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51.
Ag-Pd bimetallic nanoparticles were prepared directly in ultrathin TiO(2)-gel films by a stepwise ion-exchange/reduction approach. Ion-exchange sites were created in ultrathin films using Mg(2+) ions as template. Ag(+) ion was then incorporated by ion exchange, and converted into metallic nanoparticles by low-temperature H(2) plasma, regenerating ion-exchange sites. The same procedure was then carried out for Pd(2+) ion, producing Pd-on-Ag bimetallic nanoparticles, as TEM observation and plasmon resonance absorption indicate. By contrast, reversed metal incorporation procedure appeared to give a mixture of individual Ag and Pd nanoparticles, as confirmed by TEM, absorption spectroscopy and X-ray photoelectron spectroscopy. For hydrogenation of methyl acrylate, the catalytic activity of the Pd-on-Ag nanoparticle is 367 times as large as that of commercial Pd black and 1.6 times as large as that of Pd monometallic nanoparticle. The outstanding catalytic activity was explicable by the large fraction of the surface-exposed Pd atoms. The formation process of the bimetallic nanoparticle and their general morphological feature are discussed.  相似文献   
52.
In this review, we overview metal oxide nanostructures in which organic molecules play important roles as templates, as structural units, and, in some cases, as hosts. Their structural precision and diversity are discussed from the viewpoint of the topology of a metal-oxygen network. Supramolecular capsules of metal oxides are prepared by the self-assembly of polyoxometalates. Zeolites and mesoporous materials are synthesized by using organic molecules with their assemblies acting as templates. The topological networking of silsesquioxanes makes it possible to produce novel nanocomposites and microporous materials. In the final section, we demonstrate our recent studies into molecular imprinting, the encapsulation of a fluorescent dye, and the wrapping of individual polymer chains. Ultrathin, amorphous metal oxide films can retain the shape of organic molecules and can be used to create molecular composites by precisely wrapping individual molecules. These films are also effective in insulating molecular functions from external environments. The advantages of amorphous metal oxides are discussed in relation to the properties of the corresponding crystalline metal oxides and their potential prospects in nanotechnology.  相似文献   
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We investigate the canonical structure of the (2+1)-dimensional non-linear model in a polynomial formulation. A current density defined in the non-linear model is a vector field, which satisfies a formal flatness (or pure gauge) condition. It is the polynomial formulation in which the vector field is regarded as a dynamic variable on which the flatness condition is imposed as a constraint condition by introducing a Lagrange multiplier field. The model so formulated has gauge symmetry under a transformation of the Lagrange multiplier field. We construct the generalized Hamiltonian formalism of the model explicitly by using the Dirac method for constrained systems. We derive three types of the pre-gauge-fixing Hamiltonian systems: In the first system the current algebra is realized as the fundamental Dirac Brackets. The second one manifests the similar canonical structure as the Chern-Simons or BF theories. In the last one there appears an interesting interaction as the dynamic variables are coupled to their conjugate momenta via the covariant derivative. Received: 29 September 1998 / Published online: 14 January 1999  相似文献   
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56.
Dynamic phase transitions in a two-dimensional traffic flow model defined on a decorated square-lattice are studied numerically. The square-lattice point and the decorated site denote intersections and roads, respectively. In the present model, a car has a finite deterministic path between the origin and the destination, which is assigned to the car from the beginning. In this new model, we found a new phase between the free-flow phase and the frozen-jam phase that is absent from previous models. The new model is characterized by the persistence of a macroscopic cluster. Furthermore, the behavior in this macroscopic cluster phase is classified into three regions characterized by the shape of the cluster. The boundary of the three regions is phenomenologically estimated. When the trip length is short and the car density is high, both ends of the belt-like cluster connect to each other through the periodic boundary with some probability. This type of cluster is classified topologically as a string on a two-dimensional torus.  相似文献   
57.
Latex nanoparticles were packed inside nanometer-thick titania/polymer composite nanotubules templated by porous alumina membrane, to give free-standing particle-in-tubing "micro-beanpods" after removal of the template membrane.  相似文献   
58.
Astonishingly uniform polypyrrole nanocoating on morphologically complex cellulosic substances was achieved without disrupting the hierarchical network structures of individual cellulose fibers by means of polymerization-induced adsorption.  相似文献   
59.
We report a resonant inelastic x-ray scattering study of charge excitations in the quasi-one-dimensional Mott insulator SrCuO2. We observe a continuum of low-energy excitations, the onset of which exhibits a small dispersion of approximately 0.4 eV. Within this continuum, a highly dispersive feature with a large sinusoidal dispersion (approximately 1.1 eV) is observed. We have also measured the optical conductivity, and studied the dynamic response of the extended Hubbard model with realistic parameters, using a dynamical density-matrix renormalization group method. In contrast to earlier work, we do not find a long-lived exciton, but rather these results suggest that the excitation spectrum comprises a holon-antiholon continuum together with a broad resonance.  相似文献   
60.
A comparative analysis of electron energy-loss spectroscopy (EELS) spectra for the 1D insulating cuprate Sr2CuO3 with transferred momentum q--> axially and radially to the chain axis allows one to elucidate the structure of the charge transfer gap in in-chain response. It is determined by the superposition of two types of excitations with different magnitudes of dispersion. The low-energy response with q--> radially to the chain direction, but yet within the plane of CuO4 plaquettes, exhibits also a dispersionless peak near 2 eV. The theoretical simulation of the EELS data using exact diagonalizations of an appropriate extended Hubbard Hamiltonian for relevant clusters requires the explicit consideration of low-lying oxygen 2p pi states within the CuO4 plaquette plane beyond the standard pd sigma extended Hubbard model widely used for cuprates with corner-shared CuO4 plaquettes.  相似文献   
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