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101.
Chiral phosphoric acids and derivatives have attracted considerable attention as a powerful tool in asymmetric catalysis. Various enantioselective reactions have been developed by using these efficient Br?nsted acid organocatalysts. Although initially the activation was restricted to reactive Br?nsted basic substrates, recent reports are demonstrating the versatility of phosphoric acid catalysts in the activation of carbonyl compounds in a stereochemically controlled fashion. This tutorial review gives an overview of enantioselective Br?nsted acid catalyzed transformations with the main focus on carbonyl activation. Different activation modes, key features of the catalysts and the applied substrates are presented and discussed with the goal to elucidate the origin of stereoselectivity in these Br?nsted acid catalyzed transformations. 相似文献
102.
Pinto RC Stenlund H Hertzberg M Lundstedt T Johansson E Trygg J 《Analytica chimica acta》2011,(2):127-131
To find and ascertain phenotypic differences, minimal variation between biological replicates is always desired. Variation between the replicates can originate from genetic transformation but also from environmental effects in the greenhouse. Design of experiments (DoE) has been used in field trials for many years and proven its value but is underused within functional genomics including greenhouse experiments. We propose a strategy to estimate the effect of environmental factors with the ultimate goal of minimizing variation between biological replicates, based on DoE. DoE can be analyzed in many ways. We present a graphical solution together with solutions based on classical statistics as well as the newly developed OPLS methodology.In this study, we used DoE to evaluate the influence of plant specific factors (plant size, shoot type, plant quality, and amount of fertilizer) and rotation of plant positions on height and section area of 135 cloned wild type poplar trees grown in the greenhouse. Statistical analysis revealed that plant position was the main contributor to variability among biological replicates and applying a plant rotation scheme could reduce this variation. 相似文献
103.
Rueping M Haack K Ieawsuwan W Sundén H Blanco M Schoepke FR 《Chemical communications (Cambridge, England)》2011,47(13):3828-3830
The design of biologically inspired, multi-component cascade reactions enables the targeted synthesis of assorted structurally complex products. Similar to regulation in cells the reaction path is controlled by the substrate concentration and complex enantiopure products with high structural diversity are provided. 相似文献
104.
Ek M Borgstr?m MT Karlsson LS Hetherington CJ Wallenberg LR 《Microscopy and microanalysis》2011,17(5):752-758
The twin interface structure in twinning superlattice InP nanowires with zincblende structure has been investigated using electron exit wavefunction restoration from focal series images recorded on an aberration-corrected transmission electron microscope. By comparing the exit wavefunction phase with simulations from model structures, it was possible to determine the twin structure to be the ortho type with preserved In-P bonding order across the interface. The bending of the thin nanowires away from the intended 110 axis could be estimated locally from the calculated diffraction pattern, and this parameter was successfully taken into account in the simulations. 相似文献
105.
Niga P Johnson CM Frey JG Rutland MW 《Physical chemistry chemical physics : PCCP》2011,13(17):7939-7947
Vibrational Sum Frequency Spectroscopy (VSFS) was employed to study adsorbing films of 4-Nitro Benzo-15-Crown-5 (NB15C5) and Benzo-15-Crown-5 (B15C5) at the aqueous solution-air interface. The surface of the solution is strongly influenced by the presence of crown ether species. Changes in the orientation of NB15C5 were monitored as a function of solution concentration, by targeting the ratio of peak intensities of the CN and NO(2) vibrational modes. The water of hydration has also been probed as a function of crown concentration, salt concentration, and temperature. The latter study strongly suggests that the surface can be treated as a charged interface, and that the associated ordered water decreases with increasing ionic strength of the bulk solution. 相似文献
106.
Aniekan Magnus Ukpong 《Molecular physics》2013,111(12):1607-1618
A tight-binding molecular dynamics study of the structural evolution in tetrahedral amorphous carbon networks under dynamic hydrogen saturation is presented. The incorporation of hydrogen results in higher degrees of network disorder in second-neighbour distances, and initiates orbital re-hybridization that relaxes network stress. Using the simulated structures, numerical tests are performed to verify the effectiveness of a new structural order parameter for tetrahedrally-bonded solids. It is found that the island of accessible information, within the order parameter field shows a linear dependence between the fluctuations in first- and second-nearest-neighbour distances at a preferred orientation of 36°. A comparison with similar studies on hydrogenated amorphous silicon suggests that the local network structure of tetrahedrally-bonded amorphous solids obey the same ordering rule irrespective of differences in chemical species. 相似文献
107.
108.
109.
Woo-young So J. Magnus Wikberg Oleg Mitrofanov Theo Siegrist Arthur P. Ramirez 《Solid State Communications》2007,142(9):483-486
We report doping effects in an organic semiconductor, crystalline rubrene. Oxygen-related states are introduced (removed) by annealing in oxygen (vacuum), at an elevated temperature. Room temperature stability is found in the resulting effects: (1) about two orders of magnitude increase in carrier density at equilibrium, (2) significant modification of threshold voltages, and (3) an unchanged field-effect mobility in the on-current state. Density of states data are modeled as tunneling from the valence band in the channel region into deep-level acceptors in the adjacent region. These oxygen acceptors are the likely dopant species. 相似文献
110.