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   278113篇
  免费   3524篇
  国内免费   886篇
化学   142680篇
晶体学   3311篇
力学   11935篇
综合类   8篇
数学   32670篇
物理学   91919篇
  2020年   2306篇
  2019年   2436篇
  2018年   3197篇
  2017年   3148篇
  2016年   4956篇
  2015年   3261篇
  2014年   5009篇
  2013年   12564篇
  2012年   9371篇
  2011年   11550篇
  2010年   7909篇
  2009年   7715篇
  2008年   10738篇
  2007年   10726篇
  2006年   10119篇
  2005年   9082篇
  2004年   8235篇
  2003年   7247篇
  2002年   7144篇
  2001年   8151篇
  2000年   6262篇
  1999年   4920篇
  1998年   4134篇
  1997年   4036篇
  1996年   3694篇
  1995年   3587篇
  1994年   3545篇
  1993年   3669篇
  1992年   3729篇
  1991年   3785篇
  1990年   3553篇
  1989年   3467篇
  1988年   3502篇
  1987年   3392篇
  1986年   3212篇
  1985年   4446篇
  1984年   4613篇
  1983年   3688篇
  1982年   3912篇
  1981年   3888篇
  1980年   3760篇
  1979年   3851篇
  1978年   3975篇
  1977年   3875篇
  1976年   3960篇
  1975年   3587篇
  1974年   3610篇
  1973年   3672篇
  1972年   2470篇
  1971年   2087篇
排序方式: 共有10000条查询结果,搜索用时 225 毫秒
1.
Russian Journal of General Chemistry - Aminophosphabetaines, i.e., isobutyl {[alkyl(dimethyl)ammonio]methyl}phosphonates with higher alkyl substituents at the nitrogen atom, were obtained by a...  相似文献   
2.
Journal of Structural Chemistry - A new polymorph of the [ReCl3(MeCN)(PPh3)2] complex is synthesized and characterized by single crystal X-ray diffraction (XRD). Its recrystallization from the...  相似文献   
3.
Nitrene transfer reactions have emerged as one of the most powerful and versatile ways to insert an amine function to various kinds of hydrocarbon substrates. However, the mechanisms of nitrene generation have not been studied in depth albeit their formation is taken for granted in most cases without definitive evidence of their occurrence. In the present work, we compare the generation of tosylimido iron species and NTs transfer from FeII and FeIII precursors where the metal is embedded in a tetracarbene macrocycle. Catalytic nitrene transfer to reference substrates (thioanisole, styrene, ethylbenzene and cyclohexane) revealed that the same active species was at play, irrespective of the ferrous versus ferric nature of the precursor. Through combination of spectroscopic (UV-visible, Mössbauer), ESI-MS and DFT studies, an FeIV tosylimido species was identified as the catalytically active species and was characterized spectroscopically and computationally. Whereas its formation from the FeII precursor was expected by a two-electron oxidative addition, its formation from an FeIII precursor was unprecedented. Thanks to a combination of spectroscopic (UV-visible, EPR, Hyscore and Mössbauer), ESI-MS and DFT studies, we found that, when starting from the FeIII precursor, an FeIII tosyliodinane adduct was formed and decomposed into an FeV tosylimido species which generated the catalytically active FeIV tosylimide through a comproportionation process with the FeIII precursor.  相似文献   
4.
Journal of Experimental and Theoretical Physics - The surface current density of charge carriers in two-dimensional doubly periodic n-type semiconductor superlattices in an external magnetic field...  相似文献   
5.
Russian Journal of Applied Chemistry - A method for the disposal of hydrolysis lignin waste based on its modification with ZnCl2 and FeCl3 and subsequent carbonization at 800°C was proposed....  相似文献   
6.
Journal of Applied Spectroscopy - Three simple, sensitive, precise, and rapid spectrophotometric methods are developed and optimized for the assay of vardenafil in pharmaceutical formulations. The...  相似文献   
7.
Russian Journal of General Chemistry - The review article traces the main trends of the synthetic approach to the solution of the problem of overcoming the resistance of pathogenic bacterial...  相似文献   
8.
Journal of Optimization Theory and Applications - Direct optimization of many-revolution spacecraft trajectories is performed using an unconstrained formulation with many short-arc, embedded...  相似文献   
9.
10.
Central European Journal of Operations Research - Sustainability practice within supply chains remains in an early development phase. Enterprises still need tools that support the integration of...  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