Abstract: When strained in tension, high-manganese austenitic twinning induced plasticity (TWIP) steels achieve very high strength and elongation before necking. The main hypotheses available in the literature about the origin of their excellent work hardening include deformation twinning and dynamic strain ageing. In order to provide some answers, various experiments at different scales were conducted on FeMnC steels and the Fe28 wt%Mn3.5 wt%Al2.8 wt%Si alloy. At a macroscopic scale, tensile tests were performed on all the studied grades. It was shown that, though the FeMnAlSi based alloy retains very high elongation, the FeMnC steels properties are even more extraordinary. Tensile tests at different strain rates with the help of digital image correlation were also performed on the Fe20 wt%Mn1.2 wt%C steel to study the PLC effect occurring in this type of steel. It is suggested that supplementary hardening could come from reorientation of MnC pairs in the cores of the dislocations. At a microscopic scale, the Fe20 wt%Mn1.2 wt%C TWIP steel and the FeMnAlSi grade were thoroughly investigated by means of in situ TEM analysis. In the FeMnC steel, the formed twins could also lead to a composite effect, since they contain plenty of sessile dislocations. In the FeMnAlSi alloy, mechanical twins are thicker and contain fewer defects, leading to a lower work hardening than the other grade.

Abstract: We have investigated the magnetic depth profile of an epitaxial Fe3O4 thin film grown directly on a semiconducting ZnO substrate by soft x-ray resonant magnetic reflectometry (XRMR) and electron energy loss spectroscopy (EELS). Consistent chemical profiles at the interface between ZnO and Fe3O4 are found from both methods. Valence selective EELS and XRMR reveal independently that the first monolayer of Fe at the interface between ZnO and Fe3O4 contains only Fe3+ ions. Besides this narrow 2.5 Å interface layer, Fe3O4 shows magnetic bulk properties throughout the whole film making highly efficient spin injection in this system feasible.

Abstract: Neutron and high resolution X-ray diffraction investigations on perfect single crystals of RbH2PO4 (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric – ferroelectric phase transition, support a disorder – order character Of [PO4H2](-)-groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time-space average of orthorhombic symmetry. According to the group – subgroup relation between the tetragonal space group 142d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at T-C = 147 K. From the high resolution X-ray data it is concluded that below T-C the ferroelastic strain in the (a,b)-plane leads to micro-angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature T-C is decreased.

Abstract: The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are investigated by low-temperature neutron powder diffraction and density functional theory band structure calculations. In spite of the random distribution of the Mn3+ and Fe3+ cations, this centrosymmetric compound undergoes a transition into an incommensurate antiferromagnetically ordered state below TN≃220 K. The magnetic structure is characterized by the propagation vector k=[0,β,0] with β≃0.14 and the P221211′(0β0)0s0s magnetic superspace symmetry. It comprises antiferromagnetic helixes propagating along the b axis. The magnetic moments lie in the ac plane and rotate about π(1+β)≃204.8-deg angle between the adjacent magnetic atoms along b. The spiral magnetic structure arises from the peculiar frustrated arrangement of exchange couplings in the ab plane. The antiferromagnetic coupling along the c axis cancels the possible electric polarization and prevents ferroelectricity in BiMnFe2O6.

Abstract: Particles produced by previous laser shots may have significant influence on the analytical signal in laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma (LA-ICP) spectrometry if they remain close to the position of laser sampling. The effects of these particles on the laser-induced breakdown event are demonstrated in several ways. LIBS-experiments were conducted in an ablation cell at atmospheric conditions in argon or air applying a dual-pulse arrangement with orthogonal pre-pulse, i.e., plasma breakdown in a gas generated by a focussed laser beam parallel and close to the sample surface followed by a delayed crossing laser pulse in orthogonal direction which actually ablates material from the sample and produces the LIBS plasma. The optical emission of the LIBS plasma as well as the absorption of the pre-pulse laser was measured. In the presence of particles in the focus of the pre-pulse laser, the plasma breakdown is affected and more energy of the pre-pulse laser is absorbed than without particles. As a result, the analyte line emission from the LIBS plasma of the second laser is enhanced. It is assumed that the enhancement is not only due to an increase of mass ablated by the second laser but also to better atomization and excitation conditions favored by a reduced gas density in the pre-pulse plasma. Higher laser pulse frequencies increase the probability of particle-laser interaction and, therefore, reduce the shot-to-shot line intensity variation as compared to lower particle loadings in the cell. Additional experiments using an aerosol chamber were performed to further quantify the laser absorption by the plasma in dependence on time both with and without the presence of particles. The overall implication of laser-particle interactions for LIBS and LA-ICP-MS/OES are discussed.

