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991.
992.
Methods of analyzing and interpreting velocity-field data (both two- and three-dimensional) to understand the kinematics, dynamics, and scales of turbulence are discussed. Reynolds decomposition and vorticity are traditionally used; however, several other methods, including Galilean (constant convection velocity) and LES decompositions (low-pass filtering), in conjunction with critical-point analysis of the local velocity gradient tensor, reveal more about the structure of turbulence. Once the small-scale structures have been identified, it is necessary to assess their importance to the overall dynamics of the turbulence by visualizing the motions they induce and the stresses they impose both on other small-scale vortices and on the larger-scale field. 相似文献
993.
994.
Elisa Biasin Tim B. van Driel Gianluca Levi Mads G. Laursen Asmus O. Dohn Asbjørn Moltke Peter Vester Frederik B. K. Hansen Kasper S. Kjaer Tobias Harlang Robert Hartsock Morten Christensen Kelly J. Gaffney Niels E. Henriksen Klaus B. Møller Kristoffer Haldrup Martin M. Nielsen 《Journal of synchrotron radiation》2018,25(2):306-315
Time‐resolved X‐ray scattering patterns from photoexcited molecules in solution are in many cases anisotropic at the ultrafast time scales accessible at X‐ray free‐electron lasers (XFELs). This anisotropy arises from the interaction of a linearly polarized UV–Vis pump laser pulse with the sample, which induces anisotropic structural changes that can be captured by femtosecond X‐ray pulses. In this work, a method for quantitative analysis of the anisotropic scattering signal arising from an ensemble of molecules is described, and it is demonstrated how its use can enhance the structural sensitivity of the time‐resolved X‐ray scattering experiment. This method is applied on time‐resolved X‐ray scattering patterns measured upon photoexcitation of a solvated di‐platinum complex at an XFEL, and the key parameters involved are explored. It is shown that a combined analysis of the anisotropic and isotropic difference scattering signals in this experiment allows a more precise determination of the main photoinduced structural change in the solute, i.e. the change in Pt—Pt bond length, and yields more information on the excitation channels than the analysis of the isotropic scattering only. Finally, it is discussed how the anisotropic transient response of the solvent can enable the determination of key experimental parameters such as the instrument response function. 相似文献
995.
Jessica A. Palmer Ricardo Mejia-Alvarez James L. Best Kenneth T. Christensen 《Experiments in fluids》2012,52(3):809-829
Barchan dunes are crescentic planform-shaped dunes that are present in many natural environments, and may occur either in
isolation or in groups. This study uses high-resolution particle-image velocimetry (PIV) experiments using fixed-bed models
to examine the effects of barchan dune interaction upon the flow field structure. The barchan dune models were created from
an idealized contour map, the shape and dimensions of which were based upon previous empirical studies of dune morphology.
The experimental setup comprised two, co-axially aligned, barchan dune models that were spaced at different distances apart.
In this paper, two volumetric ratios (V
r, upstream dune: downstream dune) of 1.0 and 0.175 were examined. Models were placed in a boundary-layer wind tunnel and flow
quantification was achieved via PIV measurements of the mean and turbulent flow field in the streamwise–wall-normal plane,
along the centerline of the barchan(s), at an average flow Reynolds number of 59,000. The presence of an upstream barchan
dune induces a “sheltering effect” on the flow. Flow on the stoss side of the downstream dune is controlled by the developing
internal boundary layer from the upstream dune, as well as by the turbulent flow structures shed from the free shear layer
of the upstream dune leeside. At both volumetric ratios, enhanced turbulence is present over the downstream barchan dune leeside,
which is proposed to be caused by the interaction of shear layers from the upstream and downstream dunes. Both the size and
magnitude of the shear layer formed in the leeside of the upstream dune control this interaction, together with the proximity
of this shear layer to the stoss side of the downstream dune. Proper orthogonal decomposition (POD) analysis shows that the
distribution of turbulent kinetic energy is shifted to higher modes (i.e., smaller spatial scales) over interacting barchan
dunes, which also reflects the role of the leeside free shear layer in dominating the flow field by generation, or redistribution,
of TKE to smaller scales. 相似文献
996.
