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53.
We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.  相似文献   
54.
We show both theoretically and experimentally in an optical fiber system that a set of incoherent nonlinear waves irreversibly evolves to a specific equilibrium state, in which the individual wave packets propagate with identical group velocities. This intriguing process of velocity locking can be explained in detail by simple thermodynamic arguments based on the kinetic wave theory. Accordingly, the selection of the velocity-locked state is shown to result from the natural tendency of the isolated wave system to approach the state that maximizes the nonequilibrium entropy.  相似文献   
55.
The spinel phases Mn3-x Co x O4 are prepared directly and at low temperature (600-700°C) by means of thermal decomposition of mixed oxalate salt precursors in air or controlled atmosphere. The powders so obtained when sintered at 1200°C behave as semiconducting or insulating ceramics, depending on their cobalt content. The electrical properties of these ceramics have been found to be related to the distribution and valencies of the cations in the spinel structure. The study of their crystalline structure and stability, complemented by electrical measurements have permitted us to distinguish two types of compounds. The first type includes those given by Mn3-x Co x O4 with 0 < x < 1. These are stable at high temperature, have a tetragonal structure and are insulators. The cation distributions for them can be deduced from that of hausmannite as given by Mn2+ [Mn3+ 2]O4, by substituting Mn2+ ions with Co2+ ions. The second type of compounds correspond to the cubic phases and occur for 1 < x < 3. These are unstable at high temperature and show a lower resistivity (e.g. 360 ° cm) which is of particular interest for their application in the design of negative temperature coefficient (N.T.C.) thermistor components.  相似文献   
56.
The transamination reaction of 2,5-diaminobenzoquinonediimine (QDI) with ethylenediamine gave fluorescent 1,2,3,4-tetrahydropyrazino[2,3-g]quinoxaline (1). When the same reaction was carried out with N,N’-bis(aminoethyl)-1,3-propanediamine, a novel cationic quinoxalinium species (2) was isolated, which can be further condensed with p-cyanobenzaldehyde to afford a benzimidazolo-fused quinoxaline dye (3) that is a water-soluble fluorophore in the UV–visible range.  相似文献   
57.
Reported is the enantioselective total syntheses of mavacuran alkaloids, (+)‐taberdivarine H, (+)‐16‐hydroxymethyl‐pleiocarpamine, and (+)‐16‐epi‐pleiocarpamine, and their postulated biosynthetic precursor 16‐formyl‐pleiocarpamine. This family of monoterpene indole alkaloids is a target of choice since some of its members are subunits of intricate bisindole alkaloids such as bipleiophylline. Inspired by the biosynthetic hypothesis, an oxidative coupling approach from the geissoschizine framework to form the N1?C16 bond was explored. Quaternization of the aliphatic nitrogen center was key to achieving the oxidative coupling induced by KHMDS/I2 as it masks the nucleophilicity of the aliphatic nitrogen center and locks in the required cis conformation.  相似文献   
58.
Ligands with reactive carbon sites in the periphery of a metal center have emerged as a powerful approach for metal–ligand bond activation. These reactive carbon sites are commonly generated by deprotonation strategies. Carbon–silicon bond cleavage is a potential alternative to access such constructs. Herein, the monodesilylation of bis-silyl-substituted P,N scaffold PNSi2 in the coordination sphere of [RhI(Cl)(CO)( PNSi2 )] ( 1 ) with sodium azide is disclosed. This affords a unique dinucleating anionic κ2-C,N1-P ligand with a carbanionic methine carbon atom directly bound to rhodium as part of a four-membered Rh-N-C-C rhodacycle. This dimer undergoes meta-pyridine C−H activation facilitated by weak bases, which leads to a desymmetrization of the system and provides a σ,π-bridging 3-pyridyl fragment bound to RhI. The facile Si−C cleavage strategy may pave the way to studying the reactivity and functionalization of a variety of κ2-C,N-coordinated pyridine scaffolds for selective transformations.  相似文献   
59.
We report on the characterization of an InP/InGaAsP-material-based microdisk resonator optical filter. The originality here is constituted by the use of a localized control electrode that is used for the tuning of the resonance wavelength of the filter via the injection of a driving current. Tuning of the resonance wavelength close to 8 nm has been experimentally achieved for a drive current of 80 mA.  相似文献   
60.
The neutral complex dichloro-{diethyl[(5-phenyl-1,3,4-oxadiazol-2-ylamino)-(4-trifluoro-methylphenyl)methyl]phosphonate} (p-cymene)-ruthenium(II) was encapsulated inside a self-assembled hexameric host obtained upon reaction of 2,8,14,20-tetra-undecyl-resorcin[4]arene and water. The formation of an inclusion complex was inferred from a combination of spectral measurements (MS, UV/Vis spectroscopy, 1H and DOSY NMR). The 31P and 19F NMR spectra are consistent with motions of the ruthenium complex inside the self-assembled capsule. Molecular dynamics simulations carried out on the inclusion complex confirmed these intra-cavity movements and highlighted possible supramolecular interactions between the ruthenium first coordination sphere ligands and the inner part (aromatic rings) of the capsule. The embedded ruthenium complex was assessed in the catalytic oxidation (using NaIO4 as oxidant) of mixtures of three arylmethyl alcohols into the corresponding aldehydes. The reaction kinetics were shown to vary as a function of the substrates’ size, with the oxidation rate varying in the order benzylalcohol >4-phenyl-benzylalcohol >9-anthracenemethanol. Control experiments realized in the absence of hexameric capsule did not allow any discrimination between the substrates.  相似文献   
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