Browsing by Author "Riley, DP"
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- ItemAb initio phonon dispersion curves used to check experimentally determined elastic constants of the MAX phase Ti3SiC2(Trans. Tech Publication Inc., 2011-07-04) Kirstein, O; Zhang, JF; Kisi, EH; Riley, DPThe ternary carbide Ti3SiC2 is the archetype of MAX phases. To date, MAX phases have proven difficult to synthesize as sufficiently large single crystals from which single crystal elastic constants might be obtained. Therefore, the elastic properties not only of Ti3SiC2 but other MAX phases are extensively studied by ab initio methods. Recently single crystal elastic constants were experimentally determined for the first time using neutron diffraction. The experiment revealed extreme shear stiffness which is not only quite rare in hexagonal materials but also strongly contradicts the predictions of all published MAX phase elastic constants from ab initio calculations. In the present paper we would like to show that such shear stiffness can possibly be supported by ab initio calculations and the calculated phonon dispersion along high symmetry directions.© 2011, Trans Tech Publications
- ItemDensity functional modelling of silicate and aluminosilicate dimerisation solution chemistry(Royal Society of Chemistry, 2011-02-14) White, CE; Provis, JL; Kearley, GJ; Riley, DP; van Deventer, JSJCommon throughout sol–gel chemistry, including zeolite synthesis, aluminosilicate glass formation and geopolymerisation, is the process of inorganic oxide polymerisation and deprotonation. In this investigation, some of the fundamental reactions occurring during zeolite synthesis and geopolymerisation at high pH are investigated using density functional theory (DFT), and are compared with: (i) existing values reported in the literature, and (ii) new and previously published DFT-derived data for similar silicate reactions at near-neutral pH. From the results it is seen that the energetics of deprotonation and dimerisation reactions depend greatly on the pH value, and these results correlate well with existing experimental values and trends. Hence, this investigation exemplifies that an accurate replication of the solution environment is crucial for obtaining useful theoretical results for species dissolved in non-ideal environments. © 2011, Royal Society of Chemistry
- ItemDeuterium retention and near-surface modification of ion-irradiated diamond exposed to fusion-relevant plasma(IOP Science, 2014-04-01) Deslandes, A; Guenette, MC; Corr, CS; Karatchevtseva, I; Thomsen, L; Lumpkin, GR; Riley, DPChemical vapour deposited diamond was irradiated with 5 MeV carbon ions to simulate the damage caused by collision cascades from neutron irradiation in a fusion environment. Ion-irradiated samples were then exposed to a deuterium plasma in MAGPIE with ion flux of ~1.3 × 1021 ions m−2 s−1. Raman and near edge x-ray absorption fine structure (NEXAFS) spectroscopy were used to characterize the degree of disorder and sp2-bonding induced by the ion irradiation. The signals of sp2-bonded and disordered carbon were observed to decrease after exposure to the deuterium plasma, although sharp Raman peaks indicative of vacancy and interstitial defects induced by the MeV ions were less affected. Recovery of a diamond-like surface after plasma exposure was evident in the NEXAFS spectra. Elastic recoil detection analysis showed that the ion-damaged diamond retained more deuterium than diamond exposed only to deuterium plasma. For the case of unirradiated samples, diamond retained more deuterium than graphite. However, for the case of the ion-irradiated samples, diamond exhibited less deuterium retention than graphite. © 2014, IAEA Vienna.
- ItemDiamond structure recovery during ion irradiation at elevated temperatures(Elsevier, 2015-12-15) Deslandes, A; Guenette, MC; Belay, K; Elliman, RG; Karatchevtseva, I; Thomsen, L; Riley, DP; Lumpkin, GRCVD diamond is irradiated by 5 MeV carbon ions, with each sample held at a different temperature (300–873 K) during irradiations. The defect structures resulting from the irradiations are evident as vacancy, interstitial and amorphous carbon signals in Raman spectra. The observed variation of the full width at half maximum (FWHM) and peak position of the diamond peak suggests that disorder in the diamond lattice is reduced for high temperature irradiations. The dumbbell interstitial signal is reduced for irradiations at 873 K, which suggests this defect is unstable at these temperatures and that interstitials have migrated to crystal surfaces. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy results indicate that damage to the diamond structure at the surface has occurred for room temperature irradiations, however, this structure is at least partially recovered for irradiations performed at 473 K and above. The results suggest that, in a high temperature irradiation environment such as a nuclear fusion device, in situ annealing of radiation-created defects can maintain the diamond structure and prolong the lifetime of diamond components. © 2015 Elsevier B.V.
