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   89981篇
  免费   2390篇
  国内免费   1574篇
化学   33212篇
晶体学   922篇
力学   7287篇
综合类   92篇
数学   33109篇
物理学   19323篇
  2023年   240篇
  2022年   239篇
  2021年   361篇
  2020年   517篇
  2019年   501篇
  2018年   10740篇
  2017年   10492篇
  2016年   6650篇
  2015年   1367篇
  2014年   960篇
  2013年   1215篇
  2012年   4903篇
  2011年   11574篇
  2010年   6344篇
  2009年   6763篇
  2008年   7419篇
  2007年   9508篇
  2006年   933篇
  2005年   1945篇
  2004年   1955篇
  2003年   2283篇
  2002年   1370篇
  2001年   566篇
  2000年   537篇
  1999年   401篇
  1998年   386篇
  1997年   309篇
  1996年   362篇
  1995年   293篇
  1994年   246篇
  1993年   210篇
  1992年   149篇
  1991年   172篇
  1990年   149篇
  1989年   121篇
  1988年   115篇
  1987年   104篇
  1986年   103篇
  1985年   96篇
  1984年   71篇
  1983年   51篇
  1982年   64篇
  1981年   57篇
  1980年   53篇
  1979年   51篇
  1978年   44篇
  1914年   45篇
  1913年   41篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
31.
This work describes a novel methodology for the recognition of brandies based on direct injection of a raw sample followed by GC-MS analysis. Direct injection was chosen for its simplicity and the fact that the composition of the samples analysed remains unchanged compared to original brandy. The repeatability of the analytical procedure was evaluated by a comparison of the peak areas for randomly selected compounds obtained from 10 parallel measurements. A novel chemometric procedure was investigated in order to separate the samples studied on the basis of their geographical origin, processing technology or maturation time. In this procedure, a principal component analysis was applied to full chromatograms to select the time interval that shows the significant differences between the samples studied. It was shown that the chromatogram recorded at 36–39 min bore the maximal differences, hence it could be used to classify the brandy samples. The chromatographic peaks found within this time interval were identified and their peak areas determined. These compounds could be used as specific markers for determining geographical origin or processing technology.  相似文献   
32.
33.
34.
35.
36.
Functional π‐conjugated molecules are relevant for the preparation of new organic electronic materials with improved performance. However, their synthesis is often rendered difficult by their inherently low solubility, and the permanent attachment of solubilizing groups may change the properties of the material. Here, we introduced the chlorendylimidyl moiety as a new temporary protecting group for the straightforward large‐scale synthesis of protected quarter‐, sexi‐, octathiophene, and perylene bisimide diamine and dicarboxylic acid derivatives. The obtained chlorendylimides and chlorendylimidyl active esters were highly soluble in organic solvents, and optical spectroscopy confirmed the low tendency of the compounds to aggregate in solution. At the same time, they could be conveniently purified by recrystallization or precipitation. Single‐crystal X‐ray structures obtained for most compounds showed supramolecular motifs highlighting the role of the rigid, polychlorinated chlorendyl moieties in their crystallization. The obtained protected diamine and dicarboxylic acid derivatives were easily deprotected and converted into various amide‐substituted oligothiophenes and perylene bisimides that are of interest as new functional materials for organic electronic thin film or nanowire devices.  相似文献   
37.
Orthomodular partial algebras (OMAs) can be seen as the algebraic representation of orthomodular posets. We use Greechie diagrams for the graphical representation of OMAs and investigate characterizations for the strong embeddability of a given OMA into a Boolean OMA. We present a complete list of the Greechie diagrams of OMAs up to 24 elements, and we show that there exists an infinite OMA that is generated by 4 elements.  相似文献   
38.
39.
40.
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