Ceramic Crucible Corrosion by Heavy Metal Oxide Glasses, Part I: Phenomenological Study

Heavy metal oxide (HMO) glasses have received special attention due to their optical, electrical and magnetic properties. The problem with these glasses is their corrosive nature. In this work, three ceramic crucibles (Al 2O 3, SnO 2 and ZrO 2) were tested in the melting of the system 40 PbO-35 BiO 1.5-25 GaO 1.5 (cation-%). After glass melting, crucibles were transversally cut and analyzed by scanning electronic microscopy (SEM), coupled to microanalysis by energy dispersive spectroscopy (EDS). Results indicated that zirconia crucibles presented the highest corrosion, probably due to its smallest grain size. Tin oxide crucibles presented a low corrosion with small penetration of the glass into the crucible. This way, these crucibles are an interesting alternative to melt corrosive glasses in instead of gold or platinum crucibles. It is important to emphasize the lower cost of tin oxide crucibles, compared to gold or platinum ones.

Growth and Melting of Metallic Nanoclusters in Glass: A Review of Recent Investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-an-le X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi- or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Waoner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T(m) on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T(m) is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established.

(Table 2) Chemical composition of quenched glasses from the Mohns and Knipovich Ridges

The aim of this project was a petrogeochemical study of igneous rocks in the areas of the Mohns and Knipovich Ridges, both being the northern extensions of the Mid-Atlantic Ridge (MAR), using data available for quenching glass samples collected during Cruises 36 and 38 of R/V Akademic Mstislav Keldysh and during Cruise 15 of R/V Professor Logachev. Results of igneous rock studying from the Mohns and Knipovich Ridges at the background of evolution of the total North Atlantic Province, which had been identified earlier from tectonic and geophysical data, showed that igneous rocks of the Knipovich Ridge can be ranked as shallow tholeiites, primary melts of which were relatively rich in Na and Si and poor in Fe. This type of magma is characteristic of colder regions of the oceanic lithosphere. Its occurrence in the Knipovich Ridge and its potential propagation up to the Gakkel Ridge suggest that igneous rocks of this region originated under conditions of passive spreading in contrast to the MAR region in vicinity of Iceland and Azores, where substantial contribution of hotter material of a rising plume contributed to formation of the oceanic crust. The North Atlantic Ocean is the youngest province in terms of ocean-floor opening. Geologically and geophysically it is one of well studied regions of the World Ocean. Nevertheless some basic key items of its origin still remain to be clarified. In 1975 Scatler et al. proved specifics of this region manifested in growth of the gravity field, and also in relative height of the ocean floor in the region of 33-70°N, which was associated by them with rise of the hotter mantle, as compared with common regions of the Mid-Atlantic Ridge. Later this view was confirmed by character of magmatism, which differed in depth of generation and by melting degree of the resulting primary magma. Uniqueness of the North Atlantic region was also proved by the fact that this region was marked by extensive geochemical anomalies associated with Azores, Iceland, and Jan Mayen. All of these data allow to consider the northern part of the MAR (north of 33°N) as an united global geotectonic province. The Mohns and Knipovich Ridges located north of Iceland locate at the northern end of this province. This is the least known region. Therefore, new data for ridge areas of 73-77°N are needed for more complete geologic history of the Arctic Basin. The aim of this study was to carry out a complex comparison of magmatism at the Mohns and Knipovich Ridges with magmatism at large segments of the MAR northern province and to reconstruct mechanisms of primary magma formation, as well as conditions of their fractionation. This paper was based on results of studying quenched glasses, which reflect evolution of melt in the course of its formation.