- ItemHierarchically porous carbon–zirconium carbide spheres as potentially reusable transmutation targets(Elsevier, 2015-08-01) Scales, N; Chen, J; Hanley, TL; Riley, DP; Lumpkin, GR; Luca, VThe preparation of hierarchically porous phase-pure carbon–zirconium carbide spheres with surface areas upwards of 70 m2/g and diameters in the 1–2 mm range has been achieved. The zirconium carbide beads were prepared through carbothermal reduction of polyacrylonitrile-zirconium composites prepared via three different routes including infiltration of a zirconium precursor into preformed polyacrylonitrile (PAN) beads and two one-pot co-precipitation methods. Depending on the route used to prepare the composites, relatively high surface area phase-pure zirconium carbides with the radial macroporous internal structure of the PAN template could be prepared. The adsorption of the elements U, Mo, Cs and Sr by the zirconium carbide beads was studied as a function of pH in the range 1–13 and target element concentrations of 0.025 mmol/L. The as-prepared hierarchical zirconium carbide beads demonstrated almost quantitative uptake of Mo below pH 4 with uptake decreasing to close to zero and above pH 10.5. On the other hand, U uptake was negligible below pH 4 and became quantitative in the range 4–12. Cesium was not extracted to any significant extent in the pH range studied and Sr only to a limited extent above pH 10. The engineered hierarchical porosities, neutron transparency, thermal stability and anticipated chemical stability of the present zirconium carbide spheres makes them promising candidates as reusable transmutation targets for Minor Actinide incineration. © 2015 Elsevier Inc.
- ItemHydrogen induced vacancy formation in tungsten(Elsevier, 2014-05) Middleburgh, SC; Voskoboinikov, RE; Guenette, MC; Riley, DPAtomic scale modelling methods have been used to study the change in vacancy population when H is introduced into the W bulk matrix. Vacancy defects are predicted to dominate in pure W, and the vacancy concentration is expected to be very small. A mechanism whereby H solutes facilitate vacancy formation has been outlined and a single H interstitial is predicted to reduce the vacancy formation energy from 2.95 eV to 2.23 eV. Clustering of H interstitials in W is predicted to be unfavourable without a W vacancy. H has also been shown to affect the vacancy binding characteristics in W, changing the behaviour from a repelling interaction in pure W, retarding the formation of small vacancy clusters, to an attractive interaction when vacancy-H clusters are considered. The changes in defect behaviour predicted, will have observable implications to operational properties and the application of W in fusion reactor components. © 2014, Elsevier B.V.
- ItemInelastic neutron scattering analysis of the thermal decomposition of kaolinite to metakaolin(Elsevier, 2013-12-12) White, CE; Kearley, GJ; Provis, JL; Riley, DPUnderstanding the formation of metakaolin via kaolinite dehydroxylation is extremely important for the optimization of various industrial processes. Recent investigations have reported that the different types of hydrogen atoms in kaolinite are removed concurrently during the dehydroxylation process. Here, inelastic neutron scattering (INS) is used to analyze the location and dynamics of hydrogen atoms in kaolinite, together with the changes induced during dehydroxylation. This is achieved by using prior knowledge of how the inner and inner surface hydrogen atoms contribute to the kaolinite INS spectrum in the 200–1200 cm−1 range, in combination with a semi-quantitative analysis of the experimental INS spectra. Overall, it is seen that there is a distinct preferential loss of inner surface hydrogen-atom types during the dehydroxylation process, as determined from analysis of the Al–O–H vibrational modes (consisting of deformation and torsion) in the INS spectrum. © 2013, Elsevier B.V.
