Sidney Alterescu

Leaching of natural and nuclear waste glasses in sea water

Microtektites, some with a SiO2 content as low as 50%, exhibit a very limited extent of corrosion after having been exposed to interaction with their environment in sea-bed sediments for periods of 105a. Measurements carried out on a bottle-green microtektite glass composition in de-ionized water show that this glass has relatively high leach rates, as expected on the basis of its low SiO2 content (≈ 50%) and high modifier content (≈ 20%). However, is sea water the leach rates are smaller by at least 2 orders of magnitude, in agreement with the small extent of microtektite corrosion observed in nature (typically<5μ). Systematic studies of leachant composition effects show that the suppression of leaching in sea water is probably due to the presence of high levels of Mg. Measurements were also carried out on man-made silicate glasses related in composition to nuclear waste glasses. Although the compositions of the man-made glasses are different in many important respects from the composition of microtektites, it was found that the leach rates of the man-made glasses, too, are smaller by about 2 orders of magnitude in sea water compared with their values in de-ionized water. Surface analysis of glasses exposed to sea water shows that the predominant feature is the formation of a layer which is very rich in Mg.

A comparative study of sintered and melt-grown recrystallized YBa 2 Cu 3 O x

This paper compares the properties of yttrium barium cuprate bulk specimens prepared by means of solid-state sintering on one hand and of growth from a largely molten state on the other. The current paper focuses on magnetization measurements at 77 K which show specimens made by the latter method to have high remanence magnetization values.

Yttrium enrichment and improved magnetic properties in partially melted Y-Ba-Cu-O materials

Samples of Y-Ba-Cu-O materials with the formulation Y:Ba:Cu = x :2:3 and values of x ranging between 1 and 3 were prepared by partial melting at a maximum temperature of 1045 °C. Measurements of magnetic susceptibility and maximum (low-field) as well as remanent magnetization show highest values for x = 2. XRD and SEM/EDX analyses show that the corresponding structure involves numerous small crystals of Y 2 BaCuO 5 (211) embedded in highly ordered assemblages of continuous YBa 2 Cu 3 O 7− y (123) layers. The presence of these impurity sites is correlated with flux pinning capacity. Other impurity phases include CuO, a minor phase which shows an increase in amount when x increases, BaCuO 3 , a possibly detrimental minor phase which vanishes when x is raised from 1 to 2, and zones of intermediate composition between the 211 grains and the 123 layers, which are formed from the residual liquid upon cooling. These intermediate regions, like the 211 grains themselves, become gradually more important when x is increased from 1 to 3.

Superconducting cuprates prepared by the melt quench process and containing excess Y or additives

Bulk YBaCuO samples doped with excess Y or other additives (including rare earths and Nb) and prepared by means of the melt quench process exhibit high magnetization and diamagnetic susceptibility. These can be attributed to the formation of a microstructure which is highly aligned and has few cracks and voids. These characteristics are associated with the presence of a second phase. The importance of this study lies in its applicability to the preparation of polycrystalline superconducting oxides with very high Jc values through careful control of their microstructure, in particular through the use of non-stoichiometric formulations and the introduction of additives in conjunction with melt-based processing methods.

The low magnetic field properties of superconducting bulk yttrium barium copper oxide-sintered versus partially melted material

A comparison of the low magnetic field properties of sintered (990°C) and partially melted samples (1050°C) has been performed. Changes in the microstructure produced by recrystallization from the melt result in a significant increase in flux pinning at 77 K. Low-frequency (10–100 Hz), low-a.c. magnetic field (0.01–9.0 Oe) a.c. susceptibility data show that gross changes in the a.c. loss component accompanies the observed changes in microstructure. The effects of applied d.c. magnetic fields (10–220 Oe) on the a.c. responses of these microstructures have also been probed. Data are analyzed and critically discussed in terms of current models appropriate for granular superconductors and in terms of older models appropriate for metallic alloys and compounds. Particular attention is given to published interpretations of the in-phase or loss component of the a.c. magnetic susceptibility and to the possible roles which minority phases and sample inhomogeneities may play in determining the detailed a.c. responses of these high-Tcoxides.

Bulk YBa2Cu3O(x) superconductors through pressurized partial melt growth processing

A novel pressurized partial melt growth process has been developed for producing large pieces of bulk Y-Ba-Cu-O superconductors. During long-time partial melt growth stage, an additional driving force for solidification is obtained by using pressurized oxygen gas. The microstructure and superconducting properties of the resulting samples were investigated. It was found that this new technique can eliminate porosity and inhomogeneity, promote large-scale grain-texturing, and improve interdomain coupling as well.

Elemental fractionation and magnetic properties of melt-based Y1Ba2Cu3Oz containing excess Tb or Pt

Abstract Scanning electron microscopy of certain partially melted Y-Ba-Cu-O materials containing minority metal oxide species (Y:Tb:Ba:Cu=1:0.1:2:3 or Y:Ba:Cu with Pt impurities), accompanied by both EDX and EMP analysis, indicates that the minority species (Tb or Pt) is quantitatively concentrated in a relatively small number of 123-type grains. High magnetic susceptibility and magnetization observed for these materials indicate that such elemental distribution is not detrimental to superconducting behavior.

Interactions of silicate glasses with aqueous environments under conditions of prolonged contact and flow

This paper discusses mechanisms involving saturation and reactions that lead to the formation of altered phases in silicate glasses considered for use in geologic repositories for nuclear waste. It is shown that the rate of dissolution of silicate glasses exposed to a broad range of contact times, leachant compositions, and surface-to-volume ratios is strongly affected by the presence of reactive species such as Al, Mg, and Fe. The reactive materials may originate in the leachant or, under conditions of high surface-to-volume ratio, in the glass itself. The effects of glass composition on the course of the corrosion process can be viewed in terms of the formation of a surface layer on the leached glass; the type, composition, and structure of this layer control the dissolution behavior of the glass.