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Inhibition of the reduction of Cr(VI) at the magnetite–water interface by calcium carbonate coatings
The effect of calcium carbonate coatings on the reduction of aqueous chromate on the magnetite(1 1 1) surface has been investigated using a combination of synchrotron based X-ray photoemission spectroscopy (PES) and X-ray absorption near edge structure (XANES) spectroscopy, along with laboratory-based powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). CaCO3 coatings (dominantly calcite with minor quantities of aragonite and vaterite) of thicknesses ranging from 10 Å to 20 m were grown on magnetite(1 1 1) surfaces by exposure to supersaturated aqueous solutions followed by evaporation of the solution—a process that mimics pore-water evaporation in vadose zones leading to the formation of caliche and calcium carbonate coatings on mineral grains. Coating thicknesses were determined from attenuation of the Fe 2p photoemission signal by the carbonate coating. For coatings less than 15 Å thick, Cr 2p photoemission and Cr LII, LIII-edge XANES spectra show that chromate is reduced by the underlying magnetite surface; however, as the minimum coating thickness increases beyond 15 Å, the magnetite surface becomes passivated and further chromate reduction ceases. Our findings suggest that carbonate coatings on natural magnetite grains can significantly reduce or eliminate their ability to reduce Cr(VI), which is a toxic and highly mobile environmental contaminant. 相似文献
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Raymond G. Plevey Richard W. Rendell John Colin Tatlow 《Journal of fluorine chemistry》1982,21(2):159-169
Pyridine has been fluorinated over caesium tetrafluorocobaltate(III) (CsCoIIIF4) at 300–400°C to give a mixture of undecafluoro-N-methylpyrrolidine, bis(trifluoromethyl)amine, pentafluoropyridine and several polyfluoropyridines; the product composition depended to some extent on the geometry of the reactor. The fluorinations of pentafluoropyridine, piperidine and undecafluoropiperidine were also investigated. 相似文献
7.
Peter G. Doyle Colin Mallows Alon Orlitsky Larry Shepp 《Israel Journal of Mathematics》1989,67(1):34-42
Letn cells be arranged in a ring, or alternatively, in a row. Initially, all cells are unmarked. Sequentially, one of the unmarked
cells is chosen at random and marked until, aftern steps, each cell is marked. After thekth cell has been marked the configuration of marked cells defines some number of islands: maximal sets of adjacent marked
cells. Let ξ
k
denote the random number of islands afterk cells have been marked.
We give explicit expressions for moments of products of ξ
k
’s and for moments of products of 1/ξ
k
’s.
These are used in a companion paper to prove that if a random graph on the natural number is made by drawing an edge betweeni≧1 andj>i with probabilityλ/j, then the graph is almost surely connected ifλ>1/4 and almost surely disconnected ifλ≦1/4. 相似文献
8.
We see that the entropy method yields strong concentration results for general self‐bounding functions of independent random variables. These give an improvement of a concentration result of Talagrand much used in discrete mathematics. © 2006 Wiley Periodicals, Inc. Random Struct. Alg., 2006 相似文献
9.
Summary This paper uses Hamiltonian structures to study the problem of the limit of three-dimensional (3D) elastic models to shell
and rod models. In the case of shells, we show that the Hamiltonian structure for a three-dimensional elastic body converges,
in a sense made precise, to that for a shell model described by a one-director Cosserat surface as the thickness goes to zero.
We study limiting procedures that give rise to unconstrained as well as constrained Cosserat director models. The case of
a rod is also considered and similar convergence results are established, with the limiting model being a geometrically exact
director rod model (in the framework developed by Antman, Simo, and coworkers). The resulting model may or may not have constraints,
depending on the nature of the constitutive relations and their behavior under the limiting procedure.
The closeness of Hamiltonian structures is measured by the closeness of Poisson brackets on certain classes of functions,
as well as the Hamiltonians. This provides one way of justifying the dynamic one-director model for shells. Another way of
stating the convergence result is that there is an almost-Poisson embedding from the phase space of the shell to the phase
space of the 3D elastic body, which implies that, in the sense of Hamiltonian structures, the dynamics of the elastic body
is close to that of the shell. The constitutive equations of the 3D model and their behavior as the thickness tends to zero
dictates whether the limiting 2D model is a constrained or an unconstrained director model.
We apply our theory in the specific case of a 3D Saint Venant-Kirchhoff material andderive the corresponding limiting shell and rod theories. The limiting shell model is an interesting Kirchhoff-like shell model
in which the stored energy function is explicitly derived in terms of the shell curvature. For rods, one gets (with an additional
inextensibility constraint) a one-director Kirchhoff elastic rod model, which reduces to the well-known Euler elastica if
one adds an additional single constraint that the director lines up with the Frenet frame.
This paper is dedicated to the memory of Juan C. Simo
This paper was solicited by the editors to be part of a volume dedicated to the memory of Juan Simo. 相似文献
10.
We describe a method for searching for ideal symmetric solutions to the Prouhet-Tarry-Escott Problem. We report results of extensive searches for solutions of sizes up to 12. We found two solutions of size 10 that are smaller by two orders of magnitude than the solution found by A. Letac in the 1940s, which was the smallest size 10 solution known before our search.