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181.
Martin Schulz-Dobrick Thorsten Metzroth Hans Wolfgang Spiess Jürgen Gauss Ingo Schnell 《Chemphyschem》2005,6(2):315-327
The structure of multiply hydrogen-bonded systems is determined with picometer accuracy by a combined solid-state NMR and quantum-chemical approach. On the experimental side, advanced 1H-15N dipolar recoupling NMR techniques are capable of providing proton-nitrogen distances of up to about 250 pm with an accuracy level of +/-1 pm for short distances (i.e., around 100 pm) and +/-5 pm for longer ones (i.e., 180 to 250 pm). The experiments were performed under fast magic-angle spinning, which ensures sufficient dipolar decoupling and spectral resolution of the 1H resonance lines. On the quantum-chemical side, the structures of the hydrogen-bonded systems were computationally optimised, yielding complete sets of nitrogen-proton and proton-proton distances, which are essential for correctly interpreting the experimental NMR data. In this way, nitrogen-proton distances were determined with picometer accuracy, so that vibrational averaging effects on dipole-dipole couplings need to be considered. The obtained structures were finally confirmed by the complete agreement of computed and experimental 'H and '5N chemical shifts. This demonstrates that solid-state NMR and quantum-chemical methods ideally complement each other and, in a combined manner, represent a powerful approach for reliable, high-precision structure determination whenever scattering techniques are inapplicable. 相似文献
182.
Beeke Gerken Christoph Mahr Jakob Stahl Tim Grieb Marco Schowalter Florian F. Krause Thorsten Mehrtens Lutz Mädler Andreas Rosenauer 《Particle & Particle Systems Characterization》2023,40(9):2300048
Hetero-contacts are interfaces between different materials at the nanoscale leading to novel functional properties. In hetero-aggregates, primary particles of at least two different materials are mixed at primary particle or cluster level. Double flame spray pyrolysis (DFSP) is a versatile technique for the controlled synthesis of such materials. Characterization of hetero-aggregates by scanning transmission electron microscopy (STEM) requires acquisition and evaluation of many aggregate images in order to derive statistically significant results. Usually, STEM energy dispersive X-ray spectroscopy (EDXS) is used to acquire elemental maps providing the material distribution of the primary particles within hetero-aggregates. However, the acquisition of a single EDXS map takes up to several minutes. For this reason, determination of material types of primary particles from the intensity in high-angle annular dark field STEM images alone is desirable. These images can be acquired within a couple of seconds. In the present work, a method is suggested which allows for achieving this objective. It can be applied to distinguish materials with a significant difference in their atomic number and hence sufficient material contrast in the STEM images. 相似文献
183.
Thorsten Daubenfeld 《Nachrichten aus der Chemie》2016,64(7-8):760-760
184.
Understanding the role of shuttle vibrations in pore fluid distribution is an essential task in the exploration of plant growth
in root modules aboard space flights. Results from experimental investigations are reported in this paper on the distribution
of immiscible fluid phases in glass beads under vibrations. Hexadecane, a petroleum compound immiscible with and lighter than
water, was used in the experiments. The higher freezing point of Hexadecane (18 °C) allowed the solidification of the entrapped
blobs in the presence of water in porous media, so that their size distribution can be obtained. van Genuchten function, commonly
used to express moisture retention curves, is found to be an adequate fit for blob size distribution at residual saturation.
The effect of vibrations on the fate (mobilization, stranding, or breakup) of a solitary ganglion in porous media was studied
using a network model. A mobility criterion considering viscous, gravity, and capillary forces was developed to determine
the fate of a solitary ganglion in a porous medium. It is concluded that the effect of vibrations is to increase the likelihood
of breakup and mobilization of blobs entrapped in porous media at residual saturation. The pore fluid distributions after
vibrations are less uniform than those before vibrations. 相似文献
185.
Metal‐Doped Nitrogenated Carbon as an Efficient Catalyst for Direct CO2 Electroreduction to CO and Hydrocarbons 下载免费PDF全文
Dr. Ana Sofia Varela Dr. Nastaran Ranjbar Sahraie Julian Steinberg Wen Ju Dr. Hyung‐Suk Oh Prof. Dr. Peter Strasser 《Angewandte Chemie (International ed. in English)》2015,54(37):10758-10762
This study explores the kinetics, mechanism, and active sites of the CO2 electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon‐based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO2 to CO/H2 mixtures outperforming a low‐area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low‐cost, abundant electrocatalysts for synthetic fuel production from CO2. 相似文献
186.
Dr. James R. Frost Prof. Dr. Stefan M. Huber Dr. Stefan Breitenlechner M. Sc. Christoph Bannwarth Prof. Dr. Thorsten Bach 《Angewandte Chemie (International ed. in English)》2015,54(2):691-695
Spirocyclic oxindoles undergo an enantioselective oxygenation reaction (nine examples; e.r. up to 97:3) upon catalysis by a chiral ruthenium porphyrin complex (1 mol %). The catalyst exhibits a lactam ring, which is responsible for substrate association through hydrogen bonds, and an active ruthenium center, which is in a defined spatial relationship to the oxygenation substrate. DFT calculations illustrate the perfect alignment of the active site with the reactive C? H bond and suggest—in line with the kinetic isotope effect—an oxygen rebound mechanism for the reaction. 相似文献
187.
Thorsten vomStein Tim denHartog Julien Buendia Spas Stoychev Jakob Mottweiler Carsten Bolm Jürgen Klankermayer Walter Leitner 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(20):5957-5961
Ruthenium–triphos complexes exhibited unprecedented catalytic activity and selectivity in the redox‐neutral C C bond cleavage of the β‐O‐4 lignin linkage of 1,3‐dilignol model compounds. A mechanistic pathway involving a dehydrogenation‐initiated retro‐aldol reaction for the C C bond cleavage was proposed in line with experimental data and DFT calculations. 相似文献
188.
189.
Germain Rousseaux Shahar Seifer Victor Steinberg Alexander Wiebel 《Experiments in fluids》2007,42(2):291-299
This work is an attempt to test the concept of the hydrodynamic charge (analogous to the electric charge in electromagnetism)
in the simple case of a coherent structure such as the Burgers vortex. We provide experimental measurements of both the so-called
Lamb vector and its divergence (the charge) by two-dimensional particles images velocimetry. In addition, we perform a Helmholtz–Hodge
decomposition of the Lamb vector in order to explore its topological features. We compare the charge with the well-known Q-criterion in order to assess its interest in detecting and characterizing coherent structure. Usefulness of this concept
in studies of vortex dynamics is demonstrated.
相似文献
Germain RousseauxEmail: |
190.
This work features an analysis for the acceleration technique DIIS that is standardly used in most of the important quantum
chemistry codes, e.g. in DFT and Hartree–Fock calculations and in the Coupled Cluster method. Taking up results from Harrison
(J Comput Chem 25:328, 2003), we show that for the general nonlinear case, DIIS corresponds to a projected quasi-Newton/secant method. For linear systems,
we establish connections to the well-known GMRES solver and transfer according (positive as well as negative) convergence
results to DIIS. In particular, we discuss the circumstances under which DIIS exhibits superlinear convergence behaviour.
For the general nonlinear case, we then use these results to show that a DIIS step can be interpreted as step of a quasi-Newton
method in which the Jacobian used in the Newton step is approximated by finite differences and in which the according linear
system is solved by a GMRES procedure, and give according convergence estimates. 相似文献