Search

• Development of titanium-palladium based systems as potential high-temperature shape memory alloys. • Parameterization was effective to develop the potential parameters for titanium, palladium and ruthenium. • The relative stability of the alloy system was deduced from derived sets of potential parameters. • Lattice expansions predicted the transformation temperature of ternary alloys at high temperature. • Addition of Ru reduces the transformation temperature minimally. Titanium-palladium based systems are in high demand currently, particularly for the development of high-temperature shape memory alloys with a unique application in the aerospace industry. In particular, the Ti 50 Pd 50-X Ru X is found to be one of the promising alloys. This is due to their shape memory effect and superelasticity. Previous studies revealed that these alloys are mechanically stable above 25 at. % Ru with a positive C ′ at 0 K. So, an understanding of its transformation temperature behavior is required, which is currently not clear. The self-consistent-charge density functional tight-binding (SCC-DFTB) approach was used to develop parameters for Ti 50 Pd 50-X Ru X employing the parameterization technique. The structural, electronic and elastic properties were determined to check their relative stability on derived potentials using DFTB + code. As part of the validation for the developed set, the results were reproducible to within 5 % agreement as compared with other theoretical findings. Furthermore, the SCC sets of parameters were also used to determine the lattice expansions and check the transformation temperature of ternary Ti 50 Pd 50-X Ru X. It was observed that the addition of Ru reduces the transformation temperature. The findings imply that the SCC-DFTB approach was able to predict the stability and transformation behavior of Ti 50 Pd 50-X Ru X alloys. [ABSTRACT FROM AUTHOR]

Abstract: Density functional theory based ab- initio total energy calculations are used to determine the concentration dependences of equilibrium lattice parameters, densities, mixing energies (phase stability) and elastic constants of binary random Mg1−x Li x alloys spanning the entire concentration range, in the HCP, FCC and BCC structures. A model for predicting phase stability and elastic properties of random alloys is proposed. The current model is applied to binary Mg–Li system as a test study. The predicted phase stability trends at 0 K resemble those observed experimentally in Mg–Li phase diagram, more especially at lower temperatures. The proposed model for random alloys is not only accurate but also less computationally expensive compared to traditional supercell approach. [Copyright &y& Elsevier]

Abstract: The first principles pseudopotential calculations based on the Perdew–Burke–Ernzerhof (PBE) form of generalized gradient approximation (GGA) within density functional theory (DFT) have been utilized to investigate the structural and elastic properties of cubic-based Mg–Li alloys. The heats of formation and elastic moduli were used in predicting structural stability profile, and their results are consistent with each other. In terms of phase stability, an interesting correlation between the calculated tetragonal shear modulus (C′) and formation energy of corresponding bcc and fcc ordered compounds relative to hcp Mg and Li lattices is drawn. The predicted stability trend due to structural energy difference was further confirmed by electronic structure calculations based on Jones-type analysis. [Copyright &y& Elsevier]

Energetics of the anode system LiC6 compared to metallic lithium are calculated within the framework of local-density functional theory (LDA-DFT) techniques. Our results suggest that the energy of anode intercalation results in a small change to the cell voltage. [Copyright &y& Elsevier]

Abstract: In the current study, we report for the first time a new Ti rhombohedral (trigonal) structure induced by HEBM and subsequent sintering. During ball milling of Ti powder, solid-state transformation does not only depend on the grain refinement but also on the successful deformation of the nano-sized crystallites due to high energy ball impacts. Thermal stability of Ti-nanocrystalline in FCC allotrope was investigated. Upon sintering, the unstable FCC restored back to the rhombohedral phase rather than to HCP. The appearance of HCP Ti after sintering could suggest that prolonged milling leads to dispersion of hard particles (HCP) into more ductile particles belonging to allotropic phases, and hence possibility of resurfacing on sintering. [Copyright &y& Elsevier]

