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991.
Heat capacity values for melts of polystyrene, poly(methyl methacrylate), polypropylene, and polyethylene are calculated on the assumption that the total constant-volume heat capacity Cv is made up of two parts: one associated with molecular vibrations, and one, with holes. Numerical values of both components are given for a wide range of temperatures and compared with experimental data. For poly-1-butene insufficient data for complete evaluation are available, so that only the vibrational contribution could be discussed. 相似文献
992.
Heat capacities of molten polyethylene, polypropylene, poly-1-butene, polystyrene, and poly(methyl methacrylate) were measured over a wide range of temperature by using a differential scanning calorimeter. The upper limit of temperature was established for each polymer at about 10°K below the beginning of thermal decomposition. For poly-1-butene and poly(methyl methacrylate) the solid-state heat capacity was also measured starting from room temperature. Several samples of each polymer were used so that average values of heat capacities could be established (reported in 10°K intervals). The data revealed for all polymers a nearly linear increase of heat capacity with increasing temperature over the whole temperature range investigated. 相似文献
993.
994.
Vera Mardešić 《Geometriae Dedicata》1989,29(3):279-291
In the rational plane there exist bounded segmentally closed convex bodies with no support lines. This and other examples exhibited in this paper show that in vector spaces over non-complete ordered fields support hyperplanes of convex sets behave in a way very different from the way they behave in vector spaces over the field of real numbers.This paper was written during 1987 while the author, on leave from the University of Zagreb, was visiting the University of Washington in Seattle. 相似文献
995.
996.
997.
M. A. Goberna G. A. Lancho M. I. Todorov V. N. Vera de Serio 《Applied Mathematics and Optimization》2011,63(2):239-256
The concept of implicit active constraints at a given point provides useful local information about the solution set of linear
semi-infinite systems and about the optimal set in linear semi-infinite programming provided the set of gradient vectors of
the constraints is bounded, commonly under the additional assumption that there exists some strong Slater point. This paper
shows that the mentioned global boundedness condition can be replaced by a weaker local condition (LUB) based on locally active
constraints (active in a ball of small radius whose center is some nominal point), providing geometric information about the
solution set and Karush-Kuhn-Tucker type conditions for the optimal solution to be strongly unique. The maintaining of the
latter property under sufficiently small perturbations of all the data is also analyzed, giving a characterization of its
stability with respect to these perturbations in terms of the strong Slater condition, the so-called Extended-Nürnberger condition,
and the LUB condition. 相似文献
998.
Liu Y Nappi M Arceo E Vera S Melchiorre P 《Journal of the American Chemical Society》2011,133(38):15212-15218
The Diels-Alder reaction is probably the most powerful technology in the synthetic repertoire for single-step constructions of complex chiral molecules. The synthetic power of this fundamental pericyclic transformation has greatly increased with the emergence of asymmetric catalytic variants, and research aimed at further expanding its potential is still exciting and fascinating the chemical community. Here, we document the first asymmetric catalytic Diels-Alder reaction of in situ generated heterocyclic ortho-quinodimethanes (oQDMs), reactive diene species that have never before succumbed to a catalytic approach. Asymmetric aminocatalysis, that uses chiral amines as catalysts, is the enabling strategy to induce the transient generation of indole-, pyrrole- or furan-based oQDMs from simple starting materials, while directing the pericyclic reactions with nitroolefins and methyleneindolinones toward a highly stereoselective pathway. The approach provides straightforward access to polycyclic heteroaromatic compounds, which would be difficult to synthesize by other catalytic methods, and should open new synthetic pathways to complex chiral molecules using nontraditional disconnections. 相似文献
999.
High-performance thin-layer chromatography (HPTLC) coupled with bioluminescence detection using Vibrio fischeri bacteria can be used for screening for unknown substances. This is accomplished by dipping the HPTLC plate in an aqueous bacteria solution. Especially polar substances, however, can start to dissolve during this process, which leads to blurring and tailing of the zones on the plate. To overcome this disadvantage, we applied the bacteria solution by rolling. This method has been described for chemical derivatizations, but is very rarely used. The rolling device was made of commercially available household articles. Using octhilinone and methylparaben as test compounds, rolling was compared with dipping. Despite of performing the rolling process manually, the results were reproducible. Depending on the substance and its amount on the HPTLC plate, peaks were narrower, up to a factor of 4 higher and with a higher signal-to-noise ratio than after dipping. 相似文献
1000.
Luciano Tormen Raul A. Gil Vera L.A. Frescura Luis Dante Martinez Adilson J. Curtius 《Analytica chimica acta》2012
A fast method for the determination of As, Co, Cu, Fe, Mn, Ni, Se and V in biological samples by ETV-ICP-MS, after a simple sample treatment with formic acid, is proposed. Approximately 75 mg of each sample is mixed with 5 mL of formic acid, kept at 90 °C for 1 h and then diluted with nitric acid aqueous solution to a 5% (v/v) formic acid and 1% (v/v) nitric acid final concentrations. A palladium solution was used as a chemical modifier. The instrumental conditions, such as carrier gas flow rate, RF power, pyrolysis and vaporization temperatures and argon internal flow rate during vaporization were optimized. The formic acid causes a slight decrease of the analytes signal intensities, but does not increase the signal of the mainly polyatomic ions (14N35Cl+, 14N12C+, 40Ar12C+, 13C37Cl+, 40Ar36Ar+, 40Ar35Cl+, 35Cl16O+, 40Ar18O+) that affect the analytes signals. The effect of charge transfer reactions, that could increase the ionization efficiency of some elements with high ionization potentials was not observed due to the elimination of most of the organic compounds during the pyrolysis step. External calibration with aqueous standard solutions containing 5% (v/v) formic acid allows the simultaneous determination of all analytes with high accuracy. The detection limits in the samples were between 0.01 (Co) and 850 μg kg−1 (Fe and Se) and the precision expressed by the relative standard deviations (RSD) were between 0.1% (Mn) and 10% (Ni). Accuracy was validated by the analysis of four certified reference biological materials of animal tissues (lobster hepatopancreas, dogfish muscle, oyster tissue and bovine liver). The recommended procedure avoids plasma instability, carbon deposit on the cones and does not require sample digestion. 相似文献