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61.
Low-valent cobalt complexes were used in Reformatsky-type additions of α-haloesters and in aldol-type reactions of α-haloketones to (R)-2,3-O-cyclohexylideneglyceraldehyde. Addition products were obtained in good yield with reasonable to high diastereoselection. The addition of methyl 4-bromocrotonate, which took place exclusively at the γ-carbon, was regioselective. 相似文献
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Nunzio Motta Pierre D. Szkutnik Massimo Tomellini Anna Sgarlata Massimo Fanfoni Fulvia Patella Adalberto Balzarotti 《Comptes Rendus Physique》2006,7(9-10):1046
Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forthcoming nanotechnology era. New frontiers in nanodevice technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. In this article we will review some recent studies regarding the control of the nucleation process on semiconductor surfaces. A few approaches to form ordered patterns on surfaces are described: natural patterning induced by surface instabilities (as step bunching or step meandering), in situ substrate patterning by Scanning Tunneling Microscopy (STM), high resolution patterning by Focused Ion Beam (FIB). Growth of epitaxial layers of semiconductors (Ge/Si(100) and InAs/GaAs(100)) on patterned surfaces has been studied by STM or Atomic Force Microscopy (AFM) unveiling the way in which the first atoms start to aggregate and identifying their exact nucleation site. Control of the dot size to match the patterning typical wavelength has been achieved by using surfactants on misoriented substrates. STM images acquired in real time allows one to identify the mechanism of Ge cluster formation on patterned Si(100), and to follow the island transition from pre-pyramid to pyramid. Nucleation of ordered Ge dots on SiO2 substrates has been obtained thanks to FIB tight patterning, achieving island densities of 3.5×1010/cm2. To cite this article: N. Motta et al., C. R. Physique 7 (2006). 相似文献
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Fulvia Felluga Franco Ghelfi Giuliana Pitacco Fabrizio Roncaglia Ennio Valentin Cesare Daniele Venneri 《Tetrahedron: Asymmetry》2010,21(17):2183-2191
Esterase from Horse Liver (HLAP) was able to hydrolyze a series of linear and cyclic β,β-dialkyl-γ-nitroesters, in spite of the well-known reluctance of hydrolytic enzymes to recognize and transform hindered substrates, such as those possessing a stereogenic quaternary carbon atom next to the reaction site. The resulting optically active γ-nitroesters gave access to optically active β,β-disubstituted γ-aminoacids as well as α,α-disubstituted succinic acids, both being biologically relevant compounds. 相似文献
65.
Single- and Dual-domain Models of Solute Transport in Alluvial Sediments: the Effects of Heterogeneity Structure and Spatial Scale 总被引:1,自引:0,他引:1
Fine-scale heterogeneity of alluvial aquifers controls solute transport in groundwater at the scales relevant for practical applications: the architecture of sedimentary structures might create preferential flow paths (PFPs) or hydraulic barriers, which affect the breakthrough curves (BTCs). Objective of this paper was the assessment of the relevance of single- and dual-domain models for different heterogeneity patterns and scale lengths in alluvial sediments. Three case studies have been analysed with a classical single-domain model (SDM) and with three dual-domain models (DDMs): a dual-porosity model (DPorM) and two dual-permeability models (DPerM), which differ for the presence or the absence of solute exchange between the two domains. The first case study includes numerical tracer tests in metre-scale blocks of alluvial sediments; the second is a laboratory experiment of tracer injection in a decimetre-scale column of homogeneous sand; the third is a field tracer test performed at hectometre scale at the Cape Cod site. The relevance of the solute exchange in the DDMs is analysed with the characteristic advection and exchange times and with the Péclet and Damköhler numbers. The SDM is satisfactory for alluvial sediments with unstructured heterogeneity. The uncoupled DPerM is shown to be a better approach than the DPorM in sediments with PFPs; in this case, the coupled DPerM does not improve significantly the results of the uncoupled DPerM. A minor difference between the results of the three DDMs is observed for sediments in which the non-Fickian behaviour is not clearly determined by the presence of PFPs. 相似文献
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Andrea Cornia Fulvia Felluga Vincenzo Frenna Franco Ghelfi Andrew F. Parsons Mariella Pattarozzi Fabrizio Roncaglia Domenico Spinelli 《Tetrahedron》2012,68(29):5863-5881
The copper-catalyzed radical cyclization (RC) of N-α-perchloroacyl cyclic ketene-N,X(X=O, NR, S)-acetals was studied. While the RC of N-acyl ketene-N,O-acetals was unsuccessful, the 5-endo cyclization of the other ketene acetals provided much better results, with the following order of cyclization efficiency: hexa-atomic cyclic ketene-N,NR-acetals<penta-atomic cyclic ketene-N,S-acetals<hexa-atomic cyclic ketene-N,S-acetals. Invariably the catalytic cycle begins with the formation of a carbamoyl methyl radical. This leads to a cascade of reactions, including a radical polar crossover step, which ends with the formation of the maleimide nucleus, or precursors of this. Products from the RC of the hexa-atomic cyclic ketene-N,S-acetals, were efficiently transformed into disubstituted maleic anhydrides. 相似文献
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We consider three infinite families of cyclic presentations of groups, depending on a finite set of integers and having the same polynomial. Then we prove that the corresponding groups with the same parameters are isomorphic, and that the groups are almost all infinite. Finally, we completely compute the maximal Abelian quotients of such groups, and show that their HNN extensions are high-dimensional knot groups. Our results contain as particular cases the main theorems obtained in two nice articles: Johnson et al. (1999) and Havas et al. (2001). 相似文献