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71.
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
72.
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
73.
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
74.
Singlet-singlet resonance energy transfers from DODCI (donor) to malachite green and to DQOCI in ethanolic solutions have been investigated with a real-time picosecond streak-camera (time-resolution 3–5 ps) operating at a repetition rate of 140 MHz, in synchronism with the pulse train from a mode-locked cw dye laser used as excitation source. Acceptor concentrations from 10?5 M to 7.5 × 10?3 M were studied and average Ro values of 45.6 Å for malachite green and 61.8 Å for DQOCI are obtained from the directly measured fluorescence decay curves. Departure from Förster kinetics is seen at the lower acceptor concentrations employed. 相似文献
75.
Variously cross-linked poly(methylmethacrylates) (PMMAs) are synthesized with three additives incorporated at theoretically 1000 microg of the additive per gram of prepared polymer. The additives are Irganox 1010, Irganox 1076, and Irgafos 168. The in-house" synthesized polyacrylates are then subjected to supercritical fluid extraction (SFE) to determine if additive recovery is a function of percent cross-linking. Although considerable work in this regard has been performed with non-cross-linked polyolefins, the literature is lacking regarding polyacrylates. Some additive degradation apparently occurs during the synthesis, as judged by the increased complexity of the extract high-performance liquid chromatographic trace and the low percent recoveries observed especially for the Irganoxes. For low polymer cross-linking (1%), it appears that both PMMA synthetic reproducibility and readily observed polymer swelling during SFE are serious issues that adversely affect additive percent recovery and precision of results. Higher percent cross-linking yields more consistent analytical data than low percent cross-linking, even though the amount of additive extracted in all PMMA samples (regardless of cross-linking percentage) is essentially the same whether the extraction is via SFE or liquid-solid extraction with methylene chloride. Results for comparably cross-linked poly(ethylmethacrylate) and poly(butylmethacrylate) are similar to PMMA. 相似文献
76.
Darwish AD Avent AG Abdul-Sada AK Taylor R 《Chemical communications (Cambridge, England)》2003,(12):1374-1375
Trifluoromethylation of [60]- and [70]fullerenes occurs across both 6:6- and 5:6-bonds giving unsymmetrical tetramethyl adducts having four contiguous CF3 groups; both fullerenes give bis adducts which do not involve 6:6-addition, and unsymmetrical hexa-adducts (with contiguous CF3 groups) are also obtained from [60]fullerene. 相似文献
77.
The enantioselective cyclization of N-acyliminium ions generated in situ from tryptamine is promoted with high enantioselectivity by a new chiral thiourea catalyst. This represents the first successful system for asymmetric catalysis of the Pictet-Spengler reaction. 相似文献
78.
An efficient synthetic sequence toward the C8-C19 region of peloruside A has been developed. The route is highlighted by a selective electrophilic cyclization reaction, a single-step epoxide ring-opening/methylation sequence, and a stereoselective Mukaiyama aldol reaction. [reaction: see text] 相似文献
79.
W. S. Taylor J. G. Dulak S. N. Ketkar 《Journal of the American Society for Mass Spectrometry》1990,1(6):448-454
Plasma diagnostic studies have been carried out on the discharge source of a commercial glow discharge quadrupole mass spectrometer. Plasma parameters were determined using an electrostatic probe with the objective of determining the dependence (if any) of these parameters on the voltage placed on an auxiliary electrode immersed in the plasma. The biased electrode utilized in this study was the sampling orifice element itself. Our results indicate that, for positive orifice voltages with respect to the grounded anode, variations in the plasma potential and ion energy can be correlated directly to the bias placed on the sampling orifice. The dependence of the electron temperature on this parameter is observed to be more complex in nature, and electron number densities show little significant variation with respect to sampling orifice bias. In addition, increased orifice voltages result in an increase in the ion signal intensity measured with the mass spectrometer. Based on the results obtained here, we feel that this increase is due primarily to an increase in ion transmission to the quadrupole arising from the increased ion energy. 相似文献
80.
Design, synthesis, and binding affinities of pyrrolinone-based somatostatin mimetics 总被引:1,自引:0,他引:1
Smith AB Charnley AK Mesaros EF Kikuchi O Wang W Benowitz A Chu CL Feng JJ Chen KH Lin A Cheng FC Taylor L Hirschmann R 《Organic letters》2005,7(3):399-402
[structure: see text] Tetrapyrrolinone somatostatin (SRIF) mimetics (cf. 1), based on a heterochiral (D,L-mixed) pyrrolinone scaffold, were designed, synthesized, and evaluated for biological activity. The iterative synthetic sequence, incorporating the requisite functionalized coded and noncoded amino acid side chains, comprised a longest linear synthetic sequence of 23 steps. Binding affinities at two somatostatin receptor subtypes (hsst 4 and 5) reveal micromolar activity, demonstrating that the d,l-mixed pyrrolinone scaffold can be employed to generate functional mimetics of peptide beta-turns. 相似文献