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Static positional disorder in ulvöspinel: A single-crystal neutron diffraction study
No full textA single-crystal neutron diffraction study of a synthetic ulvöspinel sample of composition Fe3+0.40Fe2+1.80Ti0.80O4 was performed to investigate the static positional disorder at the octahedrally coordinated M site. Anisotropic structural refinement was performed in the space group Fd3̄m against neutron Laue diffraction data collected at 298 K from two millimetric-sized crystals. Initial structure refinements were conducted with Fe and Ti sharing the M site (at 1/2, 1/2, 1/2), and their partial site occupancy was refined. The tetrahedrally coordinated T site (at 1/8, 1/8, 1/8) was modeled as fully occupied by Fe. For both crystals, the final R1 index was about 3% for 9 refined parameters and 129 unitetrahedrallyque reflections, with no significant residuals.
As the atomic displacement factors obtained were anomalously high, according to the previous experimental findings, Fobs- and (Fobs–Fcal)-difference Fourier maps of the nuclear density were generated. Fourier maps showed a significant minimum located out-of-center of the M site, and indicating a displacement of the Ti4+ from the center of the octahedron. A further test refinement was successfully conducted with two mutually exclusive sites: MTi out-of-center (at 0.49, 0.49, 0.49) and MFe on the center (at 1/2, 1/2, 1/2). The resulting displacement of Ti from the octahedral center appears to be shorter than 0.15 Å.
Using bond-valence theory, the out-of-center distortion of MTi4+ is interpreted as a result of intrinsic distortions in the ulvöspinel structure. The potential implication of the octahedral distortion on the behavior of ulvöspinel at non-ambient conditions is discussed.© 2014, Mineralogical Society of America
The Influence of Austenite Grain Size during Welding Simulations of Ferritic Steels
No full textIn recent years, considerable progress has been made in the simulation of ferritic steel welding processes. The successful validation of a single-pass autogenous TIG beam weld in SA508 Gr.3 Cl.1 steel has identified key simulation variables required for the accurate prediction of post-weld residual stress in ferritic weldments. The present work outlines a sensitivity study performed to examine the influence of austenite grain growth on predicted solid-state phase transformation kinetics and consequently, residual stress predictions.© 2014, Trans Tech Publications
Superconductivity on a crossover phenomenon of spin–ladder system SrCa13Cu24O41 single crystals
No full textElectrical resistivity and alternating current susceptibility measurements were performed for the spin–ladder compound SrCa13Cu24O41 under hydrostatic pressure up to 8 GPa. The superconducting transition was observed below TC = 14.7 K at pressures above 2 GPa. By using high-quality single crystals, the bulk superconductivity with a volume fraction of 50% and temperature quadratic behavior of the normal state resistivity were observed above 3.7 GPa. They suggest that a strong interladder interaction induces the bulk superconductivity and transforms the system into a Fermi-liquid-like state. The optimal pressure was in a crossover region and the highest TC was observed without going through the bulk superconducting state. Our results suggest that the superconducting state on the crossover phenomenon presents the feature of the doped two-leg spin–ladder system because the spin gap was observed in this compound. © 2014, The Physical Society of Japan
Surface modifications of TiO2 by ion implantation
No full textRutile is a good candidate for total water oxidation and hydrogen production due mainly to its corrosion resistance in water. In its native form, the direct band gap of 3.02eV, with the absorption edge in the visible part of the solar spectrum at 416nm, but close to the UV region, results in poor photo-catalytic efficiency of around 3%, well below the commercial threshold of around 10%. One option to enhance the photo-catalytic efficiency is band gap engineering, through the reduction of the Fermi level, which may be achieve by either changing the oxygen activity, or the incorporation of aliovalent foreign ions. The incorporation of donor and acceptor-type of ions may lead to a shift in the Fermi level close to the conduction or valence bands. Single crystals of TiO2-Rutile (SG: P4 2/mnm), with (001) orientation were implanted along at 10keV and 2MeV, where the range of C in TiO2 is 21nm and 1.6μm, respectively. The ion implantation was carried out at various equivalent C concentrations, and characterised in-situ by ERDA with 82MeV Iodine, and PIXE with 2.5MeV protons. Optic band gap was assessed by ground state band structure calculations based on CASTEP code, with geometry optimisation by general gradient approximation and PBE functional, and energy optimisation by local density approximation and CA-PZ functional. © 2014, Trans Tech Publications
CLIC proteins, ezrin, radixin, moesin and the coupling of membranes to the actin cytoskeleton: A smoking gun?
