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Residual Stresses Distribution Measured by Neutron Diffraction in Fabricated Square High Strength Steel Tubes
No full textEngineers are increasingly encouraged to consider sustainability in the design and construction of new civil engineering infrastructure. Sustainability can be achieved through the use of high strength materials thereby reducing quantity of materials required in construction where possible. Knowledge of residual stresses in fabricated columns is important in identifying whether the fabricated columns can be classified as heavily welded (HW) or lightly welded (LW). The determination of residual stresses can be used to determine the local buckling of stub columns. Residual stress magnitudes are also essential in the numerical modelling of buckling behaviour of columns. This paper outlines the challenges in measurement of residual stresses using neutron diffraction in fabricated high strength steel square tubes. The residual stress line scans and maps were measured using the Kowari Strain Scanner located at the Australian Nuclear and Science Organisation (ANSTO) in Australia. © 2014, Trans Tech Publications
Relationships between electron density and magnetic properties in water-bridged dimetal complexes
No full textThe experimental and theoretical electron density distributions in two structurally similar transition metal dimers (M = Ni, Co; see Figure) were analyzed using the atoms-in-molecules (AIM) approach, and selected properties related to the chemical bonding are compared to measured intramolecular magnetic exchange interaction parameters.The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)(tBuCOO)4(tBuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal–metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal–oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers. © 2014, American Chemical Society
Evidence of Solid-Solution Reaction upon Lithium Insertion into Cryptomelane K0.25Mn2O4 Material
No full textCryptomelane-type K0.25Mn2O4 material is prepared via a template-free, one-step hydrothermal method. Cryptomelane K0.25Mn2O4 adopts an I4/m tetragonal structure with a distinct tunnel feature built from MnO6 units. Its structural stability arises from the inherent stability of the MnO6 framework which hosts potassium ions, which in turn permits faster ionic diffusion, making the material attractive for application as a cathode in lithium-ion batteries. Despite this potential use, the phase transitions and structural evolution of cryptomelane during lithiation and delithiation remain unclear. The coexistence of Mn3+ and Mn4+ in the compound during lithiation and delithiation processes induces different levels of Jahn–Teller distortion, further complicating the lattice evolution. In this work, the lattice evolution of the cryptomelane K0.25Mn2O4 during its function as a cathode within a lithium-ion battery is measured in a customized coin cell using in situ synchrotron X-ray diffraction. We find that the lithiation–delithiation of cryptomelane cathode proceeds through a solid-solution reaction, associated with variations of the a and c lattice parameters and a reversible strain effect induced by Jahn–Teller distortion of Mn3+. The lattice parameter changes and the strain are quantified in this work, with the results demonstrating that cryptomelane is a relatively good candidate cathode material for lithium-ion battery use. © 2014, American Chemical Society
Fluorinated lamellar phases: structural characterisation and use as templates for highly ordered silica materials
No full textHighly ordered silica was synthesised by using a lamellar phase comprising the anionic fluorinated surfactant sodium perfluorooctanoate and the partially-fluorinated co-surfactant/oil 1H,1H,2H,2H-perfluorooctan-1-ol in water. The phase behaviour of this system was thoroughly analysed, and it was found that even low levels of the alcohol (<0.5 mol%) were sufficient to induce a phase change from normal micelles to a lamellar phase, rationalised as a result of geometric and electrostatic effects. The properties of these phases were compared to their hydrocarbon analogues, demonstrating the unique and valuable properties exhibited by fluorocarbons, directly related with the observed nanostructure. Small-angle neutron scattering was used to analyse the internal structure of the systems, providing information on the inter-lamellar spacing, bilayer thickness and membrane elasticity. The potential for these phases to act as shear-thinning lubricants was assessed using oscillatory rheology, obtaining shear-dependent viscosity along with storage and loss moduli.© 2014, The Royal Society of Chemistry
Investigation of Polarisation in CdTe using TCT
No full textThe polarisation effect in CdTe:Cl has been studied using the Transient Current Technique (TCT) in order to quantitatively evaluate the subsequent changes in the charge transport properties as well as the electric field distribution in the sensor volume. The electric field is calculated from TCT pulses using the Schockley-Ramo theorem. The mobility of the charge carriers as well as their average drift velocity in the CdTe material are determined using the TCT pulse width. Infrared illumination demonstrated a temporary restoration of the electric field. However after a few minutes the polarization effect is resumed, even under constant IR illumination. © 2014, IOP Publishing Ltd and Sissa Medialab srl
Structural dependence of threshold displacement energies in rutile, anatase and brookite TiO2
