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211.
Election beam induced perturbations of CO chemisorbed on Ir(111) have been measured using LEED and AES. The total interaction cross-section for electron-stimulated desorption and dissociation is found to be 0.8 to 1.7 × 10?17 cm2 near monolayer coverage at a beam energy of 86 eV. This total cross-section is estimated to be 1 × 10?17 cm2 when defined with respect to the primary flux of a 2.5 keV beam. Electron-stimulated dissociation is found to occur at 1–2% of the rate of stimulated desorption. 相似文献
212.
An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface. 相似文献
213.
The heterogeneously catalyzed reaction of CO and O2 to form CO2 over Ir(110) has been studied through measurements of the transient kinetics of the various elementary reactions that may limit the steady state rate. Rate expressions for these elementary reactions — the desorption of CO, the oxidation of CO via the Langmuir-Hinshelwood mechanism, the adsorption of CO and the adsorption of oxygen — were developed using thermal desorption mass spectrometry. Several phenomena were observed: (1) the activation energies for CO desorption and CO oxidation depend markedly upon the composition of the adlayer; (2) diffusion in the adlayer may limit the rates of CO desorption and CO oxidation; (3) the formation of a surface oxide modifies these four rate processes; and (4) chemisorbed CO blocks sites for oxygen adsorption, but chemisorbed oxygen does not block sites for CO adsorption. 相似文献
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Calculation of gauge couplings and compact circumferences from self-consistent dimensional reduction
We consider a system of gravity plus free massless matter fields in 4 + N dimensions, and look for solutions in which N dimensions form a compact curved manifold, with the energy-momentum tensor responsible for the curvature produced by quantum fluctuations in the matter fields. For manifolds of sufficient symmetry (including spheres, CPN, and manifolds of simple Lie groups) the metric depends on only a single multiplicative parameter ?2, and the field equations reduce to an algebraic equation for ?, involving the potential of the matter fields in the metric of the manifold. With a large number of species of matter fields, the manifold will be larger than the Planck length, and the potential can be calculated using just one-loop graphs. In odd dimensions these are finite, and give a potential of form CN/?4. Also there are induced Yang-Mills and Einstein-Hilbert terms in the effective 4-dimensional action, proportional to additional numerical coefficients, DN and EN. General formulas are given for the gauge coupling g2 in terms of CN and DN, and the ratio ?2/8πG in terms of CN and EN. Numerical values for CN, DN, and EN are obtained for scalar and spinor fields on spheres of odd dimensionality N. It is found that the potential, g2 and ?2/8πG can all be positive but only when the compact manifold has N = 3 + 4 k dimensions. (The positivity of the potential is needed for stability of the sphere against uniform dilations or contractions). In this case, solutions exist either for spinor fields alone or for suitable mixes of spinor and scalar fields provided the ratio of the number of scalar fields to the number of fermion fields is not too large. Numerical values of the O(N + 1) gauge couplings and 8φG/?2 are calculated for illustrative values of the numbers of spinor fields. It turns out that large numbers of matter fields are needed to make these parameters reasonably small. 相似文献
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The crystallization behaviors of gels, gel-derived glasses and melted glasses are reviewed, and comparisons are effected among the observed behaviors. These comparisons are related to the behavior expected on the basis of models for the nucleation frequency and crystal growth rate, as well as the anticipated differences in material characteristics among gels and the two types of glasses. Different temperature regimes are identified where different comparisons between melted and gel-derived materials are expected. It is concluded that a need exists for more definitive, detailed data on the crystallization behavior of gel-derived materials. 相似文献
218.
Precision tests of quantum mechanics 总被引:1,自引:0,他引:1
Weinberg S 《Physical review letters》1989,62(5):485-488
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