Composition of matrix glasses and melt inclusions of fallout tephra from ODP Hole 125-782A

We studied the systematics of Cl, F and H2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ~400-4000 ppm and F ~70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (~6600-8600 ppm) and F (~780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ~2 to ~10 wt% H2O. The Izu magmas should be undersaturated in H2O and the halogens at their preferred levels of crystallization in the middle to lower crust (~3 to ~11 kbar, ~820° to ~1200°C). A substantial portion of the original H2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94+/-0.25 wt%, F = 990+/-270 ppm and H2O = 25+/-7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (~77-129%). Conversely, H2O (~13-22% recycled at arc) and F (~4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H2O, the calculated percentage of Cl and H2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (~37000 km), the global arc outflux of fluid-recycled Cl and H2O at subduction zones amounts to Cl ~2.9-3.8 mln ton/yr and H2O ~70-100 mln ton/yr, respectively - comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ~0.3-0.4 mln ton/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (~9.5), slab melting will still produce components with low Cl/F ratios (~0.9), similar to those characteristic of the upper continental crust (Cl/F ~0.3-0.9).

Chemical and isotopic compositions of basaltic glasses and crusts from the Mid-Atlantic Ridge rift valley

Data presented in the paper suggest significant differences between thermodynamic conditions, under which magmatic complexes were formed in MAR at 29°-34°N and 12°-18°N. Melts occurring at 29°-34°N were derived by melting of a mantle source with homogeneous distribution of volatile components and arrived at the surface without significant fractionation, likely, due to their rapid ascent. The MAR segments between 12° and 18°N combine contrasting geodynamic environments of magmatism, which predetermined development of a large plume region with widespread mixing of melting products of geochemically distinct mantle sources. At the same time, this region is characterized by conditions favorable for origin of localized zones of anomalous plume magmatism. These sporadic magmatic sources were spatially restricted to MAR fragments with the Hess crust, whose compositional and mechanical properties were, perhaps, favorable for focusing and localization of plume magmatism. The plume source between 12° and 18°N beneath MAR may be geochemically heterogeneous.

Influence of Water Content and Temperature on Molecular Mobility and Intracellular Glasses in Seeds and Pollen1

Although the occurrence of intracellular glasses in seeds and pollen has been established, physical properties such as rotational correlation times and viscosity have not been studied extensively. Using electron paramagnetic resonance spectroscopy, we examined changes in the molecular mobility of the hydrophilic nitroxide spin probe 3-carboxy-proxyl during melting of intracellular glasses in axes of pea (Pisum sativum L.) seeds and cattail (Typha latifolia L.) pollen. The rotational correlation time of the spin probe in intracellular glasses of both organisms was approximately 10−3 s. Using the distance between the outer extrema of the electron paramagnetic resonance spectrum (2Azz) as a measure of molecular mobility, we found a sharp increase in mobility at a definite temperature during heating. This temperature increased with decreasing water content of the samples. Differential scanning calorimetry data on these samples indicated that this sharp increase corresponded to melting of the glassy matrix. Molecular mobility was found to be inversely correlated with storage stability. With decreasing water content, the molecular mobility reached a minimum, and increased again at very low water content. Minimum mobility and maximum storage stability occurred at a similar water content. This correlation suggests that storage stability might be at least partially controlled by molecular mobility. At low temperatures, when storage longevity cannot be determined on a realistic time scale, 2Azz measurements can provide an estimate of the optimum storage conditions.

Re-melting of Bi-2212/Ag laminated tapes by partial melting process

Bi-2212 tapes are prepared by a combination of dip-coating and partial melt processing. We investigate the effect of re-melting of those tapes by partial melting followed by slow cooling on the structure and superconducting properties. Microstructural studies of re-melted samples show that they have the same overall composition as partially melted tapes. However, the fractional volumes of the secondary phases differ and the amounts and distribution of the secondary phases have a significant effect on the critical current. Critical current of Bi-2212/Ag tapes strongly depends on the maximum processing temperature. Initial J(c)'s of the tapes, which are partially melted, then slowly solidified at optimum conditions and finally post-annealed in an inert atmosphere, are up to 10.4 x 10(3) A/cm(2). It is found that the maximum processing temperature at initial partial melting has an influence on the optimum re-heat treatment conditions for the tapes. Re-melted tapes processed at optimum conditions recover superconducting properties after post-annealing in an inert atmosphere: the J(c) values of the tapes are about 80-110% of initial J(c)'s of those tapes.

