Browsing by Author "Cortie, MB"

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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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    Copper diffusion rates and hopping pathways in superionic Cu 2Se: implications for thermoelectricity
    (SSRN, 2020-10-21) Nazrul Islam, SMK; Mayank, P; Ouyang, Y; Chen, J; Sagotra, AK; Li, M; Cortie, MB; Mole, RA; Cazorla, C; Yu, DH; Wang, XL; Robinson, RA; Cortie, DL
    The ultra-low thermal conductivity of Cu2Se is well established, but there is so far no consensus on the underlying mechanism. One proposal is that the fast-ionic diffusion of copper suppresses the acoustic phonons. The diffusion coefficients reported previously, however, differ by two orders of magnitude between the various studies and it remains unclear whether the diffusion is fast enough to impact the heat-bearing phonons. Here, a two-fold approach is used to accurately re-determine the diffusion rates. Ab-initio molecular dynamics simulations, incorporating landmark analysis techniques, were closely compared with experimental quasielastic/inelastic neutron spectroscopy. Reasonable agreement was found between these approaches, consistent with the experimental coefficient of 3.1 ± 1.3 10-5 cm2.s-1 and an activation barrier of 140 ± 60 meV. The hopping mechanism includes short 2 Å hops between tetragonal and interstitial octahedral sites. This process forms dynamic Frenkel defects, however, there is no indication of additional broadening in the density-of-states indicating the intrinsic anharmonic interactions dictate the phonon lifetimes. © Preprint article - 2023 Elsevier Inc.
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    Copper diffusion rates and hopping pathways in superionic Cu2Se
    (Elsevier, 2021-08-15) Nazrul Islam, SMK; Mayank, P; Ouyang, Y; Chen, J; Sagotra, AK; Li, M; Cortie, MB; Mole, RA; Cazorla, C; Yu, DH; Wang, XL; Robinson, RA; Cortie, DL
    The ultra-low thermal conductivity of Cu2Se is well established, but so far there is no consensus on the underlying mechanism. One proposal is that the fast-ionic diffusion of copper suppresses the acoustic phonons. The diffusion coefficients reported previously, however, differ by two orders of magnitude between the various studies and it remains unclear whether the diffusion is fast enough to impact the heat-bearing phonons. Here, a two-fold approach is used to accurately re-determine the diffusion rates. Ab-initio molecular dynamics simulations, incorporating landmark analysis techniques, were closely compared with experimental quasielastic/inelastic neutron scattering. Reasonable agreement was found between these approaches, consistent with a diffusion coefficient of 3.1 ± 1.3 x 10−5 cm2.s−1 at 675 K and an activation barrier of 140 ± 60 meV. The hopping mechanism includes short 2 Å hops between tetrahedral and interstitial octahedral sites. This process forms dynamic Frenkel defects. Despite the latter processes, there is no major loss of the phonon mode intensity in the superionic state, and there is no strong correlation between the phonon spectra and the increased diffusion rates. Instead, intrinsic anharmonic phonon interactions appear to dictate the thermal conductivity above and below the superionic transition, and there is only subtle mode broadening associated with the monoclinic-cubic structural transition point, with the phonon density-of-states remaining almost constant at higher temperatures. © 2021 Acta Materialia Inc.
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    Determination of martensite structures of the Au7Cu5Al4 and Au7Cu5.7Al3.3 shape-memory alloys
    (Elsevier, 2014-10-15) Elcombe, MM; Kealley, CS; Bhatia, VK; Thorogood, GJ; Carter, DJ; Avdeev, M; Cortie, MB
    The β-phase of Au7Cu5Al4 undergoes a reversible shape-memory phase transformation for which several conflicting martensite phases have been reported. Here we show the significance of the cooling temperature used to obtain the martensite. If Au7Cu5Al4 is cooled from the parent phase condition to cryogenic temperatures, e.g. below 200 K, the martensitic phase is orthorhombic (space group Pcmn, a = 4.4841 Å, b = 5.8996 Å, c = 17.8130 Å); however, when this composition is cooled to only ∼260 K it will in general consist of a mixture of orthorhombic and monoclinic phase (the latter has space group P21/m, a = 4.4742 Å, b = 5.9265 Å, c = 13.3370 Å, β = 91.425°). In contrast, a sample with decreased Al content (Au7Cu5.7Al3.3) transforms fully to monoclinic phase if cooled to ∼260 K. © 2014 Acta Materialia Inc.
