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951.
Dr. Pavlína Košinová Dr. Radek Gažák Prof. Jean‐Luc Duroux Prof. Roberto Lazzaroni Prof. Vladimír Křen Prof. Xavier Assfeld Dr. Patrick Trouillas 《Chemphyschem》2011,12(6):1135-1142
Natural polyphenols are known to be oxidized by free radicals, which partially explains the antioxidant properties of a number of these compounds. This oxidation may also be used to synthesise new compounds of biological interest, for example, dimers. The present theoretical study describes the existing experimental evidence showing that silybin and dehydrosilybin [natural polyphenols isolated from milk thistle (Silybum marianum)] form dimers regioselectively. Based on DFT calculations, thermodynamic and kinetic values were computed in order to better understand the physicochemical behaviour of these dimerisation processes. Calculations showed that after H‐atom transfer (from polyphenol to radical), dimerisation could proceed in two steps: 1) bond formation and, when possible, 2) tautomerisation reorganisation. The former step is the limiting step, while the latter is crucial for the process to be thermodynamically favourable (ΔG<0). If this rearrangement is impossible then dimerisation is not feasible, or at least becomes a minor process (e.g., dehydrosilybin dimerisation after H‐atom abstraction from the 3‐OH group). This explains the regioselectivity of polyphenol dimerisation. 相似文献
952.
Trithiaazapentalene derivatives were prepared by the reaction of 2-alkylidene-4-oxothiazolidines with Lawesson's reagent. They are classified as two structurally different trithiaazapentalene compounds that have different contributions of monocyclic 1,2-dithiole and 1,2,4-dithiazole structures and degrees of aromaticity of the bicyclic trithiaazapentalene system. The electron-donating ability of substituents at the C(5) position of the trithiaazapentalene system is recognized as the main cause for changes in π-electron distribution. This is the first complete study of substituent effects on the structure of trithiapentalenes. 相似文献
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955.
Bojan Janković 《国际化学动力学杂志》2010,42(11):641-658
The new procedure for identification of the effective distribution function for determination of the distributed activation energy models, which is based on the Bayesian statistics, has been established. The five different continuous probability functions (exponential, logistic, normal, gamma and Weibull probability functions (the extended set of distributions)) were used for searching the most appropriate reactivity model for two heterogeneous processes: (a) the isothermal reduction process of nickel oxide under hydrogen atmosphere and (b) the isothermal degradation process of bisphenol‐A polycarbonate (Lexan) under nitrogen atmosphere. Using the Bayes weights, it was shown that for both processes, the most suitable distributed reactivity model is the Weibull distribution model. The kinetic parameters (ln A, Ea) attached with the Weibull distribution model were calculated for both investigated processes, using three different computational methods (the maximum likelihood method (MLM), the nonlinear regression analysis (NRA), and the posterior mean (the expected value of scale parameter η, E(η)). It was shown that there is an excellent agreement between the values of kinetic parameters calculated by the MLM, NRA, and E(η) approaches. Using Bayes weights, it is possible to discriminate between different probability models and to quantify how well a distribution fits the experimental data. For the formal reactivity model comparison, the use of the (nonnormalized) Jeffreys prior is recommended. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 641–658, 2010 相似文献
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Miroslav Petrović 《Graphs and Combinatorics》1999,15(2):221-232
In this paper, by means of computer checking, all simple graphs with at most two nonnegative eigenvalues, and all minimal
simple graphs with exactly two (respectively, three) nonnegative eigenvalues are determined.
Received: April 5, 1996 / Revised: May 2, 1997 相似文献
958.
J. Kierzek B. Małożewska-Bućko P. Bukowski J. L. Parus A. Ciurapiński S. Zaraś B. Kunach K. Wiland 《Journal of Radioanalytical and Nuclear Chemistry》1999,240(1):39-45
Gamma-ray spectrometry (GS), energy dispersive X-ray fluorescence (EDXRF) analysis methods and wavelength dispersive X-ray
fluorescence (WDXRF) were applied for the studies of some coal components, e.g., sulphur, light and heavy metal element concentrations
and naturally occurring radioactive isotope contents. Hundred fifty coal samples originating mostly from eight different coal
mines from Upper Silesian Coal Basin and 150 samples of ash obtained from these coal samples in laboratory by total combustion
at final temperature of 820°C, were analyzed. Such comparitive analyses can be helpful in selection of most suitable kind
of coal for burning in electrical power and heat plants to minimize the environmental pollution. 相似文献
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