well with the field strengths of these ions.