Abstract: Atomistic simulation methods were used to investigate the surface structures and stability of pyrolusite and ramsdellite polymorphs of electrolytic manganese dioxide (EMD). The interactions between the atoms were described using the Born model of Solids. This model was used to calculate the structures and energies of the low index surfaces {001}, {010}, {011}, {100}, {101} and {110} for both pyrolusite and ramsdellite. Pyrolusite is isostructural with rutile and similar to rutile the {110} surface is found to be the most stable with the relaxed surface energy 2.07Jm−2. In contrast, for ramsdellite the {101} surface is the most stable with a surface energy of 1.52Jm−2. Pyrolusite {100} and ramsdellite {100}b surfaces have equivalent energies of 2.43Jm−2 and 2.45Jm−2, respectively and similar surface areas and hence are the likely source for the intergrowths. Finally, comparison of the energies of reduction suggests that the more stable surfaces of pyrolusite are more easily reduced. [Copyright &y& Elsevier]

• Co20 wt.% Al powder was milled for 30 h. • A complete structural transformation was evidenced annealing at 600 °C. • Cubic spinel with a = 8.066 Å of ZnAl 2 O 4 prototype was formed. • A fibrous mesoporous morphology was developed on CoAl after annealing. Ball milling (BM) of elemental Co and Al was conducted for 30 h (h). This process has altered the original morphologies of both elemental Co and Al powders. Spherical Al powder particles has changed into a smooth pancake-shape while the original irregular-shaped Co agglomerates became disintegrated ultrafine particles. BM of the Co20 wt.% Al powder mixture after 30 h yielded thin irregular-shaped particles. XRD analysis revealed a partial structural transformation after BM and further structural change upon annealing the powder at 600 °C, as confirmed by the SEM images. The formation of the cubic spinel superstructure with lattice parameter a = 8.066 Å (ZnAl 2 O 4 prototype) and Fd-3 m # 227 space group was realized. By means of LEO 1525 field-emission scanning electron microscope (FE-SEM), a fibrous mesoporous structure was revealed on the milled powder after annealing the Co-Al mixture at 600 °C. The EDS analysis confirmed a minor N impurity on the annealed powder. [ABSTRACT FROM AUTHOR]

A combination of both Raman and Brillouin scattering experiments as well as Molecular Dynamics (MD) was used to study the superionic behaviour of BaF 2 doped with a wide range of LaF 3 concentrations (0 ⩽ x ⩽ 50 mol%). Raman spectroscopy reveals that for undoped BaF 2 and those doped with 5% and 10% LaF 3 , the room temperature spectra show the usual T 2g symmetry mode at 241 cm −1 whereas for those doped with 20%, 30% and 50% LaF 3 , the dominant Raman mode is of the E g symmetry situated at ∼263, 275 and 286 cm −1 , respectively. The Raman linewidths show near linear increases with temperature followed by rapid increases above the characteristic transition temperatures ( T c ), being at 1200, 850, 800, 975, 950 and 920 K for LaF 3 concentrations of 0, 5, 10, 20, 30 and 50; respectively. The temperature dependence of the squares of the Brillouin frequencies (Δ ω B ) 2 of the LA and TA acoustic modes respectively related to elastic constants C 11 and C 44 showed linear decreases followed by significant deviations around the same temperatures ( T c ), at which the Raman linewidths start to show substantial increases. The complementary studies using MD simulations show that the diffusion coefficients increase markedly above the same temperatures observed experimentally. The extrinsic fluorine ion trajectories were also determined from the MD simulations to better understand the mechanisms of diffusion. [ABSTRACT FROM AUTHOR]

• Amorphisation and recrystallisation technique was employed for simulated synthesis. • Composite nanosheets (2D) and nanoporous (3D) materials were generated. • XRDs and microstructures showed presence of layered-spinel intergrowths. • Bulk and nanoporous LiMn 2 O 4 can withstand stresses associated with cycling. Mechanical stresses originate from various factors, such as mechanical load, operational vibration, diffusion during cycling, and thermal expansion coefficients. Li-ion intercalation causes lattice expansion that will impart stress on neighbouring regions and ion mechanical degradation within the layers, which leads to weaker battery performance and eventual failure. The current study reports on the advances in the simulated synthesis of nanosheet, nanoporous, and bulk ternary LiMn 2 O 4 spinel from an atomic perspective, during which amorphisation and recrystallisation techniques were employed and the microstructures reflect the spinel and layered heterostructures. Analysis of mechanical and structural properties is carried out via stress-strain, x-ray diffraction (XRD), radial distribution functions (RDF) graphs, and molecular graphics systems before and after subjection to strain. Subsequently, molecular graphics for the nanoporous material indicated a rigid tunnel framework, whilst the bulk framework showed tilting upon compression. These structural deformations within the bulk might prevent efficient Li transport into/out of the electrode materials. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the phase stability from B2 to L10/B19/B19′ in equiatomic PtTi alloy using the density functional theory within the generalized gradient approximation. Their structural, energetic, electronic and the mechanical properties were determined to show the stability trend of the shape memory binary alloy. The lattice parameters are well reproduced and agree to within 2% with the available experimental data. We found that the calculated heats of formation and density of states predicted B19 and B19′ PtTi to be the most stable structures compared to B2 and L10. On the other hand, their mechanical stability predictions show consistency with regard to the elastic constants and phonon dispersion curves. The elastic instability (negative C ij) and soft modes were observed in the B2, B19 and B19′ structures and not found in the L10 structure. [Copyright &y& Elsevier]

