Determination of the Hyperfine Parameters of Iron Silicides by Angle Dependent Conversion Electron Mössbauer Spectroscopy on Single Crystals

Hyperfine Interactions - The Mössbauer spectra of single crystals of the iron silicides ε-FeSi, β-FeSi2 and α-FeSi2 are collected by conversion electron Mössbauer...     

Lattice dynamics and ordering effects in high-concentrated α′-NbD x -alloys

The phonon dispersion of NbD _x (0.57≦x≦0.77) in [110]-direction has been determined by inelastic neutron scattering after in situ loading of the sample. Two quasioptical excitations with Γ-point frequencies of ν=3.8 THz and ν=4.2 THz, respectively, are observed along with a threefold splitting of the longitudinal acoustic branch. At the zone boundary two of these branches are getting soft when increasing the deuterium concentration. The elastic constants extracted from the initial slopes of acoustic phonon branches exhibit an anomalous behavior in the concentration range under consideration.     

Dual Activation of Unsaturated Amides with Schwartz's Reagent: A Diastereoselective Access to Cyclopentanols and N,O-Dimethylcyclopentylhydroxylamines.

The diastereoselective access to cyclopentanols and N,O-dimethylcyclopentylhydroxylamines from 4-pentenoic acid-derived Weinreb amides is described. Based on the concomitant generation of both the nucleophilic and the electrophilic poles by hydrozirconation of the amide and the C=C double bond, the cyclisation may be tuned towards cyclopentanols or N,O-dimethylcyclopentylhydroxylamines depending on the ring-closure promotor. An extension to cis 3-substituted N,O-dimethylcyclohexylhydroxylamines is also presented.     

Dewatering Green Sapwood Using Carbon Dioxide Undergoing Cyclical Phase Change between Supercritical Fluid and Gas

Conventional kiln drying of wood operates by the evaporation of water at elevated temperature. In the initial stage of drying, mobile water in the wood cell lumen evaporates. More slowly, water bound in the wood cell walls evaporates, requiring the breaking of hydrogen bonds between water molecules and cellulose and hemicellulose polymers in the cell wall. An alternative for wood kiln drying is a patented process for green wood dewatering through the molecular interaction of supercritical carbon dioxide with water of wood cell sap. When the system pressure is reduced to below the critical point, phase change from supercritical fluid to gas occurs with a consequent large change in CO₂ volume. This results in the efficient, rapid, mechanical expulsion of liquid sap from wood. The end-point of this cyclical phase-change process is wood dewatered to the cell wall fibre saturation point. This paper describes dewatering over a range of green wood specimen sizes, from laboratory physical chemistry studies to pilot-plant trials. Magnetic resonance imaging and nuclear magnetic resonance spectroscopy were applied to study the fundamental mechanisms of the process, which were contrasted with similar studies of conventional thermal wood drying. In conclusion, opportunities and impediments towards the commercialisation of the green wood dewatering process are discussed.     

Crystal Growth of High‐melting Multi‐component Rare Earth‐Transition Metal Intermetallic Compounds from the Melt

The crystal growth of different classes of high melting multi‐component Rare Earth‐Transition Metal‐Compounds by vertical floating zone melting with inductive heating has been investigated. Phase diagram features of the multi‐component systems relevant for the crystallization process have been revealed. The critical zone travelling rate for RENi₂B₂C crystal growth is one order of magnitude smaller than for RE₂TMSi₃ compounds. This is attributed to different solidification modes of both classes of compounds, peritectic and congruent melting, respectively. The crystal perfection, element segregation and selected properties of the bulk crystals such as critical temperature of superconductivity were studied as function of the axis co‐ordinate. In the case of RE₂TMSi₂ plate‐like RESi or RESi₂ precipitates were detected in the single crystalline matrix. They were partially dissolved by annealing and subsequent quenching. The anisotropy of various superconducting and magnetic properties was determined at YNi₂B₂C and TbNi₂B₂C single crystalline specimens.