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   68956篇
  免费   285篇
  国内免费   576篇
系统科学   1270篇
丛书文集   855篇
教育与普及   319篇
理论与方法论   554篇
现状及发展   35990篇
研究方法   1422篇
综合类   27217篇
自然研究   2190篇
  2013年   827篇
  2012年   769篇
  2011年   2874篇
  2009年   667篇
  2008年   1026篇
  2007年   1116篇
  2006年   1228篇
  2005年   1424篇
  2004年   2501篇
  2003年   2132篇
  2002年   1834篇
  2001年   1894篇
  2000年   1350篇
  1999年   1219篇
  1998年   655篇
  1997年   784篇
  1994年   691篇
  1993年   698篇
  1992年   1154篇
  1991年   991篇
  1990年   1070篇
  1989年   886篇
  1988年   867篇
  1987年   861篇
  1986年   906篇
  1985年   1094篇
  1984年   949篇
  1983年   822篇
  1982年   889篇
  1981年   911篇
  1980年   1055篇
  1979年   1807篇
  1978年   1641篇
  1977年   1624篇
  1976年   1294篇
  1975年   1335篇
  1974年   1451篇
  1973年   1544篇
  1972年   1610篇
  1971年   1789篇
  1970年   2076篇
  1969年   1723篇
  1968年   1655篇
  1967年   1595篇
  1966年   1350篇
  1965年   1059篇
  1959年   622篇
  1958年   826篇
  1957年   709篇
  1956年   606篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
European blackcaps,Sylvia atricapilla, with one breeding season per year, have a single-peaked annual testes cycle. However, African conspecifics from the Cape Verde Islands with two breeding seasons per annum demonstrate a two-peaked cycle. Both population-specific cycles reflect differences in the respective endogenous circannual rhythms. Experimental hybridization of birds of the two populations resulted in an intermediate pattern of testes cycle, thus demonstrating that there are genetic components for some temporal aspects in an avian reproductive cycle. Another characteristic of the African birds, their extremely rapid juvenile development and early sexual maturity (at an age of 5–6 months) proved largely to be a photoperiodic (short-day) effect in birds hatched in autumn. The same effect could also be induced in European conspecifics exposed to correspondingly short day-lengths.  相似文献   
192.
J M Nunnari  D L Zimmerman  S C Ogg  P Walter 《Nature》1991,352(6336):638-640
The rough endoplasmic reticulum membranes of mammalian cells contain specific ribosome-binding sites. A purification to apparent homogeneity of a negatively charged protein (ERp180) of relative molecular mass 180,000 (180 K) was reported which was proposed to function as a rough endoplasmic reticulum ribosome receptor. We report here that ribosome-binding site activity quantitatively solubilized from rough endoplasmic reticulum membranes does not cofractionate with ERp180. By contrast, ribosome-binding site activity fractionates as a much smaller, positively charged protein.  相似文献   
193.
There is considerable debate among scholars over whether Descartes allowed for genuine body–body interaction. I begin by considering Michael Della Rocca’s recent claim that Descartes accepted such interaction, and that his doctrine of the creation of the eternal truths indicates how this interaction could be acceptable to him. Though I agree that Descartes was inclined to accept real bodily causes of motion, I differ from Della Rocca in emphasizing that his ontology ultimately does not allow for them. This is not the end of the story however, since two of Descartes’s successors offered incompatible ways of developing his conflicted account of motion. I contrast the occasionalist view of Nicolas Malebranche that changes in motion derive directly from divine volitions with the non-occasionalist claim of Pierre-Sylvain Regis that such changes derive from a nature distinct from God. In light of Della Rocca’s interpretation, it is noteworthy that the issue of eternal truths is relevant to both alternative accounts. Indeed, Regis took the doctrine that such truths are created to provide crucial support for his alternative to an occasionalist account of body–body interaction. What does not help Della Rocca, however, is that Regis’s view of motion requires a fundamental revision of Descartes’s ontology.  相似文献   
194.
基于Windows NT的HRPS-Ⅱ设备的控制系统设计   总被引:1,自引:0,他引:1  
介绍了华中科技大学开发的HRPS-Ⅱ型选择性激光烧结设备的体系结构,根据选择性激光烧结的工作原理并结合该系统的硬件结构开发了基于NT操作系统下的控制软件.该控制软件是一种分层的结构,各个不同优先级的任务在不同的层面上运行,实时性强的部分运行在硬件中断级别上,对控制软件的关键性技术进行了描述.通过试验运行,该设备成功地加工出了复杂的原型件,证明控制系统开发是成功的.  相似文献   
195.
BARNOTHY J 《Nature》1948,161(4096):681
  相似文献   
196.
Summary (1) Doubly refracting lipoid droplets can be demonstrated in normal and pathological sera by means of alcohol ether extraction. This is not possible with ether extraction alone.(2) These lipoid droplets can be demonstrated only, or almost only, when they are precipitated out of alcohol or ether alcohol  相似文献   
197.
198.
199.
Progress on nuclear liquid gas phase transition (LGPT) or critical behavior has been simply reviewed and some signals of LGPT in heavy ion collisions, especially in NIMROD data, are focused. These signals include the power-law charge distribution, the largest fluctuation of the fragment observables, the nuclear Zipf law, caloric curve and critical exponent analysis etc.  相似文献   
200.
Summary By means of intratracheal injection, particles of metalloid silicium about five microns in size are introduced into the lungs of guinea pigs, and the secondary parenchymatous reactions are studied from 10 minutes to 105 days. In the dust cells, the silicium is progressively broken up into very small particles of less than 1 micron in size. The cells do not show degeneration or mummification as the ordinary silica cells (Mavrogordato) do.  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