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71.
In the framework of the Einstein–Palatini formalism, even though the projective transformation connecting the arbitrary connection
with the Levi-Civita connection has been floating in the literature for a long time and perhaps the result was implicitly
known in the affine gravity community, yet as far as we know Julia and Silva were the first to realise its gauge character.
We rederive this result by using the Rosenfeld–Dirac–Bergmann approach to constrained Hamiltonian systems and do a comprehensive
self contained analysis establishing the equivalence of the Einstein–Palatini and the metric formulations without having to
impose the gauge choice that the connection is symmetric. We also make contact with the the Einstein–Cartan theory when the
matter Lagrangian has fermions. 相似文献
72.
73.
The unambiguous evolution operator K was recently introduced in the theory of constrained systems. By viewing K as a vector field over the Legendre transformation, we give an intrinsic characterization of it through simple and intuitive properties. Some immediate consequences are explored. 相似文献
74.
Reductive coupling of methyl-vinyl ketone with TiCl4-Mg gives pinacol (25%). According to the reducing agents, mesityl oxide yields 2,4,5,7-tetramethyl-octa-2,4,6-triene (with 4TiCl3-LialH4), triene or 2,4,5,7-tetramethyl-octa-2,6-dien-4,5-diol (with TiCl4-Mg), pinacol (with VCl3-Mg), and 2-acetyl-1,3,3,4,4-pentamethyl-cyclopentane (with CrCl3-Mg or FeCl3-Mg or ZrCl4 or ZrCl4-Mg) as major products. 相似文献
75.
Photolysis of dichloromethane solutions of M(CO)6 (M = Cr, W) at low temperature in the presence of hydrogen gas affords W(CO)5(H2) (1) and Cr(CO)5(H2) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T1 values for the hydride resonances and a large HD coupling of 35.3 Hz (W) and 35.8 Hz (Cr) in the HD derivatives. A cationic analogue, [Re(CO)5(H2)]+ (3), was prepared by reaction of Re(CO)5Cl with [Et3Si][B(C6F5)4] in fluorobenzene under hydrogen. Complex 3-d1 exhibits JHD = 33.9 Hz. Complex 3 is strongly acidic, with complete deprotonation by diethyl ether; complexes 1 and 2 are moderately acidic. Deprotonation of 1 is complete in the presence of one equivalent of triethylamine. 相似文献
76.
Multiple solid-phase microextraction (SPME) is an useful technique for the direct quantification of solid samples removing any matrix effect. The volatile organic compounds formed in the extrusion-coating process of multilayer packaging materials have already been quantified by multiple HS-SPME coupled to gas chromatography (GC)-mass spectrometry (MS) using volatile organic compound (VOC) solutions in hexadecane for calibration. In this article, water is proposed as solvent to prepare the calibration solutions because it provides a shorter calibration time, better linearity, better reproducibility, and lower detection limits than hexadecane. Besides, the extraction of VOCs from aqueous solutions is exhaustive and avoids the extrapolations needed to calculate the total peak areas, as they can be calculated as the sum of the individual areas of each extraction. Finally, it is checked whether the two solvents provide the same mean values for the total peak areas. 相似文献
77.
Oxygen vacancies are common to most metal oxides, whether intentionally incorporated or otherwise, and the study of these defects is of increasing interest for solar water splitting. In this work, we examine nanostructured WO3 photoanodes of varying oxygen content to determine how the concentration of bulk oxygen-vacancy states affects the photocatalytic performance for water oxidation. Using transient optical spectroscopy, we follow the charge carrier recombination kinetics in these samples, from picoseconds to seconds, and examine how differing oxygen vacancy concentrations impact upon these kinetics. We find that samples with an intermediate concentration of vacancies (∼2% of oxygen atoms) afford the greatest photoinduced charge carrier densities, and the slowest recombination kinetics across all timescales studied. This increased yield of photogenerated charges correlates with improved photocurrent densities under simulated sunlight, with both greater and lesser oxygen vacancy concentrations resulting in enhanced recombination losses and poorer J–V performances. Our conclusion, that an optimal – neither too high nor too low – concentration of oxygen vacancies is required for optimum photoelectrochemical performance, is discussed in terms of the competing beneficial and detrimental impact these defects have on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation.A medium concentration of oxygen vacancies (VO ≈ 2%) is critical to the performance of WO3 photoanodes for solar water oxidation, enhancing charge separation and reducing recombination across all timescales examined. 相似文献
78.
