Cooperative shear model for the rheology of glass-forming metallic liquids

A rheological law based on the concept of cooperatively sheared flow zones is presented, in which the effective thermodynamic state variable controlling flow is identified to be the isoconfigurational shear modulus of the liquid. The law captures Newtonian as well as non-Newtonian viscosity data for glass-forming metallic liquids over a broad range of fragility. Acoustic measurements on specimens deformed at a constant strain rate correlate well with the measured steady-state viscosities, hence verifying that viscosity has a unique functional relationship with the isoconfigurational shear modulus.     

Band structure of cubic NaxWO3

The band structures of cubic WO₃ and NaWO₃ have been calculated, and they are shown to be nearly identical, demonstrating the validity of the rigid band model for different sodium concentrations. To elucidate the nature of the bonding, the density of states is decomposed into its s, p, and d contributions from each atomic site.     

Quantum oscillations without quantum coherence

We study numerically the damping of quantum oscillations and the dynamics of the density matrix in model many-spin systems decohered by a spin bath. We show that oscillations of some density matrix elements can persist with considerable amplitude long after other elements, along with the entropy, have come close to saturation, i.e., when the system has been decohered almost completely. The oscillations exhibit very slow decay, and may be observable in experiments.     

Nonaqueous Phase Liquid Dissolution in Porous Media: Current State of Knowledge and Research Needs

Our understanding of nonaqueous phase liquid (NAPL) dissolution in the subsurface environment has been increasing rapidly over the past decade. This knowledge has provided the basis for recent developments in the area of NAPL recovery, including cosolvent and surfactant flushing. Despite these advances toward feasible remediation technologies, there remain a number of unresolved issues to motivate environmental researchers in this area. For example, the lack of an effective NAPLlocation methodology precludes effective deployment of NAPL recovery technologies. The objectives of this paper are to critically review the state of knowledge in the area of stationary NAPL dissolution in porous media and to identify specific research needs. The review first compares NAPL dissolutionbased mass transfer correlations reported for environmental systems with more fundamental results from the literature involving model systems. This comparison suggests that our current understanding of NAPL dissolution in smallscale (on the order of cm) systems is reasonably consistent with fundamental mass transfer theory. The discussion then expands to encompass several issues currently under investigation in NAPL dissolution research, including: characterizing NAPL morphology (i.e. effective size and surface area); multicomponent mixtures; scale-related issues (dispersion, flow by-passing); locating NAPL in the subsurface and enhanced NAPL recovery. Research needs and potential approaches are discussed throughout the paper. This review supports the following conclusions: (1) Our knowledge related to local dissolution and remediation issues is maturing, but should be brought to closure with respect to the link between NAPL emplacement theory (as it impacts NAPL morphology) and NAPL dissolution; (2) The role of nonideal NAPL mixtures, and intra-NAPL mass transfer processes must be clarified; (3) Valid models for quantifying and designing NAPL recovery schemes with chemical additives need to be refined with respect to chemical equilibria, mass transfer and chemical delivery issues; (4) Computational and large-scale experimental studies should begin to address parameter up-scaling issues in support of model application at the field scale; and (5) Inverse modeling efforts aimed at exploiting the previous developments should be expanded to support field-scale characterization of NAPL location and strength as a dissolving source.     

Anelastic to plastic transition in metallic glass-forming liquids

The configurational properties associated with the transition from anelasticity to plasticity in a transiently deforming metallic glass-forming liquid are studied. The data reveal that the underlying transition kinetics for flow can be separated into reversible and irreversible configurational hopping across the liquid energy landscape, identified with beta and alpha relaxation processes, respectively. A critical stress characterizing the transition is recognized as an effective Eshelby "backstress," revealing a link between the apparent anelasticity and the "confinement stress" of the elastic matrix surrounding the plastic core of a shear transformation zone.     

Polymerization reaction engineering: past,present and future

In this short review we describe some of the main developments of polymer reaction engineering since the early days of polymer science in the 1930's to the current challenges of today.     

Hydrogen bonding. Part 75. Infrared, titrametric and molecular orbital study of the structure, solubility and reaction with fluoride ion of 2,3-dicarboxypyrazine

2,3-Dicarboxypyrazine (2,3-H₂DCPz), unlike 2,3-dicarboxypyridine (2,3-H₂DCPy), does not exist as a zwitterion in the solid state. This is due to the remarkably low base strength of nitrogen in the pyrazine ring relative to that of pyridine. The decrease in the base strength of pyrazine relative to pyridine may result from greater disruption of the aromatic π-system of pyrazine on protonation than is the case with pyridine. We propose that 2,3-H₂DCPz has a structure of C_s symmetry in the solid state in which one carboxyl group forms an internal hydrogen bond to carbonyl of adjacent carboxyl in the same molecule, and the second carboxyl forms an intermolecular hydrogen bond to carbonyl in an adjacent molecule. The monoanion in 2,3-NaHDCPz has a strong O–H–O covalent three-center hydrogen bond between carboxylate groups. When saturated solutions of 2,3-H₂DCPz are treated with F⁻, one additional diacid molecule is taken into solution for each 1.5 F⁻ added. As is the case with 2,3-H₂DCPy, the high solubility and acid strength of 2,3-H₂DCPz (pK_a=2.87) leads to formation of the hydrogen-rich H₂F₃⁻ ion instead of HF₂⁻.     

Spin-flip induced magnetoresistance in positionally disordered organic solids

A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up "spin-blocked" pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.     

Fe K-edge X-ray resonant magnetic scattering from Ba(Fe1?xCox)2As2 superconductors

We present an X-ray resonant magnetic scattering study at the Fe-K absorption edge of the BaFe₂As₂ compound. The energy spectrum of the resonant scattering, together with our calculation using the full-potential linear-augmented plane wave method with a local density functional suggests that the observed resonant scattering arises from electric dipole (E1) transitions. We discuss the role of Fe K-edge X-ray resonant magnetic scattering in understanding the relationship between the structure and the antiferromagnetic transition in the doped Ba(Fe_1−x Co _x )₂As₂ superconductors.