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   12620篇
  免费   2401篇
  国内免费   2962篇
测绘学   708篇
大气科学   1913篇
地球物理   3529篇
地质学   6761篇
海洋学   994篇
天文学   849篇
综合类   866篇
自然地理   2363篇
  2024年   11篇
  2023年   28篇
  2022年   63篇
  2021年   71篇
  2020年   64篇
  2019年   120篇
  2018年   86篇
  2017年   84篇
  2016年   92篇
  2015年   89篇
  2014年   224篇
  2013年   391篇
  2012年   352篇
  2011年   230篇
  2010年   293篇
  2009年   321篇
  2008年   495篇
  2007年   788篇
  2006年   916篇
  2005年   1066篇
  2004年   1080篇
  2003年   850篇
  2002年   870篇
  2001年   829篇
  2000年   789篇
  1999年   941篇
  1998年   1138篇
  1997年   740篇
  1996年   734篇
  1995年   684篇
  1994年   621篇
  1993年   740篇
  1992年   582篇
  1991年   497篇
  1990年   327篇
  1989年   196篇
  1988年   174篇
  1987年   88篇
  1986年   22篇
  1985年   45篇
  1984年   31篇
  1983年   46篇
  1982年   28篇
  1981年   20篇
  1980年   22篇
  1979年   16篇
  1978年   16篇
  1977年   11篇
  1976年   13篇
  1877年   7篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
本文报告一种用半导体激光器作光源,用子脉冲宽度100ns,长度为511的周期伪随机码对半导体激光器的输出作幅度调制;APD作光电检测器,高速数字信号处理器TMS320C25作信号处理的激光雷达组成方案和试作样机的实验结果。  相似文献   
52.
53.
54.
55.
56.
57.
Eighteen gametophytes including L.japonica ,L.ochotensis and L.longissima,were verified with random amplified polymorphic DNA (RAPD) technique.Eighteen ten-base primers were chosen from 100 primers selected for final amplification test.Among the total of 205 bands amplified ,181(88.3%) were polymorphic ,The genetic distance among different strains ranged from 0.072 to 0.391,The dendrogram constructed by unweighted pair -group method with arithmetic(UPGMA) method showed that the female and male gamtophytes of the same cell lines could be grouped in pairs respectively,It indicated that RAPD analysis could be used not only to distin-gusih different strains of Laminariz ,but also to distinguish male and female gametophyte within the same cell lines,There is ambiguous systematic relationship if judged mereely by the present data It seems that the use of RAPD marker is limited to elucidstion of the phylogenetic relationship among the species of Laminaria.  相似文献   
58.
The collisional instability of the drift wave in a multi-component plasma is investigated. It is shown that when the electron and ion density gradients are different, e.g., due to the presence of a static third component or due to neutral drag effects, the drift mode becomes unstable. The instability is caused by the simultaneous action of the electron collisions with all other plasma species and the spatial difference of the density of the plasma components. This instability may be expected as a natural consequence of the stratification of a multi-component plasma placed in an external gravity field where it can operate for any amount of charge on heavy particles. Therefore it could develop in weakly ionized cold interstellar regions for example, when the heavy particles, i.e. charged grains, are a few tens of Å in size, and carry typically ±1,±2 charge. In the solar atmosphere, it may appear in the weakly ionized photospheric layers due to the convective motion of the neutral component.  相似文献   
59.
V838 Mon is the prototype of a new class of objects. Understanding the nature of its multistage outburst and similar systems is challenging. So far, several scenarios have been invoked to explain this group of stars. In this work, the planets-swallowing model for V838 Mon is further investigated, taking into account the findings that the progenitor is most likely a massive B-type star. We find that the super-Eddington luminosity during the eruption can explain the fast rising times of the three peaks in the optical light curve. We used two different methods to estimate the location where the planets were consumed. There is a nice agreement between the values obtained from the luminosities of the peaks and from their rising time-scale. We estimate that the planets were stopped at a typical distance of one solar radius from the centre of the host giant star. The planets-devouring model seems to give a satisfying explanation to the differences in the luminosities and rising times of the three peaks in the optical light curve of V838 Mon. The peaks may be explained by the consumption of three planets or alternatively by three steps in the terminal falling process of a single planet. We argue that only the binary merger and the planets-swallowing models are consistent with the observations of the new type of stars defined by V838 Mon.  相似文献   
60.
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