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991.
We examine the kinematic and dynamic properties of fluid spacetimes in higher order gravity. In particular we extend the general equations of Ehlers and Ellis governing relativistic fluid dynamics from general relativity to the higher order theory. We find exact results for the evolution of shear in Bianchi spacetimes with isotropic surfaces, thus generalising the general relativity results. Furthermore we show that the vanishing of vorticity, shear and acceleration does not imply FRW geometry inR + R
2 gravity without the further assumption of a barotropic equation of state,p = p(p), p(p) 0. In particular, this result means that the Ehlers-Geren-Sachs theorem on cosmic background radiation also holds in the higher order theory. 相似文献
992.
993.
994.
This paper is part of a program investigating symmetries that are defined at a physical or observational level rather than purely geometrically. Here we generalize previous work on dynamical matter symmetries of relativistic gases. If the matter symmetry vector is surface-forming with the dynamical Liouville vector, then Einstein's equations reduce it to a Killing symmetry of the metric. We show that this conclusion is unaltered if the gas particles are subject to a nongravitational force (including the electromagnetic force on charged particles) or if the gravitational field obeys higher-order field equations. In the Brans-Dicke theory, the matter symmetry reduces to a homothetic symmetry of the metric. This is also the case for a generalized conformal symmetry in Einstein's theory. We consider the problem of relaxing the surface-forming assumption in an attempt to determine whether there are dynamical symmetries that do not necessarily reduce to geometrical symmetries of the metric. 相似文献
995.
A. Kamangerpour M. Ashraf-Khorassani L. T. Taylor H. M. McNair L. Chorida 《Chromatographia》2002,55(7-8):417-421
Summary Supercritical Fluid Chromatography with a packed column on a mixture of eight polyphenols has been optimized. Carbon dioxide
which was modified with methanol which contained less than 1% (w/w) citric acid as a secondary additive served as the mobile
phase. Two tandem diol columns were used sequentially. The optimized method was applied to a supercritical fluid grape seed
extract. Various components in the extract could be identified by retention time and ultraviolet spectral comparison with
our synthetic mixture of polyphenols. 相似文献
996.
The microwave spectra of the normal and the 2-13C isotopic species of 1-cyanobicyclo[1.1.0]butane have been observed and assigned, leading to the following rotational constants: (normal), A=8807.202 ± 0.004, B=2924.334 ± 0.002, C=2509.322 ± 0.002 and (isotope), A=8608.85 ± 0.85, B=2902.88 ± 0.02, and C=2478.56 ± 0.02 MHz. Measurements of the second-order Stark effect led to
T=4.08 ± 0.01 D. Based on the available microwave data coupled with geometryoptimizedab initio molecular orbital structures for cyanocyclopropane and 1-cyanobicyclo[1.1.0]-butane, a molecular structure for the latter molecule is proposed. Analysis of the dipole moments of these molecules and of bicyclo[1.1.0]butane has led to the conclusion that the bicyclobutyl group is a better electron donor than is cyclopropyl. In addition, a simple frontier molecular orbital model is not sufficient for explaining all of the structural changes that occur on substituting cyano at the bridgehead of bicyclo[1.1.0]butane. 相似文献
997.
998.
H. P. Qi M. Berglund P. D. P. Taylor F. Hendrickx A. Verbruggen P. De Bièvre 《Analytical and bioanalytical chemistry》1998,361(8):767-773
Re-certification of the absolute isotopic composition of the natural lithium isotopic reference material (IRM), IRMM-016, requires measurements calibrated by means of synthetic mixtures of highly enriched lithium isotopes. Ten such mixtures were prepared by weighing and mixing of two well characterised, isotopically enriched, Li2CO3 compounds. The starting materials, 99.9981% enriched 6Li, and 99.9937% enriched 7Li, were purified by ion exchange, and the purified materials converted from LiOH to Li2CO3 by reaction with CO2. Ten new mixtures were prepared by mixing different weighed amounts of these dissolved Li2CO3 carrier compounds. The compounds had an estimated level of impurities of 100 ± 100 μg · g–1 (expanded uncertainty with a coverage factor of 2). In the ten mixtures, the n(6Li)/n(7Li) ratio varies from 0.025 to 14 and the achieved expanded relative uncertainty on the amount ratio prepared is typically 2 · 10–4. These mixtures were then used to determine the correction factor, K, for mass discrimination of the measurement procedure and instrument concerned. 相似文献
999.
Abstract The development of a methodology which optimizes the light dosage in tissue and improves the tailoring of the light with consequent sparing of normal adjacent tissue, will enhance the possibility for routine clinical photodynamic therapy. Specific, important goals for the clinical use of PDT are (a) efficient distribution of light flux to all parts of the tumor at sufficient level to effect eradication. (b) avoidance of the destruction of adjacent normal tissue, and (c) ability to tailor the treatment field, taking into account the varied shapes of tumors. Dividing the available power among several fibers is a promising method of achieving these goals. This is accomplished by (1) extending the volume, and by (2) increasing the flux spatial uniformity. This latter defocussing of the flux field is especially important because it may help to avoid concentrating a high intensity field from an implanted fiber near an essential structure of the normal tissue. The question arises how best to orient these multiple fibers for maximum coverage and uniformity. Hence, theoretical and experimental investigations were made to determine optimal fiber placements. A series of intensity distributions were generated using two and three fibers positioned at various separations within a postulated tumor volume. A criterion for uniformity was defined. Iterative computation produced optimal fiber separation for the given constraints. In the two fiber case, for small values of attenuation coefficient (μ. ≦ 0.2 mm?1), optimal fiber separation ranged from 0.6 to 0.7 times the diameter of the defined volume. For large values of attenuation coefficient (μ. ≧ 0.8), fiber separation was about 0.5 to 0.55 times the region diameter. The effects of fiber separation on volume of treatment were also determined. Maximal treatment volume was found to be dependent on the attenuation coefficient. With μ, = 0.50, a 40% increase in treatment volume over single fiber insertion of equivalent energy input was shown to be obtainable with a dual fiber configuration of 24 mm separation. Experiments using two fibers in vitro in mammalian tissue were performed to substantiate these results. The multiple fiber system is a promising method for delivering optimum light dosage to targeted PDT tissue. 相似文献
1000.