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排序方式: 共有497条查询结果,搜索用时 515 毫秒
491.
The nanostructure of a nanocrystalline Fe73.5Si13.5B9Nb3Cu alloy has been studied by means of Mössbauer spectroscopy, 1D and 3D atom probes. After 6 h at 520 °C, the crystallized fraction of the alloy is about 53%. α-Fe(Si) nanocrystals, 10–20 nm in diameter, are embedded in a retained amorphous matrix. They have a near Fe80Si20 composition and exhibit a DO3 structure. The very low number density of Cu-rich particles does not match with the hypothesis of a systematic heterogeneous precipitation of α-Fe close to Cu-rich particles. A detailed analysis of the distribution of alloying elements in the retained amorphous matrix reveals the formation of a Nb-rich shell around α-Fe(Si) nanocrystals. It is proposed that this shell is formed during the growth of nanocrystals and the decomposition of the matrix, which tends toward a Fe3B composition. The Nb-rich shell behaves as a diffusion barrier which inhibits the growth of nanocrystals. However, despite the presence of a Nb-rich shell, the junction between adjacent nanocrystals may occur, even if this phenomenon is rare. 相似文献
492.
Hatnean JA Raturi R Lefebvre J Leznoff DB Lawes G Johnson SA 《Journal of the American Chemical Society》2006,128(46):14992-14999
The reactions of 2 equiv of the ligand precursor P(CH2NHPh)3 or P[CH2NH-3,5-(CF3)2C6H3]3 with 3 equiv of Mn[N(SiMe3)2]2 provide high-yielding routes to the triangular trinuclear Mn(II) complexes [P(CH2NPh)3]2Mn3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mn3(THF)3. The solid-state structures of these paramagnetic complexes have approximate C3 symmetry. The magnetic moments from 300 to 1.8 K could be fit as a magnetic Jahn-Teller distorted isosceles triangle. These complexes exhibit spin frustration and possess an S = 1/2 ground state, as revealed by a plot of magnetization versus field at 1.8 K; at fields above 3.8 T, the occupation of an excited state with S = 3/2 becomes significant. The diamagnetic magnesium analogues were prepared by the reaction of the ligand precursor P(CH2NHPh)3, P[CH2NH-3,5-(CF3)2C6H3]3, or P(CH2NH-3,5-Me2C6H3)3 with nBu2Mg. The solid-state structures of [P(CH2NPh)3]2Mg3(THF)3.1.5THF and [P(CH2N-3,5-(CF3)2C6H3)3]2Mg3(THF)3 were determined. Solution 1H NMR spectroscopy was used to demonstrate that the solid-state structures are maintained in solution. The aryl group of the terminal amido donor exhibits slow rotation on the NMR time scale, and this was found to be an electronic effect. Solution 31P{1H} NMR spectroscopy revealed an unexpected 15 Hz coupling between phosphorus nuclei in these complexes. Calculations on a model complex using density functional theory demonstrates that this coupling occurs via a combined through-space, through-bond pathway. 相似文献
493.
B. Hardy M. Raybaut J. B. Dherbecourt J. M. Melkonian A. Godard A. K. Mohamed M. Lefebvre 《Applied physics. B, Lasers and optics》2012,107(3):643-647
We present novel frequency tuning methods for broadband mid-infrared spectroscopy that take advantage of the unique frequency sampling properties of our recently developed “nested dual-cavity doubly resonant optical parametric oscillator” (NesCOPO). These methods, referred to as Vernier frequency sampling, enable mode-hop tuning with an adjustable virtual-cavity intermode spacing. Both frequency resolution and span are widely adjustable and can be tailored to fulfill the requirements either for broadband spectroscopy (>50 cm?1 spectral coverage) at low resolution or for high resolution (<0.01 cm?1) narrow band spectroscopy. The technique is applied to short-range (10 to 30 m) atmospheric CO2 measurements at 4.2 µm using integrated path differential absorption LIDAR. 相似文献
494.
Sylla F Flacco A Kahaly S Veltcheva M Lifschitz A Sanchez-Arriaga G Lefebvre E Malka V 《Physical review letters》2012,108(11):115003
In laser-plasma experiments, we observed that ion acceleration from the Coulomb explosion of the plasma channel bored by the laser is prevented when multiple plasma instabilities, such as filamentation and hosing, and nonlinear coherent structures (vortices or postsolitons) appear in the wake of an ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp allows us to control the onset of these instabilities. We deduced that the laser pulse is depleted into these structures in our conditions, when a plasma at about 10% of the critical density exhibits a gradient on the order of 250 μm (Gaussian fit), thus hindering the acceleration. A promising experimental setup with a long pulse is demonstrated enabling the excitation of an isolated coherent structure for polarimetric measurements and, in further perspectives, parametric studies of ion plasma acceleration efficiency. 相似文献
495.
采用射频溅射法,通过预先制备Ti和Ni 2种过渡层的工艺在3Cr13马氏体不锈钢基片上制备了硫化钨薄膜,通过与无过渡层的射频溅射WS2薄膜对比,研究了2种过渡层对薄膜性能的影响.3种样品的物相分析,表面形貌观察,微区化学成分测定,膜基结合力和薄膜摩擦系数测试等结果表明,Ti过渡层对硫化钨薄膜结构和S/W原子比没有明显的影响,对膜层的摩擦系数影响也不大,但薄膜耐磨性有所提高.而Ni过渡层虽对S/W原子比无明显影响,但却明显地提高薄膜Ⅱ型织构(基面取向)的强度,从而降低了薄膜的摩擦系数,提高了薄膜耐磨性. 相似文献
496.
Lucas Fortier Dr. Cyrille Gosset Dr. Corentin Lefebvre Dr. Sylvain Pellegrini Prof. Lydie Pélinski Dr. Till Bousquet 《European journal of organic chemistry》2023,26(4):e202201247
An efficient organophotocatalyzed protocol was developed for the preparation of 3-substituted phthalides. The presented transformation was performed under particularly mild conditions within 6 h and was ultimately applied to a precursor of the herbaric acid. 相似文献
497.
Quentin Lefebvre Mina Bocu Christophe Giorgiutti Thomas C. Fessard 《Helvetica chimica acta》2023,106(4):e202200189
Bicycloalkanes are well-known isosteres of alkynes and aromatic rings and their use in the field of medicinal chemistry and drug discovery has been on the rise in the past ten years. Herein, we report the modular and practical synthesis of bicycloalkane-containing fragments using nickel-photoredox catalysis. The comparison of their relevant physical-chemical properties with their aromatic counterparts shows a substantial improvement in LogP and aqueous solubility for bicyclo[1.1.1]pentane derivatives, but not for bicyclo[2.2.2]octane derivatives. 相似文献