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51.
Moura Ramos JJ Correia NT Diogo HP Descamps M 《The journal of physical chemistry. B》2006,110(16):8268-8273
Molecular mobility in crystalline anhydrous caffeine was studied by the dielectric technique of thermally stimulated depolarization currents (TSDC). Two relaxational processes were found, one appearing at approximately -10 degrees C that is ascribed to a reorientational glass transition, and a higher temperature one that probably arises from local molecular motions that are precursors of diffusion and sublimation. The experimental results suggest that both crystalline phases II and I of caffeine, that have distinct crystal structures, are solid rotator phases. Furthermore, this dynamic reorientational disorder shows a reorientational glass transition at the same temperature in phase II and in metastable phase I. 相似文献
52.
Renata S. Amais Stephen E. Long Joaquim A. Nóbrega Steven J. Christopher 《Analytica chimica acta》2014
A method is described for quantification of sulfur at low concentrations on the order of mg kg−1 in biodiesel and diesel fuels using isotope dilution and sector field inductively coupled plasma mass spectrometry (ID-SF-ICP-MS). Closed vessel microwave-assisted digestion was employed using a diluted nitric acid and hydrogen peroxide decomposition medium to reduce sample dilution volumes. Medium resolution mode was employed to eliminate isobaric interferences at 32S and 34S related to polyatomic phosphorus and oxygen species, and sulfur hydride species. The method outlined yielded respective limits of detection (LOD) and limits of quantification (LOQ) of 0.7 mg kg−1 S and 2.5 mg kg−1 S (in the sample). The LOD was constrained by instrument background counts at 32S but was sufficient to facilitate value assignment of total S mass fraction in NIST SRM 2723b Sulfur in Diesel Fuel Oil at 9.06 ± 0.13 mg kg−1. No statistically significant difference at a 95% confidence level was observed between the measured and certified values for certified reference materials NIST SRM 2773 B100 Biodiesel (Animal-Based), CENAM DRM 272b and NIST SRM 2723a Sulfur in Diesel Fuel Oil, validating method accuracy. 相似文献
53.
Joaquim Messeguer Marisol Montolio Eduardo Soriano Angel Messeguer 《Tetrahedron》2010,66(13):2444-5027
A study on the preparation of N-alkylglycines (peptoids) that contain tertiary amino residues on the N-alkyl side chains is reported. The appropriate combination of the submonomer strategy with N-alkylglycine monomer couplings depending upon the structure of the N-alkyl side chain that must be incorporated into the peptoid is determinant for the efficiency of the synthetic pathway. The application of this strategy to the preparation of SICHI, an N-alkyglycine trimer containing tertiary amino residues in the three N-alkyl branches, and that has been identified as a potent Semaphorin 3A inhibitor, is presented. 相似文献
54.
Cherta L Beltran J Portolés T Hernández F 《Analytical and bioanalytical chemistry》2012,402(7):2301-2314
A multiresidue method has been developed for quantification and identification of 66 multiclass priority organic pollutants
in water by fast gas chromatography (GC) coupled to mass spectrometry (MS). Capabilities and limitations of single quadrupole
mass spectrometer as detector in fast GC were studied evaluating the chromatographic responses in terms of sensitivity and
chromatographic peak shapes, as they were influenced by scan time. The number of monitored ions in a selected ion monitoring
(SIM) group strongly conditioned the scan time and subsequently the number of data points per peak. A compromise between peak
shape and scan time was adopted in order to reach the proper conditions for quantitative analysis. An average of 10–15 points
per peak was attained for most compounds, involving scan times between 0.1 and 0.22 s. The method was validated for mineral,
surface, and groundwater. A solid-phase extraction pre-concentration step using C18 cartridges was applied. Four isotopically labeled standards were added to the samples before extraction and used as surrogates
to ensure a reliable quantification. Analyses were performed by GC–MS in electron ionization mode, monitoring the three most
abundant and/or specific ions for each compound and using the intensity ratios as a confirmatory parameter. With a chromatographic
run of less than 10 min, SIM mode provided excellent sensitivity and identification capability due to the monitoring of three
ions and the evaluation of their intensity ratio. Limits of detection below 10 ng/L were reached for most of the 66 compounds
in the three matrices studied. Accuracy and precision of the method were evaluated by means of recovery experiments at two
fortification levels (10 and 100 ng/L), obtaining recoveries between 70% and 120% in most cases and relative standard deviations
below 20%. The possibilities of a simultaneous SIM scan method have also been explored for non-target qualitative analysis.
