全文获取类型
收费全文 | 67073篇 |
免费 | 804篇 |
国内免费 | 1215篇 |
专业分类
测绘学 | 2409篇 |
大气科学 | 4747篇 |
地球物理 | 13106篇 |
地质学 | 26216篇 |
海洋学 | 4726篇 |
天文学 | 12309篇 |
综合类 | 2242篇 |
自然地理 | 3337篇 |
出版年
2021年 | 407篇 |
2020年 | 450篇 |
2019年 | 453篇 |
2018年 | 5626篇 |
2017年 | 4859篇 |
2016年 | 3643篇 |
2015年 | 870篇 |
2014年 | 1202篇 |
2013年 | 2187篇 |
2012年 | 2223篇 |
2011年 | 4317篇 |
2010年 | 3457篇 |
2009年 | 4357篇 |
2008年 | 3607篇 |
2007年 | 4066篇 |
2006年 | 1728篇 |
2005年 | 1442篇 |
2004年 | 1642篇 |
2003年 | 1563篇 |
2002年 | 1361篇 |
2001年 | 1052篇 |
2000年 | 1005篇 |
1999年 | 838篇 |
1998年 | 867篇 |
1997年 | 816篇 |
1996年 | 691篇 |
1995年 | 702篇 |
1994年 | 623篇 |
1993年 | 536篇 |
1992年 | 501篇 |
1991年 | 509篇 |
1990年 | 581篇 |
1989年 | 498篇 |
1988年 | 457篇 |
1987年 | 584篇 |
1986年 | 484篇 |
1985年 | 610篇 |
1984年 | 690篇 |
1983年 | 656篇 |
1982年 | 573篇 |
1981年 | 622篇 |
1980年 | 515篇 |
1979年 | 472篇 |
1978年 | 476篇 |
1977年 | 434篇 |
1976年 | 426篇 |
1975年 | 417篇 |
1974年 | 396篇 |
1973年 | 426篇 |
1971年 | 260篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
232.
233.
M. R. Bate S. H. Lubow G. I. Ogilvie K. A. Miller 《Monthly notices of the Royal Astronomical Society》2003,341(1):213-229
We analyse the non-linear, three-dimensional response of a gaseous, viscous protoplanetary disc to the presence of a planet of mass ranging from 1 Earth mass (1 M⊕ ) to 1 Jupiter mass (1 MJ ) by using the zeus hydrodynamics code. We determine the gas flow pattern, and the accretion and migration rates of the planet. The planet is assumed to be in a fixed circular orbit about the central star. It is also assumed to be able to accrete gas without expansion on the scale of its Roche radius. Only planets with masses M p ≳ 0.1 MJ produce significant perturbations in the surface density of the disc. The flow within the Roche lobe of the planet is fully three-dimensional. Gas streams generally enter the Roche lobe close to the disc mid-plane, but produce much weaker shocks than the streams in two-dimensional models. The streams supply material to a circumplanetary disc that rotates in the same sense as the orbit of the planet. Much of the mass supply to the circumplanetary disc comes from non-coplanar flow. The accretion rate peaks with a planet mass of approximately 0.1 MJ and is highly efficient, occurring at the local viscous rate. The migration time-scales for planets of mass less than 0.1 MJ , based on torques from disc material outside the Roche lobes of the planets, are in excellent agreement with the linear theory of type I (non-gap) migration for three-dimensional discs. The transition from type I to type II (gap) migration is smooth, with changes in migration times of about a factor of 2. Starting with a core which can undergo runaway growth, a planet can gain up to a few MJ with little migration. Planets with final masses of the order of 10 MJ would undergo large migration, which makes formation and survival difficult. 相似文献
234.
235.
We present radio interferometric observations of the Algol-type binary system RZ Cassiopeiae made with the VLA and MERLIN arrays at 6 cm over an incomplete orbital cycle of the system (1.195 d). We detected RZ Cas with both instruments. The images were unresolved in both cases, with angular extents comparable to the synthesized beams. The peak flux density in the VLA image was 1.14 mJy beam−1 and in the MERLIN image it was 0.93 mJy beam−1 . The derived brightness temperatures are 4.02 × 108 and 4.35 × 108 K and the effective electron energies are 0.347 and 0.346 MeV for the MERLIN and VLA data respectively. The radio light curve shows an interesting modulation centred close to the primary eclipse which seems to correlate with ASCA SIS observations of the system. The results can be interpreted as an emitting region on the outer hemisphere of the cool component aligned along the centroid axis of the binary system. 相似文献
236.
Dolmatova L. S. Eliseykina M. G. Timchenko N. F. Kovaleva A. L. Shitkova O. A. 《中国海洋湖沼学报》2003,21(4):293-304
Pure fraction (92%–95%) of phagocytes (FP) and a mixture of amoebocytes (62%) and morula cells (38%)-FPMC- of the holothurianEupentacta fraudatrix' (Holothuroidea, Dendrochirota) were obtained by using ficoll-verographine step gradient. Basal production of reactive oxygen species (ROS) in FP quantified
by using reduction of nitroblue tetrazolium (NBT) was more than twice that in FPMC. Thermostable toxin ofYersinia pseudotuberculosis (TST) at different concentrations (0.2; 0.5; 2.5 μg/ml, but not 0.1 μg/ml) stimulated NBT reduction in FPMC after 24 h incubation.
In FP, TST at concentrations of 0.1 and 0.2 μg/ml inhibited and at concentrations of 0.5 and 2.5 μg/ml stimulated NBT reduction
after 24 h incubation. Maximal effect was observed in FP and FPMC at TST concentrations of 0.5 and 0.2 μg/ml, respectively.
Addition of catalase (0.7 μg/ml) to the cells treated with TST (2.5 μg/ml) was followed by a decrease in NBT reduction compared
to that under toxin treatment alone. TST stimulated superoxide dismutase activity in concentration-dependent manner (maximum
at 0.5 μg/ml concentration in FP) after 24 h treatment, and this stimulation was prevented by a commercial catalase. Plant
lectin concanavalin A stimulated NBT reduction more than 5-fold in FPMC compared to the control. With addition of TST, lectin
stimulated ROS to lesser extent than that with lectin alone. When catalase, TST, and lectin were added into the FPMC simultaneously,
ROS increase was similar to that under lectin treatment alone. On the whole, data obtained indicated that ROS generation in
holothurian coelomocytes especially occurs in both stimulated and not stimulated phagocytes, and that changes in ROS production
by these cells may be one of the mechanisms of antibacterial protection of holothurians.
This work was financially supported by the Russian Foundation for Fundamental Research Grant (No. 00-04-48949). 相似文献
237.
The present paper reports 9 species of pontoniine shrimps collected from Hainan IS-land ,South China Sea ,by the Joint Chinese-German Marine Biology Expeditions,in which Peri-climenaeus arabicus (Calman,1939) and Periclimenaeus hecate (Nobili,1904) are recorded for the first time from Hainan Island. 相似文献
238.
The problem of perfect fluid distribution in spatially homogeneous and anisotropic Bianchi type VI0 space-time is considered in a scalar tensor theory of gravitation proposed by Saez and Ballester (1985). Exact solutions
of the field equations are derived when the metric potentials are functions of cosmic time only. Some physical and geometrical
properties of the solutions are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
239.
Stellar population synthesis at the resolution of 2003 总被引:2,自引:0,他引:2
240.