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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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    Formation and structure of V–Zr amorphous alloy thin films
    (Elsevier B.V., 2015-01-15) King, DJM; Middleburgh, SC; Liu, ACY; Tahini, HA; Lumpkin, GR; Cortie, MB
    Although the equilibrium phase diagram predicts that alloys in the central part of the V–Zr system should consist of V2Zr Laves phase with partial segregation of one element, it is known that under non-equilibrium conditions these materials can form amorphous structures. Here we examine the structures and stabilities of thin film V–Zr alloys deposited at room temperature by magnetron sputtering. The films were characterized by X-ray diffraction, transmission electron microscopy and computational methods. Atomic-scale modelling was used to investigate the enthalpies of formation of the various competing structures. The calculations confirmed that an amorphous solid solution would be significantly more stable than a random body-centred solid solution of the elements, in agreement with the experimental results. In addition, the modelling effort provided insight into the probable atomic configurations of the amorphous structures allowing predictions of the average distance to the first and second nearest neighbours in the system. © 2014 Elsevier Ltd.
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    Fracture mechanics of mollusc shells
    (The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Cortie, MB; McBean, KE; Elcombe, MM
    The shape and structure of the shells of molluscs has attracted considerable attention. One aspect of interest is the comparatively high resistance to fracture of these shells. It is known that they are composite structures of aragonite, other calcereous materials, and up to 5% by volume of protein ‘glue’. A large component of their toughening derives from crack tip blunting, deflection and closure, concepts well-known from the field of fracture mechanics. However, the possibility that they might also derive a measure of toughening from a residual stress distribution has been generally overlooked, although Illert first raised this over a decade ago. The optimum situation would be when the inner surface of the shell is maintained in a state of tensile stress, while the outer layers are in the necessarily counter-balancing compressive state. We have examined this hypothesis using a combination of neutron diffraction and scanning electron microscopy and find that it is certainly feasible. However, a definitive proof will require a diffraction study. © The Authors
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    Fracture mechanics of mollusc shells
    (Elsevier B. V., 2006-11-15) Cortie, MB; McBean, KE; Elcombe, MM
    The shape and structure of the shells of molluscs has attracted considerable attention. One aspect of interest is the comparatively high resistance to fracture of these shells. It is known that they are composite structures of aragonite, other calcereous materials, and up to 5% by volume of protein ‘glue’. A large component of their toughening derives from crack tip blunting, deflection and closure, concepts well-known from the field of fracture mechanics. However, the possibility that they might also derive a measure of toughening from a residual stress distribution has been generally overlooked, although Illert first raised this over a decade ago. The optimum situation would be when the inner surface of the shell is maintained in a state of tensile stress, while the outer layers are in the necessarily counter-balancing compressive state. We have examined this hypothesis using a combination of neutron diffraction and scanning electron microscopy and find that it is certainly feasible. However, a definitive proof will require a diffraction study at higher resolution. © 2006 Elsevier B.V.
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    High temperature transformations of the Au7Cu5Al4 shape-memory alloy
    (Elsevier, 2011-02-24) Cortie, MB; Kealley, CS; Bhatia, VK; Thorogood, GJ; Elcombe, MM; Avdeev, M
    The β-phase of Au7Cu5Al4 undergoes a reversible shape-memory phase transformation, however there has been some uncertainty regarding the crystal structure or structures of the parent phase. Here we show that, under equilibrium conditions, the parent phase possesses the L21 structure between its Ap (about 79°C) and 630°C, and the B2 primitive cubic structure between 630°C and its melting point. It melts directly from B2 into the liquid state and hence never achieves the random bcc A2 structure that has been previously mooted. Splat-cast samples of the alloy are martensitic, proving that development of equilibrium order and defect concentration are not pre-requisites for the A → M transformation to occur. © 2011, Elsevier Ltd.
