Browsing by Author "Dowd, A"

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    AuCuAl shape memory alloys for use in nano-actuators
    (Australian Institute of Physics, 2010-02-02) Bhatia, VK; Kealley, CS; Thorogood, GJ; Dowd, A; Cortie, MB
    Although Al, Au and Cu each has the face centered cubic structure in elemental solid form, they do not readily alloy with one another, and instead form a series of binary and ternary intermetallic compounds. The ternary system is very interesting and contains, amongst other features, an 18-carat shape memory electron compound sometimes called ‘Spangold’ [1] ( Figure 1). Shape memory alloys are remarkable in that they have the ability to return to their initial state and shape after being deformed. This ability arises from a martensitic (displacive) phase transformation (see Figure 2), which is strongly influenced by temperature, crystal structure and degree of ordering. It is already known that the parent phase of ‘Spangold’ must contain at least a minimum degree of ternary ordering before the reversible displacive transformation needed for the shape memory effect can take place [3]. Furthermore, the parent phase has been found to have the L21 ordered body-centered cubic packing arrangement [4], while it has been reported that the martensite can be described using a monoclinic unit cell [5]. Here we examine the possibility of using this compound as a nano-actuator. Magnetron sputtering was used to deposit the Au, Cu and Al. The films were then characterized using x-ray reflectometry (see figure 3), grazing incidence X-ray diffraction, scanning electron microscopy and atomic force microscopy. The properties of these films are compared with that of bulk samples.
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    Electrochemical energy storage on nanoporous copper sponge
    (Springer Nature, 2022-03-24) McPherson, DJ; Dowd, A; Arnold, MD; Gentle, A; Cortie, MB
    A proof-of-principle double-layer symmetrical supercapacitor with nanoporous copper/copper oxide electrodes and an aqueous electrolyte is investigated. The electrodes are manufactured by selective dissolution of Al from a eutectic composition of Cu17.5Al82.5 using 5 M NaOH. The ostensible (i.e., net external) capacitance of a symmetrical two-electrode cell with 0.1 M KNO3 electrolyte is assessed over a series of charge/discharge cycles and is about 2 F per gram of Cu in this simple prototype. Capacitance varies during a discharge cycle due evidently to the deeply buried surfaces and pseudocapacitive reactions contributing charge toward the end of a discharge cycle. In principle such a device should have very low ohmic losses due to its highly conductive backbone and would be suitable for applications requiring maximum energy efficiency over repeated cycling. The aqueous electrolyte ensures fire safety but this comes at the cost of lower energy content. © The Authors - Open Access under a Creative Commons Attribution 4.0 International License
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    Residual stress in mollusc shells
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Cortie, MB; Dowd, A; He, KYH; Choi, A; Luzin, V
    The aragonite-protein composite material out of which the shells of most molluscs are made has a fracture toughness of about 8 MPa√m. This is surprisingly high considering that the aragonite phase has a bulk fracture toughness of only 0.9 MPa√m. One reason for the improved performance of the shell relative to pure aragonite is that it is comprised of a strongly textured array of aragonite platelets in an organic matrix. It is well known that cracks that initiate in an aragonite platelet are deflected, blunted or arrested when they reach the more ductile organic phase. We speculate that a compressive residual stress at strategic locations in the shell may further improve its resistance to crack propagation. To investigate this hypothesis we used neutron diffraction, X-ray diffraction and Raman spectroscopy in an attempt to identify whether a detectable stress distribution exists in a large mollusc shell and, if so, whether this stress state can provide enhanced fracture toughness. Freshly collected shells of the gastropod Ninella torquata (family Turbinidae), which has a diameter of about 10 cm, were used. The texture of the samples was readily extracted using neutron diffraction and an apparent residual stress gradient of several MPa identified. This effect was not evident in X-ray diffraction of powder samples taken in layers spaced through the wall thickness. The possible existence and implications of a non-uniform stress distribution through the shell are analyzed and discussed.
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    Strategies to control the spectral properties of Au–Ni thin films
    (Elsevier, 2014-01-31) McPherson, DJ; Supansomboon, S; Zwan, B; Keast, VJ; Cortie, DL; Gentle, A; Dowd, A; Cortie, MB
    Gold and nickel have quite different dielectric functions. Here we use a combination of calculation and sample manufacture to assess two strategies by which thin films of these elements can be produced with a controlled range of far-field optical properties. In the first approach, control can be achieved by manipulating the density of states of metastable solid solutions, which in turn controls the dielectric function. In the second approach the optical properties of the films are controlled by varying the geometry of stacks fabricated from the constituent elements. We show that the two approaches can produce equivalent results so both are viable options in practice. Modeling is used to reveal how the structure controls the optical properties and to map out the possible color gamut. Predictions are tested with thin film samples fabricated by magnetron sputtering. © 2013, Elsevier B.V.
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    Thin films of AuCuAl shape memory alloy for use in plasmonic nano-actuators
    (Cambridge University Press/Springer Nature, 2011-02-04) Bhatia, VK; Thorogood, GJ; Dowd, A; Cortie, MB
    We describe the fabrication and structure of nanoscale thin films of β phase shape memory alloys with the nominal atomic stoichiometry Au7Cu5Al4 (corresponding to 5.8 wt% Al). These alloys possess properties that suggest they could be used in nanoscale actuators. The films described here are between 20 and 50 nm thick which is below the thickness at which some other shape memory alloys cease to transform. However, microstructural and X-ray studies confirm that the coatings still exhibit the displacive transformations that are a prerequisite for the shape memory effect. © Materials Research Society 2011