P. B. Macedo

Intrinsic and impurity infrared absorption in As2Se3 glass

Abstract A quantitative study of infrared absorption in the 250–4000 cm −1 region of As 2 Se 3 glasses doped with small amounts of As 2 O 3 or purified by various procedures has been carried out with particular attention to absorption in the wavelength regions of the CO 2 and CO lasers. The dependence of the relative intensities of the oxide impurity bands in the 650–1340 cm −1 region on the total amount of As 2 O 3 added to the glass indicates the existence of three distinct oxide-impurity species. A number of higher-frequency impurity bands which are due to the presence of hydrogen in the glass and whose intensities are highly dependent on the glass-melting conditions have been observed and classified. Intrinsic multiphonon absorption in the 400–1100 cm −1 region has been interpreted in terms of combination and overtone bands of the two highest-frequency fundamental vibrational modes. Absorption coefficients of As 2 Se 3 glass in the 920–1090 cm −1 CO 2 laser region are limited by intrinsic multiphonon absorption to values of around 10 −2 cm −1 . The lowest absorption coefficients measured in the 1700–2000 cm −1 CO laser region were around 2 × 10 −3 cm −1 and may contain contributions from hydrogen-impurity bands.

Application of the Environmental Relaxation Model to the Temperature Dependence of the Viscosity

An equation relating viscous flow processes to local microstructure is applied to the viscosity of nine materials belonging to different classes of glass formers. It is seen that the model proposes the correct general behavior for the viscosity and the associated activation energy distributions. The model further explains this behavior in terms of a temperature‐dependent ordering process in the structure of the material occurring at low temperatures. Finally, a simultaneous fit of the viscosity and the conductivity of a molten salt yields parameters whose relationships are physically meaningful in terms of the viscous relaxation and conduction relaxation processes.

Ultrasonic Relaxations in a Vitreous Ca(NO3)2–KNO3 Mixture

Ultrasonic relaxations have been studied in a vitreous 60% KNO3–40% Ca(NO3)2 mixture (mol%) from 1 to 185 MHz in the temperature range 90–130°C. The spectrum of longitudinal relaxation times has been found to be symmetric, fitting a Gaussian distribution in the logarithm of relaxation times, and having a temperature dependent width which narrows remarkably at the high‐temperature end. The occurrence of a high‐temperature sharp distribution of relaxation times in a molten salt system raises a major objection to the currently held belief that the origin of such sharp, high‐temperature distributions, found in molten oxides for example, originate from the covalent nature of the molecular bonds. It has been shown in this paper that a topological model previously proposed by Simmons and Macedo, correlating the distribution of relaxation times to the distribution of molecular environments, provides an excellent qualitative description of the characteristics of the observed relaxation spectra.

Multiphonon absorption in As2S3As2Se3 glasses

Abstract Multiphonon absorption in As2S3 and As2Se3 glasses is well explained by a molecular model in terms of combination bands of high frequency vibrational modes of pyramidal AsY3 and bent AsYAs groups (Y = S or Se). Multiphonon absorption coefficients in mixed As2S3As2Se3 glasses are nearly additive in terms of the pure components, suggesting a high degree of non-random mixing.

The chemical durability of tektites—A laboratory study and correlation with long-term corrosion behavior

Leach tests carried out on tektite specimens (indochinites and australites) under high-dilution conditions show a common behavior characterized by low leach rates (1.8 × 10−5g · m−2 · d−1or 7.2 × 10−12m · d−1 at 23°C) and an activation energy of (79.6 ± 0.7) × 103J · mol−1. The extent of selective leaching is very small, of the order of 10−8 m. Extrapolation of test results over the lifetime of the tektites gives an excellent agreement with field observations on the extent of corrosion, and this is an important step in establishing the validity of laboratory tests as a basis for the development of models and predictions concerning long-term durabilities at least in the limiting case of high dilution or rapid flow. The results are also shown to be in agreement with various previous observations on the corrosion resistance of tektites. The chemical durability of tektites is observed to be consistent with their composition, highlighting requirements for high corrosion resistance in glasses; these requirements include a silica content in excess of 67 mol%, an extremely low water content and an alkali content which is low both absolutely and relative to the di- and poly-valent metal oxide levels. It is shown that artificial glasses which fulfil these criteria are no less corrosion-resistant than the corresponding natural glasses. These conclusions have bearing on the development as well as on the evaluation of glasses intended for very long service, such as radioactive waste vitrification media.

