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101.
Water vapor can be a significant interference in the analysis of air for non-methane volatile organic compounds (NMVOCs) using solid-adsorbent sampling techniques. The adsorbent materials used in sampling cartridges have different hydrophobic characteristics, and it is therefore necessary to characterize solid-adsorbent cartridges over a wide range of humidity. Controlled humidity experiments were performed to assess the extent of water vapor interference when samples are collected onto AirToxics solid-adsorbent cartridges. It was found that elevating the temperature of the cartridge to 10 degrees C above the temperature of the air sample greatly reduced water vapor adsorption and interferences and resulted in > or = 90% recovery of NMVOCs, biogenic VOCs and chlorofluorocarbons. Similar collection efficiencies were obtained at ambient temperature by reducing the relative humidity to > or = 60% in the sample by dilution with dry, scrubbed ambient air. A procedure also was developed and optimized for dry-purging cartridges prior to analysis. However, under optimized conditions, significant losses of C3-C5 compounds still occurred under highly humid conditions. It was determined that these losses were due to reduced retention during sampling rather than loss during the dry purge procedure. The dry purge method was shown to be adequate at high humidities for sampling NMVOCs with retention indices greater than 500. 相似文献
102.
Hoffman BJ Taylor LT Rumbelow S Goff L Pinkston JD 《Journal of chromatography. A》2004,1043(2):285-290
Supercritical fluid chromatography (SFC) was used for the analysis of ethoxylated and propoxylated surfactants. Samples were derivatized to phenylated silyl ethers with a disilazane-chlorosilane mixture. Addition of a phenyl group to the surfactant allowed UV-absorbance detection of each oligomer. Acetonitrile and methanol were evaluated as mobile phase modifiers. Better peak shape was realized with methanol-modified CO2 on an octadecyl silica bonded phase than with acetonitrile-modified CO2. Peak assignments were made via SFC coupled with electrospray ionization-mass spectrometry (ESI-MS) in the positive ion mode. A sulfonamide-embedded alkyl stationary phase was also evaluated for separation of the derivatized samples. SFC-UV and SFC-ESI-MS data were jointly used for calculation of average molar oligomer values which were then compared to values calculated from 1H NMR data of non-derivatized samples. The derivatization or separation method using the sulfonamide embedded phase required no preliminary cleanup and yielded reproducible oligomer values that were consistent with those of the manufacturer's nominal values. 相似文献
103.
A Java applet that predicts solute losses during evaporation from a binary or ternary mixture has been developed which gives good agreement with experiment and can be used to estimate the boiling points of solutes, making use of information which is often collected and then discarded (http://www.ch.cam.ac.uk/magnus/rotavap/). 相似文献
104.
105.
Karen V. Baker John M. Brown Neil A. Cooley Gareth D. Hughes Richard J. Taylor 《Journal of organometallic chemistry》1989,370(1-3):397-406
The palladium dibromide complexes of (S,R)-(1,1′-bis-diphenylphosphino)-2-ferrocencylthyldimethylamine and (S,R)-(1-diphenylphosphino)-2-ferrocenylethyldimethylamine have been reduced with dilithiocyclooctatetraene to form the corresponding Pd0 cyclooctatetraene complexes. Their reactions with E-4-methoxy-2′-bromophenylethene, and then benzylmagnesium chloride at −60 to −30°C, provide information on the structure of intermediates in asymmetric cross-coupling. 相似文献
106.
Summary The capabilities of sulfur hexafluoride as a mobile phase for supercritical-fluid chromatography are investigated. An evaluation
of its overall utility on the basis of separations of standard aromatic hydrocarbon odel mixtures performed on a variety of
bonded-phase, packed columns with UV detection is presented. The dependence of separation performance upon operational parameters
is also examined. A comparative evaluation of the chroamtographic properties of supercitrical sulfur hexafluoride and those
of supercritical carbon dioxide is developed from these separations under corresponding supercritical state conditions. 相似文献
107.
