Mechanistic Insights into Enantioselective C?H Photooxygenation of Aldehydes via Enamine Catalysis

Organocatalytic photooxygenation of aldehydes at the α‐position proceeds via enamine catalysis, though enamines should be easily oxidized by singlet oxygen respectively to amides and carbonyl compounds. Moreover, the formation of a zwitterionic enamine peroxide intermediate was postulated based on experimental and theoretical data. The reaction affords desired diols (after in situ reduction) in a decent yield and (S)‐ or (R)‐enantioselectivity depending on a catalyst used. The (S)‐enantiomer predominated in imidazolidinone‐catalyzed reactions, while prolineamides assured the formation of the (R)‐stereoisomer. DFT calculation suggests that the enamine–oxygen complex with the lowest energy has the E, s‐cis conformation for the prolineamide derivative and E, s‐trans for the imidazolidinone catalyst, explaining the opposite stereoselectivity in the photooxygenation reaction.

     

Fe-Al phase formation around SHS reactions under isothermal conditions

This study investigates the phenomena preceding and accompanying the SHS reaction between Fe and Al elemental powders during sintering. SEM and XRD analysis were used to observe the mechanisms of formation of Fe-Al intermetallic phases. The analysis of sintered material just before the SHS reaction demonstrates that in addition to the well-known Fe₂Al₅ phase and the low-aluminum solid solution of iron, the high-aluminum phases FeAl₂ and FeAl₃ are formed. The kinetics of phase transformations under isothermal conditions were investigated by DSC using the JMA (Johnson-Mehl-Avrami) model. This approach allowed us to calculate Avrami coefficients, which characterize the speed and the manner of particular phase transformations.     

Luminescence of CdMnTe/CdMgTe structures with periodically arranged narrow-gap inclusions

The study is concerned with the excitonic and intracenter emission spectra of planar structures with periodically arranged Cd_0.9Mn_0.1Te layers (with a nominal thickness corresponding to one monolayer) separated by Cd_0.5Mg_0.5Te barriers with different thicknesses. The complex structure of the spectra and their variations in the temperature range from 5 to 80 K are indicative of the inhomogeneity of the Cd_0.9Mn_0.1Te layers, the important role of vertical diffusion, and the correlation between irregularities in the direction of growth at a small spacing between the narrow-gap Cd_0.9Mn_0.1Te layers. A substantial factor controlling the spectrum and quantum efficiency of excitonic emission is the relation between the exciton radius and the period of the structure.     

原位聚合抗磨损PA6/纳米SiO2复合材料制备及其性能研究

为提高聚酰胺6（PA6）的抗磨损性能,采用原位聚合法合成并制备了PA6/纳米SiO2复合材料,研究了该材料的抗磨损性能、耐热性能、力学性能和结晶性能。结果表明,原位聚合PA6/纳米SiO2复合材料具有良好的抗磨损特性,当纳米SiO2含量为1 %(质量分数,下同)时,复合材料抗磨损性能最佳,该材料的热变形温度、拉伸强度、弹性模量和断裂伸长率均明显高于原料PA6;当纳米SiO2含量为3 %时,复合材料热变形温度由原料PA6的64.6 ℃提高到130 ℃。采用原位聚合母料法制备的PA6/纳米SiO2复合材料同样具有理想的抗磨损性能,并可获得更好的力学性能,且可大幅降低材料制备成本。     

Low temperature deposition of dense MnCo2O4 protective coatings for steel interconnects of solid oxide cells

In this work manganese cobalt spinel (MnCo₂O₄) coatings were deposited on steel substrates by spray pyrolysis at 390?°C. This is at much lower temperatures than previously reported (typically in excess of 900?°C). It was possible to produce coatings with well controlled thickness (2-5-10?μm). The as-deposited coatings were evaluated for their microstructural changes and electrical conductivity up to 800?°C. Results confirm the formation of a single phase spinel with high density and electrical conductivity. Based on the obtained results, it might be concluded that spray pyrolysis is a very promising method to develop protective coatings for steel substrates at low temperatures overcoming limitations of many other methods.     

