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   283609篇
  免费   5361篇
  国内免费   1446篇
化学   162752篇
晶体学   3553篇
力学   10831篇
综合类   61篇
数学   35866篇
物理学   77353篇
  2021年   2294篇
  2020年   2690篇
  2019年   2854篇
  2018年   3776篇
  2017年   3587篇
  2016年   5789篇
  2015年   4223篇
  2014年   5422篇
  2013年   12708篇
  2012年   11927篇
  2011年   13780篇
  2010年   9289篇
  2009年   8764篇
  2008年   12559篇
  2007年   12661篇
  2006年   11798篇
  2005年   10857篇
  2004年   9652篇
  2003年   8273篇
  2002年   7982篇
  2001年   7802篇
  2000年   6027篇
  1999年   4395篇
  1998年   3648篇
  1997年   3564篇
  1996年   3699篇
  1995年   3095篇
  1994年   3266篇
  1993年   3090篇
  1992年   3356篇
  1991年   3319篇
  1990年   3142篇
  1989年   3013篇
  1988年   2884篇
  1987年   2833篇
  1986年   2811篇
  1985年   3786篇
  1984年   3849篇
  1983年   3127篇
  1982年   3465篇
  1981年   3209篇
  1980年   2974篇
  1979年   3118篇
  1978年   3315篇
  1977年   3312篇
  1976年   3367篇
  1975年   3063篇
  1974年   3169篇
  1973年   3210篇
  1972年   2418篇
排序方式: 共有10000条查询结果,搜索用时 796 毫秒
1.
Kinetics and Catalysis - The catalytic properties of a nickel phosphide catalyst supported on silica gel in the reductive amination of carbonyl compounds were studied in a flow reactor using...  相似文献   
2.
Yang  T. L.  Kao  C. L.  Kuo  C. E.  Yeh  H. C.  Li  W. J.  Li  H. T.  Chen  C. Y. 《Chemistry of Natural Compounds》2022,58(5):825-827
Chemistry of Natural Compounds - A novel bibenzyl derivative, hydrangchinenin (1), was isolated from the stems of Hydrangea chinensis Maxim. (Hydrangeaceae). Its structure was determined on the...  相似文献   
3.
Journal of Dynamics and Differential Equations - In this paper we focus on a class of symmetric vector fields in the context of singularly perturbed fast-slow dynamical systems. Our main question...  相似文献   
4.
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   
5.
Journal of Analytical Chemistry - The present development involved a flow injection strategy using a mini-column of multiwalled carbon nanotubes and fluorescent detection (λex of 283 nm,...  相似文献   
6.
Meccanica - In this work we study, from the numerical point of view, a one-dimensional thermoelastic problem where the thermal law is of type III. Quasi-static microvoids are also assumed within...  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.
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   
9.
Physics of Atomic Nuclei - In this paper, the analysis of the relation between the multiplication factor of neutrons of the multiplying system measured in critical experiments and the system...  相似文献   
10.
Set-Valued and Variational Analysis - In a Hilbert framework ℌ, we study the convergence properties of a Newton-like inertial dynamical system governed by a general maximally monotone...  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