Elastic heterogeneity in metallic glasses

When a stress is applied on a metallic glass it deforms following Hook's law. Therefore it may appear obvious that a metallic glass deforms elastically. Using x-ray diffraction and anisotropic pair-density function analysis we show that only about 3/4 in volume fraction of metallic glasses deforms elastically, whereas the rest of the volume is anelastic and in the experimental time scale deform without resistance. We suggest that this anelastic portion represents residual liquidity in the glassy state. Many theories, such as the free-volume theory, assume the density of defects in the glassy state to be of the order of 1%, but this result shows that it is as much as a quarter.     

Loop grafting of Bacillus subtilis lipase A: inversion of enantioselectivity

Lipases are successfully applied in enantioselective biocatalysis. Most lipases contain a lid domain controlling access to the active site, but Bacillus subtilis Lipase A (LipA) is a notable exception: its active site is solvent exposed. To improve the enantioselectivity of LipA in the kinetic resolution of 1,2-O-isopropylidene-sn-glycerol (IPG) esters, we replaced a loop near the active-site entrance by longer loops originating from Fusarium solani cutinase and Penicillium purpurogenum acetylxylan esterase, thereby aiming to increase the interaction surface for the substrate. The resulting loop hybrids showed enantioselectivities inverted toward the desired enantiomer of IPG. The acetylxylan esterase-derived variant showed an inversion in enantiomeric excess (ee) from -12.9% to +6.0%, whereas the cutinase-derived variant was improved to an ee of +26.5%. The enantioselectivity of the cutinase-derived variant was further improved by directed evolution to an ee of +57.4%.     

Efficient simultaneous reverse Monte Carlo modeling of pair-distribution functions and extended x-ray-absorption fine structure spectra of crystalline disordered materials

An efficient implementation of simultaneous reverse Monte Carlo (RMC) modeling of pair distribution function (PDF) and EXAFS spectra is reported. This implementation is an extension of the technique established by Krayzman et al. [J. Appl. Cryst. 42, 867 (2009)] in the sense that it enables simultaneous real-space fitting of x-ray PDF with accurate treatment of Q-dependence of the scattering cross-sections and EXAFS with multiple photoelectron scattering included. The extension also allows for atom swaps during EXAFS fits thereby enabling modeling the effects of chemical disorder, such as migrating atoms and vacancies. Significant acceleration of EXAFS computation is achieved via discretization of effective path lengths and subsequent reduction of operation counts. The validity and accuracy of the approach is illustrated on small atomic clusters and on 5500-9000 atom models of bcc-Fe and α-Fe(2)O(3). The accuracy gains of combined simultaneous EXAFS and PDF fits are pointed out against PDF-only and EXAFS-only RMC fits. Our modeling approach may be widely used in PDF and EXAFS based investigations of disordered materials.     

A new approach to stereospecific synthesis of P-chiral phosphorothioates. Preparation of diastereomeric dithymidyl-(3'-5') phosphorothioates

A new method for stereospecific synthesis of P-chiral phosphorothioates based on intramolecular nucleophile catalysis was developed.     

In Situ High‐Energy Synchrotron Radiation Study of Boehmite Formation,Growth, and Phase Transformation to Alumina in Sub‐ and Supercritical Water

Boehmite (AlOOH) nanoparticles have been synthesized in subcritical (300 bar, 350 °C) and supercritical (300 bar, 400 °C) water. The formation and growth of AlOOH nanoparticles were studied in situ by small‐ and wide‐angle X‐ray scattering (SAXS and WAXS) using 80 keV synchrotron radiation. The SAXS/WAXS data were measured simultaneously with a time resolution greater than 10 s and revealed the initial nucleation of amorphous particles takes place within 10 s with subsequent crystallization after 30 s. No diffraction signals were observed from Al(OH)₃ within the time resolution of the experiment, which shows that the dehydration step of the reaction is fast and the hydrolysis step rate‐determining. The sizes of the crystalline particles were determined as a function of time. The overall size evolution patterns are similar in sub‐ and supercritical water, but the growth is faster and the final particle size larger under supercritical conditions. After approximately 5 min, the rate of particle growth decreases in both sub‐ and supercritical water. Heating of the boehmite nanoparticle suspension allowed an in situ X‐ray investigation of the phase transformation of boehmite to aluminium oxide. Under the wet conditions used in this work, the transition starts at 530 °C and gives a two‐phase product of hydrated and non‐hydrated aluminium oxide.     

Non-Destructive Characterization of Engineering Materials Using High-Energy X-rays at the Advanced Photon Source

Engineering materials are typically made from transition metals and exist in polycrystalline form. The ability to characterize this class of materials, both ex situ and in situ, is particularly important as the demand for efficient, environment-friendly machines rises and the need to understand the material response at a smaller length scale increases. In the past decade, the use of high-energy synchrotron X-rays to characterize bulk engineering materials has increased significantly, with several high-energy beamlines and new characterization techniques coming online around the world.     

Non-invasive characterization of manufacturing techniques and corrosion of ancient Chinese bronzes and a later replica using synchrotron X-ray diffraction

Three bronze vessels from the ancient Chinese art collection at the Art Institute of Chicago (AIC) were examined—with the advanced non-invasive characterization capabilities of high-energy synchrotron X-ray diffraction performed at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL)—to create a comprehensive overview of each object’s manufacture as well as subsequent corrosion processes. Findings were also complemented with traditional non-invasive characterization techniques, including optical imaging, X-ray radiographic imaging, and X-ray fluorescence (XRF) spectrometry. The results—obtained without sampling—allow a clear distinction between genuinely ancient Chinese bronzes from those with modern restorations and from “archaistic” objects made many centuries later, in emulation of ancient styles.     

Emerging Opportunities in High-Energy X-ray Science: The Diffraction-Limited Storage Ring Frontier

The worldwide march to electron storage rings with diffraction-limited photon properties in the X-ray regime is well underway. First out of the gate is MAX-IV in Sweden, scheduled for operation in 2016, followed by SIRIUS in Brazil in 2018; both are greenfield rings operating at 3 GeV with ~520 m circumference and emittances of ~250 pm-rad. They will be followed by the upgrade of ESRF, operating at 6 GeV with a target emittance of 150 pm-rad and operational date of 2020. The upgrade of the APS at Argonne National Laboratory (6 GeV, 60 pm-rad) is anticipated to follow shortly thereafter.     

Microstructure and thermal stability of arc-evaporated Cr–C–N coatings

The role of C incorporation in the microstructure and thermal stability of arc-evaporated Cr–C–N coatings is explored via reactive growth in a mixed C₂H₄–N₂ environment. C is found to react more readily than N at both the Cr cathode and the coating surfaces, so that a C₂H₄-to-N₂ flow ratio of only 1% yields a C-to-N ratio of approximately 10% within the coatings. The as-deposited microstructures consist primarily of the δ-Cr(C,?N) phase and possess high compressive residual stresses, which decrease with increasing C content. Post-deposition annealing up to 700°C results in depletion of lattice defects, and concomitant reductions in stress and coating hardness, together with phase transformations which suggest metastable phase formation during growth. Apparent activation energies for this lattice defect are found to be in the range expected for bulk diffusion of N and C (2.4–2.8?eV). The results suggest that inclusion of small amounts of C in this system offers the ability to reduce internal stresses while maintaining defect-related hardness increases, permitting growth of thicker and thus more wear-resistant coatings.