- ItemInitial damage processes for diamond film exposure to hydrogen plasma(Elsevier Science, 2013-12-01) Deslandes, A; Guenette, MC; Samuell, CM; Karatchevtseva, I; Ionescu, M; Cohen, DD; Blackwell, B; Corr, CS; Riley, DPDiamond is considered to be a possible alternative to other carbon based materials as a plasma facing material in nuclear fusion devices due to its high thermal conductivity and resistance to chemical erosion. In this work CVD diamond films were exposed to hydrogen plasma in the MAGnetized Plasma Interaction Experiment (MAGPIE): a linear plasma device at the Australian National University which simulates plasma conditions relevant to nuclear fusion. Various negative sample stage biases of magnitude less than 500 V were applied to control the energies of impinging ions. Characterisation results from SEM, Raman spectroscopy and ERDA are presented. No measureable quantity of hydrogen retention was observed, this is either due to no incorporation of hydrogen into the diamond structure or due to initial incorporation as a hydrocarbon followed by subsequent etching back into the plasma. A model is presented for the initial stages of diamond erosion in fusion relevant hydrogen plasma that involves chemical erosion of non-diamond material from the surface by hydrogen radicals and damage to the subsurface region from energetic hydrogen ions. These results show that the initial damage processes in this plasma regime are comparable to previous studies of the fundamental processes as reported for less extreme plasma such as in the development of diamond films. © 2013, Elsevier Ltd.
- ItemIon irradiated graphite exposed to fusion-relevant deuterium plasma(Elsevier, 2014-12-01) Deslandes, A; Guenette, MC; Corr, CS; Karatchevtseva, I; Thomsen, L; Ionescu, M; Lumpkin, GR; Riley, DPGraphite samples were irradiated with 5 MeV carbon ions to simulate the damage caused by collision cascades from neutron irradiation in a fusion environment. The ion irradiated graphite samples were then exposed to a deuterium plasma in the linear plasma device, MAGPIE, for a total ion fluence of ∼1 × 1024 ions m−2. Raman and near edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to characterize modifications to the graphitic structure. Ion irradiation was observed to decrease the graphitic content and induce disorder in the graphite. Subsequent plasma exposure decreased the graphitic content further. Structural and surface chemistry changes were observed to be greatest for the sample irradiated with the greatest fluence of MeV ions. D retention was measured using elastic recoil detection analysis and showed that ion irradiation increased the amount of retained deuterium in graphite by a factor of four. © 2014, Elsevier B.V.
- ItemM(n+1)AXn phases are they tolerant/resistant to damage(Australian Institute of Physics, 2011-02-03) Whittle, KR; Riley, DP; Blackford, MG; Aughterson, RD; Moricca, SA; Lumpkin, GR; Zaluzec, NJTernary carbide materials have been proposed as having applications within the future nuclear technologies, both fusion (ITER/DEMO) and fission (Gen IV). These new designs require a material to have the ability to tolerate radiation damage to high levels, with a high level of predictability. As part of such a process two systems, specifically Ti3AlC2 and Ti3SiC2 have been studied to determine their radiation tolerance, using in-situ ion beam irradiation with 1 MeV Xe ions, coupled with transmission electron microscopy. Irradiations have shown that both systems show little amorphisation at 300K up to doses of at least 6.25 x 1015 ions cm-2 (~28-30 dpa). However, there is a subtle difference between Ti3AlC2 and Ti3SiC2, with Ti3SiC2 showing more evidence for damage. Further irradiations using 500 KeV Xe to fluences equivalent to 100 dpa have also been undertaken, with crystalline material visible and evidence of recrystallisation. Explanations and possible mechanisms for recovery from damage are presented, along with implications for future potential uses.
- ItemMechanical properties of tungsten copper composites: direct measurement by neutron diffraction(Australian Institute of Physics, 2014-02-04) Mignone, PJ; Finlayson, TR; Kabra, S; Zhang, SY; Franks, GV; Riley, DPThe composite W-10 wt%Cu (19.35% by volume, assuming negligible porosity) has been studied using the ENGIN-X beamline at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory. An as-machined, compression sample was initially measured in order to check for the presence of residual stresses in the composite, using a mixed powder sample of the same elemental weight fractions as the “zero stress” comparison. Then a series of mechanical tests were carried out on the composite for applied compressive loads up to 250 MPa at both room temperature and 100ºC and compared with similar tests carried out on pure tungsten and copper samples. Residual stress values of -280 MPa (for the tungsten matrix) and 480 MPa (for the copper particulate phase) were measured for the as-machined sample. This is a surprising result, given that the yield stress for copper is typically less than 100 MPa but is not inconsistent with residual stresses reported in the literature for W-Cu composites. The mechanical properties for the composite have also been determined from the results of these in-situ, mechanical tests and compared with finite element calculations based on microstructural models for the composite material.
