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   265446篇
  免费   5232篇
  国内免费   3464篇
测绘学   7214篇
大气科学   19602篇
地球物理   55859篇
地质学   92882篇
海洋学   22095篇
天文学   57116篇
综合类   1050篇
自然地理   18324篇
  2021年   2246篇
  2020年   2653篇
  2019年   2886篇
  2018年   4319篇
  2017年   3908篇
  2016年   6185篇
  2015年   4306篇
  2014年   7016篇
  2013年   14336篇
  2012年   6850篇
  2011年   8556篇
  2010年   7437篇
  2009年   10108篇
  2008年   8973篇
  2007年   8473篇
  2006年   9832篇
  2005年   7962篇
  2004年   7896篇
  2003年   7384篇
  2002年   6938篇
  2001年   6145篇
  2000年   6039篇
  1999年   5229篇
  1998年   5261篇
  1997年   5079篇
  1996年   4714篇
  1995年   4459篇
  1994年   4142篇
  1993年   3890篇
  1992年   3659篇
  1991年   3607篇
  1990年   3781篇
  1989年   3534篇
  1988年   3317篇
  1987年   3872篇
  1986年   3431篇
  1985年   4265篇
  1984年   4761篇
  1983年   4434篇
  1982年   4347篇
  1981年   3946篇
  1980年   3661篇
  1979年   3518篇
  1978年   3496篇
  1977年   3294篇
  1976年   3065篇
  1975年   2967篇
  1974年   2928篇
  1973年   3087篇
  1972年   2031篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
791.
792.
Progress in the introduction of coulometry for the analysis of total carbon dioxide (TCO2) in marine waters is described. An extractor—stripper removes CO2 that is measured coulometrically by the quantity of electricity (coulombs) used to electrogenerate OH? ions for the titration of the acid formed by the reaction of CO2 and ethanolamine. The equivalence point is detected photometrically with thymolphthalein as the indicator, and Faraday's Law relates coulombs to equivalents of titrant generated and CO2 determined so that there are no standard curves needed or titrants to standardize or store. Accuracy was determined by adding gelatin capsules containing 100–1500 μg C of pure CaCO3 into the stripper, and accuracies of better than ± 1 μg C were achieved. The best precision for natural seawater (± 1 standard error) of ± 0.5 μmol l?1 was found for 17 samples of Bermuda coastal waters having a mean TCO2 of 2007.2 μmol l?1 (0.05% CV). Sources of error and precautions are discussed. This method, which has been used successfully at sea, can be used to study a variety of marine phenomena involving TCO2.  相似文献   
793.
794.
The steady flow expansions of monatomic gases consisting of Maxwellia molecules into a vacuum moving under external forces with a source of spherical symmetry are investigated. The analysis is based on the B-G-K model of the Boltzmann equation with the approximation in hypersonic limit. The kinetic equation is solved by using the moment method. Analytical forms for the density and temperature are obtained for small and large distances from the source. The results show that the temperature in free expansion is less than that in the case of the expansion under the influence of external field of forces.  相似文献   
795.
Skylab EUV observations of an active region near the solar limb were analyzed. Both cool (T < 106 K) and hot (T > 106 K) loops were observed in this region. For the hot loops the observed intensity variations were small, typically a few percent over a period of 30 min. The cool loops exhibited stronger variations, sometimes appearing and disappearing in 5 to 10 min. Most of the cool material observed in the loops appeared to be caused by the downward flow of coronal rain and by the upward ejection of chromospheric material in surges. The frequent EUV brightenings observed near the loop footpoints appear to have been produced by both in situ transient energy releases (e.g. subflares) and the infall/impact of coronal rain. The physical conditions in the loops (temperatures, densities, radiative and conducting cooling rates, cooling times) were determined. The mean energy required to balance the radiative and conductive cooling of the hot loops is approximately 3 × 10–3 erg cm–3 s–1. One coronal heating mechanism that can account for the observed behavior of the EUV emission from McMath region 12634 is heating by the dissipation of fast mode MHD waves.  相似文献   
796.
797.
Summary. Surface wave behaviour in flat anisotropic structures is first illustrated by performing an exact computation on a simple two-layer model. The variational procedure of Smith & Dahlen is then used to compute the partial derivatives of surface wave phase velocities with respect to the elastic parameters in more realistic earth models. Linear relationships between the partial derivatives for a general anisotropic structure and those for a transversely isotropic structure are derived. When considering waves propagating in a fixed direction, there are only four independent derivatives for Rayleigh waves, and two for Love waves. To avoid the lack of resolution in an inverse method, we propose to use physically constrained models. These results are illustrated by using a model with hexagonal symmetry and a symmetry axis oriented either vertically or horizontally. Quasi-Love- and quasi-Rayleigh-wave partial derivatives are computed for both axis orientations. Modes up to the second overtone and periods ranging between 45 and 130 s have been considered. Finally, anomalies of phase velocity are computed in an oceanic model made of 1/6 oriented olivine crystals with horizontal or vertical preferred orientations of the a -axis.  相似文献   
798.
The lifetimes, characteristics of the shapes as well as lengths and perimeters of artificial solar granules (Nordlund, 1982, 1984a) are compared with data from the literature and parameters determined from two different sets of observed granules. No significant differences of the parameters for these sets of granules are detectable.Mitteilungen aus dem Kiepenheuer-Institut Nr. 251.  相似文献   
799.
800.
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