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Muhammad Arshad M. Alejandro Fernández Eoghan M. McGarrigle Varinder K. Aggarwal 《Tetrahedron: Asymmetry》2010,21(13-14):1771-1776
The epoxidation of meroquinene aldehyde with a chiral sulfur ylide as the key step in the synthesis of quinine and quinidine is described. The epoxidation reactions proceed under reagent control with high selectivity and good yield. The effect of sulfide and ylide substituents on the stereochemical outcome of the reaction is discussed. 相似文献
114.
An integrated mechanical model for fiber-laden membranes is presented and representative predictions of relevance to cellulose
ordering and orientation in the plant cell wall are presented. The model describes nematic liquid crystalline self-assembly
of rigid fibers on an arbitrarily curved fluid membrane. The mechanics of the fluid membrane is described by the Helfrich
bending-torsion model, the fiber self-assembly is described by the 2D Landau-de Gennes quadrupolar Q-tensor order parameter model, and the fiber-membrane interactions (inspired by an extension of the 2D Maier-Saupe model to
curved surfaces) include competing curvo-philic (curvature-seeking) and curvo-phobic (curvature-avoiding) effects. Analysis
of the free energy reveals three fiber orientation regimes: (a) along the major curvature, (b) along the minor curvature,
(c) away from the principal curvatures, according to the competing curvo-philic and curvo-phobic interactions. The derived
shape equation (normal stress balance) now includes curvature-nematic ordering contributions, with both bending and torsion
renormalizations. Integration of the shape and nematic order equations gives a complete model whose solution describes the
coupled membrane shape/fiber order state. Applications to cylindrical membranes, relevant to the plant cell wall, shows how
growth decreases the fiber order parameter and moves the fibers’ director from the axial direction towards the azimuthal orientation,
eventually leading to a state of stress predicted by pure membranes. The ubiquitous 54.7° cellulose fibril orientation with
respect to the long axis in a cylindrical plant cell wall is shown to be predicted by the preset model when the ratio of curvo-phobic
and curvo-philic interactions is in the range of the cylinder radius. 相似文献
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Lucas P. Tendela Alejandro FedericoGuillermo H. Kaufmann 《Optics and Lasers in Engineering》2011,49(2):281-284
This paper presents an interferometric measurement of the out-of-plane deflections produced by a piezoelectric transducer, manufactured by thick-film deposition of a ceramic paste over an alumina substrate, when is subjected to a DC electric voltage. It is shown that a digital speckle pattern interferometer with an incorporated phase-shifting facility allows the measurement of nanometer displacements generated by the piezoelectric device. These measurements are used to evaluate the effective piezoelectric charge constant along the polarization direction (d33)eff that characterizes the thick-film transducer. 相似文献
118.
Clemente-Juan JM Coronado E Gaita-Ariño A Giménez-Saiz C Chaboussant G Güdel HU Burriel R Mutka H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(24):5701-5708
The ground-state properties of a Co3II moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2].40D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as H = -2 sigma a = x,y,z (Ja12 S1a S2a + Ja23 S2a S3a), where Ja are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: Jx12 = Jy23, Jy12 = Jx23, Jz12 = Jz23. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: Jx12 = Jy23 = 1.37, Jy12 = Jx23 = 0.218, and Jz12 = Jz23 = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3. 相似文献
119.
The nature of superacid electrophilic species in HF/SbF(5): a density functional theory study 总被引:1,自引:0,他引:1
A density functional theory study at the B3LYP/6-31++G** + RECP(Sb) level of the HF/SbF(5) superacid system was carried out. The geometries of possible electrophilic species, such as H(2)F(+).Sb(2)F(11)(-) and H(3)F(2)(+).Sb(2)F(11)(-), were calculated and correspond with available experimental results. Calculations of different equilibrium reactions involving HF and SbF(5) allowed the relative concentration of the most energetically favorable species present in 1:1 HF/SbF(5) solutions to be estimated. These species are H(+).Sb(2)F(11)(-), H(2)F(+).Sb(2)F(11)(-), H(3)F(2)(+).Sb(2)F(11)(-), and H(4)F(3)(+).Sb(2)F(11)(-), which correspond to 36.9, 16.8, 36.9, and 9.4%, respectively. Calculations of the acid strength of the electrophilic species were also performed and indicated that, for the same anion, the acid strength increases with the solvation degree. The entropic term also plays a significant role in proton-transfer reactions in superacid systems. 相似文献
120.
García-Herrero A Montero E Muñoz JL Espinosa JF Vián A García JL Asensio JL Cañada FJ Jiménez-Barbero J 《Journal of the American Chemical Society》2002,124(17):4804-4810
We show that the conformational features of the molecular complexes of E. coli beta-galactosidase and O-glycosides may differ from those formed with closely related compounds in their chemical nature, such as C- and S-glycosyl analogues. In the particular case presented here, NMR and ab initio quantum mechanical results show that the 3D-shapes of the ligand/inhibitor within the enzyme binding site depend on the chemical nature of the compounds. In fact, they depend on the relative size of the stereoelectronic barriers for chair deformation or for rotation around Phi glycosidic linkage. 相似文献