Abstract: Composition and temperature dependence of the crystal structure of IrTi with Ti content of 46–61 at.% was investigated using high-temperature X-ray diffraction analysis. Single-point energy calculations and geometry optimization were carried out based on first-principles calculations to understand the phase stability and transformation. Using X-ray diffraction analyses, a CsCl-type B2 structure was found in Ir-61 at.% Ti at room temperature; on the other hand, an orthorhombic structure in the alloys with Ti content below 58 at.%. The atomic positions obtained through first-principles calculations suggested a distorted L10 structure refined as a new orthorhombic (Cmmm #65) structure. The orthorhombic structure changed to almost a tetragonal structure with rising temperature and finally to a distorted B2 structure above 873 K in Ir-46–58Ti alloys. [Copyright &y& Elsevier]

Abstract: The deleterious low-temperature tetragonal phases in prototypical Pt-based superalloys have variously been reported as taking the tI16-U3Si (DOc), tI16-Ir3Si (DOc′) and tP16-Pt3Ga structure-types in contrast to the high-temperature cubic cP4-Cu3Au (L12) phase. We have investigated the relative stability of these four structure-types at absolute zero by using density functional theory. We find that the ground state of stoichiometric Pt3Al is tP16-Pt3Ga and that the other three lattices are mechanically unstable at absolute zero. Experiments are needed to measure the internal displacement parameters of these three competing tetragonal phases. [Copyright &y& Elsevier]

Nano-architectured Li x TiO 2 are promising as anode electrode materials for lithium rechargeable batteries due to their ability to accommodate and transport lithium atoms at low and elevated temperatures. In the current study, lithiated nanosheets with concentrations, Li 0. 03 TiO 2 and Li 0. 07 TiO 2 , were crystallised from amorphous precursors at 2000K, using large scale molecular dynamics (MD) simulation method, which was followed by a cooling process towards 0 K. The cooled nanosheets, Li 0. 03 TiO 2 and Li 0. 07 TiO 2 , were subsequently heated at temperature intervals of 100 K from 100 K to 500 K with a Nose-Hoover thermostat. The calculated Ti-O radial distribution functions (RDFs), were used to confirm the extent of crystallisation after cooling. The simulated X-ray diffraction (XRD) spectra, at low (0 K) and high (500 K) temperatures, exhibit peaks associated with both rutile and brookite polymorphs. Microstructural features also depicted diffusion pathways with rutile and brookite character, consistent with XRD results. These results confirm that the Li 0. 03 TiO 2 and Li 0. 07 TiO 2 nanosheets are good candidates for anode materials in lithium ion batteries (LIB), since they can contain and transport lithium ions well, even under higher temperature conditions. Lithiated snapshots of evolving microstructures of nanosheets (Li 0. 03 TiO 2) at 0 K and 500 K with different types of defects. Image 1 • We report first time atomistic simulation of TiO 2 nanosheets materials intercalated with 0.03 and 0.07 Li ions. • The L 0.03 TiO 2 and L 0.07 TiO 2 nanosheets shows wide open channels after recrystallisation synthesis. • The crystalline L 0.03 TiO 2 and L 0.07 TiO 2 nanosheets show closed channels after cooling process. • The materials shows similar characteristics at low and high temperature which are then excellent anode electrode materials. [ABSTRACT FROM AUTHOR]