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   38788篇
  免费   19372篇
  国内免费   44084篇
测绘学   3702篇
大气科学   21783篇
地球物理   13206篇
地质学   37710篇
海洋学   16772篇
天文学   936篇
综合类   4915篇
自然地理   3220篇
  2024年   220篇
  2023年   443篇
  2022年   870篇
  2021年   1226篇
  2020年   2307篇
  2019年   5649篇
  2018年   5890篇
  2017年   5534篇
  2016年   5605篇
  2015年   5017篇
  2014年   4490篇
  2013年   5154篇
  2012年   5035篇
  2011年   4801篇
  2010年   4742篇
  2009年   4021篇
  2008年   3370篇
  2007年   3420篇
  2006年   3011篇
  2005年   2788篇
  2004年   2983篇
  2003年   2654篇
  2002年   2378篇
  2001年   2151篇
  2000年   1807篇
  1999年   1771篇
  1998年   1772篇
  1997年   1851篇
  1996年   1418篇
  1995年   1362篇
  1994年   1228篇
  1993年   1194篇
  1992年   1007篇
  1991年   756篇
  1990年   721篇
  1989年   590篇
  1988年   500篇
  1987年   390篇
  1986年   313篇
  1985年   248篇
  1984年   281篇
  1983年   169篇
  1982年   204篇
  1981年   144篇
  1980年   108篇
  1979年   110篇
  1978年   48篇
  1977年   45篇
  1971年   53篇
  1970年   44篇
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
221.
九脊游仆虫(Euplotes novemcarinata Wang、1930)是土壤及淡水中生活的原生动物。本文对其重要的分类特征诸如银线系及纤毛下器做了形态学研究,确认了其分类地位,并比较了国外迄今的研究工作,在此基础上,将同物异名苔藓游仆虫(E.muscicolaKahl、1932)作了归并。  相似文献   
222.
The oceans have rich resources of microbes. In recent years, searching for antibiotic of new structure and special biological activity from marine microbes has received great attention. This paper reports the results of the studies on the distribution and the antibacterial properties of Streptomyces in the intertidal zone of Southern Fujian. These studies have important significance to further investigate  相似文献   
223.
224.
Recently, some results have been acquired with the Monte- Carlo statistical experiments in the design of ocean en gineering. The results show that Monte-Carlo statistical experiments can be widely used in estimating the parameters of wave statistical distributions, checking the probability model of the long- term wave extreme value distribution under a typhoon condition and calculating the failure probability of the ocean platforms.  相似文献   
225.
海上不同高度风速换算关系的研究   总被引:1,自引:0,他引:1  
本文用海上石油平台上观测的风梯度资料,计算了在不同风况下海面摩擦速度和粗糙高度。结果表明,海面摩擦速度u_*与海上10m高度处风速u_(10)的关系为u_*=0.055u_(10)-0.058。在4—5级风时,海面平均摩擦速度为50cm/s,粗糙度为0.022m,它相当于陆上的平均粗糙高度。在风速为2—3级时,二者与海上通常采用的数据接近。文中最后给出了上述两种风况下海上不同高度的换算系数。  相似文献   
226.
By analysing the scatter diagrams of characteristic the wave height H and the period T on the basis of instrumental data from various ocean wave stations, we found that the conditional expectation and standard deviation of wave period for a given wave height can be better predicted by using the equations of normal linear regression rather than by those based on the log- normal law. The latter was implied in Ochi' s bivariate log-normal model(Ochi. 1978) for the long-term joint distribution of H and T. With the expectation and standard deviation predicted by the normal linear regression equations and applying proper types of distribution, we have obtained the conditional distribution of T for given H. Then combining this conditional P(T / H) with long-term marginal distribution of the wave height P(H) we establish a new parameterized model for the long-term joint distribution P(H,T). As an example of the application of the new model we give a method for estimating wave period associated with an extreme w  相似文献   
227.
In this study the zinc exchange process and the reason for the non-additive exchange of δ-MnO2/montmorrillonite have been investigated. The results suggest that with pH increasing from 2 to 8, the stepwise exchange of zinc occurs on δ-MnO. and montmorrillonite in their mixture.At low pH zinc ions are bound on δ-MnO2, while at high pH, the exchange on montmorrillonite dominates the surface interaction in the mixture. The study demonstrates that the stepwise exchange and the heterogeneity of the binding sites on δ-MnO2 surface are the reason for the non-additive exchange. It is also shown that when δ-MnO2 amounts to 5 %, zinc exchange on sea sediments is controlled by 8-MnO2.  相似文献   
228.
The vertical migration of diatoms in mud-flats was studied in littoral of Xibian, Longhai County and Xiao'ao, Lianjiang County, the dominant species being Gyrosigma spencerii and Hantzschia virgata respectively. Both light and tide are important factors affecting the vertical migration of diatoms. With the advent of tide, the diatom community disappears from the surface. It reappears gradually when exposed to light. In the dark, the diatoms never appear in great quantity on the surface in laboratory or in littoral. Whether in the light or in the dark, the diatoms aggregate in maximum numbers in the upper 2 mm of the sediment. But when the tide has covered the sediments for 4 hr, Hantz. virgata penetrates the mud down to a depth of 14 mm, Gy. spencerii, 12 mm. We have found that Gy. spencerii posseses phototaxis, but no tide rhythm and no day-night rhythm in our laboratory.  相似文献   
229.
By using occanographic data (luring 1955-1978, we have analysed aspects of volume transport variation of the Kuroshio tor PN-linc in the East China Sea, and relationships between volume transport variation of the Kuroshio and the subtropical high pressure (SHP) of the Pacific. We have found that volume transport of the Kuroshio is closely related to the long-period oscillations of the subtropical high pressure for 8-9 years and 2-3 years, respectively. The subtropical high pressure has a phase lag behind compared with volume transport of the Kuroshio. In addition, the seasonal volume transport of the Kuroshio is closely related to the subtropical high pressure, too. The results show that volume transport variation of the Kuroshio has certain influence on future variation of the subtropical high pressure. Therefore, the volume transport of the Kuroshio may be an important parameter lor long-period forecasting of the subtropical high pressure.  相似文献   
230.
1Introduction Aerobic anoxygenic phototrophic bacteria(AAPB) are facultative heterotrophs that can utilizelight for photosynthesis. These unique-functioningbacteria are widely distributed in marine environ-ments, such as coastal waters (Shiba et al., 1991…  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