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751.
Budny A Novak F Plumeré N Schetter B Speiser B Straub D Mayer HA Reginek M 《Langmuir : the ACS journal of surfaces and colloids》2006,22(25):10605-10611
Nonporous spherical silica nanoparticles resulting from a controlled St?ber process are covalently surface modified with redox-active molecules. Ferrocene, a ruthenium(II) complex with an N2P2Cl2 ligand set, and a sterically hindered biphenylamine are used as modifiers. The resulting materials are characterized by physical, spectroscopic, electrochemical, and chemical methods. The cyclic voltammetric behavior is studied in detail and reveals effects of charge transport by electron hopping along the surface of particles adsorbed on a Pt electrode. The ruthenium(II) complex remains catalytically active with respect to hydrogenation upon immobilization on the particles. Thus, the respective material provides a heterogenized homogeneous hydrogenation catalyst on a solid support. 相似文献
752.
In this work we present the formation of micrometre-sized lipid vesicles and tubes with perfectly homogeneous diameter and extraordinary length. The method is a novel approach for unconventional fabrication of soft-matter microstructured devices based on the combination of top-down and bottom-up fabrication processes. Photolithography techniques are applied to fabricate microsized apertures that provide the requirements to form lipid structures with predictable size and to align and guide the vesicles and tubes in microstructured channels. The formation is facilitated by self-assembly of polar lipids to a lipid membrane that is afterwards forced to undergo a shape transformation by extrusion through a microsized aperture. Both the geometrical restriction by the small aperture and the pressure difference between the top and bottom sides of the aperture determine the form and length of the vesicles and tubes. A strong pressure difference favors the formation of lipid tubes, while a low pressure difference results in the formation of vesicle bunches with spherical and cylindrical shapes. Potential applications for the formed lipid structures could be as microreactors and transport channels as well as in the construction of flexible microfluidic networks. 相似文献
753.
Photocurrent transients were used to investigate electron transport in mesoporous, nanocrystalline TiO2 films immersed into aqueous electrolyte, a regime where recombination cannot be neglected. Laser intensity and potential-dependent measurements show a decreased transient time of the current peak, which is explained by trap filling and electron loss from trap states into the electrolyte. A strongly enhanced recombination is furthermore observed, when the pH of the electrolyte is increased, while the current peak shifts toward longer transient times. Numerical simulations were used to decouple the impact of recombination and trapping on the transient response. We show that enhanced recombination in the absence of trapping accelerates the transfer of a current signal, while increased recombination slows down the transient current in the presence of electron trap sites. 相似文献
754.
A method for isotachophoretic determination of sweeteners of different character in candies and chewing gums was developed. A capillary of 0.8 mm ID and 90 mm effective length made of fluorinated ethylene-propylene copolymer is filled with an electrolyte system consisting of 10 mM HCl + 14 mM Tris, pH 7.7 (leading electrolyte) and 5 mM L-histidine + 5 mM Tris, pH 8.3 (terminating electrolyte). The analysis is performed at a driving current of 200 microA and for detection current is decreased to 100 microA. Boric acid is added to the aqueous sample solution to form borate complexes with substances of polyhydroxyl nature and make them migrate isotachophoretically. Using conductivity detection, the calibration curves in the tested concentration range up to 2.5 mM were linear for all components of interest: acesulfame K, saccharine, aspartame, cyclamate, sorbitol, mannitol, lactitol, and xylitol. The concentration detection limits ranged between 0.024 and 0.081 mM. Good precision of the ITP method is evidenced by favorable RSD values ranging from 0.8 to 2.8% obtained at the analyte concentration of 1.0 mM (n = 6). The analysis time was about 20 min. Simplicity, accuracy, and low cost of analyses make ITP an alternative procedure to methods used so far for the determination of ionizable sweeteners. 相似文献
755.
Martín-Matute B Edin M Bäckvall JE 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(23):6053-6061
Highly efficient synthesis of enantiopure diacetates of 2,4-pentanediol and 2,5-hexanediol starting from commercially available mixtures of the diols (dl/meso approximately 1:1) has been realized by combining a fast ruthenium-catalyzed epimerization with an enzymatic transesterification. The in situ coupling of these two processes produces the diacetates in high yield in >99 % enantiomeric excess. 相似文献
756.
Jochmann MA Blessing M Haderlein SB Schmidt TC 《Rapid communications in mass spectrometry : RCM》2006,20(24):3639-3648
Compound-specific isotope analysis (CSIA) has been established as a useful tool in the field of environmental science, in particular in the assessment of contaminated sites. What limits the use of gas chromatography/isotope ratio mass spectrometry (GC/IRMS) is the low sensitivity of the method compared with GC/MS analysis; however, the development of suitable extraction and enrichment techniques for important groundwater contaminants will extend the fields of application for GC/IRMS. So far, purge and trap (P&T) is the most effective, known preconcentration technique for on-line CSIA with the lowest reported method detection limits (MDLs in the low microg/L range). With the goal of improving the sensitivity of a fully automated GC/IRMS analysis method, a commercially available P&T system was modified. The method was evaluated for ten monoaromatic compounds (benzene, toluene, para-xylene, ethylbenzene, propylbenzene, isopropylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, fluorobenzene) and ten halogenated volatile organic compounds (VOCs) (dichloromethane, cis-1,2-dichloroethene, trans-1,2-dichloroethene, carbon tetrachloride, chloroform, 1,2-dichloroethane, trichloroethene, tetrachlorethene, 1,2-dibromoethane, bromoform). The influence of method parameters, including purge gas flow rates and purge times, on delta13C values of target compounds was evaluated. The P&T method showed good reproducibility, high linearity and small isotopic fractionation. MDLs were determined by consecutive calculation of the delta13C mean values. The last concentration for which the delta13C value was within this iterative interval and for which the standard deviation was lower than +/-0.5 per thousand for triplicate measurements was defined as the MDL. MDLs for monoaromatic compounds between 0.07 and 0.35 microg/L are the lowest values reported so far for continuous-flow isotope ratio measurements using an automated system. MDLs for halogenated hydrocarbons were between 0.76 and 27 microg/L. The environmental applicability of the P&T-GC/IRMS method in the low-microg/L range was demonstrated in a case study on groundwater samples from a former military air field contaminated with VOCs. 相似文献
757.