The International Measurement Evaluation Programme (IMEP) is an interlaboratory comparison scheme, founded, owned and coordinated by the Institute for Reference Materials and Measurements (IRMM) since 1988. IMEP-9 is the third round of trace elements in water evaluation following IMEP-3 and IMEP-6. Reference values for 15 elements stating total concentrations and combined uncertainties (according to GUM) were established. The reference values were established mainly by isotope dilution mass spectrometry (IDMS) as a primary method of measurement, and values traceable to the SI were obtained. The four elements that could not be certified by IDMS were assigned values by means of other measurement techniques. Results from 201 laboratories from 35 countries and four continents were evaluated against the reference values and the comparability between the laboratories is presented graphically. 相似文献
108.
The interaction of a nonionic polymeric surfactant with an anionic surfactant at the oil–water interface has been studied
by its effects on the droplet size, stability and rheology of emulsions. Oil-in-water (o/w) emulsions were prepared using
isoparaffinic oil and mixtures of a nonionic polymeric surfactant with an anionic surfactant. The macro-molecular surfactant
was a graft copolymer with a backbone of polymethyl methacrylate and grafted polyethylene oxide (a graft copolymer with PEO
chains of MW=750). The anionic surfactant was sodium dodecyl sulfate (SDS). The stabiliza-tion of the emulsion droplets was
found to be different when using one or the other surfactant. The mechanism of stabilization of emulsion droplets by the macro-molecular
surfactant is of the steric type while the stabilization by anionic surfactant is of the electrostatic repulsion type. Emulsions
stabilized with mixtures present both types of stabilization. Other effects on the preparation and stabilization of emulsions
were found to be dependent on properties associated with the surfactant molecular weight such as the Marangoni effect and
Gibbs elasticity. The initial droplet size of the emulsions showed a synergistic effect of the surfactant combination, showing
a minimum for the mixtures compared to the pure components. Emulsion stability also shows a synergistic interaction of both
surfactants. Rheological measurements allow for the estimation of the interparticle interaction when measured as a function
of volume fraction. Most of the effects observed can be attributed to the differences in interfacial tension and droplet radius
produced by both surfactants and their mixtures. The elastic moduli are well explained on the basis of droplet deformation.
Ionic versus steric stabilization produce little difference in the observed rheology, the only important differences observed
concerned the extent of the linear viscoelasticity region.
Received: 22 November 1996 Accepted: 24 March 1997 相似文献
109.
The CD spectra of L-2-methylpyrrolidine, L-prolinol, and their N-methylated derivatives have been determined. As in the 2-substituted piperidines, N-methylation results in an inversion of the sign of the Cotton effects (CE). However, the sign of the long-wavelength CE does not follow the simple helicity rule found for 2-substituted piperidines, since the pyrrolidine ring is itself chiral and makes its own contributions to the observed CE's. The rotational contribution due to pyrrolidine ring chirality appears to be opposite in sign to and larger in magnitude than that due to the 2-substituent in both the secondary and the tertiary amines. 相似文献
110.
The reduction of 1-methyl-, 3-methyl- and 1,3-dimethyl-5-nitrouracil (Ia-c) to the corresponding 5-aminouracils (IIa-c) is described. Diazotization of 5-amino-1-methyluracil (IIa) and 5-amino-1,3-dimethyluracil (IIc) gave 5-diazouracils which were characterized as thermally stable C6 covalent hydrates (III and XIII). Diazotization of 5-amino-3-methyluracil (IIb) gave anhydro 5-diazo-3-methyluracil (X) which underwent covalent methanolation and thermally reversible covalent hydration. Treatment of III and XIII with hot methanol resulted in solvent exchange of the C6 hydroxyl groups by a mechanism which may involve initial formation of diazoethers. Treatment of the methanolates (IV, XI and XIV) with dimethylamine resulted in coupling at the diazo group with a concomitant expulsion of the C6 methoxyl groups to give 5-(3,3-dimethyl-1-triazeno)uracils (XVa-c). 相似文献