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231.
Smith CD Baxendale IR Lanners S Hayward JJ Smith SC Ley SV 《Organic & biomolecular chemistry》2007,5(10):1559-1561
The cycloaddition of acetylenes with azides to give the corresponding 1,4-disubstituted 1,2,3-triazoles is reported using immobilised reagents and scavengers in pre-packed glass tubes in a modular flow reactor. 相似文献
232.
233.
Guo T Geis S Hedman C Arndt M Krick W Sonzogni W 《Journal of the American Society for Mass Spectrometry》2007,18(5):817-825
Beta-methylamino-L-alanine (BMAA) is a neurotoxic amino acid that can be produced by cyanobacteria in aqueous environments. To analyze this compound by gas chromatography/mass spectrometry (GC/MS), BMAA must be derivatized to a nonpolar, volatile compound. This can be accomplished by reacting BMAA with ethyl chloroformate. While carrying out electron ionization (EI) mass spectrometric analysis on the (13)C-labeled derivative, it was discovered that the formation of an ion with a peak at m/z 245.12 is the result of [CH(3)CH(2)O.] loss from the amino groups resulting from alpha-cleavage. This differs from previous reports that attributed this peak to alpha-cleavage of the carboxylic ester portion of the BMAA derivative. This finding is important for understanding BMAA derivative mass spectrometric fragmentation patterns and ultimately to properly identifying and quantifying BMAA. Fragmentation pathways for the formation of other major peaks observed in the EI mass spectra are also proposed. 相似文献
234.
Correlated ab initio calculations are used to analyze the interaction between nitrosyl hydride (HNO) and CH3X (X = F, Cl, Br). Three minima are located on the potential energy surface of each complex. The more strongly bound contains a NH...X bond, along with CH...O; CH...O and CH...N bonds occur in the less stable minimum. Binding energies of the global minimum lie in the range of 11-13 kJ/mol, and there is little sensitivity to the identity of the halogen atom. Unlike most other such hydrogen bonds, the NH covalent bond in this set of complexes becomes shorter, and its stretching frequency shifts to the blue, upon forming the NH...X hydrogen bond. The amount of this blue shift varies in the order F > Cl > Br. 相似文献
235.
Vandevyver CD Chauvin AS Comby S Bünzli JC 《Chemical communications (Cambridge, England)》2007,(17):1716-1718
Water-soluble triple-stranded [Ln(2)(L)(3)] helicates have been successfully tested as imaging probes in human cervical adenocarcinoma cells (HeLa), the complex being not toxic and clearly staining their cytoplasm in a concentration-dependent manner. 相似文献
236.
Eckers C Laures AM Giles K Major H Pringle S 《Rapid communications in mass spectrometry : RCM》2007,21(7):1255-1263
Many formulated products contain complex polymeric excipients such as polyethylene glycols (PEGs). Such excipients can be readily ionized by electrospray and may be present at very high concentrations, thus making it very difficult to identify trace level impurities such as degradants in samples, even if hyphenated techniques such as liquid chromatography/mass spectrometry (LC/MS) are used. Ion mobility (IM) spectrometry is a very rapid gas-phase separation technique and offers additional separation capability within the LC timeframe. This work investigates the use of an IM separator in combination with high-pressure liquid chromatography (HPLC) and MS, to improve the separation of drug-related materials from excipients, thus aiding the identification of trace-level impurities in an anti-HIV medication, Combivir. 相似文献
237.
Pandey D Zemlyanov DY Bevan K Reifenberger RG Dirk SM Howell SW Wheeler DR 《Langmuir : the ACS journal of surfaces and colloids》2007,23(9):4700-4708
Molecular layers formed from 4-trifluoromethylbenzenediazonium tetrafluoroborate and 4-Methylbenzenediazonium tetrafluoroborate have been assembled on H-passivated Si(111) and studied by UHV STM and XPS. STM imaging shows well-developed Si(111) step edges and terraces both on Si(111):H and Si(111) substrates covered with a molecular layer. STM I(V) data acquired at different tip-substrate separations reveals a factor of approximately 10 enhancement in current for positive bias voltage when current flows through the 4-trifluoromethyl molecule when compared to the 4-methyl variant. The observed current enhancement in I(V) can be understood by comparing the projected density of states of the two molecule-Si systems calculated using a density functional theory local density approximation after geometry optimization was performed via the conjugate gradient method. XPS data independently confirm that H-passivated Si(111) remains oxygen free for short exposures to ambient conditions and provide evidence that the molecules chemically react with the silicon surface. 相似文献
238.
