Observation of ε′-Ag17 Mg54 phase induced by plasma immersion ion implantation

This paper provides a confirmation of the effectiveness of the recently suggested ab initio approach to the theoretical prediction of phase transformations which may be induced in metallic alloys by metal plasma immersion and ion implantation processing. The approach is based on an assumption that at certain concentrations of the implanted species, the relaxation of the exited electronic state of the implanted structure should be accompanied by the rearrangement of atoms leading to the formation of a new phase. Recently, on the basis of density functional theory calculations of the energetic characteristics of the electronic subsystems of the implanted Mg–Ag system, it was predicted that concentrations of the implanted Ag ions within the range from ～18 to 23 at% Ag, favor transition to the phase ε′-Ag₁₇Mg₅₄. Our transmission electron microscopy observations and electron diffraction analysis of the Mg-based alloy subjected to the implantation of Ag ions at dose of ～5×10¹⁵ ion/cm² confirmed that the formation of the ε′-Ag₁₇Mg₅₄ phase indeed takes place.     

Kinetic simulation of the 3D growth of subsurface impurity nanoclusters during cobalt deposition onto a copper surface

A physical model of the 3D growth of a subsurface cobalt cluster during the epitaxy of cobalt atoms on a copper substrate is developed. Time dependences are established for the cobalt cluster radius and height.     

Homo‐Diels–Alder reaction of a very inactive diene,bicyclo[2,2,1]hepta‐2,5‐diene,with the most active dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione. Solvent,temperature, and high pressure influence on the reaction rate

Solvent, temperature, and high pressure influence on the rate constant of homo‐Diels–Alder cycloaddition reactions of the very active hetero‐dienophile, 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione (1), with the very inactive unconjugated diene, bicyclo[2,2,1]hepta‐2,5‐diene (2), and of 1 with some substituted anthracenes have been studied. The rate constants change amounts to about seven orders of magnitude: from 3.95^.10^?3 for reaction (1+2) to 12200 L mol^?1 s^?1 for reaction of 1 with 9,10‐dimethylanthracene (4e) in toluene solution at 298 K. A comparison of the reactivity (ln k₂) and the heat of reactions (?_r‐nH) of maleic anhydride, tetracyanoethylene and of 1 with several dienes has been performed. The heat of reaction (1+2) is ?218 ± 2 kJ mol^?1, of 1 with 9,10‐dimethylanthracene ?117.8 ± 0.7 kJ mol^?1, and of 1 with 9,10‐dimethoxyanthracene ?91.6 ±0.2 kJ mol^?1. From these data, it follows that the exothermicity of reaction (1+2) is higher than that with 1,3‐butadiene. However, the heat of reaction of 9,10‐dimethylanthracene with 1 (?117.8 kJ mol^?1) is nearly the same as that found for the reaction with the structural C=C counterpart, N‐phenylmaleimide (?117.0 kJ mol^?1). Since the energy of the N=N bond is considerably lower (418 kJ/bond) than that of the C=C bond (611 kJ/bond), it was proposed that this difference in the bond energy can generate a lower barrier of activation in the Diels–Alder cycloaddition reaction with 1. Linear correlation (R = 0.94) of the solvent effect on the rate constants of reaction (1+2) and on the heat of solution of 1 has been observed. The ratio of the volume of activation (?V^≠) and the volume of reaction (?V_r‐n) of the homo‐Diels–Alder reaction (1+2) is considered as “normal”: ?V^≠/?V_r‐n = ?25.1/?30.95 = 0.81. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Individual Si and C adatoms, as well as SiC clusters, on a Si surface are simulated by the molecular dynamics method in the course of investigation of the initial stages of formation of a SiC layer on silicon with the help of molecular beam epitaxy. The potential energy surfaces for Si and C adatoms on the (2 × 1) reconstructed Si(001) surface and on the nonreconstructed Si(111) surface, as well as on the Si(111) surface with a SiC cluster, are calculated and analyzed. The values of migration barriers for adatoms on these surfaces are calculated. The effect of the SiC cluster on deformation of the surface region of Si(111) and on the migration of adatoms is investigated. The deep minima observed on the potential energy surfaces immediately above a cluster and at its boundaries can trap diffusing adatoms. The distributions of stresses and strains in the silicon lattice under a cluster on the surface are studied and described.     

