The Experts below are selected from a list of 195 Experts worldwide ranked by ideXlab platform

Michael L. Falk - One of the best experts on this subject based on the ideXlab platform.

  • Thermal effects in the shear-transformation-zone theory of amorphous plasticity: comparisons to metallic glass data.
    Physical Review E, 2004
    Co-Authors: Michael L. Falk, James S. Langer, Leonid Pechenik
    Abstract:

    We extend our earlier shear-transformation-zone theory of amorphous plasticity to include the effects of thermally assisted molecular rearrangements. This version of our theory is a substantial revision and generalization of conventional theories of flow in Noncrystalline Solids. As in our earlier work, it predicts a dynamic transition between jammed and flowing states at a yield stress. Below that yield stress, it now describes thermally assisted creep. We show that this theory accounts for the experimentally observed strain-rate dependence of the viscosity of metallic glasses, and that it also captures many of the details of the transient stress-strain behavior of those materials during loading. In particular, it explains the apparent onset of superplasticity at sufficiently high stress as a transition between creep at low stresses and plastic flow near the yield stress. We also argue that there are internal inconsistencies in the conventional theories of these deformation processes, and suggest ways in which further experimentation as well as theoretical analysis may lead to better understanding of a broad range of nonequilibrium phenomena.

  • Molecular-dynamics study of ductile and brittle fracture in model Noncrystalline Solids
    Physical Review B, 1999
    Co-Authors: Michael L. Falk
    Abstract:

    Molecular-dynamics simulations of fracture in systems akin to metallic glasses are observed to undergo embrittlement due to a small change in interatomic potential. This change in fracture toughness, however, is not accompanied by a corresponding change in flow stress. Theories of brittle fracture proposed by Freund and Hutchinson indicate that strain rate sensitivity is the controlling physical parameter in these cases. A recent theory of viscoplasticity in this class of Solids by Falk and Langer further suggests that the change in strain rate sensitivity corresponds to a change in the susceptibility of local shear transformation zones to applied shear stresses. A simple model of these zones is developed in order to quantify the dependence of this sensitivity on the interparticle potential.

  • Plastic Dynamics and Brittle vs. Ductile Failure in Noncrystalline Solids
    MRS Proceedings, 1998
    Co-Authors: Michael L. Falk
    Abstract:

    AbstractWe simulate fracture in two amorphous Solids with different inter-particle potentials. These small changes in potential result in significant changes in dissipation near the crack tip. While one might expect these effects to arise from a change in flow stress, measurements reveal this is not the case. To understand why, we consider the relationship between crack dynamics, rate-dependent plasticity, and molecular-level structures in the glassy solid. In particular we discuss the macro-scale continuum theory of dynamic brittle fracture in a viscoplastic solid developed by Freund and Hutchinson and the meso-scale theory of viscoplasticity proposed by Falk and Langer. We further consider a simplified model on the molecular scale as a first-step toward the construction of first-principles models of dynamic plasticity and the brittle ductile transition in Noncrystalline materials.

Raouf El-mallawany - One of the best experts on this subject based on the ideXlab platform.

  • Tellurite glasses. Part 1. Elastic properties
    Materials Chemistry and Physics, 1998
    Co-Authors: Raouf El-mallawany
    Abstract:

    Abstract Tellurite glasses (pure, binary, ternary and quaternary systems containing transition metal or rare earth oxides) are now a new type of Noncrystalline Solids with a lot of applications over a wide range of compositions, temperatures and frequencies. Elastic properties provide much information about the structures of Solids and they are directly related to the interatomic potentials. Glasses are isotropic and have only two independent elastic constant, C11 and C44. These two parameters have been collected from the longitudinal and shear sound velocities and density of the glass. The rest of the elastic constants (bulk, Young's modulus, Poisson's ratio) could be deduced. The hydrostatic and uniaxial pressure dependences of ultrasonic waves in these glasses at room temperature have been collected. The data proved both second and third order elastic constants of the glass and in consequence the shear and the longitudinal acoustic mode Gruneisen parameters have both been collected. The estimated bulk modulus and Poisson's ratio have been calculated using the bond compression model according to the cationanion bond of each oxide present in the glass. Information about the structure of the glass can be deduced after calculating the number of network bonds per unit volume, the value of the average stretching force constant, the average ring size, the structure sensitivity factor and the mean cross-link density. Comparisons between the calculated end experimental elastic moduli and Poisson's ratio have been carried out. Also, the role of halogen inside the glass network has been discussed.