Laia Haurie Ana Inés Fernández Josep Maria Chimenos Ferran Espiell 《Polymer Degradation and Stability》2006,91(5):989-994
Synthetic hydromagnesite obtained from an industrial by-product was evaluated as a non-halogenated flame retardant. It was used in combination with aluminium hydroxide (ATH) and compared with commercial flame retardants like magnesium hydroxide (MH) and natural hydromagnesite-huntite (U) in a polyolefin system of low-density polyethylene/poly(ethylene-co-vinyl acetate) (LDPE/EVA).The thermal stability and flame behaviour of the halogen free flame retarded composites were studied by thermogravimetric and differential thermal analysis (TG-DTA), limiting oxygen index (LOI), epiradiateur and cone calorimeter. It has been shown that synthetic hydromagnesite could be an alternative solution to the use of MH in non-halogenated flame retardant systems in EVA. 相似文献
79.
SMILES strings are the most compact text based molecular representations. Implicitly they contain the information needed to compute all kinds of molecular structures and, thus, molecular properties derived from these structures. We show that this implicit information can be accessed directly at SMILES string level without the need to apply explicit time-consuming conversion of the SMILES strings into molecular graphs or 3D structures with subsequent 2D or 3D QSPR calculations. Our method is based on the fragmentation of SMILES strings into overlapping substrings of a defined size that we call LINGOs. The integral set of LINGOs derived from a given SMILES string, the LINGO profile, is a hologram of the SMILES representation of the molecule described. LINGO profiles provide input for QSPR models and the calculation of intermolecular similarities at very low computational cost. The octanol/water partition coefficient (LlogP) QSPR model achieved a correlation coefficient R2=0.93, a root-mean-square error RRMS=0.49 log units, a goodness of prediction correlation coefficient Q2=0.89 and a QRMS=0.61 log units. The intrinsic aqueous solubility (LlogS) QSPR model achieved correlation coefficient values of R2=0.91, Q2=0.82, and RRMS=0.60 and QRMS=0.89 log units. Integral Tanimoto coefficients computed from LINGO profiles provided sharp discrimination between random and bioisoster pairs extracted from Accelrys Bioster Database. Average similarities (LINGOsim) were 0.07 for the random pairs and 0.36 for the bioisosteric pairs. 相似文献
80.
Espelt L Parella T Bujons J Solans C Joglar J Delgado A Clapés P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(20):4887-4899
The potential of dihydroxyacetone phosphate (DHAP)-dependent aldolases to catalyze stereoselective aldol additions is, in many instances, limited by the solubility of the acceptor aldehyde in aqueous/co-solvent mixtures. Herein, we demonstrate the efficiency of emulsion systems as reaction media for the class I fructose-1,6-bisphosphate aldolase (RAMA) and class II recombinant rhamnulose-1-phosphate aldolase from E. coli (RhuA)-catalyzed aldol addition between DHAP and N-benzyloxycarbonyl (N-Cbz) aminoaldehydes. The use of emulsions improved the RAMA-catalyzed aldol conversions by three to tenfold relative to those in conventional DMF/water mixtures. RhuA was more reactive than RAMA towards the N-Cbz aminoaldehydes regardless of the reaction medium. With (S)- or (R)-Cbz-alaninal, RAMA exhibited preference for the R enantiomer, while RhuA had no enantiomeric discrimination. The linear N-Cbz aminopolyols thus obtained were submitted to catalytic intramolecular reductive amination to afford the corresponding iminocyclitols. This reaction was diastereoselective in all cases examined; the face selectivity was controlled by the stereochemistry of the newly formed hydroxyl group originating from the aldehyde. Characterization of the resulting iminocyclitols allowed the assessment of the diastereoselectivity of the enzymatic aldol reactions with respect to the N-protected aminoaldehyde. RAMA formed single diastereoisomers from N-Cbz-glycinal and from both enantiomers of N-Cbz-alaninal, while 14 % of the epimeric product was observed from N-Cbz-3-aminopropanal. Diastereoselectivity from RhuA was lower than that observed from RAMA. Interestingly, a single diastereoisomer was formed from (S)-Cbz-alaninal, whereas only a 34 % diastereomeric excess was observed from its enantiomer (i.e., (R)-Cbz-alaninal). 相似文献