- ItemModular in-situ reaction chamber design for time resolved diffraction(Oldenbourg Verlag, 2009-09) Styles, MJ; Riley, DP; Christoforidis, J; Olsen, SRIn an effort to mitigate the expense and uncertain performance of customised environment chambers, researchers at the University of Melbourne and the Australian Nuclear Science and Technology Organisation (ANSTO) have designed and are currently constructing a modular reaction chamber, capable of separating the necessities of diffraction methodologies from those of the desired sample environment. The In-Situ Reaction Chamber (ISRC) abstracts many of the details intrinsic to the diffractometer, allowing users to design inexpensive environmental inserts that may be readily customised to their individual needs. Overall, the modularised design aims to reduce the development costs of performing in-situ diffraction experiments, while minimising the experimental setup time and overall uncertainty of ancillary performance. © 2009, Oldenbourg Verlag
- ItemNEXAFS spectroscopy of CVD diamond films exposed to fusion, relevant hydrogen plasma(Elsevier, 2013-04-01) Guenette, MC; Deslandes, A; Samuell, CM; Tadich, A; Thomsen, L; Cowie, BCC; Corr, CS; Riley, DPA series of CVD diamond films have been exposed to hydrogen plasma in the linear magnetized plasma device, MAGPIE, with various applied sample stage biases between 0 V (no applied bias) to − 500 V. The plasma-induced damage to the surface structure of the diamond films has been investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy in both the Auger electron yield (AEY) and total fluorescence yield (TFY) modes. The key diamond NEXAFS spectral features (diamond core exciton and second absolute band gap) are found to be diminished following plasma exposure as measured in the surface sensitive, AEY spectra, whilst these features remain unchanged relative to an unexposed diamond reference film as measured using the bulk sensitive, TFY spectra. These results, in conjunction with SRIM simulations, show definitively that the damage to the surface of the diamond films is restricted to the scale of the penetration depth of the H ions and no damage is induced at greater depths. The power and sensitivity of NEXAFS spectroscopy in assessing damage to the surface of diamond from fusion-relevant plasma-surface interactions are demonstrated. © 2013, Elsevier B.V.
- ItemA novel multi-scale modelling approach for determining the bulk properties of difficult-to-characterise composites(Australian Institute of Physics, 2013-02-05) Mignone, PJ; Wang, M; Finlayson, TR; Echlin, MP; Mottura, A; Pollock, TM; Riley, DP; Franks, GVA multi-scale modelling approach is presented for determining the bulk properties of copper-infiltrated Tungsten (W-Cu). A three-dimensional (3D) data-set of the W-Cu microstructure was generated using a novel serial-sectioning instrument. The image data were then reconstructed into a 3D Finite Element (FE) mesh. This made it possible to determine the bulk properties of W-Cu by simulating a representative volume of the microstructure.
- ItemProbeless tool aided friction stir welding as a fabrication technique for tungsten embedded mechanical composite of copper(The American Society of Mechanical Engineers, 2014-11-18) Ahuja, Y; Ibrahim, R; Paradowska, AM; Riley, DPFriction stir welding (FSW) is a relatively new solid state metallurgical joining technique. It flourishes on the simple principle of utilising frictional heat by the stirring motion of a non-consumable rotating tool to create the seam. Feasibility of FSW aided by a newly designed probeless tool was investigated for fabricating copper-tungsten mechanical composite. The most effective parameter combination was determined by conducting a parametric study of the probeless tool aided FSW copper. Strength of the mechanical composite fabricated at this condition was evaluated through punch shear testing. Punch shear testing established that the friction stir welded interface of the copper-tungsten composite was 87% as strong as the base metal (i.e. copper). Advantages of the designed technique have been summarised. © 2014 ASME
- ItemRadiation tolerance of M(n+1)AX(n) phases, Ti3AlC2 and Ti3SiC2(Elsevier, 2010-08-01) Whittle, KR; Blackford, MG; Aughterson, RD; Moricca, SA; Lumpkin, GR; Riley, DP; Zaluzec, NJDuring investigations of novel material types with uses in future nuclear technologies (ITER/DEMO and GenIV fission reactors), ternary carbides with compositions Ti3AlC2 and Ti3SiC2 have been irradiated with high Xe fluences, 6.25 × 1015 ions cm−2 (25–30 dpa), using the IVEM-TANDEM facility at Argonne National Laboratory. Both compositions show high tolerance to damage, and give indications that they are likely to remain crystalline to much higher fluences. There is a visible difference in tolerance between Ti3AlC2 and Ti3SiC2 that can be related to the changes in bonding within each material. These initial findings provide evidence for a novel class of materials (+200 compounds) with high radiation resistance, while, significantly, both of these materials are composed of low-Z elements and hence exhibit no long-term activation. © 2010, Elsevier Ltd.
