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   149288篇
  免费   2871篇
  国内免费   513篇
化学   68914篇
晶体学   1150篇
力学   8931篇
综合类   9篇
数学   42031篇
物理学   31637篇
  2023年   480篇
  2022年   640篇
  2021年   922篇
  2020年   1056篇
  2019年   983篇
  2018年   11331篇
  2017年   10953篇
  2016年   7916篇
  2015年   2742篇
  2014年   2345篇
  2013年   4253篇
  2012年   8292篇
  2011年   14854篇
  2010年   8261篇
  2009年   8289篇
  2008年   10476篇
  2007年   12485篇
  2006年   3997篇
  2005年   5338篇
  2004年   5278篇
  2003年   4817篇
  2002年   3336篇
  2001年   1768篇
  2000年   1621篇
  1999年   1070篇
  1998年   914篇
  1997年   815篇
  1996年   1046篇
  1995年   748篇
  1994年   746篇
  1993年   797篇
  1992年   761篇
  1991年   648篇
  1990年   588篇
  1989年   578篇
  1988年   490篇
  1987年   493篇
  1986年   457篇
  1985年   673篇
  1984年   609篇
  1983年   483篇
  1982年   574篇
  1981年   543篇
  1980年   500篇
  1979年   478篇
  1978年   473篇
  1977年   386篇
  1976年   433篇
  1974年   403篇
  1973年   392篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
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   
2.
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   
3.
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   
4.
5.

In this paper, we design a Branch and Bound algorithm based on interval arithmetic to address nonconvex robust optimization problems. This algorithm provides the exact global solution of such difficult problems arising in many real life applications. A code was developed in MatLab and was used to solve some robust nonconvex problems with few variables. This first numerical study shows the interest of this approach providing the global solution of such difficult robust nonconvex optimization problems.

  相似文献   
6.
Set-Valued and Variational Analysis - R. Tyrell Rockafellar and his collaborators introduced, in a series of works, new regression modeling methods based on the notion of superquantile (or...  相似文献   
7.
Mejjaoli  H.  Ben Saïd  S. 《Analysis Mathematica》2021,47(3):629-663
Analysis Mathematica - We introduce and study the k-Hankel Gabor transform. We investigate the localization operators for this transform. In particular, we study their trace class properties and we...  相似文献   
8.
9.
A temperature control unit was implemented to vary the temperature of samples studied on a commercial Mobile Universal Surface Explorer nuclear magnetic resonance (MOUSE-NMR) apparatus. The device was miniaturized to fit the maximum MOUSE sampling depth (25 mm). It was constituted by a sample holder sandwiched between two heat exchangers placed below and above the sample. Air was chosen as the fluid to control the temperature at the bottom of the sample, at the interface between the NMR probe and the sample holder, in order to gain space. The upper surface of the sample was regulated by the circulation of water inside a second heat exchanger placed above the sample holder. The feasibility of using such a device was demonstrated first on pure water and then on several samples of bread dough with different water contents. For this, T1 relaxation times were measured at various temperatures and depths and were then compared with those acquired with a conventional compact closed-magnet spectrometer. Discussion of results was based on biochemical transformations in bread dough (starch gelatinization and gluten heat denaturation). It was demonstrated that, within a certain water level range, and because of the low magnetic field strength of the MOUSE, a linear relationship could be established between T1 relaxation times and the local temperature in the dough sample.  相似文献   
10.
Sun  Yekai  Yuan  Jie  Vizzaccaro  Alessandra  Salles  Loïc 《Nonlinear dynamics》2021,104(4):3077-3107
Nonlinear Dynamics - The nonlinear modes of a non-conservative nonlinear system are sometimes referred to as damped nonlinear normal modes (dNNMs). Because of the non-conservative...  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