Elfed Lewis - One of the best experts on this subject based on the ideXlab platform.

  • Tellurite Glass and Its Application in Lasers
    Advanced Functional Materials, 2020
    Co-Authors: Pengfei Wang, Shijie Jia, Yuxuan Jiang, Xin Wang, Shunbin Wang, Elfed Lewis
    Abstract:

    This chapter provides expert coverage of the physical properties of new Noncrystalline Solids—tellurite glass and the latest laser applications of the material—offering insights into innovative applications for laser and sensing devices, among others. In particular, there is a focus on specialty optical fibers, supercontinuum generation and laser devices, and luminescence properties for laser applications. This chapter also addresses the fabrication and optical properties and uses of tellurite glasses in optical fibers and optical microcavities, the significance of from near infrared (NIR) to mid-infrared (MIR) emissions and the development of tellurite glass-based microcavity lasers. The important attributes of these tellurite glasses and their applications in lasers were discussed in this chapter.

Bradley F. Chmelka - One of the best experts on this subject based on the ideXlab platform.

  • A General Protocol for Determining the Structures of Molecularly Ordered but Noncrystalline Silicate Frameworks
    2016
    Co-Authors: Darren H. Brouwer, Sylvian Cadars, Juergen Eckert, Zheng Liu, Osamu Terasaki, Bradley F. Chmelka
    Abstract:

    A general protocol is demonstrated for determining the structures of molecularly ordered but Noncrystalline Solids, which combines constraints provided by X-ray diffraction (XRD), one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles quantum chemical calculations. The approach is used to determine the structure(s) of a surfactant-directed layered silicate with short-range order in two dimensions but without long-range periodicity in three-dimensions (3D). The absence of long-range 3D molecular order and corresponding indexable XRD reflections precludes determination of a space group for this layered silicate. Nevertheless, by combining structural constraints obtained from solid-state 29Si NMR analyses, including the types and relative populations of distinct 29Si sites, their respective 29Si–O–29Si connectivities and separation distances, with unit cell parameters (though not space group symmetry) provided by XRD, a comprehensive search of candidate framework structures leads to the identification of a small number of candidate structures that are each compatible with all of the experimental data. Subsequent refinement of the candidate structures using density functional theory calculations allows their evaluation and identification of “best” framework representations, based on their respective lattice energies and quantitative comparisons between experimental and calculated 29Si isotropic chemical shifts and 2J(29Si–O–29Si) scalar couplings. The comprehensive analysis identifies three closely related and topologically equivalent framework configurations that are in close agreement with all experimental and theoretical structural constraints. The subtle differences among such similar structural models embody the complexity of the actual framework(s), which likely contain coexisting or subtle distributions of structural order that are intrinsic to the material

  • A general protocol for determining the structures of molecularly ordered but Noncrystalline silicate frameworks.
    Journal of the American Chemical Society, 2013
    Co-Authors: Darren H. Brouwer, Sylvian Cadars, Juergen Eckert, Zheng Liu, Osamu Terasaki, Bradley F. Chmelka
    Abstract:

    A general protocol is demonstrated for determining the structures of molecularly ordered but Noncrystalline Solids, which combines constraints provided by X-ray diffraction (XRD), one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles quantum chemical calculations. The approach is used to determine the structure(s) of a surfactant-directed layered silicate with short-range order in two dimensions but without long-range periodicity in three-dimensions (3D). The absence of long-range 3D molecular order and corresponding indexable XRD reflections precludes determination of a space group for this layered silicate. Nevertheless, by combining structural constraints obtained from solid-state (29)Si NMR analyses, including the types and relative populations of distinct (29)Si sites, their respective (29)Si-O-(29)Si connectivities and separation distances, with unit cell parameters (though not space group symmetry) provided by XRD, a comprehensive search of candidate framework structures leads to the identification of a small number of candidate structures that are each compatible with all of the experimental data. Subsequent refinement of the candidate structures using density functional theory calculations allows their evaluation and identification of "best" framework representations, based on their respective lattice energies and quantitative comparisons between experimental and calculated (29)Si isotropic chemical shifts and (2)J((29)Si-O-(29)Si) scalar couplings. The comprehensive analysis identifies three closely related and topologically equivalent framework configurations that are in close agreement with all experimental and theoretical structural constraints. The subtle differences among such similar structural models embody the complexity of the actual framework(s), which likely contain coexisting or subtle distributions of structural order that are intrinsic to the material.

Yoshitaka Ishii - One of the best experts on this subject based on the ideXlab platform.