Light Scattering in Aqueous LiCl Solutions; Evidence for a Low Temperature Immiscibility

Rayleigh and Brillouin light scattering spectra have been obtained in a number of concentrated aqueous LiCl solutions. The scattering from density fluctuations shows the dispersion effects typical of viscoelastic relaxation as the temperature is decreased. In the same temperature region the Rayleigh scattering associated with fluctuations in concentration becomes quite intense. This is interpreted as portending the presence of a miscibility gap in the LiCl–H2O system. The upper consolute temperature associated with this immiscibility is quite near the glass transition region so that no large‐scale phase separation can be seen. In addition it appears there is yet another mechanism producing Rayleigh scattering in the solutions. We have tentatively identified this as arising from fluctuations in the order parameter(s) describing the progress of ionic hydration reaction(s).

Corrosion mechanisms and chemical durability of glass media proposed for the fixation of radioactive wastes

Results from leach tests conducted on simulated wastes fixated in borosilicate and high-silica glasses are presented. During the leaching period, the dissolution rates of several waste components were monitored. These components included alkali metals, alkaline earths, transition metals, rare earths, and an actinide, and it is shown how they differ and how the measurement of leach rates of some of these components can be misleading regarding long-term behavior. The problems of applying the results to the prediction of long-term dissolution behavior are discussed; models are presented for reliable long-term predictions, and the essential tests which must be conducted to allow the formation of these reliable predictions for long-term behavior are listed.

Light scattering in a number of optical grade glasses

Abstract Light scattering measurements have been made on a number of commercially available optical grade glasses. By measuring the spectrum as well as intensity of the scattered light it was found possible both to obtain the total scattered intensity and to identify the origin of the scattering. The optical attenuation coefficient due to scattering was evaluated. It was found that in all glasses studied the total scattered intensity exceeds that in fused silica by at least a factor of two.

Kinetics of concentration fluctuations in a binary alkali–silicate system

Laser light scattering was applied to a potassium–silicate system to observe the growth of supercritical concentration fluctuations. The Rayleigh and Brillouin spectra of our glass samples were measured as a function of heat treatment time duration and temperature. From the spectra we obtained the Landau–Placzek ratios and Brillouin shifts for each heat treatment interval and temperature. In glass forming melts, critical points may occur in regions of high shear viscosity; here because of the low mobility, relaxation times for concentration fluctuations are on the order of many hours. Thus, heat treatment followed by rapid quenching ’’freezes in’’ the fluctuations characteristic of the treatment temperature, and consequently the light scattering measurements detect the high temperature structure. The measurements reported here provide the first direct evidence for a phase transition in a potassium–silicate system. From the equilibrium Landau–Placzek ratios, the spinodal temperatures were calculated. We al...

Degenerate Excited State in the Structure of B2O3

A degenerate excited state is essential to the description of structural rearrangements which account for the relaxational heat capacity, thermal expansion, and compressibility of vitreous B2O3. These relaxational responses cannot be simultaneously described by a two‐state model, or by any model involving a single ordering parameter, as shown by the magnitude of the Prigogine—Defay ratio (π≠1). The apparent two‐state behavior of the temperature derivatives, Δα and Δcp, and failure of a two‐state model to predict a sufficiently large compressibility (pressure derivative), suggest an analysis of structural rearrangements in vitreous B2O3 in terms of a single ground state and a twofold degenerate excited state which is characterized by different volumes.