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    Martensite destabilization in Au7Cu5Al4 shape-memory alloy
    (Elsevier, 2011-03) Bhatia, VK; Kealley, CS; Prior, MJ; Cortie, MB
    Aging-induced changes in the austenite peak (Aρ) temperature of Au7Cu5Al4 shape-memory alloy are investigated. Whereas heat treating the parent phase at temperatures >140°C or aging the martensite for long times at room temperature both stabilized the Aρ to ~80°C, low-temperature excursions into the parent phase caused the subsequent Aρ to drop to ~60°C and the transformation hysteresis to decrease. The evidence indicates that this destabilization of the martensite is caused by time-dependent relaxation of elastic constraint due to parent-phase lath migration during the preceding low-temperature austenitizing treatment. This mechanism of aging is different from that of the better-known symmetry-conforming short-range order phenomenon. © 2011, Elsevier Ltd.
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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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    Segregation and migration of species in the CrCoFeNi high entropy alloy
    (Elsevier, 2014-06-25) Middleburgh, SC; King, DJM; Lumpkin, GR; Cortie, MB; Edwards, L
    The formation and migration of intrinsic defects in the CrCoFeNi high entropy alloy has been investigated using ab-initio modeling. The ease of vacancy formation was found to vary depending on the element: vacancies formed by the removal of Fe, Ni and Co are positive while Cr has a negative vacancy formation energy and will precipitate out of the CrCoFeNi alloy. This mechanism may allow the formation of a corrosion passivating oxide layer, analogous to Cr behaviour in stainless steels. The migration and behaviour of intrinsic interstitial defects has also been investigated. © Elsevier B.V.
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    Spontaneous emergence of optically polarizing nanoscale structures by co-deposition of aluminum with refractory metals: implications for high-temperature polarizers
    (American Chemical Society, 2022-03-25) Tai, MC; Arnold, MD; Estherby, C; de Silva, KSB; Gentle, AR; Cortie, DL; Mitchell, DRG; Westerhausen, MT; Cortie, MB
    The unexpected growth of highly aligned and optically polarizing metallic fins during physical vapor deposition under modestly oblique conditions is investigated. The fins exhibit nanoscale dimensions and are formed when Al is co-sputtered with any of V, Cr, Nb, Mo, Ta, W, Ru, Fe, Ni, Pt, Zr, Mg, and Ti. It is proposed that the phenomenon is caused by anomalously low atomic mobility in the alloys and intermetallic compounds formed by co-depositing with Al. In contrast, when Cu, Ag, and Au (which diffuse more rapidly in Al) are deposited, no fins form. There is a sharp visible transition in optical properties as the ratio of Al to other element is decreased: the color of the sample changes from black to silver-white for compositions containing less than about 55 atom % Al. The region over which the color change occurs is associated with a very strongly polarized reflectance. Cross-sectional elemental mapping and Monte Carlo simulations suggest that growth of the fins may be nucleated by Al hillocks and enhanced by shadowing effects. The diversity of suitable metals makes this a versatile technique for producing nanoscale polarizing surfaces suitable for high-flux and high-temperature applications. © American Chemical Society
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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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    Structure and magnetism of ultra-small cobalt particles assembled at titania surfaces by ion beam synthesis
    (Elsevier, 2021-12) Bake, A; Rezoanur Rahman, M; Evans, PJ; Cortie, MB; Nancarrow, M; Abrudan, R; Radu, F; Khaydukov, Y; Causer, GL; Callori, SJ; Livesey, KL; Mitchell, DRG; Pastuovic, Z; Wang, XL; Cortie, DL
    Metallic cobalt nanoparticles offer attractive magnetic properties but are vulnerable to oxidation, which suppresses their magnetization. In this article, we report the use of ion beam synthesis to produce ultra-small, oxidation-resistant, cobalt nanoparticles embedded within substoichiometric TiO2-δ thin films. Using high fluence implantation of cobalt at 20–60 keV, the particles were assembled with an average size of 1.5 ± 1 nm. The geometry and structure of the nanoparticles were studied using scanning transmission electron microscopy. Near-edge X-ray fluorescence spectroscopy on the L2,3 Co edges confirms that the majority of the particles beneath the surface are metallic, unoxidised cobalt. Further evidence of the metallic nature of the small particles is provided via their high magnetization and superparamagnetic response between 3 and 300 K with a low blocking temperature of 4.5 K. The magnetic properties were studied using a combination of vibrating sample magnetometry, element-resolved X-ray magnetic circular dichroism, and depth-resolved polarised neutron reflectometry. These techniques provide a unified picture of the magnetic metallic Co particles. We argue, based on these experimental observations and thermodynamic calculations, that the cobalt is protected against oxidation beneath the surface of titania owing to the enthalpic stability of TiO2 over CoO which inhibits solid state reactions. Crown Copyright © 2021 Published by Elsevier B.V.