H. Christensen 《Fresenius' Journal of Analytical Chemistry》1955,144(1):50
Ohne Zusammenfassung 相似文献
997.
Automation of solid-phase microextraction-gas chromatography-mass spectrometry extraction of eucalyptus volatiles 总被引:1,自引:0,他引:1
Zini CA Lord H Christensen E de AT Caramão EB Pawliszyn J 《Journal of chromatographic science》2002,40(3):140-146
Solid-phase microextraction (SPME) coupled with gas chromatography (GC)-ion-trap mass spectrometry (ITMS) is employed to analyze fragrance compounds from different species of eucalyptus trees: Eucalyptus dunnii, Eucalyptus saligna, Eucalyptus grandis, and hybrids of other species. The analyses are performed using an automated system for preincubation, extraction, injection, and analysis of samples. The autosampler used is a CombiPAL and has much flexibility for the development of SPME methods and accommodates a variety of vial sizes. For automated fragrance analysis the 10- and 20-mL vials are the most appropriate. The chromatographic separation and identification of the analytes are performed with a Varian Saturn 4D GC-ITMS using an HP-5MS capillary column. Several compounds of eucalyptus volatiles are identified, with good reproducibility for both the peak areas and retention times. Equilibrium extraction provides maximal sensitivity but requires additional consideration for the effect of carryover. Preequilibrium extraction allows good sensitivity with minimal carryover. 相似文献
998.
A model has been developed for determining the time history of piston slap impact force. This model takes into account the influence of the oil film on the impact behaviour, which was found to be an important factor. However, it was also found that entrapped gas bubbles in the oil are equally significant. Three test rigs were designed and built to study these effects on the impact phenomenon and extensive tests were carried out. The impact force time history has been determined using Reynolds' theory. Results have shown that Reynolds' theory for fluid film squeezing can be applied for oil film damping determination. However, the experimental results have also shown that when gas is entrapped during the impact, this theory considerably overpredicts the magnitude of the impact. An eight-degree-of-freedom lumped parameter model was developed through the dynamic analysis of each component of an internal combustion engine's reciprocating system. The effective damping factor derived from this model was found to be inversely proportional to the oil film thickness cubed, as expected from Reynolds' theory. A dynamic model has been proposed, where the oil film mixed with bubbles is considered to be analogous to a serial spring and damping system. By incorporating a spring in series with this damper, the effect of the bubbles can also be predicted. 相似文献
999.
Michael Pittelkow Jrn B. Christensen E. W. Meijer 《Journal of polymer science. Part A, Polymer chemistry》2004,42(15):3792-3799
A new type of guest has been designed and synthesized for the exo‐type supramolecular functionalization of adamantyl‐urea‐terminated poly(propylene imine) dendrimers. This new type of guest motif features a uriedo methane sulfonic acid moiety that binds very selectively to the surfaces of dendrimers via a combination of noncovalent interactions forming well‐defined complexes. The guest–host properties have been examined for a fifth‐generation adamantyl‐urea‐functionalized poly(propylene imine) dendrimer capable of binding 32 guest molecules and for a model host molecule that can bind only one guest molecule. The guest–host chemistry has been studied with 1H NMR spectroscopy, nuclear Overhauser enhancement spectroscopy NMR spectroscopy, T1‐relaxation NMR experiments, and IR spectroscopy. The 1:32 ratio with the dendrimer has been confirmed unambiguously from 1H NMR spectra of the complex after size exclusion chromatography. Competition experiments with guests bearing a carboxylic acid instead of a sulfonic acid in the binding motif have demonstrated that the sulfonic acid has superior binding strength. Also, the importance of a combination of noncovalent interactions has been shown via competition experiments with a guest lacking the uriedo moiety. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3792–3799, 2004 相似文献
1000.
One of the few methods for synthesis of 1,6‐dioxapyrenes is the acid catalyzed cyclization of 2,6‐disubstituted 1,5‐bis(2‐oxoalkoxy)naphthalenes. The scope and limitations of this reaction has been investigated and 11 new 2,7‐disubstituted 1,6‐dioxapyrenes have been prepared and characterized. Most of the compounds undergo two reversible oxidations to give the corresponding radical as well as di‐ cations. 相似文献