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   47942篇
  免费   270篇
  国内免费   579篇
系统科学   1225篇
丛书文集   169篇
教育与普及   307篇
理论与方法论   512篇
现状及发展   30280篇
研究方法   251篇
综合类   13859篇
自然研究   2188篇
  2013年   809篇
  2012年   454篇
  2011年   2452篇
  2009年   640篇
  2008年   657篇
  2007年   720篇
  2006年   812篇
  2005年   1036篇
  2004年   2172篇
  2003年   1739篇
  2002年   1397篇
  2001年   1137篇
  2000年   643篇
  1999年   829篇
  1998年   645篇
  1997年   778篇
  1996年   531篇
  1995年   399篇
  1994年   681篇
  1993年   688篇
  1992年   744篇
  1991年   662篇
  1990年   719篇
  1989年   545篇
  1988年   533篇
  1987年   524篇
  1986年   594篇
  1985年   733篇
  1984年   676篇
  1983年   575篇
  1982年   739篇
  1981年   777篇
  1980年   845篇
  1979年   1175篇
  1978年   1090篇
  1977年   1098篇
  1976年   967篇
  1975年   931篇
  1974年   709篇
  1973年   1028篇
  1972年   1097篇
  1971年   1048篇
  1970年   1009篇
  1969年   994篇
  1968年   963篇
  1967年   815篇
  1966年   672篇
  1965年   570篇
  1964年   460篇
  1963年   414篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
1 Results Acid-doped polybenzimidazoles[1] are particularly appealing because of high proton conductivity with no or low humidification and promising fuel cells performances. PBI, in fact, contains basic functional groups which can easily interact with strong oxo-acids, such as H3PO4 and H2SO4. The acid partially protonates the polymer and partially is freely dispersed in the polymer backbone, so allowing proton migration via Grotthuss mechanism along the anionic chains[2]. Anyway, a technological limit...  相似文献   
42.
1 Results In the coordination system by using complexation with organic ligand, the ff emission of lanthanide(Ⅲ) (Ln(Ⅲ)) is induced the excitation energy transfer form the organic chromophore under the light-irradiation. However, there are not so much number of reports to discuss the energy relaxation mechanism in such complexes with Ln(Ⅲ). Recently, we succeeded firstly to estimate the rate constant of the energy transfer between the ligand and Ln(Ⅲ) in Pr(Ⅲ)-phenanthroline analogs[1]. Here, we will di...  相似文献   
43.
44.
45.
分子间的范德华力是十分微弱的,但可以说普遍的存在着,在很多体系中起着巨大的作用,在物理学、化学、生物学等学科中得到了广泛的研究。但怎样计算范德华力?如何考虑各种条件对范德华力的影响?  相似文献   
46.
Summary The depressor action of small doses of adrenaline on the blood pressure (B.P.) of the decapitated cat may be abolished by pretreatment with pilocarpine; atropine restores the original depressor action. The fall in B.P. due to adrenaline after ergotamine once more becomes an increase when pilocarpine has been given earlier. After atropine a fall in B.P. again occurs. This effect of pilocarpine and atropine on a depressor response has already been described in the case of Aludrine (N-isopropyl-noradrenaline) byFromherz.  相似文献   
47.
Summary (1) By precipitation with ammonium sulfate, streptomycin, or calcium salts, we obtained from the pulp of tomatoes a substance containing carotinoids. The behaviour of this substance was analogous to that of chloroplst-substance and to that of animal cytoplasmatic nucleoproteins. Like these it contains proteins, lipids, and very probably nucleic acids. We regard this substance as achromoplastine.(2) Experiments with slices of green tomatoes show that the changing over of the chlorophyll content into the carotinoid content is inhibited by the presence of streptomycin.(3) Streptomycin inhibits the formation of chlorophyll in etiolated separated cabbage leafs, just as this drug inhibits the formation of chlorophyll in growing seeds.(4) The development of anthocyanides is not influenced by streptomycin.  相似文献   
48.
Summary Intravenous or subcutaneous administration of pyribenzamine and other antihistaminics is able to decrease, and to delay the appearance of chemosis produced by mustard oils in the eye of the rabbit. Simultaneously occurring vascular reactions, on the other hand, are but slightly affected by antihistaminics. Intravenous or subcutaneous administration of rutin has no clear-cut effect on chemosis; it is however able to inhibit the vascular reaction.  相似文献   
49.
Summary The disappearance of thrombin—formed in the blood, or added to serum-follows a manomolecular reaction-type. Heparin increases the reaction-velocity of this thrombin-inactivating process.Our investigation established that toluidine blue or kinase, which, according to the literature, bind heparin, strongly reduce the speed of thrombin-inactivation too. Therefore the heparin-binding capacity of these substances is also manifested in the decrease of thrombin-inactivation.  相似文献   
50.
莱钢特钢厂20CrMnTiH钢材成分波动较大,淬透性带HRC8~10,范围较大,制约了齿轮钢质量的进一步提高.通过研究,发现铸坯中心偏析尤其是碳偏析为主要问题.通过工艺优化采取各项措施,尤其增加电磁搅拌后,中心碳偏析得到明显改善;C控制在0.19%~0.20%的炉次达到了95.6%;淬透性带控制在HRC6内的达到了92.1%;在HRC4内的达到了86.2%;提高了产品质量,满足了用户需求.  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