Jens Hartung Irina Kempter Thomas Gottwald Michaela Schwarz Rainer Kneuer 《Tetrahedron: Asymmetry》2009,20(18):2097-2104
Esterification of cyclic thiohydroxamic acids, for example, N-hydroxypyridine-2(1H)-thione, N-hydroxy-4-methylthiazole-2(3H)-thione, and N-hydroxy-4-(p-chlorophenyl)-thiazole-2(3H)-thione, occurred with inversion of configuration at the attacked stereocenter, as evident from the use of chiral alcohols, alkyl p-toluene sulfonates, and cyclic sulfates. Stereochemical analysis of enantiomerically pure O-alkyl thiohydroxamates was performed on the basis of CD-spectroscopy and chemical derivatization. The assignment of the relative configuration in cyclic O-esters was feasible via NMR spectroscopy, whereas chiral aliphatic glycolato monoesters required hydroxyl group derivatization with chloro-(4R,5R)-bis[(1R,2S,5R)-menth-1-yloxycarbonyl)]-1,3,2-dioxaphospholane for this purpose. 相似文献
758.
We consider the problem of finding integrals of motion for quantum elliptic Calogero-Moser systems with arbitrary number of
particles extended by introducing spinexchange interaction. By direct calculation, after making certain ansatz, we found first
two integrals — quite probably, lowest nontrivial members of the whole commutative ring. This result might be considered as
the first step in constructing this ring of the operators which commute with the Hamiltonian of the model.
相似文献
759.
Photobleaching and stabilization of. fluorophores used for single-molecule analysis. with one- and two-photon excitation 总被引:2,自引:0,他引:2
Fluorescence spectroscopic measurements at the single-molecule level usually require large absorption cross sections and fluorescence
quantum yields for the dyes under study. In addition to these parameters, the collectable number of fluorescence photons and,
thus, the signal-to-noise ratio of the measurement, is influenced by processes like triplet-state population and photobleaching,
shifting the saturation threshold of the dye to lower excitation intensities. Confocal fluorescence correlation spectroscopy
(FCS) is a versatile method to precisely determine photon emission rates of single molecules but also gives access to rate
constants of dynamic bleaching and intersystem crossing. In recent FCS studies in solution and living cells, two-photon excitation
with its inherent spatial sectioning has proven to be a very valuable alternative to minimize background and cumulative signal
loss. However, there is evidence that in many dye systems, the photobleaching rates with two-photon excitation are significantly
enhanced with respect to one-photon excitation at comparable photon-emission yields. The reasons have so far remained mainly
speculative. In the present study, potential photobleaching pathways are investigated by adding chemical stabilizers and by
working at different oxygen concentrations. The results suggest that the population of triplet states does not appear to be
responsible for the limited emission rate with two-photon excitation. Rather, photobleaching pathways via the formation of
radicals seem to be plausible causes for the signal limitation. Favorable conditions are discussed to maximize the overall
photon-collection yield in two-photon experiments.
Received: 4 July 2001 / Published online: 23 November 2001 相似文献
760.
Michael Bartke Sebastian Kröner Annette Wittebrock Karl-Heinz Reichert Ilias Illiopoulus Christoph Johannes Dittrich 《Macromolecular Symposia》2007,259(1):327-336
Summary: Sorption experiments of ethylene and propylene in different polypropylene powder samples, both homopolymer and heterophasic copolymers with different rubber content, have been carried out in a high-pressure magnetic suspension balance at 10 bars pressure and 70 °C. The gross solubilities measured can be well correlated with the rubber content of the polymer samples. Solubility of ethylene and propylene in the rubber phase differ from solubility in the amorphous fraction of the homopolymer, especially the concentration ratio of propylene to ethylene differs significantly between rubber phase and amorphous fraction of the homopolymer. From the slope of monomer uptake, information on kinetics of mass-transfer can be gained. No significant differences were observed in terms of mass-transfer for ethylene and propylene. With increasing rubber content, effective diffusion coefficients increased slightly. By combined sorption studies with powder samples and compressed films, information about both effective diffusion coefficients and the effective length scale of diffusion could be gained. It could be shown, that the particle radius is not the characteristic length of diffusion in the studied powder samples. Mass transfer of nearly all samples could be described by a constant diffusion length of 120 to 130 µm, independently on particle size. This indicates that the effective scale of diffusion in polymer particles is in between microparticle and macroparticle scale used in classical particle modeling. 相似文献