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111.
A cluster of many small holes with negative imaginary surface impedances may generate a negative refraction index 下载免费PDF全文
Ahmed Alsaedi Bashir Ahmad Durga Prasad Challa Mokhtar Kirane Mourad Sini 《Mathematical Methods in the Applied Sciences》2016,39(13):3607-3622
We deal with the scattering of an acoustic medium modeled by an index of refraction n varying in a bounded region Ω of and equal to unity outside Ω. This region is perforated with an extremely large number of small holes Dm's of maximum radius a, a << 1, modeled by surface impedance functions. Precisely, we are in the regime described by the number of holes of the order M:=O(aβ ? 2), the minimum distance between the holes is d ~ at, and the surface impedance functions of the form λm~λm,0a?β with β > 0 and λm,0 being constants and eventually complex numbers. Under some natural conditions on the parameters β,t, and λm,0, we characterize the equivalent medium generating approximately the same scattered waves as the original perforated acoustic medium. We give an explicit error estimate between the scattered waves generated by the perforated medium and the equivalent one, respectively, as a→0. As applications of these results, we discuss the following findings:
112.
Ramana CV Nageswara Reddy C Gonnade RG 《Chemical communications (Cambridge, England)》2008,(27):3151-3153
Herein we describe a one-step assembly of structurally complex small molecules representing the central skeleton of integrastatins by employing a simple pinacol transform. 相似文献
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Attachment of phosphites to styrene copolymers is described which are used as rhodium hydroformylation catalysts. The influence of the chain loading on the activity and complex formation of three types of copolymer-bound rhodium hydroformylation catalysts in comparison with their low molecular weight analogues has been studied. The catalytic activity of the polystyrene-bound system with the most bulky phosphite, the first system studied, is identical to that of the low molecular weight analogue. The catalysts show a high activity towards the hydroformylation of the otherwise unreactive cyclooctene. It was found that only one phosphite is coordinated to the rhodium complex in its active form. An equilibrium between this complex and an inactive complex without phosphite ligands prohibits its use in continuous flow reactors. Secondly, as polymer support a perfectly random copolymer of styrene and less bulky 3,3′,5,5′-tetra-tert-butylbiphenyl-2,2′-diyl p-vinylphenylphosphite was used. The chain loading α of this copolymer with phosphite ligands has a large influence on the complex formation of the catalyst. With high chain loadings moderately active bis-phosphite catalysts are formed. Low chain loadings give active, easily accessible, monophosphite complexes. The active species in the hydroformylation of sterically hindered alkenes is a mono-phosphite rhodium complex. The activity of the copolymer-bound catalyst towards the hydroformylation of cyclooctene is found to be as high as the activity of its low molecular weight analogue. For styrene, this polymer catalyst yields a slower catalyst than the low-molecular weight analogue. The third part demonstrates that silica-grafted polymer-bound phosphite modified rhodium complexes can be used in continuous flow reactors. The hydroformylation of styrene was carried out at moderate pressure (pCO/H2 = 3 MPa) and temperature (T = 100°C), yielding constant conversions over a period of at least ten days. These positive results were obtained in benzene as a solvent and for a ligand to rhodium ratio of only four. 相似文献
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R. Buter Y. Y. Tan G. Challa 《Journal of polymer science. Part A, Polymer chemistry》1973,11(11):2975-2989
Methyl methacrylate (MMA) was polymerized by radical initiation at 25°C or 35°C in various solvents in the presence of stereoregular poly(methyl methacrylate) (PMMA). The occurrence of stereospecific replica polymerization appeared to be related to the capability of stereoassociation of isotactic and syndiotactic PMMA. The solvents can be roughly divided into three types. Type A solvents are polar solvents, which promote stereoassociation resulting in gelation and precipitation. Examples are dimethylformamide, dimethyl sulfoxide, and acetone. Type B solvents are nonpolar aromatic solvents like benzene and toluene, wherein stereoassociation is weaker but still leads to gelation. Type C solvents are very good solvents, in which stereoassociation does not occur. Chloroform and dichloromethane belong to this class. In solvents of type A as well as type B, polymerization in the presence of i-PMMA as a polymer matrix was syndiospecific. However, in the presence of s-PMMA as a polymer matrix the polymerization was isospecific only in type A solvents. The syndiotactic or isotactic triad contents of the polymer formed could be as high as ca. 90% at low conversions. In solvents of type C, polymerization in the presence of stereoregular PMMA proceeds according to a normal radical mechanism. Syndiotacticity was always less than 70%. Stereocomplexes formed in situ during replica polymerization were partly crystalline as detected by x-ray diffraction. The highest crystallinity was detected in those formed in type A solvents. 相似文献
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Monodisperse hydrogel microspheres by forced droplet formation in aqueous two-phase systems 总被引:1,自引:0,他引:1
Ziemecka I van Steijn V Koper GJ Rosso M Brizard AM van Esch JH Kreutzer MT 《Lab on a chip》2011,11(4):620-624
This paper presents a method to form micron-sized droplets in an aqueous two-phase system (ATPS) and to subsequently polymerize the droplets to produce hydrogel beads. Owing to the low interfacial tension in ATPS, droplets do not easily form spontaneously. We enforce the formation of drops by perturbing an otherwise stable jet that forms at the junction where the two aqueous streams meet. This is done by actuating a piezo-electric bending disc integrated in our device. The influence of forcing amplitude and frequency on jet breakup is described and related to the size of monodisperse droplets with a diameter in the range between 30 and 60 μm. Rapid on-chip polymerization of derivatized dextran inside the droplets created monodisperse hydrogel particles. This work shows how droplet-based microfluidics can be used in all-aqueous, surfactant-free, organic-solvent-free biocompatible two-phase environment. 相似文献
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The surface temperature and surface mole fractions are calculated for a catalytic hydrogen oxidation reaction over a Pt/Al2O3 catalyst pellet. The thermodynamics of irreversible processes was used in order to ensure the correct introduction of coupled heat and mass transfer. Two pathways, one using the 4 x 4 resistivity matrix and the other using a simplified effective conductivity matrix, were proven to yield equivalent results. By using expressions for the thermal diffusion coefficients, heats of transfer, and the Maxwell-Stefan diffusion coefficients given in the literature, available experimental data could be reproduced. The Dufour effect was found to be negligible for the prediction of the surface temperature. Neglecting the Soret effect would increase the predicted value of the surface temperature significantly-more than 30 K out of an average of about 400 K. It is found that the reaction rate can be used to predict the surface temperature. 相似文献