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31.
Large polymer filaments can form when drag reducing polymers are injected through wall slots. The presence of these structures enhances the performance of the drag reducing function by mechanisms which are not understood. This paper shows how particle image velocimetry (PIV) techniques can be used to study changes in the configuration of the injected polymer and in the structure of the velocity field with increasing drag reduction. The filaments are found to behave as solid bodies which break up in high shear regions close to a boundary. The breakup process provides an explanation of why the filaments are not observed close to a wall and offers the possibility of providing a heterogeneous distribution of small aggregates of polymers which could be more effective than uniformly distributed molecules as suggested by Hoyer and Gyr (J Non-Newton Fluid Mech 65:221–240, 1996; J Fluids Eng 120:818–823, 1998), Dunlop and Cox (Phys Fluids 20:203–213, 1977) and Vlachogiannis et al. (Phys Fluid 15:3786–3794, 2004). PIV measurements show dramatic qualitative changes in the velocity patterns at maximum drag reduction. 相似文献
32.
Douglas W. Crandell Shivnath Mazumder P. Andrew Evans Mu-Hyun Baik 《Chemical science》2015,6(12):6896-6900
Density functional theory calculations demonstrate that the reversal of regiochemical outcome of the addition for substituted methyl propiolates in the rhodium-catalyzed [(2 + 2) + 2] carbocyclization with PPh3 and (S)-xyl-binap as ligands is both electronically and sterically controlled. For example, the ester functionality polarizes the alkyne π* orbital to favor overlap of the methyl-substituted terminus of the alkyne with the pπ-orbital of the alkenyl fragment of the rhodacycle during alkyne insertion with PPh3 as the ligand. In contrast, the sterically demanding xyl-binap ligand cannot accommodate the analogous alkyne orientation, thereby forcing insertion to occur at the sterically preferred ester terminus, overriding the electronically preferred orientation for alkyne insertion. 相似文献
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Boerner LJ Mazumder S Pink M Baik MH Zaleski JM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(51):14539-14551
The synthesis of a new series of free‐base, NiII and ZnII 2,3,12,13‐tetra(ethynyl)‐5,10,15,20‐tetraphenyl porphyrins is described. Upon heating, two of the four ethynyl moieties undergo Bergman cyclization to afford the monocyclized 2,3‐diethynyl‐5,20‐diphenylpiceno[10,11,12,13,14,15‐jklmn]porphyrin in 30 %, 10 %, and trace yields, respectively. The structures of all products were investigated by using quantum chemical calculations and the free‐base analogue was isolated and crystallized; all compounds show significant deviation from the idealized planar structure. No fully‐cyclized bispiceno[20,1,2,3,4,5,10,11,12,13,14,15‐fghij]porphyrin was isolated from the reaction mixture. To understand why only two of the four enthynyl groups undergo Bergman cyclization, the reaction coordinates were examined by using DFT at the PWPW91/cc‐pVTZ(‐f) level coupled to a continuum solvation model. The barrier to cyclization of the second pair of ethynyl groups was found to be 5.5 kcal mol?1 higher than the first, suggesting a negative cooperative effect and significantly slower rate for the second cyclization. Cyclization reactions for model porphyrin–enediynes with ethene‐ and H‐functionality substitutions at the meso‐phenyl rings were also examined, and found to have a similar barrier to diradical formation for the second cyclization event as for the first in these highly planar molecules. By enforcing an artificial 30° cant in two of the pyrrole rings of the porphyrin, the second barrier was increased by 2 kcal mol?1 in the ethene model system; this suggests that the disruption of the π conjugation of the extended porphyrin structure is the cause of the increased barrier to the second cyclization event. 相似文献
35.
Baik MH Mazumder S Ricci P Sawyer JR Song YG Wang H Evans PA 《Journal of the American Chemical Society》2011,133(20):7621-7623
The computational analysis of the rhodium-catalyzed Pauson-Khand reaction indicates that the key transition state is highly charge-polarized, wherein different diastereoisomers have distinctively different charge polarization patterns. Experimental studies demonstrate that chloro-enynes provide the optimal σ-electron-withdrawing group to promote polarization and thereby reduce the activation barrier to provide a highly diastereoselective reaction at room temperature. 相似文献
36.
Lee BR Oh KT Oh YT Baik HJ Park SY Youn YS Lee ES 《Chemical communications (Cambridge, England)》2011,47(13):3852-3854
We report charge-switching ionic nanocomplexes comprised of glycol chitosan grafted with 2,3-dimethylmaleic acid (DMA) (denoted as 'GCS-g-DMA' hereafter) and a proapoptotic peptide. This system allowed for improved peptide delivery to tumor sites via a mechanism of selective peptide release when the pH was dropped from 7.4 to 6.8. 相似文献
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A dye‐sensitized solar cell (DSSC) containing a TiO2 film treated with COOH‐functionalized germanium nanoparticles (Ge COOH Nps) exhibited a higher short‐circuit photocurrent density (Jsc; 15.4 mA cm−2) compared to the corresponding untreated DSSC (13.4 mA cm−2) using N719 and a 12 μm thick TiO2 film at 100 mW cm−2. The amount of N719 attached to the treated TiO2 film was 21 % greater than that attached to the untreated TiO2 film. Enhancement of the Jsc value by 15 % was attributed mostly to an intramolecular charge transfer from N719 attached to the Ge COOH Nps to the TiO2 conduction band through the Ge COOH Nps. 相似文献
40.
Byoung Sun Jung In Su Baik Ii Sub Song Gi-Dong Lee Seung Hee Lee 《Liquid crystals》2006,33(9):1077-1082
The degree of colour shift depending on viewing direction in the in-plane switching (IPS) mode has been investigated. The IPS cell with pure twist deformation exhibits a stronger colour shift than that of the IPS cell with tilt as well as twist deformation, although the former has better luminance uniformity in the bright state than the latter. Furthermore, the IPS cell with multi-directional LC orientation, with tilt as well as twist deformation, shows the least colour shift. 相似文献