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   53700篇
  免费   700篇
  国内免费   565篇
测绘学   1452篇
大气科学   3806篇
地球物理   9922篇
地质学   19232篇
海洋学   4912篇
天文学   12911篇
综合类   189篇
自然地理   2541篇
  2022年   365篇
  2021年   633篇
  2020年   659篇
  2019年   710篇
  2018年   1589篇
  2017年   1508篇
  2016年   1876篇
  2015年   990篇
  2014年   1747篇
  2013年   2871篇
  2012年   1859篇
  2011年   2376篇
  2010年   2073篇
  2009年   2685篇
  2008年   2299篇
  2007年   2356篇
  2006年   2196篇
  2005年   1624篇
  2004年   1641篇
  2003年   1548篇
  2002年   1475篇
  2001年   1299篇
  2000年   1221篇
  1999年   994篇
  1998年   1041篇
  1997年   951篇
  1996年   817篇
  1995年   783篇
  1994年   685篇
  1993年   604篇
  1992年   594篇
  1991年   593篇
  1990年   623篇
  1989年   493篇
  1988年   504篇
  1987年   528篇
  1986年   487篇
  1985年   610篇
  1984年   673篇
  1983年   590篇
  1982年   561篇
  1981年   499篇
  1980年   469篇
  1979年   478篇
  1978年   458篇
  1977年   368篇
  1976年   346篇
  1975年   357篇
  1974年   308篇
  1973年   342篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
The possible correlation noted between the intrinsic quasar luminosity and the absorption line expulsion velocity is re-examined using homogeneous data sets for metal and L only line systems. The method of analysis is chosen to enable any reasonable form of correlation to be found. No correlation is detected at a confidence level >10%.  相似文献   
73.
74.
F.A. Gifford  R.P. Hosker  K.S. Rao 《Icarus》1978,36(1):133-146
The spreading angle of a number of light and dark Martian streaks is determined from selected Mariner 9 images. The resulting frequency distributions of spreading half-angles have maxima at ~5° for light, and ~7° for dark streaks; however the dark streaks have a secondary maximum spreading angle at ~14°. The smaller values, which include most streaks, are interpreted as crater-wake spreading phenomena. The larger value, found in only a few dark streaks or “tails,” may result from atmospheric diffusion and subsequent deposition of material from isolated sources such as vents or blowouts. An atmospheric diffusion-deposition analysis is presented, assuming this streak origin, from which it is possible to deduce the eddy diffusivity, K, in Mars' boudary layer. Calculated K values are found to agree with various theoretical estimates. They lie in the range 107 and 109 cm2 sec?1 and exhibit the proper scale dependence. Thus it appears that, in addition to streak-derived wind direction patterns and speed information, it is possible in a few cases to derive information on Mars' boundary-layer turbulence from streak-spreading measurements.  相似文献   
75.
The effects of a large magnetic field on the H and K lines of Caii are investigated incorporating both the linear and quadratic Zeeman effects for fields in the range 106–108 G. For these fields the linear term is the dominant one, resulting in a Paschen-Back effect with a smaller quadratic Zeeman shift superimposed on all components. Due to the partial cancellation of the quadratic effect between the upper and lower states of the lines, a much smaller quadratic Zeeman shift occurs than in the lines of other atoms previously studied at similar fields.  相似文献   
76.
77.
78.
79.
80.
The reservoir quality of Jurassic and Triassic fluvial and lacustrine-deltaic sandstones of the Yanchang Oil Field in the Ordos Basin is strongly influenced by the burial history and facies-related diagenetic events. The fluvial sandstones have a higher average porosity (14.8%) and a higher permeability (12.7×10?3 ?m2) than those of the deltaic sandstones (9.8% and 5.8 ×10?3 ?m2, respectively). The burial compaction, which resulted in 15% and 20% porosity loss for Jurassic and Triassic sandstones, respectively, is the main factor causing the loss of porosity both for the Jurassic and Triassic sandstones. Among the cements, carbonate is the main one that reduced the reservoir quality of the sandstones. The organic acidic fluid derived from organic matter in the source rocks, the inorganic fluid from rock-water reaction during the late diagenesis, and meteoric waters during the epidiagenesis resulted in the formation of dissolution porosity, which is the main reason for the enhancement of reservoir-quality.  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