Abstract: The most efficient use of spatial volume and the lowest potential energies in the metal oxide structures are based on cubic close packing (ccp) or hexagonal close packing (hcp) of anions with cations occupying the interstices. A promising way to tune the composition of close packed oxides and design new compounds is related to fragmenting the parent structure into modules by periodically spaced planar interfaces, such as twin planes at the unit cell scale. The unique crystal chemistry properties of cations with a lone electron pair, such as Bi3+ or Pb2+, when located at interfaces, enables them to act as chemical scissors, to help relieve configurational strain. With this approach, we synthesized a new oxide, BiMnFe2O6, where fragments of the hypothetical hcp oxygen-based MO structure (the NiAs structure type), for the first time, serve as the building modules in a complex transition metal oxide. Mn3+ and Fe3+ ions are randomly distributed in two crystallographically independent sites (M1 and M2). The structure consists of quasi two-dimensional blocks of the 2H hexagonal close packed MO structure cut along the (114) crystal plane of the hcp lattice and stacked along the c axis. The blocks are related by a mirror operation that allows BiMnFe2O6 to be considered as a polysynthetically twinned 2H hcp MO structure. The transition to an AFM state with an incommensurate spin configuration at [similar] 212 K is established by 57Fe Mössbauer spectroscopy, magnetic susceptibility, specific heat and low temperature powder neutron diffraction.

Abstract: For many years, photocatalysis has been proposed as one of the promising techniques to abate environmental pollutants. To improve these reactions it is vital to know the reaction mechanisms of the photocatalytic degradation. This new reactor will make it possible to study the catalytic surface at the moment the reactions occur. By the means of UV LED illumination there is no need of an external UV lamp and thus lowers the cost. The validation of this newly developed reactor is done by investigating the photocatalytic reaction mechanism of nitric oxide (NO) and comparing these findings with those already discussed in literature. From these results, it became clear that the newly developed FTIR in situ reactor allows real time study of photocatalytic degradations.

Abstract: It is well known that, in bulk, the solution of the Bogoliubovde Gennes equations is the same whether or not the HartreeFock term is included. Here the HartreeFock potential is position independent and so gives the same contribution to both the single-electron energies and the Fermi level (the chemical potential). Thus, the single-electron energies measured from the Fermi level (they control the solution) stay the same. This is not the case for nanostructured superconductors, where quantum confinement breaks the translational symmetry and results in a position-dependent HartreeFock potential. In this case its contribution to the single-electron energies depends on the relevant quantum numbers. We numerically solved the Bogoliubovde Gennes equations with the HartreeFock term for a clean superconducting nanocylinder and found a shift of the curve representing the thickness-dependent oscillations of the critical superconducting temperature to larger diameters.

Abstract: Vortex charges in high-temperature superconductor (HTS) are studied by solving the Bogoliubovde Gennes equations based on a model Hamiltonian with antiferromagnetic (AF) and d-wave orders in the presence of the long-range Coulomb repulsion. For a sufficient strength of the AF order, the negative vortex charge is found. A sign change between negative and positive may occur by tuning the long-range Coulomb repulsion strength or the doping parameter. Recent NMR experiments are hopefully understood. We show that the charged vortex can induce a spin-orbit coupling that is important for superconductors with a short coherence length and a large value of the energy gap over the Fermi-level ratio. Fractional flux quanta are possible for HTS.

Abstract: In a numerical experiment based on Gross-Pitaevskii formalism, we demonstrate unique topological quantum coherence in optically trapped Bose-Einstein condensates (BECs). Exploring the fact that vortices in a rotating BEC can be pinned by a geometric arrangement of laser beams, we show the parameter range in which vortex-antivortex molecules or multiquantum vortices are formed as a consequence of the optically imposed symmetry. Being low-energy states, we discuss the conditions for spontaneous nucleation of these unique molecules and their direct experimental observation, and provoke the potential use of the phase print of an antivortex or a multiquantum vortex when realized in unconventional circumstances.

Abstract: Particularly favourable properties of glass ceramics are developed on the basis of two key advantages of these materials: the variation of chemical composition and of microstructure. Therefore, detailed structural and chemical information are necessary to get insight in novel glass ceramic materials. We present here two examples of Er-doped Li2O-Al2O3-SiO2, with different quantities of ZrO2, both obtained with sol-gel synthesis. Different transmission electron microscopy techniques: conventional TEM, HRTEM, and EELS are used and the results are compared with those previously obtained with XRD and Rietveld analysis. We also demonstrate the 3D reconstruction, obtained from HAADF-STEM imaging, to determine the morphology of nanosize precipitates in these composites. (c) 2007 Elsevier B.V. All rights reserved.