Motoaki Kimura Steve Tung James Lew Chih-Ming Ho Fukang Jiang Yu-Chong Tai 《Fluid Dynamics Research》1999,24(6):679-342
Measurements of wall shear-stress streaks of a turbulent boundary layer in the channel flow were carried out using a MEMS-based micro-shear-stress imaging chip, which contains about 100 sensors. The chip is designed and fabricated by surface micromachining technology. One arrray of 25 micro-shear-stress sensors in the chip that covers a length of 7.5 mm is used to measure the instantaneous spanwise distribution of the surface shear stress. The statistics of high shear-stress streaks were established. Based on the measurement, the physical quantities associated with the high shear-stress streaks, such as their length, width and peak shear-stress level, were obtained. We found out that a high correlation exists between the peak shear-stress level and front-end shear-stress slope of a high shear-stress streak. This important property is currently being applied to the deisgn of a real-time flow control logic. 相似文献
239.
Rafa Wysokiski Wiktor Zierkiewicz Mariusz Michalczyk Steve Scheiner 《Molecules (Basel, Switzerland)》2021,26(21)
Molecules of the type XYT = Ch (T = C, Si, Ge; Ch = S, Se; X,Y = H, CH3, Cl, Br, I) contain a σ-hole along the T = Ch bond extension. This hole can engage with the N lone pair of NCH and NCCH3 so as to form a chalcogen bond. In the case of T = C, these bonds are rather weak, less than 3 kcal/mol, and are slightly weakened in acetone or water. They owe their stability to attractive electrostatic energy, supplemented by dispersion, and a much smaller polarization term. Immersion in solvent reverses the electrostatic interaction to repulsive, while amplifying the polarization energy. The σ-holes are smaller for T = Si and Ge, even negative in many cases. These Lewis acids can nonetheless engage in a weak chalcogen bond. This bond owes its stability to dispersion in the gas phase, but it is polarization that dominates in solution. 相似文献
240.
Yoon-Kyoung Cho Ali Dhinojwala Steve Granick 《Journal of Polymer Science.Polymer Physics》1997,35(17):2961-2968
We study the drainage of a near-theta solvent through densely grafted polymer layers and compare to recent notions that these layers display little permeability to solvent flow at surface separations less than a “hydrodynamic thickness.” The solvent is trans-decalin (a near-theta solvent at the experimental temperature of 24°C). The polymer is polystyrene (PS) end-attached to two opposed mica surfaces via the selective adsorption of the polyvinylpyridine (PVP) block of a PS-PVP diblock copolymer. The experimental probe was a surface forces apparatus modified to apply small-amplitude oscillatory displacements in the normal direction. Out-of-phase responses reflected viscous flow of solvent alone—the PS chains did not appear to contribute to dissipation over the oscillation frequencies studied. The value of the hydrodynamic thickness (RH) was less than the coil thickness (Lo) measured independently from the onset of surface–surface interactions in the force-distance profile, implying significant penetration of the velocity field into the polymer layer. As the surface–surface separation was reduced from 3Lo to 0.3Lo, the apparent hydrodynamic thickness (R) decreased monotonically to values R ≪ RH. Physically, this indicates that the “slip plane” moved progressively closer to the solid surfaces with decreasing surface–surface separation. This was accompanied by augmentation of the effective viscosity by a factor of up to approximately 5, indicating somewhat diminished permeability of solvent through the overlapping polymer layers. Similar results hold for the flow through surface-anchored polymers in a good solvent. It is interesting to note the strong stretching of densely end-grafted polymers in a theta solvent. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2961–2968, 1997 相似文献