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141.
The physical origin of tip-induced motion of Cu adatoms on anisotropic Cu surfaces is investigated by means of total energy calculations which are based on three different semi-empirical potentials. The calculations show that for certain tip–adatom distances the activation barrier for the adatom to move towards the tip disappears completely, whereas the barrier in the opposite direction increases and the adatom experiences an attractive force towards the tip. The general trends do not depend on the shape and chemical nature of the tip, but quantitatively there appear differences. 相似文献
142.
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144.
Maria Bruma Corneliu Hamciuc Elena Hamciuc Frank Mercer Nataliya Belomoina Burkhard Schulz 《Macromolecular Symposia》1997,122(1):303-311
New heterocyclic polyamides have been synthesized by solution polycondensation of aromatic diamines containing phenylquinoxaline units with diacid chlorides having both imide and hexafluoroisopropylidene (6F) groups. These polymers are soluble in polar aprotic solvents, such as N-methylpyrrolidone (NMP) or N, N-dimethylformamide (DMF), and can be cast into flexible thin films from solutions. They show high thermooxidative stability with decomposition temperatures above 400°C and glass transition temperatures in the range of 225-300°C. The polymer films exhibit good chemical resistance towards diluted acids and good electrical insulating properties with dielectric constants in the range of 3.2–3.7. 相似文献
145.
Günter Reck Ingo Orgzall Burkhard Schulz Birgit Dietzel 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(11):o634-o635
The title compound [systematic name: dimethyl 4,4′‐(1,3,4‐oxadiazole‐2,5‐diyl)diphenylenedicarboxylate], C18H14N2O5, crystallizes under similar conditions in two different orthorhombic crystalline forms. In both forms, the molecule consists of two equivalent parts. In form 1, these parts are related by a twofold axis of space group Pbcn, and in form 2, by a mirror plane of space group Cmc21. The O atom of the oxadiazole ring occupies a special position on the twofold axis and on the mirror plane in forms 1 and 2, respectively. 相似文献
146.
Wolfgang Fraenk Thomas M. Klaptke Burkhard Krumm Peter Mayer Holger Piotrowski Martin Vogt 《无机化学与普通化学杂志》2002,628(4):745-750
The Lewis acid (C6F5)3B was reacted with ICN, NH2CN, C3N3X3 (X = H, Cl, F). The resulting Lewis acid base adducts ( 1—5 ) were fully characterized by analytic and spectroscopic methods. Additionally, the structures of the adducts 1—4 were determined by single crystal X‐ray analyses. It has been qualitatively shown, that a high field shift of the 11B as well as the 19F NMR resonances of the o‐F atoms of the C6F5‐substituents suggests a longer B—N distance. 相似文献
147.
Heiko Schirmer Michael Duetsch Frank Stein Thomas Labahn Burkhard Knieriem Armin de Meijere 《Angewandte Chemie (International ed. in English)》1999,38(9):1285-1287
A novel mode of reaction towards arylethynes is shown by the β-trimethylsilyl-substituted α,β-unsaturated Fischer carbene complexes 1 . A mixture of the isomeric, highly substituted spiro[4.4]nonatrienes 2 and 3 is formed by the formal insertion of three alkyne molecules and subsequent cyclization (see scheme). Such selective triple insertions of alkynes into ethenylcarbene complexes have not been previously observed. 相似文献
148.
Sandra M. Meyers Cornelia Laule Irene M. Vavasour Shannon H. Kolind Burkhard Mädler Roger Tam Anthony L. Traboulsee Jimmy Lee David K.B. Li Alex L. MacKay 《Magnetic resonance imaging》2009
This study compared region of interest (ROI) and voxel-based analysis (VBA) methods to determine the optimal method of myelin water fraction (MWF) analysis. Twenty healthy controls were scanned twice using a multi-echo T2 relaxation sequence and ROIs were drawn in white and grey matter. MWF was defined as the fractional signal from 15 to 40 ms in the T2 distribution. For ROI analysis, the mean intensity of voxels within an ROI was fit using non-negative least squares. For VBA, MWF was obtained for each voxel and the mean and median values within an ROI were calculated. There was a slightly higher correlation between Scan 1 and 2 for the VBA method (R2=0.98) relative to the ROI method (R2=0.95), and the VBA mean square difference between scans was 300% lower, indicating VBA was the most consistent between scans. For the VBA method, mean MWF was found to be more reproducible than median MWF. As the VBA method is more reproducible and gives more options for visualization and analysis of MWF, it is recommended over the ROI method of MWF analysis. 相似文献
149.
Burkhard Stahlmecke Sandra Wagener Christof Asbach Heinz Kaminski Heinz Fissan Thomas A. J. Kuhlbusch 《Journal of nanoparticle research》2009,11(7):1625-1635
The stability of agglomerates is not only an important material parameter of powders but also of interest for estimating the
particle size upon accidental release into the atmosphere. This is especially important when the size of primary particles
is well below the agglomerate size, which is usually the case when the size of primary particles is below 100 nm. During production
or airborne transportation in pipes, high particle concentrations lead to particle coagulation and the formation of agglomerates
in a size range of up to some micrometers. Binding between the primary particles in the agglomerates is usually due to van
der Waals forces. In the case of a leak in a pressurized vessel (e.g. reactor, transport pipe, etc.), these agglomerates can
be emitted and shear forces within the leak can cause agglomerates to breakup. In order to simulate such shear forces and
study their effect on agglomerate stability within the airborne state, a method was developed where agglomerate powders can
be aerosolized and passed through an orifice under various differential pressure conditions. First results show that a higher
differential pressure across the orifice causes a stronger fragmentation of the agglomerates, which furthermore seems to be
material dependent. 相似文献
150.