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21.
22.
Larry I. Shoer Kerrie I. Gell Jeffrey Schwartz 《Journal of organometallic chemistry》1977,136(2):c19-c22
Dialkylaluminum hydrides and Cp2ZrX2 (Cp = π5-C5H5; X = Cl, H, alkyl) were found to react by aggregation to give mixed ZrAl trihydride complexes containing ZrHAl bridges. Reactions of these mixed-metal hydrides are discussed and structural parallels between them and known aluminum hydride complexes are drawn. 相似文献
23.
[reaction: see text] Ester-containing nitrones, including 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide 5, have been reported to be robust spin traps for superoxide (O2*-). Using a chiral column, we have been able to isolate the two enantiomers of nitrone 5. With enantiomerically pure nitrone 5a and 5b we explored whether one of these isomers was solely responsible for the EPR spectrum of aminoxyl 6. Data obtained demonstrate that the spin trapping of O2*- by nitrone 5a and nitrone 5b affords the identical EPR spectra and lifetimes in homogeneous aqueous solution and exhibits the same ratio of cis and trans isomers. Quantum chemical modeling in vacuo also finds no difference, aside from the expected optical activity, arising from the difference in stereochemistry. 相似文献
24.
G. A. Jeffrey 《Journal of inclusion phenomena and macrocyclic chemistry》1984,1(3):211-222
There are three general classes of hydrate inclusion compounds: the gas hydrates, the per-alkyl onium salt hydrates, and the alkylamine hydrates. The first are clathrates, the second are ionic inclusion compounds, the third are semi-clathrates. Crystallization occurs because the H2O molecules, like SiO2, can form three-dimensional four-connected nets. With water alone, these are the ices. In the inclusion hydrates, nets with larger voids are stabilized by including other guest molecules. Anions and hydrogen-bonding functional groups can replace water molecules in these nets, in which case the guest species are cations or hydrophobic moieties of organic molecules. The guest must satisfy two criteria. One is dimensional, to ensure a comfortable fit within the voids. The other is functional. The guest molecules cannot have either a single strong hydrogen-bonding group, such as an amide or a carboxylate, or a number of moderately strong hydrogen-bonding groups, as in a polyol or a carbohydrate.The common topological feature of these nets is the pentagonal dodecahedra: i.e., 512-hedron. These are combined with 51262-hedra, 51263-hedra, 51264-hedra and combinations of these polyhedra, to from five known nets. Two of these are the well-known 12 and 17 Å cubic gas hydrate structures,Pm3n, Fd3m; one is tetragonal,P4
2/mnm, and two are hexagonal,P6
3/mmc andP6/mmm. The clathrate hydrates provide examples of the two cubic and the tetragonal structures. The alkyl onium salt hydrates have distorted versions of thePm3n cubic, the tetragonal, and one of the hexagonal structures. The alkylamine hydrate structures hitherto determined provide examples of distorted versions of the two hexagonal structures.There are also three hydrate inclusion structures, represented by single examples, which do not involve the 512-hedra. These are 4(CH3)3CHNH2·39H2O which is a clathrate; HPF6·6H2O and (CH3)4NOH·5H2O which are ionic-water inclusion hydrates. In the monoclinic 6(CH3CH2CH2NH2)·105H2O and the orthorhombic 3(CH2CH2)2NH·26H2O, the water structure is more complex. The idealization of these nets in terms of the close-packing of semi-regular polyhedra becomes difficult and artificial. There is an approach towards the complexity of the water salt structures found in the crystals of proteins. 相似文献
25.
The creation and propagation of jump discontinuities in the solutions of semilinear strictly hyperbolic systems is studied in the case where the initial data has a discrete set, {x i } i =1n , of jump discontinuities. LetS be the smallest closed set which satisfies:
- S is a union of forward characteristics.
- S contains all the forward characteristics from the points {x i } i =1n .
- if two forward characteristics inS intersect, then all forward characteristics from the point of intersection lie inS.
26.
The impact of cellulose structure on binding interactions with hemicellulose and pectin 总被引:1,自引:0,他引:1
Four cellulose substrates including highly crystalline cellulose nanowhiskers (CNWs) from Gluconacetobacter xylinus (cellulose Iα) or cotton (cellulose Iβ) and amorphous cellulose derived from CNWs (phosphoric acid swollen cellulose nanowhiskers, PASCNWs) were used to explore the interaction between cellulose and well-defined xyloglucan, xylan, arabinogalactan and pectin. The binding behavior was characterized by adsorption isotherm and Langmuir models. The maximum adsorption and the binding constant of xyloglucan, xylan and pectin to any CNWs were always higher than to PASCNWs derived from the same source. The binding affinity of xyloglucan, xylan and pectin to G. xylinus cellulose was generally higher than to cotton cellulose, showing that binding interactions depended on the biological origin of cellulose and associated differences in its structure. The surface area, porosity, crystal plane and degree of order of cellulose substrate may all impact the interactions. 相似文献
27.
28.
29.
Jeffrey Rauch 《Communications in Mathematical Physics》1986,106(3):481-484
We show that for most non-scalar systems of conservation laws in dimension greater than one, one does not have BV estimates of the form $$\begin{gathered} \parallel \overline V u(\overline t )\parallel _{T.V.} \leqq F(\parallel \overline V u(0)\parallel _{T.V.} ), \hfill \\ F \in C(\mathbb{R}),F(0) = 0,F Lipshitzean at 0, \hfill \\ \end{gathered} $$ even for smooth solutions close to constants. Analogous estimates forL p norms $$\parallel u(\overline t ) - \overline u \parallel _{L^p } \leqq F(\parallel u(0) - \overline u \parallel _{L^p } ),p \ne 2$$ withF as above are also false. In one dimension such estimates are the backbone of the existing theory. 相似文献
30.
In this Letter, we introduce a simple metal waveguide array for realizing all-angle wide frequency bandwidth negative refraction from the visible to infrared frequencies. Theoretical analysis from the rigorous coupled-wave theory reveals that the negative coupling constant resulting from the anomalous coupling of guided surface plasmon polariton modes contributes to the negative refraction. The analytical results are confirmed by finite-difference time-domain numerical simulations. Our result provides an alternative way to construct robust all-angle negative refractive materials operating in a wide range of frequency from the near-infrared to the visible range. 相似文献