Lithium aminoborohydrides 16. Synthesis and reactions of monomeric and dimeric aminoboranes

Aminoboranes are synthesized in situ from the reaction of the corresponding lithium aminoborohydrides (LABs) with methyl iodide, trimethylsilylchloride (TMS-Cl), or benzyl chloride under ambient conditions. In hexanes, the reaction using methyl iodide produces aminoborane and methane, whereas in tetrahydro-furan (THF) this reaction produces amine-boranes (R1R2HN:BH3) as the major product. The reaction of iPr-LAB with TMS-Cl or benzyl chloride yields exclusively diisopropylaminoborane [BH2-N(iPr)2] in THF as well as in hexanes at 25 degrees C. Diisopropylaminoborane and dicyclohexylaminoborane exist as monomers due to the steric requirement of the alkyl group. All other aminoboranes studied are not sterically hindered enough to be monomers in solution, but instead exist as a mixture of monomers and dimers. The dimers are four-membered rings formed through boron-nitrogen coordination. In general aminoboranes are not hydroborating reagents. However, monomeric aminoboranes, such as BH2-N(iPr)2, can reduce nitriles in the presence of catalytic amounts of LiBH4. This BH2-N(iPr)2/LiBH4 reducing system also re-duces ketones, aldehydes, and esters. Diisopropylaminoborane, synthesized from iPr-LAB, can be converted into boronic acids by a palladium-catalyzed reaction with aryl bromides. Aminoboranes derived from heterocyclic amines, such as pyrrole, pyrazole, and imidazole, can be prepared by the direct reaction of borane/tetrahydrofuran (BH3:THF) with these heterocyclic amines. It has been reported that pyrazole-derived aminoborane forms a six-membered dimer through boron-nitrogen coordination, where as, pyrrolylborane forms a dimer through boron-hydrogen coordination. Pyrrolylborane monohydroborates both alkenes and alkynes at ambient temperatures. Hydroboration of styrene with pyrrolylborane followed by hydrolysis gives the corresponding boronic acid, 2-phenylethylboronic acid, in 40% yield. Similarly phenylacetylene is mono-hydroborated by pyrrolylborane, to give E-2-phenylethenylboronic acid in 50% yield.     

Structural Phase Transitions in Niobium Hydrogen Thin Films: Mechanical Stress,Phase Equilibria and Critical Temperatures

Metal−hydrogen (M−H) systems offer grand opportunities for studies on fundamental aspects of thermodynamics and kinetics. When the system size is reduced to the nanoscale, microstructural defects as well as mechanical stress affect the systems’ properties. This is contemplated for the model system of epitaxial niobium−hydrogen (Nb−H) thin films. Hydrogen absorption in metals commonly leads to lattice expansion which is hindered when the metal adheres to a flat rigid substrate. Consequently, high mechanical stress of about −10 GPa for 1 H/Nb are predicted, in theory. However, metals cannot yield such high stresses and respond with plastic deformation, commonly limiting measured stresses to −2 to −3 GPa for 100 nm Nb−H films. It will be shown that the coherency state changes with film thickness reduction, shifting the onset of plastic deformation to larger hydrogen concentrations. Below critical film thicknesses, plastic deformation is fully absent. The system then behaves purely elastic and ultra-high stress of about −10 (±2) GPa can be obtained. Arising stress controls the phase stability of M−H systems, and the coherency state strongly affects the nucleation and growth dynamics of the phase transition. In case of Nb−H thin films of less than 8 nm thickness the common phase transformation from the α-phase solid solution to the hydride phase is completely suppressed at 300 K. Related effects can be utilised to optimise metal−hydrides used in applications.