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41.
[formula: see text] The segment-coupling Prins cyclization avoids two of the problems common to other Prins cyclization protocols: side-chain exchange and partial racemization by reversible 2-oxonia Cope rearrangement. Model studies demonstrate the stereochemical fidelity of Prins cyclizations using alpha-acetoxy ethers compared with direct aldehyde-alcohol Prins reactions. Furthermore, we propose a mechanism for the racemization observed in some intermolecular Prins cyclizations. Two straightforward syntheses of optically pure (-)-centrolobine highlight the utility of Prins cyclizations. 相似文献
42.
Paola Vitale Cinzia D’Introno Filippo Maria Perna Maria Grazia Perrone Antonio Scilimati 《Tetrahedron: Asymmetry》2013,24(7):389-394
The use of microorganism growing cells is a well recognized methodology in biocatalyzed organic reactions. A non-conventional thermotolerant Kluyveromyces marxianus yeast strain was used for the bio-reduction of different arylketones. Differently substituted ketones were converted into the corresponding (S)-alcohols with up to 96% enantiomeric excess under very mild reaction conditions. Kluyveromyces marxianus represents a promising biocatalyst for the production of optically active 1-arylethanols. 相似文献
43.
44.
Andrea Irace Luigi Sirleto Paolo Spirito Giovanni F. Vitale Antonello Cutolo S. Campopiano Luigi Zeni 《Optics and Lasers in Engineering》2003,39(2):890
In this paper we present various approaches to the measurement of recombinative parameters in silicon wafers, epitaxial layers and solar cells. In particular our techniques are able to measure the bulk lifetime and the surface recombination velocity at low injection levels. We also show that the techniques presented are a reliable tool to monitor the fabrication process of a standard crystalline silicon solar cell. In all the methodologies presented there are no stringent requirements concerning the state of wafer surface. 相似文献
45.
46.
The phase transition of the Gross-Neveu model with N fermions is investigated by means of a nonperturbative evolution equation for the scale dependence of the effective average action. The critical exponents and scaling amplitudes are calculated for various values of N in d = 3. It is also explicitly verified that the Neveu-Yukawa model belongs to the same universality class as the Gross-Neveu model. 相似文献
47.
The existence theorem of Minkowski for a polytope with given facet normals and areas is adapted to a data-analytic context.
More precisely, we show that a centered, random point sample arising from an absolutely continuous distribution in R
d
can be uniquely mapped into such a polytope almost surely. With increasing sample size, the sequence of (scaled) polytopes
converges almost surely to a limiting convex body that is associated with the underlying distribution. An accompanying central
limit theorem is proved using methods from the theory of empirical processes.
Received January 28, 1999, and in revised form April 12, 1999. 相似文献
48.
We define the notions of Azumaya category and Brauer group in category theory enriched over some very general base category V. We prove the equivalence of various definitions, in particular in terms of separable categories or progenerating bimodules. When V is the category of modules over a commutative ring R with unit, we recapture the classical notions of Azumaya algebra and Brauer group and provide an alternative, purely categorical treatment of those theories. But our theory applies as well to the cases of topological, metric or Banach modules, to the sheaves of such structures or graded such structures, and many other examples. 相似文献
49.
50.
The decomposition of L2Ir(CO)R, R = n-alkyl) prepared in situ by the reacton of n-alkyllithium or -magnesium reagents with L2Ir(CO)Cl(2) produces a mixture of n-alkane and isomerized alkene, the ratio of which is strongly dependent on the concentration [L] of triphenylphosphine as well as certain other additives. When [L] = 0, positional isomerism and isotope scrabling are extensive as is the isomerism of added olefin, suggesting that β-hydride elimination is rapid and reversible and that any olefins participating in an iridium hydride addition-elimination sequence are also capable of exchange with free olefin in solution. When [Ph3P]/[2] >/ 1, the principal product (>90%) is the 1-alkene. No positional isomerism or isotope scrambling is observed only a minor amount of alkane is produced. A mechanistic scheme consistent these observations is proposed. 相似文献