Size-dependent melting of nanoparticles: Hundred years of thermodynamic model

Thermodynamic model first published in 1909, is being used extensively to understand the size-dependent melting of nanoparticles. Pawlow deduced an expression for the size-dependent melting temperature of small particles based on the thermodynamic model which was then modified and applied to different nanostructures such as nanowires, prism-shaped nanoparticles, etc. The model has also been modified to understand the melting of supported nanoparticles and superheating of embedded nanoparticles. In this article, we have reviewed the melting behaviour of nanostructures reported in the literature since 1909.

Influence of seeding on crystallization behaviour of BaNaB9O15 glasses

Transparent BaNaB9O15 (BNBO) glasses were fabricated via the conventional melt-quenching technique. X-ray powder diffraction (XRD) followed by differential scanning calorimetric (DSC) studies confirmed the amorphous and glassy nature of the as-quenched samples, respectively. The effect of seeding on the crystallization of BNBO glasses was studied by non-isothermal DSC method and was modeled using the Johnson-Mehl-Avrami and Ozawa equations. The activation energy for seeded glasses decreased with the increase in fraction of crystallization. The values for the onset of crystallization and Avrami exponent were found to be lower for seeded samples which were associated with the heterogeneous nucleation and epitaxial processes.

X-ray photoelectron spectroscopic studies of surface oxidation of metallic glasses

Surface oxidation of the metallic glass Fe40Ni38Mo4B18 has been studied by X-ray photoelectron spectroscopy. The oxidation behaviour of the metallic glass has been compared with a crystallized sample of the same composition. A similar study has been carried out on the metallic glass Ni76Si12B12,which shows the importance of chemical composition in determining the surface oxidation behaviour of these alloys.

Small Polaron Hopping And High-Temperature Dc Conductivity Of Chalcogenide Glasses

D.C. conductivity behaviour of a variety of chalcogenide glasses have been analysed using ln σ vs Image plots as suggested in the multiphonon assisted polaron hopping model of Triberis and Friedman. The agreement with the model is very satisfactory and further analysis of the model using c.

Axisymmetric melting of a long cylinder due to an infinite flux

By employing a new embedding technique, a short-time analytical solution for the axisymmetric melting of a long cylinder due to an infinite flux is presented in this paper. The sufficient condition for starting the instantaneous melting of the cylinder has been derived. The melt is removed as soon as it is formed. The method of solution is simple and straightforward and consists of assuming fictitious initial temperature for some fictitious extension of the actual region.

Structural, dielectric, impedance and optical properties of CaBi2B2O7 glasses and glass-nanocrystal composites

Optically clear glasses were fabricated by quenching the melt of CaCO3-Bi2O3-B2O3 (in equimolecular ratio). The amorphous and glassy characteristics of the as-quenched samples were confirmed via the X-ray powder diffraction (XRD) and differential scanning calorimetric (DSC) studies These glasses were found to. have high thermal stability parameter (S). The optical transmission studies carried out in the 200-2500 nm wavelength range confirmed both the as-quenched and heat-treated samples to be transparent between 400 nm and 2500 nm. The glass-plates that were heat-treated just above the glass transition temperature (723 K) for 6 h retained approximate to 60% transparency despite having nano-crystallites (approximate to 50-100 nm) of CaBi2B2O7 (CBBO) as confirmed by both the XRD and transmission electron microscopy (TEM) studies. The dielectric properties and impedance characteristics of the as-quenched and heat-treated (723 K/6 h) samples were studied as a function of frequency at different temperatures. Cole-Cole equation was employed to rationalize the impedance data.