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Jeffrey J. Jackson Dr. Hiroyuki Kobayashi Sophia D. Steffens Prof. Dr. Armen Zakarian 《Angewandte Chemie (International ed. in English)》2015,54(34):9971-9975
The asymmetric synthesis of dragmacidin D ( 1 ) was completed in 10 steps. Its sole stereocenter was set by using direct asymmetric alkylation enabled by a C2‐symmetric tetramine and lithium N‐(trimethylsilyl)‐tert‐butylamide as the enolization reagent. A central Larock indole synthesis was employed in a convergent assembly of the heterocyclic subunits. The stereochemical evidence from this work strongly supports the predicted S configuration at the 6′′′ position, which is consistent with other members of the dragmacidin family of natural products. 相似文献
4.
Josep Cornella Evan P. Jackson Ruben Martin 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(13):4147-4150
We report the first enantioselective C C bond formation through C O bond cleavage using aryl ester counterparts. This method is characterized by its wide substrate scope and results in the formation of quaternary stereogenic centers with high yields and asymmetric induction. 相似文献
5.
Sabrina V. Kirner Christian Henkel Dirk M. Guldi Jackson D. Megiatto Jr David I. Schuster 《Chemical science》2015,6(12):7293-7304
A new set of [Cu(phen)2]+ based rotaxanes, featuring [60]-fullerene as an electron acceptor and a variety of electron donating moieties, namely zinc porphyrin (ZnP), zinc phthalocyanine (ZnPc) and ferrocene (Fc), has been synthesized and fully characterized with respect to electrochemical and photophysical properties. The assembly of the rotaxanes has been achieved using a slight variation of our previously reported synthetic strategy that combines the Cu(i)-catalyzed azide–alkyne cycloaddition reaction (the “click” or CuAAC reaction) with Sauvage''s metal-template protocol. To underline our results, complementary model rotaxanes and catenanes have been prepared using the same strategy and their electrochemistry and photo-induced processes have been investigated. Insights into excited state interactions have been afforded from steady state and time resolved emission spectroscopy as well as transient absorption spectroscopy. It has been found that photo-excitation of the present rotaxanes triggers a cascade of multi-step energy and electron transfer events that ultimately leads to remarkably long-lived charge separated states featuring one-electron reduced C60 radical anion (C60˙–) and either one-electron oxidized porphyrin (ZnP˙+) or one-electron oxidized ferrocene (Fc˙+) with lifetimes up to 61 microseconds. In addition, shorter-lived charge separated states involving one-electron oxidized copper complexes ([Cu(phen)2]2+ (τ < 100 ns)), one-electron oxidized zinc phthalocyanine (ZnPc˙+; τ = 380–560 ns), or ZnP˙+ (τ = 2.3–8.4 μs), and C60˙– have been identified as intermediates during the sequence. Detailed energy diagrams illustrate the sequence and rate constants of the photophysical events occurring with the mechanically-linked chromophores. This work pioneers the exploration of mechanically-linked systems as platforms to position three distinct chromophores, which are able to absorb light over a very wide range of the visible region, triggering a cascade of short-range energy and electron transfer processes to afford long-lived charge separated states. 相似文献
6.
Leonardo Dalseno Antonino Júlia Rocha Gouveia Rogrio Ramos de Sousa Júnior Guilherme Elias Saltarelli Garcia Luara Carneiro Gobbo Lara Basílio Tavares Demetrio Jackson dos Santos 《Molecules (Basel, Switzerland)》2021,26(8)
Several efforts have been dedicated to the development of lignin-based polyurethanes (PU) in recent years. The low and heterogeneous reactivity of lignin hydroxyl groups towards diisocyanates, arising from their highly complex chemical structure, limits the application of this biopolymer in PU synthesis. Besides the well-known differences in the reactivity of aliphatic and aromatic hydroxyl groups, experimental work in which the reactivity of both types of hydroxyl, especially the aromatic ones present in syringyl (S-unit), guaiacyl (G-unit), and p-hydroxyphenyl (H-unit) building units are considered and compared, is still lacking in the literature. In this work, the hydroxyl reactivity of two kraft lignin grades towards 4,4′-diphenylmethane diisocyanate (MDI) was investigated. 31P NMR allowed the monitoring of the reactivity of each hydroxyl group in the lignin structure. FTIR spectra revealed the evolution of peaks related to hydroxyl consumption and urethane formation. These results might support new PU developments, including the use of unmodified lignin and the synthesis of MDI-functionalized biopolymers or prepolymers. 相似文献
7.
