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71.
72.
The aim of this article is to propose a simple analytical model that can describe the isothermal crystallization process in materials when the formation of a stable crystalline phase is preceded by the formation of a metastable phase. This model explains deviations from the well-known Johnson-Mehl-Avrami-Kolmogorov kinetics theory and predicts the three slopes in Avrami’s plot. The model predictions were compared with experimental results obtained from X-ray measurements in the chalcogenide glasses with composition of Ge2Sb2Te5 (thin films) and in aqueous solutions of methylhydrazine monohydrate during isothermal phase transformations. In order to validate the proposed model to represent experimental results, a computer program was developed. This program uses experimental data from measurements of the total volume fraction at different times during isothermal transformations and fits the model parameters that best represent the kinetic behavior of the system. 相似文献
73.
Dr. Claudio Dalvit Dr. Anna Vulpetti 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(22):7592-7601
It is known that strong hydrogen‐bonding interactions play an important role in many chemical and biological systems. However, weak or very weak hydrogen bonds, which are often difficult to detect and characterize, may also be relevant in many recognition and reaction processes. Fluorine serving as a hydrogen‐bond acceptor has been the subject of many controversial discussions and there are different opinions about it. It now appears that there is compelling experimental evidence for the involvement of fluorine in weak intramolecular or intermolecular hydrogen bonds. Using established NMR methods, we have previously characterized and measured the strengths of intermolecular hydrogen‐bond complexes involving the fluorine moieties CH2F, CHF2, and CF3, and have compared them with the well‐known hydrogen‐bond complex formed between acetophenone and the strong hydrogen‐bond donor p‐fluorophenol. We now report evidence for the formation of hydrogen bonds involving fluorine with significantly weaker donors, namely 5‐fluoroindole and water. A simple NMR method is proposed for the simultaneous measurement of the strengths of hydrogen bonds between an acceptor and a donor or water. Important implications of these results for enzymatic/chemical reactions involving fluorine, for chemical and physical properties, and for ligand/protein 19F NMR screening are analyzed through experiments and theoretical simulations. 相似文献
74.
Irati Lapuerta Dr. Silvia Vera Prof. Mikel Oiarbide Prof. Claudio Palomo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(21):7229-7237
Direct coupling of enolizable aldehydes with C‐alkynyl imines is realized affording the corresponding propargylic Mannich adducts of syn configuration, thus complementing previous methods that gave access to the anti‐isomers. The combination of proline and a urea Brønsted base cocatalyst is key for the reactions to proceed under very mild conditions (3–10 mol % catalyst loading, dichloromethane as solvent, ?20 °C, 1.2 molar equivalents of aldehyde) and with virtually total stereocontrol (syn/anti ratio up to 99:1; ee up to 99 %). Some possibilities of further chemical elaboration of adducts are also briefly illustrated. 相似文献
75.
76.
77.
Sources of small quantities of hydrogen chloride (nmol s-1 and pmol s-1), based on molecular diffusion through a membrane from a reservoir of strong hydrochloric acid, are described. Accurate procedures of standardizing and checking the sources are reported. Extension to other reagents is possible. This standardized source provides a reliable means for the preparation of gaseous reference mixtures as well as standard solutions of hydrochloric acid. 相似文献
78.
Giovanni C. Petrucelli Mauricio A. Meirinho Thais R. Macedo Claudio Airoldi 《Thermochimica Acta》2006,450(1-2):16-21
Synthesized hydrated lamellar acidic crystalline magadiite (H2Si14O29·2H2O) nanocompound was used as host for intercalation of polar n-alkylmonoamine molecules of the general formula H3C(CH2)nNH2 (n = 1–6) in aqueous solution. The original interlayer distance (d) of 1500 pm, determined by X-ray powder diffraction patterns, increases after intercalation. The values correlated with the number of aliphatic amine carbon (nc) atoms: d = [(1312 ± 11) + (21 ± 2)]nc. The amount of intercalated amines (Ns), decreased as nc increased: Ns = [(5.82 ± 0.04) − (0.45 ± 0.01)]nc. The acidic layered nanocompound was calorimetrically titrated with the amines and the thermodynamic data gave exothermic values for all guest molecules, as shown by the correlation: ΔintH = −[(24.45 ± 0.49) − (1.91 ± 0.10)]nc and d = [(1576 ± 16) − (10.8 ± 1.0)]ΔintH. The negative values of the Gibbs energies and the positive entropies also presented the correlations: ΔintG = −[(22.8 ± 0.2) − (0.2 ± 0.1)]nc and ΔintS = [(6 ± 1) + (5 ± 1)]nc, respectively. 相似文献
79.
Machulek A Vautier-Giongo C Moraes JE Nascimento CA Quina FH 《Photochemistry and photobiology》2006,82(1):208-212
The photo-Fenton reaction (Fe2+/Fe3+, H2O2, UV light) is strongly inhibited by high concentrations of added chloride ion. In this work, the effect of added chloride ion on the photocatalytic step that converts Fe(III) back to Fe(II) is studied by nanosecond laser flash photolysis over a wide range of pH (1.0-3.3) and concentrations of Fe(III) (0.1-1.0 mM) and chloride ion (0.05-0.75 M). An explicit mechanistic model based on the preferential formation of the less-reactive Cl2*- radical anion via two routes (competitive photolysis of the iron(III)-chloride complex to chlorine atoms instead of the desired hydroxyl radical and pH-dependent scavenging of the hydroxyl radical by chloride ion) is proposed. This model, which fits the laser flash photolysis data for the production and decay of Cl2*- over the entire range of conditions investigated, suggests that inhibition of the photocatalytic step of the photo-Fenton process in the presence of chloride ion can be circumvented by maintaining the pH of the medium at or slightly above 3.0 throughout the reaction. 相似文献
80.
Espinoza M Olea-Azar C Speisky H Rodríguez J 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2009,71(5):1638-1643
Four different types of Chilean wines (Cabernet Sauvignon, Merlot, Carmenere and Syrah) were selected and examined in their free radical scavenging capacities by electron spin resonance (ESR) and spectrophotometric methods. The free radical scavenging properties were evaluated against 2,2-diphenyl-1-picrylhydrazyl (DPPH*) radical, 2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy (Galvinoxyl) radical and hydroxyl radical (HO*). The possible effect on these scavenging properties of added transition metals to these wines was evaluated. Among the wines evaluated, Cabernet Sauvignon was the one with the highest activity against all radicals tested. The presence of added copper or iron to wines resulted in a reduced free radical scavenging capacity for all type of wines studied. The formation of redox inactive complexes between polyphenols of wine and transition metals is the possible cause of this reduction in antioxidant activity. 相似文献