Browsing by Author "Keller, Thomas"

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    Ductile-to-semiductile transition in PP-MWNT nanocomposites
    (Weinheim : Wiley-VCH, 2007) Satapathy, Bhabani K.; Ganß, Martin; Weidisch, Roland; Pötschke, Petra; Jehnichen, Dieter; Keller, Thomas; Jandt, Klaus D.
    A ductile-to-semiductile transition in the crack resistance behaviour of PP/MWNT composites is discussed, using an essential work of fracture approach based on a post yield fracture mechanics concept and its possible interrelation to the structural attributes studied by TEM, SEM, and WAXD. A maximum in the non-essential work of fracture is observed at 0.5 wt-% MWNT content, which demonstrates the enhanced resistance to crack propagation compared to pure PP, followed by a sharp decline with the increase in MWNT content to 1.5 wt.-%, which reveals a ductile-to-semiductile transition. Fracture kinetic studies present a qualitative picture of the nature of such a transition in terms of a) switch over from non-steady (in pure PP) to steady-state crack tip opening displacement rate (in nanocomposites), and b) a ductile-to-semiductile transition largely as a result of delayed- yielding of the nanocomposites.
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    Hidden Charge Order in an Iron Oxide Square-Lattice Compound
    (College Park, Md. : APS, 2021) Kim, Jung-Hwa; Peets, Darren C.; Reehuis, Manfred; Adler, Peter; Maljuk, Andrey; Ritschel, Tobias; Allison, Morgan C.; Geck, Jochen; Mardegan, Jose R. L.; Bereciartua Perez, Pablo J.; Francoual, Sonia; Walters, Andrew C.; Keller, Thomas; Abdala, Paula M.; Pattison, Philip; Dosanjh, Pinder; Keimer, Bernhard
    Since the discovery of charge disproportionation in the FeO2 square-lattice compound Sr3Fe2O7 by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained “hidden” to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO2 planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty of detecting them by conventional probes. We discuss the implications of these findings for research on “hidden order” in other materials.