In this study, flexible silk fibroin protein and biocompatible barium hexaferrite (BaM) nanoparticles were combined and electrospun into nanofibers, and their physical properties could be tuned through the mixing ratios and a water annealing process. Structural analysis indicates that the protein structure of the materials is fully controllable by the annealing process. The mechanical properties of the electrospun composites can be significantly improved by annealing, while the magnetic properties of barium hexaferrite are maintained in the composite. Notably, in the absence of a magnetic field, cell growth increased slightly with increasing BaM content. Application of an external magnetic field during in vitro cell biocompatibility study of the materials demonstrated significantly larger cell growth. We propose a mechanism to explain the effects of water annealing and magnetic field on cell growth. This study indicates that these composite electrospun fibers may be widely used in the biomedical field for controllable cell response through applying different external magnetic fields. 相似文献
8.
The relaxation of slightly poly-disperse linear flexible polymers has been expressed in a simplified blending rule which is presumed to be a weighted linear superposition of the relaxation spectra of mono-disperse components which constitute the blend. Discrete components are characterized by their molecular weight M
i,weight fraction w
i,and relaxation time spectrum H
i(). ). In contrast to broadly distributed blends in which the small molecules mobilize the large ones and vice versa, we introduce the term slightly polydisperse for blends with molecular weight distributions narrow enough to have very little change in the longest relaxation times of each molecular weight component. The properties of this blending rule are analyzed and dynamic data is calculated for slightly poly-disperse polystyrene. As an application, the blending rule is used to determine the characteristic mono-disperse parameters (BSW parameters) of two materials, poly (vinyl methyl ether) and polycarbonate, for which we could not determine their BSW parameters directly since they were not available in nearly monodisperse form. The proposed blending rule can only be applied to systems in which all components are above the entanglement molecular weight, i.e. M
iM
c.Dedicated to the memory of Professor Tasos C. Papanastasiou 相似文献
9.
G. Erlebacher M.Y. Hussaini T.L. Jackson 《Theoretical and Computational Fluid Dynamics》1998,11(1):1-29
This paper addresses nonlinear effects which result from the interaction of shock waves with vortices. A series of experiments
are carried out, which involve the interaction of a strong shock wave with a single plane vorticity wave and a randomly distributed
wave system. These experiments are first conducted in the linear regime to obtain a mutual verification of theory and computation.
They are subsequently extended into the nonlinear regime. A systematic study of the interaction of a plane shock wave and
a single vortex is then conducted. Specifically, we investigate the conditions under which nonlinear effects become important,
both as a function of shock Mach number, M
1, and incident vortex strength (characterized by its circulation Γ). The shock Mach number is varied from 2 to 8, while the
circulation of the vortex is varied from infinitesimally small values (linear theory) to unity. Budgets of vorticity, dilatation,
and pressure are obtained. They indicate that nonlinear effects become more significant as both the shock Mach number and
the circulation increase. For Mach numbers equal to 5 and above, the dilatation in the vortex core grows quadratically with
circulation. An acoustic wave propagates radially outward from the vortex center. As circulation increases, its upstream-facing
front steepens at low Mach numbers, and its downstream-facing front steepens at high Mach numbers. A high Mach number asymptotic
expansion of the Rankine--Hugoniot conditions reveals that nonlinear effects dominate both the shock motion and the downstream
flow for ΓM
1 > 1.
Received 28 June 1997 and accepted 25 November 1997 相似文献
10.
A similarity solution for a low Mach number nonorthogonal flow impinging on a hot or cold plate is presented. For the constant-density case, it is known that the stagnation point shifts in the direction of the incoming flow and that this shift increases as the angle of attack decreases. When the effects of density variations are included, a critical plate temperature exists; above this temperature the stagnation point shifts away from the incoming stream as the angle is decreased. This flow field is believed to have applications to the reattachment zone of certain separated flows or to a lifting body at a high angle of attack. Finally, we examine the stability of this nonorthogonal flow to self-similar, three-dimensional disturbances. Stability characteristics of the flow are given as a function of the parameters of this study: ratio of the plate temperature to that of the outer potential flow and angle of attack. In particular, it is shown that the angle of attack can be scaled out by a suitable definition of an equivalent wave number and temporal growth rate, and the stability problem for the nonorthogonal case is identical to the stability problem for the orthogonal case. By use of this scaling, it can be shown that decreasing the angle of attack decreases the wave number and the magnitude of the temporal decay rate, thus making nonlinear effects important. For small wave numbers, it is shown that cooling the plate decreases the temporal decay of the least-stable mode, while heating the plate has the opposite effect. For moderate to large wave numbers, density variations have little effect except that there exists a range of cool plate temperatures for which these disturbances are extremely stable.This work was supported by the National Aeronautics and Space Administration under NASA Contract NAS1-18605 while the authors were in residence at the Institute for Compute Applications in Science and Engineering, NASA Langley Research Center, Hampton, VA 23665, U.S.A. 相似文献