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981.
982.
Sandholt P.E. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1992,20(6):715-725
Attention is focused on a specific category of auroral event occurring predominantly during a southward directed interplanetary magnetic field (IMF). Coordinated observations from the ground and satellites in polar orbit have been used to study the temporal/spatial development of the events in relation to the background patterns of particle precipitation and ionospheric convection as well as the field-aligned current and ion drift characteristics of the individual events. Both prenoon and postnoon sector events are reported. In one case the auroral event was observed near the zero point potential line separating the morning and postnoon convection cells. The available data indicate that this auroral event sequence is a signature of quasi-periodic bursts of particle entry from the magnetosheath along flux tubes convecting into the polar cap, representing structures of enhanced IMF B Y-related convection poleward of the persistent cleft aurora. Such events may be initiated by localized patches of enhanced merging rate at the dayside magnetopause 相似文献
983.
Lu KangAlison R. Keimowitz Michaeleen R. MunrowStewart E. Novick 《Journal of Molecular Spectroscopy》2002,213(2):122-129
The rotational spectra of the argon acetone weakly bound complex was studied by pulsed jet Fabry-Perot Fourier transform microwave spectroscopy. Over 500 transitions of the complex were measured between 5.5 and 26 GHz from J=2-1 to J=12-11. The two methyl groups undergo hindered internal rotation resulting in four or five internal rotation states. The microwave transitions are within these states, resulting in a splitting of each rotational transition into four and sometimes five distinct transitions. The three-fold barrier to internal rotation is determined to be 260 cm−1, 2% less than the 266 cm−1 barrier in acetone itself. The structure of the complex has the argon atom above the heavy atom plane of the acetone, 3.52 Å from the CO bond and approximately in the Cs plane, which is perpendicular to the CCC plane of acetone. 相似文献
984.
The photoconductivity and electrical conductivity, the thermally stimulated polarization and depolarization currents, the
permittivity, and the dielectric losses of noncentrosymmetric Ca3Ga2Ge4O14 crystals of a structural type of trigonal calcium gallogermanate (CGG) have been studied for the first time in the temperature
interval 100–700 K in air and in vacuum. The features of the temperature behavior of CGG are associated, in particular, with
the effect of electrically active defects and partial disordering of the cation sublattice of the crystal structure, which
generates polar formations (dipoles) of several kinds. Certain parameters of the electromigration and of the electrically
active defects that cause charge-relaxation and polarization effects are determined. It is concluded that dipole relaxation
in CGG makes it a new representative of a class of materials in which reorientation of the dipoles as a system of mobile local
distortions of the structure has a substantial effect on the physical properties.
Fiz. Tverd. Tela (St. Petersburg) 39, 871–876 (May 1997) 相似文献
985.
986.
Time-resolved pulsed spectroscopy was used to measure the luminescence spectra of calcium fluoride. Characteristic features
of the luminescence of self-trapped excitons are discussed. It is shown that various configurations of self-trapped excitons
incorporating hole nuclei of a more complex structure, may be formed in CaF2 crystals.
Fiz. Tverd. Tela (St. Petersburg) 40, 1226–1227 (July 1998) 相似文献
987.
988.
Differentiation between acutely ischemic myocardium and zones of completed infarction in dogs on the basis of frequency-dependent backscatter 总被引:1,自引:0,他引:1
K A Wear M R Milunski S A Wickline J E Perez B E Sobel J G Miller 《The Journal of the Acoustical Society of America》1989,85(6):2634-2641
The goal of this work was to determine whether the frequency dependence of apparent backscatter coefficient (not corrected for attenuation within the myocardium) could differentiate completed, remote infarction from acute myocardial injury in vivo. Myocardial infarcts were produced in six dogs by coronary artery occlusion. One to 12 months later, acute ischemic injury was induced in each dog by ligation of a coronary artery that supplied a region of myocardium adjacent to the established infarct. Infarct, ischemic, and normal regions were interrogated with a 5-MHz, circular, 0.5-in. diam, broadband, focused, piezoelectric transducer mounted in a water-filled stand-off device placed against the exposed, beating heart. Apparent backscatter coefficients were measured over the range of frequencies from 3-7 MHz. The frequency dependence was obtained from the slope of log apparent backscatter coefficient versus log frequency. No significant difference in frequency dependence was found between normal and acutely ischemic myocardium for periods of up to 2 h of ischemia. In contrast, frequency dependence in regions of remote infarct (1.8 +/- 0.1, mean +/- standard error) was significantly lower than that in acutely ischemic or nonischemic regions (2.3 +/- 0.1) (p less than 0.01). These results suggest that remote myocardial infarction can be differentiated from acutely injured but still potentially salvageable myocardium in vivo on the basis of the frequency dependence of backscatter. 相似文献
989.
990.