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561.
Manuela Ramos Silva Vnia M. Moreira Cludia Cardoso Ana Matos Beja Jorge A. R. Salvador 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(9):o529-o531
In the title compounds, C22H29ClO3, (I), and C21H29ClO2, (II), respectively, the B rings adopt a half‐chair conformation and the D rings adopt an envelope conformation. A twist of the steroid skeleton of both compounds is observed. There is a positional disorder of the acetoxy group of (II), with the terminal atoms disordered over two positions with near equal occupancy. Quantum‐mechanical ab initio calculations using a molecular orbital Hartree–Fock method were performed for the isolated molecules, thus allowing the distinction within the structural features of these two androstane derivatives of which characteristics are intrinsic to the molecules and which are due to packing effects. The skeletal twisting was found to be innate to the molecules, while the acetoxy disorder is due to packing effects. 相似文献
562.
The derivatives of 1,4-benzodioxan are found widely spread in nature and have great biomedical importance. The present work reports an experimental and computational study on the thermochemistry of 1,4-benzodioxan and several of its 6-R derivatives in the gaseous phase, at T = 298.15 K. Our current results were obtained from measurements of combustion energies, at T = 298.15 K, using a static bomb calorimeter. The standard molar enthalpies of vaporization/sublimation were measured by Calvet microcalorimetry and corrected to T = 298.15 K. Additionally, estimates were performed of the enthalpies of formation of all the studied compounds in the gas phase, using DFT and other more accurate correlated calculations, together with appropriate isodesmic or homodesmic reactions. There is a reasonable agreement between computational and experimental results. 相似文献
563.
Bernhard Beckermann Ana C. Matos Franck Wielonsky 《Journal of Computational and Applied Mathematics》2008,219(2):329
Recently, Brezinski has proposed to use Wynn's ε-algorithm in order to reduce the Gibbs phenomenon for partial Fourier sums of smooth functions with jumps, by displaying very convincing numerical experiments. In the present paper we derive analytic estimates for the error corresponding to a particular class of hypergeometric functions, and obtain the rate of column convergence for such functions, possibly perturbed by another sufficiently differentiable function. We also analyze the connection to Padé–Fourier and Padé–Chebyshev approximants, including those recently studied by Kaber and Maday. 相似文献
564.
Peroxidase immobilized on Amberlite IRA-743 resin for on-line spectrophotometric detection of hydrogen peroxide in rainwater 总被引:1,自引:0,他引:1
The importance of atmospheric hydrogen peroxide (H2O2) in the oxidation of SO2 and other compounds has been well established. A spectrophotometric method for the determination of hydrogen peroxide in rainwater is proposed. This method is based on selective oxidation of hydrogen peroxide using an on-line tubular reactor containing peroxidase immobilized on Amberlite IRA-743 resin. The hydrogen peroxide in the presence of phenol, 4-aminoantipyrine and peroxidase, produces a red compound (λ = 505 nm). Beer's law is obeyed in a concentration range of 1–100 μmol l−1 hydrogen peroxide with an excellent correlation coefficient (r = 0.9991), at pH 7.0, with a relative standard deviation (R.S.D.) <2%. The detection limit of the method is 0.7 μmol l−1 (4.8 ng of H2O2 in a 200 μl sample). Measurements of hydrogen peroxide in rain samples were carried out over the period from November 2003 to January 2005, in the central area of the Juiz de Fora city, Brazil. The concentration of H2O2 varied from values lower than the detection limit to 92.5 μmol l−1. The effects of the presence of nonseasalt (NSS) SO42−, NO3− and H+ in the concentration of hydrogen peroxide in the rainwater had been evaluated. The average concentrations of H2O2, NO3−, NSS SO42− and SO42− are 23.4, 18.9, 7.9 and 10.3 μmol l−1, respectively. The pH values for 82% of the collected samples are greater than 5.0. The spectrophotometeric method developed in this work that uses enzyme immobilized on the resin ion-exchange compared with the amperometric method did not present any significant difference in the results. 相似文献
565.
W. K. Yoshito J. R. Matos V. Ussui D. R. R. Lazar J. O. A. Paschoal 《Journal of Thermal Analysis and Calorimetry》2009,97(1):303-308
A porous nickel–8 mol% yttria stabilized zirconia (Ni–8YSZ) composite, used as anode for solid oxide fuel cell, was obtained
by reduction of NiO–8YSZ cermet. The first goal was the evaluation of the temperature effect of powder processing by thermogravimetry.
In addition, the influence of porosity in the reduction kinetic of the sample sintered at 1450 °C was evaluated. The final
porosity produced in NiO–8YSZ composite by pore former was 30.4 and 37.9 vol.%, respectively, for 10 and 15 mass% of corn
starch. The sample with 15 mass% of corn starch promotes a reduction rate almost twice higher than sample with 10 mass% of
corn starch. The porosity introduced by the reduction of NiO was 23 vol.%. 相似文献
566.
Bruno Matos Paz Dr. Hao Jiang Prof. Dr. Karl Anker Jørgensen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(5):1846-1853
The stereochemical outcome of most aminocatalytic transformations is determined by the steric‐shielding or hydrogen‐directing approach. However, in recent years several reactions have appeared in which the stereochemical outcome is beyond these two approaches. This Concept article will highlight such reactions and postulate that the stereochemical outcome can be accounted for by electrostatic interactions between the catalyst‐bound substrate and the reagent in the transition state. 相似文献
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