Ferromagnetism in (Zn,Cr)Se Layers Grown by Molecular Beam Epitaxy

Magnetization measurements of MBE grown epilayers of (Zn,Cr)Se with relatively large Cr content of 0.014 and 0.021 are presented. We evidence the presence of a strong ferromagnetic coupling between Cr ions, but also suggest a significant clustering due to a pronounced superparamagnetic behavior found in the layers. We estimate the intraparticle Curie temperature to be about 100 K, which combined with other arguments appears to indicate that some magnetic properties of Cr-rich layers might be dominated by the presence of small grains of A^II–Cr₂–B₄ ^VI spinels.     

Mechanism of intra-shell recombination of transition metal and rare earth ions in nanostructures of II–VI compounds

Based on the results of optically detected magnetic resonance (ODMR) and time-resolved investigations we relate the observed lifetime shortening of intra-shell Mn²⁺ emission to spin dependent magnetic interactions between localized spins of Mn²⁺ ions and spins/magnetic moments of free carriers. We show that this mechanism is active both in bulk and in low dimensional structures, such as quantum wells (QWs), quantum dots (QDs) and nanostructures.     

Structural and electrical transport properties of Pr-doped SrTi 0.93 Co 0.07 O 3-δ a novel SOEC fuel electrode materials

Solid Oxide Electrolyzer Cells (SOECs) are very promising electrochemical devices for the production of syngas (H2/CO) by H2O and CO2 co-electrolysis. The structure, microstructure and electrical properties of the fuel electrode material play a crucial role in the performance of the whole cell and efficiency of electrocatalytic reduction of steam into hydrogen. In the present work, a novel Co and Pr co-doped SrTiO3-δ material attracted attention as a potential fuel electrode for SOFC/SOEC. Materials with different praseodymium content were prepared by a solid-state reaction process. XRD confirmed cubic perovskite structure in all obtained samples. SEM results showed porosity in doped materials and EDX proved ABO3 stoichiometry. TEC values were about 1.17–1.26•10−5 K−1 very close to the YSZ electrolyte value. XPS studies turn out that a praseodymium can be multivalent and exist on mixed +3 and + 4 oxidation state. Electrical conductivity of samples was measured by DC 4-wire method in range of 100-800 °C. Highest value of total conductivity was achieved for Sr0.7Pr0.3Ti0.93Co0.07O3-δ and reached 23.7 S∙cm−1. The obtained results were discussed and analyzed in term of defect chemistry.     

Spin properties of trions in a dense quasi-2D electron gas

The reflection and photoluminescence spectra of modulation-doped CdTe/CdMgTe quantum-well structures have been studied. It was found that the magnitude and sign of the Zeeman splitting of the trion reflection line depend on the electron concentration in the quantum well, whereas the magnitude and sign of the splitting of the exciton line are absolutely the same for all the electron concentrations under study. In the photoluminescence spectra, the magnitude and sign of the Zeeman splittings for the exciton and trion were the same. This ??renormalization?? of the trion g factor is explained in terms of the model of combined exciton-electron processes.     

Radiative recombination in the CdMgTe matrix with ultrathin narrow-gap CdMnTe layers

Exitonic emission from three structures consisting of CdMnTe layers separated by thick wide-gap CdMgTe layers are studied in the temperature range from 13 to 300 K. The thickness of the CdMnTe layers corresponds to the 0.5, 1.5, and 3.0 monolayers. To calculate the excitonic density-of-state spectra in these structures, a simple model that takes into account variations in the elemental composition of the alloy inside the exciton sphere under variations in its position with respect to the narrow-gap layer is used. It is shown that the energy extent of the emission and density-of-state spectra and the positions of the characteristic features of the spectra are in agreement. On low-intensity excitation (1–10 W/cm²), heating of the samples from 13 K yields a relative decrease in the emission intensity of excitions localized in the CdMgTe layers, so that this emission at 80 K is much lower in intensity than the emission of excitions bound at the narrow-gap layers. In the conditions of high-intensity excitation (10³–10⁵ W/cm²) at 80 K, the excitonic emission from the CdMgTe layers prevails; however, at room temperature, the emission from the narrow-gap layers dominates over the emission from the wide-gap layers. The intracenter luminescence of the Mn²⁺ ions in the sample with three monolayers CdMnTe presents a doublet structure, with the components corresponding to the internal and external layers of the three-layer planar inclusions containing manganese.