No full textThe CLIC proteins are a highly conserved family of metazoan proteins with the unusual ability to adopt both soluble and integral membrane forms. The physiological functions of CLIC proteins may include enzymatic activity in the soluble form and anion channel activity in the integral membrane form. CLIC proteins are associated with the ERM proteins: ezrin, radixin and moesin. ERM proteins act as cross-linkers between membranes and the cortical actin cytoskeleton. Both CLIC and ERM proteins are controlled by Rho family small GTPases. CLIC proteins, ERM and Rho GTPases act in a concerted manner to control active membrane processes including the maintenance of microvillar structures, phagocytosis and vesicle trafficking. All of these processes involve the interaction of membranes with the underlying cortical actin cytoskeleton. The relationships between Rho GTPases, CLIC proteins, ERM proteins and the membrane:actin cytoskeleton interface are reviewed. Speculative models are proposed involving the formation of localised multi-protein complexes on the membrane surface that assemble via multiple weak interactions. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters.© 2013, Elsevier B.V
Retreat history of the East Antarctic ice sheet since the last glacial maximum
Get PDFThe East Antarctic Ice Sheet (EAIS) is the largest continental ice mass on Earth, and documenting its evolution since the Last Glacial Maximum (LGM) is important for understanding its present-day and future behaviour. As part of a community effort, we review geological evidence from East Antarctica that constrains the ice sheet history throughout this period (∼30,000 years ago to present). This includes terrestrial cosmogenic nuclide dates from previously glaciated regions, 14C chronologies from glacial and post-glacial deposits onshore and on the continental shelf, and ice sheet thickness changes inferred from ice cores and continental-scale ice sheet models. We also include new 14C dates from the George V Land – Terre Adélie Coast shelf. We show that the EAIS advanced to the continental shelf margin in some parts of East Antarctica, and that the ice sheet characteristically thickened by 300–400 m near the present-day coastline at these sites. This advance was associated with the formation of low-gradient ice streams that grounded at depths of >1 km below sea level on the inner continental shelf. The Lambert/Amery system thickened by a greater amount (800 m) near its present-day grounding zone, but did not advance beyond the inner continental shelf. At other sites in coastal East Antarctica (e.g. Bunger Hills, Larsemann Hills), very little change in the ice sheet margin occurred at the LGM, perhaps because ice streams accommodated any excess ice build up, leaving adjacent, ice-free areas relatively unaffected. Evidence from nunataks indicates that the amount of ice sheet thickening diminished inland at the LGM, an observation supported by ice cores, which suggest that interior ice sheet domes were ∼100 m lower than present at this time. Ice sheet recession may have started ∼18,000 years ago in the Lambert/Amery glacial system, and by ∼14,000 years ago in Mac.Robertson Land. These early pulses of deglaciation may have been responses to abrupt sea-level rise events such as Meltwater Pulse 1a, destabilising the margins of the ice sheet. It is unlikely, however, that East Antarctica contributed more than ∼1 m of eustatic sea-level equivalent to post-glacial meltwater pulses. The majority of ice sheet recession occurred after Meltwater Pulse 1a, between ∼12,000 and ∼6000 years ago, during a period when the adjacent ocean warmed significantly. Large tracts of East Antarctica remain poorly studied, and further work is required to develop a robust understanding of the LGM ice sheet expansion, and its subsequent contraction. Further work will also allow the contribution of the EAIS to post-glacial sea-level rise, and present-day estimates of glacio-isostatic adjustment to be refined. © 2014 The Authors
Segregation and migration of species in the CrCoFeNi high entropy alloy
No full textThe 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
Impact of Cu doping on the structure and electronic properties of LaCr1–yCuyO3
No full textOxides of the type LaCr1–yCuyO3 have been prepared using solid-state methods and their crystal structures refined using synchrotron X-ray powder diffraction. The solubility limit of Cu was found to be around y = 0.2, and such oxides are orthorhombic in space group Pbnm. X-ray absorption spectroscopy measurements at the Cr and Cu L-edges demonstrated that the Cr remains trivalent upon Cu doping, with the Cu being present as Cu(III). The oxides are found to be antiferromagnets, and the Néel temperature, TN, decreases as the Cu content is increased. The crystal and magnetic structures of one example La(Cr0.85Cu0.15)O3 have been investigated between 3 and 350 K by neutron powder diffraction. The samples are semiconductors. © 2014, American Chemical Society
pH-responsive micelles based on caprylic acid
No full textFree fatty acids play a vital role as fuel for cells and in lipid metabolism. During lipid digestion in the gastrointestinal tract, triglycerides are hydrolyzed, resulting in free fatty acid and monoglyceride amphiphilic products. These components, together with bile salts, are responsible for the transport of lipids and poorly water-soluble nutrients and xenobiotics from the intestine into the circulatory system of the body. In this study, we show that the self-assembly of digestion products from medium-chain triglycerides (tricaprylin) in combination with bile salt and phospholipid is highly pH-responsive. Individual building blocks of caprylic acid within the mixed colloidal structures are mapped using a combination of small-angle X-ray and neutron scattering combined with both solvent contrast variation and selective deuteration. Modeling of the scattering data shows transitions in the size and shape of the micelles in combination with a transfer of the caprylic acid from the core of the micelles to the shell or into the bulk water upon increasing pH. The results help to understand the process of lipid digestion with a focus on colloidal structure formation and transformation for the delivery of triglyceride lipids and other hydrophobic functional molecules. © 2014, American Chemical Society
Temperature dependence of alkali-metal rattling dynamics in the β-pyrochlores, AOs2O6 (A = K, Rb, Cs), from MD simulation
No full textWe investigate the temperature response of the alkali-metal rattling modes in β-pyrochlores,
AOs2O6 (A = K, Rb, Cs), from the results of ab initio molecular dynamics (MD) simulations
performed at 20K, 100K and 300K. Our results show that the temperature response of the T1u
mode is clearly different from that of the T2g mode for all three pyrochlores. In this regard,
two features are of particular note for both K and Rb; (1) the T1u mode exhibits a distinctly
stronger softening response with decreasing temperature compared to the T2g mode, and (2)
the T1u mode becomes stronger and sharper with decreasing temperature. These two findings
suggest that the T1u mode is significantly more anharmonic and sensitive to the cage dynamics
than the T2g mode. Examination of the local potentials around the alkali-metal atoms reveals
that K has the flattest and most anharmonic potential at all temperatures while Cs exhibits the
narrowest potential. The temperature dependence of the local potentials reveals that, for K,
the potential at a higher temperature is not a simple extrapolation to higher energy of that at
a lower temperature. Instead, we find significant reconstruction of the potential at different
temperatures. Finally, we explore the temperature response of the coupling between the alkali
metals and find a complex temperature dependence which suggests that the origin of the
coupling may be more complex than a pure Coulomb interaction. We also find an unexpected
increase in the static disorder of the system at low temperatures for the K and Rb pyrochlores. © 2014, IOP Publishing Lt