Browsing by Author "Sherwood, John N."

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    Microhardness indentation studies of 2-4-6 trinitrotoluene
    (Wiley, 2021-10-20) Gallagher, Hugh G.; Sherwood, John N.; Vrcelj, Ranko
    The microhardness of the {001} faces of 2-4-6 trinitrotoluene crystals has been investigated using both Vickers and Knoop indentation methods. The Vickers hardness number was found to be 22.5 kg mm−2 independent of crystal orientation and perfection. At ambient temperatures (∼20 °C) the Knoop hardness number varied between 20.5 kg mm−2 and 24.0 kg mm−2 with crystal orientation. At higher temperature (50 °C) the Knoop hardness anisotropy curve retained its shape, although the overall hardness decreased by 10 %. We interpret this change as reflecting a simple temperature dependant loosening of the crystal lattice rather than any change in deformation mechanism. No variation of Knoop hardness was evident with changing load. The hard direction was [010] and the soft [100]. The dominant operative slip system was defined to be {001}[010].
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    Seeded crystal growth of the acentric organic non-linear optical material Methyl-p-Hydroxybenzoate (MHB) from the vapour phase
    (American Chemical Society, 2023-06-06) Hou, Wenbo B.; Ristic, Radoljub I.; Sherwood, John N.; Vrcelj, Ranko
    Using in-situ differential interference contrast microscopy (DICM), growth morphology, structure, and step velocities of the vicinal hillocks on {110} and {111̅} faces of MHB crystal seeds growing from the vapour phase have been investigated over a supersaturation (σ) range of (0.2 < σ < 0.6). Under these conditions of supersaturation, a dislocation induced growth mechanism was identified. Ex-situ atomic force microscopy (AFM) shows that some dislocation induced hillocks exhibit hollow cores. The general observations of the {110} and {111̅} surfaces reveal that these faces follow a classical mode of layer growth, continuous generation of new layers by dislocation outcrops, which subsequently bunch and spread to cover the entire facets. A tangential step velocity of the slow and fast sides of {110} and {111̅} growth hillocks show a linear dependence with supersaturation in the region of (0.2 < σ < 0.4). Analysis of this dependence leads to the respective growth parameters for the identified growth mechanism: the activation energies for the slow and fast step motion of a growth hillock (EaS and EaF) and the corresponding kinetic coefficients (βaS and βaF), for both faces. The growth from physical vapour transport (PVT) shows that for the title material, as with a number of other polar materials, solvent poisoning is not the cause of the highly differential growth rates and is an intrinsic feature of the crystal. The results suggest that in terms of the production of large single crystals of high perfection by PVT, the supersaturation range for dislocation growth should be between 0.2 and 0.4. These findings provide a foundation for the rational design of large MHB crystals that may find applications utilizing their high optoelectronic potential.