The developed method has been applied to the analysis of surface water samples collected from the Mediterranean region of
Spain. 相似文献
55.
A Diltiazem kinetic spectrophotometric method was optimized by factorial analysis. The experimental method is based on a two-stage reaction of Diltiazem with hydroxylamine and a ferric salt: in the first stage there is a hydroxamic acid formation; and, in the second stage there is a red colour complex ferric hydroxamate formation. The variables under investigation were: solvent; hydroxylamine, sodium hydroxide and ammonium ferric sulphate concentrations; volume of perchloric acid; and, temperature. The responses of the reactional system were the maximum absorbance, the wavelength and the reaction time at maximum absorbance. Experimental design methodologies were used in the optimization. Fractional and full factorial designs followed by optimization Box-Behnken and central composite experimental designs were used. The observed optimum conditions were: methanol as reaction solvent; hydroxylamine concentration of 9.375%; sodium hydroxide concentration of 18.750%; ferric reagent concentration of 2.000%; minimum volume of perchloric acid to neutralize the sodium hydroxide; and, room temperature as reaction temperature. With this set of experimental conditions a reaction time of 10.5 s with maximum colour development at 512 nm wavelength was achieved. 相似文献
56.
Dailly A. Schneider R. Billaud D. Fort Y. Ghanbaja J. 《Journal of nanoparticle research》2003,5(3-4):389-393
A novel chemical reduction method using an activated alkaline hydride (LiH or NaH-t-BuONa) in tetrahydrofuran solvent has been applied to antimony salt reduction. X-ray diffraction and transmission electron microscopy studies have been carried out to characterize the morphology and structure of the materials. Alkali hydride nature influence has been proved. In both cases the process allows to prepare antimony particles in nanometer range from few nanometers to about 20nm which could be used as anodic materials for lithium–ion batteries. With lithium hydride well-crystallized particles inclined to agglomeration were observed whereas finely dispersed amorphous particles were pointing out after activated sodium hydride reduction. 相似文献
57.
Philippe Gros Sabine Choppin Julien Mathieu Yves Fort 《The Journal of organic chemistry》2002,67(1):234-237
The determination of the initial deprotonation site of 2-chloro- and 2-methoxypyridine during reaction with BuLi-LiDMAE has been investigated. A series of experiments on deuterated regioisomers revealed a direct lithiation at C-6 excluding a potential first classical ortholithiation and lithium equilibration in the reaction medium. These results suggested that the formation of lithium aggregates at the neighboring of the pyridinic nitrogen atom favored BuLi delivery at C-6 as well as 6-lithio intermediate stabilization. 相似文献
58.
The first direct alpha-lithiation of 4-DMAP has been performed via reaction with the BuLi-Me(2)N(CH(2))(2)OLi (BuLi-LiDMAE) reagent. This new methodology avoids the use of a activation-lithiation-regeneration sequence or halogen-metal exchange classically employed. New useful DMAP-containing synthons and polyheterocycles have been efficiently prepared. 相似文献
59.
Paula Amaral Luís M. Fernandes Joaquim Júdice Hanif D. Sherali 《Journal of Global Optimization》2009,45(4):645-666
This paper addresses the problem of finding an optimal correction of an inconsistent linear system, where only the nonzero
coefficients of the constraint matrix are allowed to be perturbed for reconstructing a consistent system. Using the Frobenius
norm as a measure of the distance to feasibility, a nonconvex minimization problem is formulated, whose objective function
is a sum of fractional functions. A branch-and-bound algorithm for solving this nonconvex program is proposed, based on suitably
overestimating the denominator function for computing lower bounds. Computational experience is presented to demonstrate the
efficacy of this approach. 相似文献
60.