- ItemShear stiffness in nanolaminar Ti3SiC2 challenges ab initio calculations(Institute of Physics, 2010-04-28) Kisi, EH; Zhang, JF; Kirstein, O; Riley, DP; Styles, MJ; Paradowska, AMNanolaminates such as the Mn + 1AXn (MAX) phases are a material class with ab initio derived elasticity tensors published for over 250 compounds. We have for the first time experimentally determined the full elasticity tensor of the archetype MAX phase, Ti3SiC2, using polycrystalline samples and in situ neutron diffraction. The experimental elastic constants show extreme shear stiffness, with c44 more than five times greater than expected for an isotropic material. Such shear stiffness is quite rare in hexagonal materials and strongly contradicts the predictions of all published MAX phase elastic constants derived from ab initio calculations. It is concluded that second order properties such as elastic moduli derived from ab initio calculations require careful experimental verification. The diffraction technique used currently provides the only method of verification for the elasticity tensor for the majority of new materials where single crystals are not available. © 2010, Institute of Physics
- ItemSingle crystal elastic constants of the MAX phase Ti3AlC 2 determined by neutron diffraction(Trans Tech Publications, 2010-01-01) Kirstein, O; Zhang, JF; Kisi, EH; Riley, DP; Styles, MJ; Paradowska, AMThe ternary ceramic Ti3AlC2 has an interesting combination of electrical, thermal and mechanical properties. Single crystal elastic constants under the Reuss approximation for the micromechanical state were obtained by analysing the shifts of neutron diffraction peaks while a polycrystalline sample was subjected to a compressive load varying from 5 to 300 MPa. The values of Young’s modulus and Poisson’s ratio computed from the single crystal compliances are in good agreement with those obtained directly from strain gauges and from the average changes in the a and c unit cell parameters.
- ItemStructure of kaolinite and influence of stacking faults: reconciling theory and experiment using inelastic neutron scattering analysis(American Institute of Physics, 2013-05-21) White, CE; Kearley, GJ; Provis, JL; Riley, DPThe structure of kaolinite at the atomic level, including the effect of stacking faults, is investigated using inelastic neutron scattering (INS) spectroscopy and density functional theory (DFT) calculations. The vibrational dynamics of the standard crystal structure of kaolinite, calculated using DFT (VASP) with normal mode analysis, gives good agreement with the experimental INS data except for distinct discrepancies, especially for the low frequency modes (200 - 400 cm(-1)). By generating several types of stacking faults (shifts in the a, b plane for one kaolinite layer relative to the adjacent layer), it is seen that these low frequency modes are affected, specifically through the emergence of longer hydrogen bonds (O-H center dot center dot center dot O) in one of the models corresponding to a stacking fault of -0.3151a - 0.3151b. The small residual disagreement between observed and calculated INS is assigned to quantum effects (which are not taken into account in the DFT calculations), in the form of translational tunneling of the proton in the hydrogen bonds, which lead to a softening of the low frequency modes. DFT-based molecular dynamics simulations show that anharmonicity does not play an important role in the structural dynamics of kaolinite. © 2013, American Institute of Physics.
- ItemThree-dimensional characterization of the permeability of W–Cu composites using a new “triBeam” technique(Elsevier, 2014-02) Echlin, MP; Mottura, A; Wang, M; Mignone, PJ; Riley, DP; Franks, GV; Pollock, TMLarge three-dimensional microstructural datasets have been gathered for two W–Cu composites of 10 and 15 wt.% Cu using the TriBeam system via in situ femtosecond laser sectioning in a scanning electron microscope. Laser ablation was performed on W–Cu samples along a 90 ° edge, milling parallel with the imaging surface. Secondary electron images for 1000 two-dimensional slices were segmented into binary images representing Cu and W components using EM/MPM (expectation–maximization/maximization of the posterior marginals) image-processing algorithms. A statistically random volume sampling approach has been employed to evaluate the microstructural and property volume element sizes necessary for the assessment of volume fraction, surface-area to volume ratio and permeability, respectively. This approach also characterizes the mean values and variability in microstructure and properties for volume elements ranging from 10 μm to 160 μm on edge. The converged values of the volume fractions of Cu closely match experimental values measured by the Archimedes technique.© 2013 Acta Materialia Inc.