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981.
We investigate the so-called critical 2nth-order Sturm–Liouville difference operators and associated symmetric banded matrices. We show that arbitrarily small (in a certain sense) negative perturbation of a non-negative critical operator leads to an operator which is no longer non-negative. 相似文献
982.
Roman Šimon Hilscher Petr Zemánek 《Journal of Difference Equations and Applications》2013,19(1):84-117
In this paper we develop the Weyl–Titchmarsh theory for discrete symplectic systems with general linear dependence on the spectral parameter. We generalize and complete several recent results concerning these systems, which have the spectral parameter only in the second equation. Our new theory includes characterizations of the Weyl discs and Weyl circles, their limiting behaviour, properties of square summable solutions including the analysis of the exact number of linearly independent square summable solutions and limit point/circle criteria. Some illustrative examples are also provided. 相似文献
983.
984.
Evangelia Kalligiannaki Markos A. Katsoulakis Petr Plecháč Dionisios G. Vlachos 《Journal of computational physics》2012,231(6):2599-2620
In this work we propose a hierarchy of Markov chain Monte Carlo methods for sampling equilibrium properties of stochastic lattice systems with competing short and long range interactions. Each Monte Carlo step is composed by two or more sub-steps efficiently coupling coarse and finer state spaces. The method can be designed to sample the exact or controlled-error approximations of the target distribution, providing information on levels of different resolutions, as well as at the microscopic level. In both strategies the method achieves significant reduction of the computational cost compared to conventional Markov chain Monte Carlo methods. Applications in phase transition and pattern formation problems confirm the efficiency of the proposed methods. 相似文献
985.
Svagera Z Hanzlíková D Simek P Hušek P 《Analytical and bioanalytical chemistry》2012,402(9):2953-2963
Four disulfide-reducing agents, dithiothreitol (DTT), 2,3-dimercaptopropanesulfonate (DMPS), and the newly tested 2-mercaptoethanesulfonate
(MESNA) and Tris(hydroxypropyl)phosphine (THP), were investigated in detail for release of sulfur amino acids in human plasma.
After protein precipitation with trichloroacetic acid (TCA), the plasma supernatant was treated with methyl, ethyl, or propyl
chloroformate via the well-proven derivatization–extraction technique and the products were subjected to gas chromatographic–mass
spectrometric (GC–MS) analysis. All the tested agents proved to be rapid and effective reducing agents for the assay of plasma
thiols. When compared with DTT, the novel reducing agents DMPS, MESNA, and THP provided much cleaner extracts and improved
analytical performance. Quantification of homocysteine, cysteine, and methionine was performed using their deuterated analogues,
whereas other analytes were quantified by means of 4-chlorophenylalanine. Precise and reliable assay of all examined analytes
was achieved, irrespective of the chloroformate reagent used. Average relative standard deviations at each analyte level were
≤6%, quantification limits were 0.1–0.2 μmol L−1, recoveries were 94–121%, and linearity was over three orders of magnitude (r
2 equal to 0.997–0.998). Validation performed with the THP agent and propyl chloroformate derivatization demonstrated the robustness
and reliability of this simple sample-preparation methodology. 相似文献
986.
987.
Fryčák P Jirkovský J Ranc V Bednář P Havlíček V Lemr K 《Journal of mass spectrometry : JMS》2012,47(6):720-726
Atmospheric pressure chemical ionization is known for producing unusual artifacts of the ionization process in some cases. In this work, processes occuring in atmospheric pressure chemical ionization/MS of orotic acid that afforded ions accompanying protonated and deprotonated orotic acid molecules in the spectra were studied. Two processes ran in parallel in the ion source: decarboxylation of neutral orotic acid and collision-induced dissociation of its protonated or deprotonated form. A procedure discerning pre-ionization decomposition and post-ionization dissociation by manipulating ion source parameters was proposed. Experiments with isotopically labeled solvents confirmed ion-molecule reactions of the product of collision-induced dissociation of protonated orotic acid with solvent molecules in the ion source and even under vacuum in the ion trap. 相似文献
988.
Luk Fojt Petr Klapetek Ludk Strak Vladimír Vetterl 《Micron (Oxford, England : 1993)》2009,40(8):918-922
Atomic force microscopy was used to distinguish changes in morphology of bacteria induced by 50 Hz 10 mT magnetic field exposure. It is known that alternating magnetic field exposure causes decrease of viability of different bacterial strains. Previously we found that the viability of rod-like bacteria exposed to magnetic field decreased twice more in comparison with the spherical ones. Motivated by this fact we carried out this study with bacterial cells of both shapes. We used Escherichia coli (rod-like) and Paracoccus denitrificans (spherical) bacteria. As a result we have not observed any change in bacterial morphology neither of rod-like nor of spherical bacteria after 1 h, 50 Hz and 10 mT magnetic field exposure. 相似文献
989.
Emanuel Makrlík Petr Vaňura Jan Budka 《Monatshefte für Chemie / Chemical Monthly》2009,287(1):583-585
Abstract
The stability constant of the dibenzo-18-crown-6·H3O+ cationic complex species dissolved in nitrobenzene saturated with water has been determined from extraction experiments in the two-phase water–nitrobenzene system and from γ-activity measurements. Various structures of protonated dibenzo-18-crown-6 are discussed. 相似文献990.
Petr Svoboda Dagmar Svobodova Dagmar Merinska Toshiaki Ougizawa 《European Polymer Journal》2009,45(8):2434-2442
A blend of poly(ε-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) containing 27.5 wt% of acrylonitrile having the critical composition (80/20 PCL/SAN) was studied. This PCL/SAN blend having a lower critical solution temperature (LCST) phase boundary at 122 °C offered an excellent opportunity to investigate, firstly the kinetics of phase separation above LCST (125-180 °C), and secondly the kinetics of phase dissolution below LCST (50-115 °C). The blend underwent a temperature-jump above LCST where spinodal decomposition (SD) proceeded, yielding a regularly phase-separated structure (SD structure). Then, it was quenched to the temperatures below LCST when the phase dissolution proceeded. Optical microscopy was used to observe the spinodal decomposition qualitatively while light scattering was used to characterize the phase separation and phase dissolution quantitatively. It was found that during phase dissolution the peak maximum moved towards a smaller angle (wavelength of concentration fluctuations increased) while the peak intensity decreased. This behavior was explained by a model. Also it was found that the fastest phase dissolution kinetics at 80 °C, which was characterized by an apparent diffusion coefficient, was about 10 times slower than the kinetics of phase separation at 180 °C. 相似文献