  • Characterization of Polymorphs and Solid-State Reactions for Paramagnetic Systems by 13C Solid-State NMR and ab Initio Calculations
    Journal of the American Chemical Society, 2007
    Co-Authors: Medhat A. Shaibat, Leah B. Casabianca, Nalinda P. Wickramasinghe, Stephen Guggenheim, And Angel C. De Dios, Yoshitaka Ishii
    Abstract:

    Despite its importance in drug and material science, spectroscopic characterization of polymorphs or supramolecular structures of paramagnetic systems often poses challenges, particularly for Noncrystalline Solids. This work demonstrates that 13C solid-state NMR (SSNMR) of paramagnetic systems under very fast magic angle spinning (VFMAS) (spinning speed of 20 kHz or higher) provides exceptionally sensitive means to probe small structural difference among polymorphs of paramagnetic complexes in Noncrystalline Solids, including drugs and materials containing paramagnetic metal ions. 13C VFMAS SSNMR experiments and corresponding ab initio shift calculations for Cu(II)(8-quilinolinol)2, anticancer agents, show that α- and β-forms of this compound can be easily distinguished by notable difference in paramagnetic relaxation times and hyperfine shifts. It is also shown that applications of the present technique allow for quantitative and chemical characterization of solid-state reactions for Cu(8-quilinolinol)2 ...

  • Elucidating connectivity and metal-binding structures of unlabeled paramagnetic complexes by 13C and 1H solid-state NMR under fast magic angle spinning.
    The journal of physical chemistry. B, 2007
    Co-Authors: Nalinda P. Wickramasinghe, Medhat A. Shaibat, Yoshitaka Ishii
    Abstract:

    Characterizing paramagnetic complexes in Solids is an essential step toward understanding their molecular functions. However, methodologies to characterize chemical and electronic structures of paramagnetic systems at the molecular level have been notably limited, particularly for Noncrystalline Solids. We present an approach to obtain connectivities of chemical groups and metal-binding structures for unlabeled paramagnetic complexes by 13C and 1H high-resolution solid-state NMR (SSNMR) using very fast magic angle spinning (VFMAS, spinning speed >or=20 kHz). It is experimentally shown for unlabeled Cu(II)(Ala-Thr) that 2D 13C/1H correlation SSNMR under VFMAS provides the connectivity of chemical groups and assignments for the characterization of unlabeled paramagnetic systems in Solids. We demonstrate that on the basis of the assignments provided by the VFMAS approach multiple 13C-metal distances can be simultaneously elucidated by a combination of measurements of 13C anisotropic hyperfine shifts and 13C T1 relaxation due to hyperfine interactions for this peptide-Cu(II) complex. It is also shown that an analysis of 1H anisotropic hyperfine shifts allows for the determination of electron-spin states in Fe(III)-chloroprotoporphyin-IX in solid states.

  • Published on Web 04/05/2005 Enhanced Sensitivity and Resolution in 1 H Solid-State NMR Spectroscopy of Paramagnetic Complexes under Very Fast Magic Angle Spinning
    2004
    Co-Authors: Nalinda P. Wickramasinghe, Medhat Shaibat, Yoshitaka Ishii
    Abstract:

    Paramagnetic complexes in Solids have attracted increasing interest due to their diverse applications in modern material science, 1,2 bioinorganic chemistry, 3 and pharmacology. 4 Characterizing these paramagnetic complexes is essential to understand their functions and design improved systems. However, the methodologies for characterizing paramagnetic systems have been limited, compared to those for diamagnetic systems, in particular, for Noncrystalline Solids. Electron paramagnetic resonance (EPR) is a standard method for analyzing paramagnetic systems. However, EPR typically requires isotope labeling to obtain structural information on ligands through a hyperfine dipolar coupling. Solution NMR, a powerful tool for organic compounds, often exhibits limited resolution and sensitivity for paramagnetic materials because of paramagnetic broadening. 5 Also, solution NMR does not provide unique characteristics in Solids such as morphologies, which can alter essential properties of materials and drugs. Solid-state NMR (SSNMR) is a powerful method for structural analysis of Noncrystalline Solids. Among various nuclei, 13 C SSNMR has been most widely applied for its excellent resolution. However, the limited sensitivity of 13 C SSNMR has required larger amount of samples (0.1-1 mmol), compared with other analysis because of low abundance of 13 C. 1 H SSNMR is an attractive alternative to 13 C SSNMR, particularly for unlabeled systems and samples in limited quantities because of its high sensitivity. 6,7 I