全文获取类型
收费全文 | 33261篇 |
免费 | 538篇 |
国内免费 | 387篇 |
专业分类
测绘学 | 1236篇 |
大气科学 | 2568篇 |
地球物理 | 6564篇 |
地质学 | 11825篇 |
海洋学 | 2591篇 |
天文学 | 7794篇 |
综合类 | 146篇 |
自然地理 | 1462篇 |
出版年
2021年 | 291篇 |
2020年 | 283篇 |
2019年 | 334篇 |
2018年 | 841篇 |
2017年 | 792篇 |
2016年 | 1043篇 |
2015年 | 606篇 |
2014年 | 991篇 |
2013年 | 1727篇 |
2012年 | 1070篇 |
2011年 | 1329篇 |
2010年 | 1110篇 |
2009年 | 1434篇 |
2008年 | 1256篇 |
2007年 | 1195篇 |
2006年 | 1218篇 |
2005年 | 1014篇 |
2004年 | 903篇 |
2003年 | 897篇 |
2002年 | 899篇 |
2001年 | 787篇 |
2000年 | 761篇 |
1999年 | 687篇 |
1998年 | 644篇 |
1997年 | 645篇 |
1996年 | 587篇 |
1995年 | 557篇 |
1994年 | 517篇 |
1993年 | 447篇 |
1992年 | 386篇 |
1991年 | 422篇 |
1990年 | 425篇 |
1989年 | 401篇 |
1988年 | 368篇 |
1987年 | 446篇 |
1986年 | 372篇 |
1985年 | 467篇 |
1984年 | 499篇 |
1983年 | 487篇 |
1982年 | 462篇 |
1981年 | 373篇 |
1980年 | 373篇 |
1979年 | 332篇 |
1978年 | 324篇 |
1977年 | 312篇 |
1976年 | 273篇 |
1975年 | 265篇 |
1974年 | 292篇 |
1973年 | 321篇 |
1972年 | 203篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
172.
P. G. Jonker G. Nelemans Z. Wang A. K. H. Kong D. Chakrabarty M. Garcia P. J. Groot M. van der Klis T. Kerr B. Mobasher M. Sullivan T. Augusteijn B. W. Stappers P. Challis R. P. Kirshner J. Hjorth A. Delsanti 《Monthly notices of the Royal Astronomical Society》2003,344(1):201-206
We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751−305. There are no stars in the 0.7-arcsec error circle (0.7 arcsec is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of R > 23.1, I > 21.6, Z > 20.6, J > 19.6 and K > 19.2 . We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disc subject to the reddening observed in X-rays for XTE J1751−305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disc or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultracompact system. 相似文献
173.
W.W. Zeilinger S. De Rijcke H. Dejonghe G.K.T. Hau 《Astrophysics and Space Science》2003,284(2):607-610
Deep imaging and long-slit spectroscopy was obtained for a sample of dwarf ellipticals in the Fornax cluster, NGC 5044 and
NGC 5898 groups using the ESO VLT. The observational data extend out to typically 1.5–2 effective radii and indicate a kinematic
dichotomy in the family of ellipticals. The observed stellar kinematics indicate a luminosity–velocity dispersion relation
largely supporting Supernova-driven stellar mass loss scenarios for the formation of dwarf ellipticals. Stellar dynamical
models favour dark matter halos with typical mass-to-light ratios in the range of 3 to 9 solar units.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
174.
175.
Pavlova viridis sp. nov. is described on the basis of light and electron microscope observations. The material was collected from the coast
of haiyang county of Shandong, China. It is characterized by 1) yellowish green to green chloroplast, 2) rudimentary short
flagellum 0.3 μm long and hook-shaped, 3) long flagellum with small spherical knob-scales, 4) absence of pyrenoid and stigma,
and 5) marine habitat. This new species andP. salina seem to be most closely related to each other but their colour, the insertion of their three appendages, the shape of their
knob scales, and their cell periplasts are quite different from one another.
Contribution No. 1879 from the Institute of Oceanology, Academia Sinica 相似文献
176.
R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(2-3):145-154
Spectral measurement of Mkn 421 were made in the hard X-ray energy band of 20–200 keV using a high sensitivity, large area
scintillation counter telescope on November 21, 2000 and these coincided with the onset of an active X-ray phase as seen in
the ASM counting rates on board RXTE. The observed spectrum can not be fitted to a single power law similar to the PDS data
of BeppoSAX. The data can be fitted both by a two component power-law function or a combination of an exponential function
with a power law component at the high energies above 80 keV. We identify these components with those arising from the synchrotron
self compton and the high energy power-law tail arising from the upgrading of the thermal photons due to multiple Compton
scattering a la Cyg X-1. A comparison with the earlier data clearly suggests a spectral variability in the hard X-ray spectrum
of the source. We propose a continuously flaring geometry for the source as the underlying mechanism for energy release. 相似文献
177.
William K. Hartmann 《Meteoritics & planetary science》1995,30(4):451-467
Abstract— This paper addresses several current issues related to use of craters in interpreting planetary surface histories. The primary goal is to test the widely adopted hypothesis of multiple populations of impactors at different times or places in the Solar System. New data presented here revise a “lunar highland” crater diameter distribution that has been widely used as evidence of an early distinct population of impactors. This curve, which has a depression of the size distribution at mid-sizes, does not, in fact, represent the lunar highlands generally. I show that it is associated with regions of intercrater plains. The more extensive the obliteration by intercrater plains, the deeper the depression. Modeling indicates that the depression of the curve is caused by the obliteration process itself. The oldest, most cratered regions of lunar highlands do not show the depression. These findings call into question earlier interpretations of multiple populations of impactors in the Solar System and of a distinctive primordial population. The present work is consistent, instead, with (1) a relatively uniform size distribution of interplanetary impactors, of mixed origins, back to 4 Ga ago and throughout the sampled Solar System; (2) fragmentation as the process that produced that size distribution; (3) saturation equilibrium on the most heavily cratered surfaces; and (4) differences in structure in the size distribution caused not by distinct impactor populations but by episodes of endogenic obliteration. If accepted, these results would modify some studies of solar system evolution, including assertions of two to five distinct populations of impactors, assumptions of lack of saturation equilibrium, and identifications of specific heliocentric or planetocentric sources for impactors within outer planet satellite systems. 相似文献
178.
In this paper we consider the time-dependent diffuse reflection and transmission problems for a homogeneous anisotropically-scattering atmosphere of finite optical depth and solve it by the principle of invariance. Also we consider the time-dependent diffuse reflection and transmission of parallel rays by a slab consisting of two anisotropic homogeneous layers, whose scattering and transmission properties are known. It is shown how to express the time-dependent reflected and transmitted intensities in terms of their components. In a manner similar to that given by Tsujita (1968), we assumed that the upward-directed intensities of radiation at the boundary of the two layers are expressed by the sum of products of some auxiliary functions depending on only one argument. Then, after some analytical manipulations, three groups of systems of simultaneous integral equations governing the auxiliary functions are obtained. 相似文献
179.
180.