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    Structure determination of gold-based shape memory alloy
    (Australian Institute of Physics, 2009-02-04) Kealley, CS; Bhatia, VK; Thorogood, GJ; Elcombe, MM; Cortie, MB
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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
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    Ultra-high thermoelectric performance in graphene incorporated Cu2Se: role of mismatching phonon modes
    (Elsevier, 2018-11) Li, M; Cortie, DL; Liu, J; Yu, DH; Islam, SMKN; Zhao, L; Mitchell, DRG; Mole, RA; Cortie, MB; Dou, SX; Wang, XL
    A thermoelectric material consisting of Cu2Se incorporated with up to 0.45 wt% of graphene nanoplates is reported. The carbon-reinforced Cu2Se exhibits an ultra-high thermoelectric figure-of-merit of zT = 2.44 ± 0.25 at 870 K. Microstructural characterization reveals dense, nanostructured grains of Cu2Se with multilayer-graphene and graphite agglomerations located at grain boundaries. High temperature X-ray diffraction shows that the graphene incorporated Cu2Se matrix retains a cubic structure and the composite microstructure is chemically stable. Based on the experimental structure, density functional theory was used to calculate the formation energy of carbon point defects and the associated phonon density of states. The isolated carbon inclusion is shown to have a high formation energy in Cu2Se whereas graphene and graphite phases are enthalpically stable relative to the solid solution. Neutron spectroscopy proves that there is a frequency mismatch in the phonon density of states between the carbon honeycomb phases and cubic Cu2Se. This provides a mechanism for the strong scattering of phonons at the composite interfaces, which significantly impedes the conduction of heat and enhances thermoelectric performance. © 2018 Elsevier Ltd. A
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    Ultra-small cobalt particles embedded in titania by ion beam synthesis: additional datasets including electron microscopy, neutron reflectometry, modelling outputs and particle size analysis
    (Elsevier, 2022-02) Bake, A; Rahman, R; Evans, PJ; Cortie, MB; Nancarrow, M; Abrudan, R; Radu, F; Khaydukov, Y; Causer, GL; Livesey, KL; Callori, SJ; Mitchell, DRG; Pastuovic, Z; Wang, XL; Cortie, DL
    This Data-in-brief article includes datasets of electron microscopy, polarised neutron reflectometry and magnetometry for ultra-small cobalt particles formed in titania thin films via ion beam synthesis. Raw data for polarised neutron reflectometry, magnetometry and the particle size distribution are included and made available on a public repository. Additional elemental maps from scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) are also presented. Data were obtained using the following types of equipment: the NREX and PLATYPUS polarised neutron reflectometers; a Quantum Design Physical Property Measurement System (14 T); a JEOL JSM-6490LV SEM, and a JEOL ARM-200F scanning transmission electron microscope (STEM). The data is provided as supporting evidence for the article in Applied Surface Science (A. Bake et al., Appl. Surf. Sci., vol. 570, p. 151068, 2021, DOI 10.1016/j.apsusc.2021.151068), where a full discussion is given. The additional supplementary reflectometry and modelling datasets are intended to assist future scientific software development of advanced fitting algorithms for magnetization gradients in thin films. Crown Copyright © 2021 - Open Access CC BY-NC-ND