首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   84227篇
  免费   14125篇
  国内免费   18391篇
测绘学   10477篇
大气科学   10740篇
地球物理   17435篇
地质学   42624篇
海洋学   9843篇
天文学   7711篇
综合类   5998篇
自然地理   11915篇
  2024年   244篇
  2023年   810篇
  2022年   2315篇
  2021年   2714篇
  2020年   2988篇
  2019年   3205篇
  2018年   2886篇
  2017年   3382篇
  2016年   3529篇
  2015年   3919篇
  2014年   4668篇
  2013年   5051篇
  2012年   5201篇
  2011年   5659篇
  2010年   4818篇
  2009年   5777篇
  2008年   5739篇
  2007年   6116篇
  2006年   5842篇
  2005年   5154篇
  2004年   4671篇
  2003年   4221篇
  2002年   3748篇
  2001年   3278篇
  2000年   3022篇
  1999年   2797篇
  1998年   2394篇
  1997年   2060篇
  1996年   1828篇
  1995年   1601篇
  1994年   1571篇
  1993年   1351篇
  1992年   979篇
  1991年   728篇
  1990年   595篇
  1989年   494篇
  1988年   386篇
  1987年   238篇
  1986年   173篇
  1985年   141篇
  1984年   70篇
  1983年   46篇
  1982年   56篇
  1981年   35篇
  1980年   45篇
  1979年   48篇
  1978年   48篇
  1977年   36篇
  1976年   7篇
  1954年   29篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
161.
个人所得税是与每个人联系都很紧密的税种,它的完善与否,直接体现了我国法制建设的完善程度及居民素质的高低。但现行税制由于内外原因,存在各种缺陷。因此有必要在理论中找到对策,并在实践中寻求答案逐步完善个人所得税制度。  相似文献   
162.
163.
164.
The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
165.
166.
The star WR 7a, also known as SPH 2, has a spectrum that resembles that of V Sagittae stars although no O  vi emission has been reported. The Temporal Variance Spectrum – TVS – analysis of our data shows weak but strongly variable emission of O  vi lines which is below the noise level in the intensity spectrum.
Contrary to what is seen in V Sagittae stars, optical photometric monitoring shows very little, if any, flickering. We found evidence of periodic variability. The most likely photometric period is   P phot= 0.227(±14) d  , while radial velocities suggest a period of   P spec= 0.204(±13) d  . One-day aliases of these periods can not be ruled out. We call attention to similarities with HD 45166 and DI Cru (= WR 46), where multiple periods are present. They may be associated to the binary motion or to non-radial oscillations.
In contrast to a previous conclusion by Pereira et al., we show that WR 7a contains hydrogen. The spectrum of the primary star seems to be detectable as the N  v 4604 Å  absorption line is visible. If so, it means that the wind is optically thin in the continuum and that it is likely to be a helium main sequence star.
Given the similarity to HD 45166, we suggests that WR 7a may be a qWR – quasi Wolf–Rayet – star. Its classification is WN4h/CE in the Smith, Shara & Moffat three-dimensional classification system.  相似文献   
167.
168.
169.
We present the results of a systematic investigation of spectral evolution in the Z source GX 349+2, using data obtained during 1998 with the Proportional Counter Array (PCA) on-board the RXTE satellite. The source traced a extended normal branch (NB) and flaring branch (FB) in the colour–colour diagram (CD) and the hardness-intensity diagram (HID) during these observations. The spectra at different positions of the Z-track were best fitted by a model consisting of a disc blackbody and a Comptonized spectrum. A broad (Gaussian) iron line at ∼6.7 keV is also required to improve the fit. The spectral parameters showed a systematic and significant variation with the position along the Z-track. The evolution in spectral parameters is discussed in view of the increasing mass accretion rate scenario, proposed to explain the motion of Z sources in the CD and the HID.  相似文献   
170.
We investigate the role of seasonal variations of Titan’s stratospheric composition on the temperature. We use a general circulation model coupled with idealized chemical tracers that reproduce variations of ethane (C2H6), acetylene (C2H2), and hydrogen cyanide (HCN). Enhancement of the mole fractions of these compounds, at high latitudes in the winter hemisphere relative to their equatorial values, induces a relative decrease in temperature above approximately 0.2 mbar, with a peak amplitude around −20 K, and a relative increase in temperature below, around 1 mbar, with a peak amplitude around +7 K. These thermal effects are mainly due to the variations of the cooling to space induced by the varying distributions. The ethane, acetylene, and hydrogen cyanide variations affect the cooling rates in a similar way, with the dominant effect being due to ethane, though its latitudinal variations are small.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号