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71.
Summary This study was aimed at optimizing the separation of non-ionic surfactants, resulting from the condensation of ethylene oxide with natural fatty alcohols in the C16 and C18 range (saturated and unsaturated), and presenting a high degree of condensation, i.e. 20 and 25 ethylene oxide units (Brij 99 and KM 25). The cation exchange stationary phase is a partially ionized silica, conditioned in different ways. We have studied the influence of the nature of cations on the separation selectivity. Cations studied included alkali metals (Li+, Na+, K+, Rb+, Cs+), alkaline earth metals (Mg2+, Ca2+, Sr2+, Ba2+), transition metals (Zn2+, Cd2+, Hg2+, Pb2+, Cu2+), and quaternary ammonium (NH
4
+
, NMe
4
+
) ions as well as a proton (acetic acid). The influences of ionic strength, pH and addition of cosolvent were examined. A study of the influence of temperature on the system selectivity evidenced a strong interdependance of these two parameters. The optimized conditions [mobile phase: CH3CN/H2O (92/8), pH 7.4, sodium acetate 5.10–3 M; temperature gradient between 25 and 50°C] allowed for the first time the distributions of Brij 99 and KM 25 to be obtained. 相似文献
72.
Summary This paper is devoted to the numerical analysis of a bidimensional two-phase Stefan problem. We approximate the enthalpy formulation byC
0 piecewise linear finite elements in space combined with a semi-implicit scheme in time. Under some restrictions related to the finite element mesh and to the timestep, we prove positivity, stability and convergence results. Various numerial tests are presented and discussed in order to show the accuracy of our scheme.This work is supported by the Fonds National Suisse pour la Recherche Scientifique. 相似文献
73.
74.
75.
We review here the results of our investigations concerning chaotic atomic scattering in the presence of a laser field. Particular emphasis is put on the existence of classical stable resonance structures, induced by the intense laser field, which are embedded in the field-free continuum. We show that phase space structures in the vicinity of a resonance island play an important role in the chaotic scattering behavior and form the basis for a mechanism to enhance the lifetimes of the collisional partners. Quantum calculations, based on a wave packet propagation method, show that quantum solutions are strongly influenced by the classical phase space structures. More specifically, a wave packet is found to spread differently in the regular and chaotic regions; in the latter case it spreads exponentially with time until saturation occurs, defining the saturation time. We also investigate the dependence of the spreading rates in both the regular and chaotic regimes. Calculations with an ensemble of classical trajectories are also presented to further illustrate the smoothing effects of varying. 相似文献
76.
77.
The solid-phase peptide synthesis of an octapeptide contained in hen egg-white lysozyme (sequence Cys (residue 64) to Gly (residue 71)) is reported. Each step of the synthesis was verified by amino acid analysis. Three main reaction products were obtained. The octapeptide (50% of the reaction products) was purified by two chromatographies on Dowex 50 × 2. The purified peptide was digested by an aminopeptidase and degradated by means of Edman' s method; this latter procedure has shown that the purified octapeptide still contained around 10% of a shorter peptide. 相似文献
78.
J T Arnason B Guèrin M M Kraml B Mehta R W Redmond J C Scaiano 《Photochemistry and photobiology》1992,55(1):35-38
Sanguinarine, a commercial drug exhibiting antimicrobial and antitumor properties, was studied with respect to its basic photochemical characteristics and also with regard to its phototoxicity to mosquito larvae (Aedes atropalpus). Sanguinarine proved to be clearly phototoxic to larvae, with an LD50 of 0.096 mg/mL with near UV exposure as compared with 23.3 mg/mL without. Flash photolysis experiments enabled the study of the triplet state of sanguinarine to be undertaken. Quenching by oxygen occurs with a rate constant of 6 x 10(9) M-1s-1 and time-resolved emission studies indicate that sanguinarine produces a significant amount of singlet oxygen (phi delta = 0.16) as does the isoquinoline alkaloid, berberine (phi delta = 0.25). These values represent the first direct quantitative measurements of photosensitization parameters of these compounds. Additionally, sanguinarine exhibits efficient electron donation properties, undergoing reaction with methyl viologen with a rate constant greater than 10(10) M-1s-1, but is a poor electron acceptor. Phototoxicity of sanguinarine can thus be explained in terms of its photosensitization properties. 相似文献
79.
In three or more dimensions (3) it is proved that if the correlations decay faster than |x|-(-20) then gauge symmetry breaking is excluded. In one and two dimensions (=1 or 2) the gauge symmetry is always preserved. 相似文献
80.
M. Devillers J. Ladrière D. Apers 《Journal of Radioanalytical and Nuclear Chemistry》1987,109(2):439-448
The incorporation of carrier-free57Co2+ by coprecipitation in Fe and Mn oxalato compounds is characterized by a double radioactive tracer method. This is achieved by labelling the host compound with a short-lived tracer,52Fe or52Mn. Activity ratios are measured at several stages, during the precipitation and, afterwards, during the dissolution of the solid. In any case, results indicate very large incorporation yields of57Co in the precipitate, independently from the intrinsic precipitation yield of the host compound. The observation of a continuous distribution of dopant atoms in the matrix in coprecipitation and dissolution experiments allows to rule out the adsorption of the doping species on a pre-existing precipitate of the host compound and accounts for isomorphous replacement. Incorporation of57Co in Fe(II) oxalate follows a logarithmic distribution law, giving rise to a concentration gradient of dopant species in the precipitate. Dopant atoms are found to be concentrated in the internal layers of the crystals. 相似文献