Chemistry of Uranium in Glass-Forming Melts: Redox Interactions of Uranium with Chromium and Iron in Aluminosilicates

The mutual interactions by internal redox reactions of uranium and chromium and uranium and iron in two aluminosilicate glass-forming melts were studied by optical spectrophotometry. Cr(VI) was found to oxidize completely all available U(V) and U(IV) to U(VI), whereas Fe(III) was not a strong enough oxidizing agent to affect the uranium redox states. Cr(II) totally reduced all the U(V) and U(VI) in the system to U(IV); Fe(II) did not reduce any of the uranium redox species. Cr(III) coexisted with U(VI), U(V), and U(IV) in the melts. The degree of mutual interaction was governed not only by the difference in redox potentials between the respective redox couples, but also by the stability of molecular associations between the redox ions within the melt microstructure.     

Fracture Induced in Al2O3 Bicrystals by Anisotropic Thermal Expansion

Bicrystals of Al₂O₃ were fabricated to study the effects of thermal expansion anisotropy on fracture in a model system containing one grain boundary. Fractures occurred perpendicular to the directions of maximum tensile stress in bicrystals with thermal expansion coefficient differences as low as 0.31×10⁻⁶⁰C⁻¹ and originated at the boundary, probably in areas of high residual stress associated with pores. Numerical stress analysis of two-dimensional model bicrystal configurations showed that stresses induced by thermal expansion anisotropy are maximal in a region localized along the bicrystal boundary.     

Chemical Strengthening of Polycrystalline Alumina

Low-expansion solid solution surface layers formed by reaction of Cr₂O₃ and Al₂O₃ were used to chemically strengthen alumina bodies. The surface layers were formed by packing the samples in Cr₂O₃ powder and heating to high temperatures. Fluorides or chlorides were added to the packing material, leading to substantial additional improvement. The flexural strengths of the control samples were increased by refiring in the presence of the products of decomposition of the fluorides. At high concentrations of fluorides in the packing material, the outside dimensions and the weights of the samples decreased during the treatments.     

Dyadic output feedback laws generated as Kronecker products : optimal and suboptimal solutions

The problem of eigenvalue assignment in the system dx/dt equals; Ax + Bu, y = Dx, using the dyadic output feedback law u = u₀ + q · p^Ty is considered via a formulation developed earlier by the author, in which p and q occur in the Kronecker product vector p?q. The equations governing the values of p and q which give an optimum approximation to a prescribed spectrum of eigenvalues are derived, and a special case is solved. Various facets of the problem of generating suboptimal solutions are discussed.     

“THERE'S NO PLACE LIKE NEW ORLEANS”: SENSE OF PLACE AND COMMUNITY RECOVERY IN THE NINTH WARD AFTER HURRICANE KATRINA

ABSTRACT: This study contributes to the literature on the strength of place attachment, identity, and dependence in low‐ and moderate‐income neighborhoods. It also engages the literature concerning the role of sense of place in community engagement and the disruption in place attachment, identity, and dependence that natural disasters can cause. By drawing on interview data collected from residents who returned to New Orleans after the storm and from former New Orleanians who evacuated to Houston but did not return, this article investigates the “sense of place” that residents in Ninth Ward New Orleans neighborhoods identify in their narratives about their pre‐ and post‐Hurricane Katrina experiences. The data considered here suggest that returning residents believe that New Orleans in general (and their Ninth Ward neighborhoods in particular) possess a unique bundle of characteristics that, when taken together, cannot be found or replicated elsewhere. While sense of place is an important motivator for returning residents, the data also suggest that complementary factors must be in place if the full potential of this social resource is to be realized.     

Interface study for stainless steel fibre-reinforced aluminium matrix composite

Regular interface zones with uniform thickness in AISI 304 stainless steel-reinforced aluminium-matrix composite have been obtained using a vacuum high-pressure diffusion-bonding technique. Extensive and intensive experiments were performed to examine the growth of interfacial compounds with the variation of hot-pressing time and temperature. In the initial stage, the overall growth rate of the interface was found to follow a parabolic law. After a certain diffusion time, the interface growth rate fell behind that predicted by the parabolic law. A modified parabolic law has been established to explain the deviation and proved to be a better model to fit the experimental data. An activation energy of 152 kJ mol^-1 was found, which was somewhat lower than that obtained by previous work. The lower value of activation energy is attributed to the pressure (70 MPa) applied during hot pressing. Energy dispersive spectroscopic analysis and microhardness measurement indicated that the interface zone consists of a mixture of intermetallic compounds Fe(Cr, Ni)Al₂, Fe(Cr, Ni)Al₃ and (Fe, Cr, Ni)₂Al₇. This revised version was published online in November 2006 with corrections to the Cover Date.