No full textSystematic molecular dynamics simulations of low energy cascades have been performed to examine how threshold displacement events are effected by changes in crystal structure. Exploiting the structural proximity of the rutile, anatase and brookite polymorphs of TiO2, a quantitative examination of defect production has been carried out including detailed defect analysis and the determination of values of the threshold displacement energy (Ed). Across all polymorphs comparable values of Ed are reported for oxygen at around 20 eV, with the value for Ti in rutile (73 ± 2 eV) significantly higher than that in brookite (34 ± 1 eV) and anatase (39 ± 1 eV). Quantifying defect formation probability as a function of Primary Knock-on Atom (PKA) energy, simulations in rutile indicate a consistent reduction in defect formation at energies higher than Ed relative to anatase and brookite. Defect cluster analysis reveals a significant proportion of di-Frenkel pairs in anatase at Ti PKA energies around Ed. These clusters, which are stabilised by the localisation of two Frenkel pairs, are associated with a recombination barrier of approximately 0.19 eV. As such, annihilation is likely under typical experimental conditions which suggests an expected increase in the measured Ti value of Ed. Identical O defect populations produced at the threshold by the O PKA in both rutile and anatase explain the comparable values of Ed. At higher O PKA energies, the commencement of defect production on both sublattices in anatase is observed in contrast to the confinement of defects to the O sublattice in rutile. The overall trends reported are consistent with in-situ irradiation experiments and thermal spike simulations, suggesting the contrasting radiation response of the polymorphs of TiO2 is apparent during the initial stages of defect production. © 2014, Elsevier B.V
Toward a mechanism of rattler coupling in the β-pyrochlores AOs2O6 (A = K, Rb, Cs)
No full textWe have applied ab initio molecular dynamics simulations to study metal–metal coupling on the alkali-metal sublattice in the β-pyrochlore osmates, AOs2O6 (A = K, Rb, Cs) at 300 K. We find that the dynamics of the alkali-metal atoms (rattlers) exhibit stronger rattler–rattler correlations than rattler–cage correlations, and that, at 300 K, this correlation is strongest for Cs. We show that the rattler–rattler correlations control the dominant dynamics in the rattling of these atoms. We provide preliminary evidence that the rattler correlated motion occurs primarily through two somewhat distinct vibrational modes: a high-energy mode (peak A) couples the rattlers to each other and a low-energy mode (peak B) couples the rattlers to the cage modes. Rattler–rattler correlated motion through the high-energy mode provides insight into the trend in spectral broadening from Cs to K. The spectral broadening is inversely proportional to the strength of the dynamical correlations on the alkali-metal sublattice which in turn depend on the atomic size of the rattler, decreasing from Cs to K. Thus, the broadest spectrum exhibited by the K is partly a consequence of the small size of this rattler which permits a greater range of motions involving combinations of both correlated and anti-correlated dynamics. We emphasize that the identification of the somewhat distinct roles of the high-energy (peak A) and low-energy (peak B) modes in rattler coupling reported in this work is a significant step toward a complete fundamental mechanism of rattler dynamical coupling in these osmates. We believe that such a mechanism will have profound implications for a broad class of cage compounds, including clathrates and skutterudites.© 2014, Springer Science+Business Media New York
Kinetics vs. thermodynamics: a unique crystal transformation from a uranyl peroxo-nanocluster to a nanoclustered uranyl polyborate.
No full textA novel method to prepare a nano-clustered uranyl polyborate in aqueous solution at room temperature has been developed. The initially formed kinetically favoured sodium uranyl peroxide yellow crystals transform, in the presence of boric acid, to the thermodynamically stable sodium uranyl polyborate in light yellow-green single crystal form. © Royal Society of Chemistry 201
Residual Stresses Associated with the Production of Coiled Automotive Springs
No full textCold coiling of high tensile steel rod into helical coil springs for the automotive industry is a new technique being implemented amongst spring manufacturers worldwide. To characterise this coil production process, the neutron strain scanning technique has been employed to non-destructively elucidate the influence production stages have on the tri-axial residual stress state. Samples investigated represented key production steps in the cold-coil forming process: Cold coiling; Tempering; Hot setting; Hot peening; Shot peening. Investigations revealed that the stress field was axi-symmetrical, that the dominant variation in all samples occurred along the hoop direction (helical circumference), whilst the radial and axial stresses are substantially lower. Accurate two-dimensional stress maps of the rod cross section have been compiled revealing key features associated with the cold coiling step. Comparison of the stress fields after each production step revealed altered stress values. The final shot peening process stage not only reduced stress concentrations at the internal bore, but contributed to the establishment of favourable surface residual stress conditions that enhance the fatigue life of the final product. © 2014, Trans Tech Publications
Force chains in monodisperse spherical particle assemblies: Three-dimensional measurements using neutrons
No full textThe full triaxial stress state within individual particles in a monodisperse spherical granular assembly has been measured. This was made possible by neutron imaging and computed tomography combined with neutron diffraction strain measurement techniques and associated stress reconstruction. The assembly in question consists of 549 precision steel ball bearings under an applied axial load of 85 MPa in a cylindrical die. Clear evidence of force chains was observed in terms of both the shape of the probability distribution function for normal stresses and the network formed by highly loaded particles. An extensive analysis of the source and magnitude of uncertainty in these measurements is also presented.©2014 American Physical Society