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   247924篇
  免费   3506篇
  国内免费   615篇
化学   129243篇
晶体学   3582篇
力学   10021篇
综合类   6篇
数学   31222篇
物理学   77971篇
  2021年   2110篇
  2020年   2448篇
  2019年   2609篇
  2018年   3617篇
  2017年   3435篇
  2016年   5398篇
  2015年   3662篇
  2014年   4930篇
  2013年   12000篇
  2012年   9965篇
  2011年   11304篇
  2010年   7756篇
  2009年   7347篇
  2008年   10183篇
  2007年   10119篇
  2006年   9535篇
  2005年   8780篇
  2004年   8059篇
  2003年   6994篇
  2002年   6684篇
  2001年   7241篇
  2000年   5783篇
  1999年   4408篇
  1998年   3533篇
  1997年   3444篇
  1996年   3340篇
  1995年   2994篇
  1994年   2925篇
  1993年   2759篇
  1992年   3138篇
  1991年   3049篇
  1990年   3003篇
  1989年   2736篇
  1988年   2793篇
  1987年   2756篇
  1986年   2584篇
  1985年   3390篇
  1984年   3613篇
  1983年   2937篇
  1982年   3112篇
  1981年   3090篇
  1980年   2893篇
  1979年   2996篇
  1978年   3080篇
  1977年   3074篇
  1976年   3152篇
  1975年   2907篇
  1974年   2895篇
  1973年   3024篇
  1972年   1912篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis.  相似文献   
43.
44.
45.
46.
There are thousands of papers published every year investigating the properties and possible applications of ionic liquids. Industrial use of these exceptional fluids requires adequate understanding of their physical properties, in order to create the ionic liquid that will optimally suit the application. Computational property prediction arose from the urgent need to minimise the time and cost that would be required to experimentally test different combinations of ions. This review discusses the use of machine learning algorithms as property prediction tools for ionic liquids (either as standalone methods or in conjunction with molecular dynamics simulations), presents common problems of training datasets and proposes ways that could lead to more accurate and efficient models.

In this review article, the authors discuss the use of machine learning algorithms as tools for the prediction of physical and chemical properties of ionic liquids.  相似文献   
47.
Russian Journal of General Chemistry - The main sources of emissions of industrial nitrogen oxides have been reviewed. A promising method for the absorption of nitrogen monoxide by a reusable...  相似文献   
48.
In this communication, we report the synthesis of small‐sized (<10 nm), water‐soluble, magnetic nanoparticles (MNPs) coated with polyhedral oligomeric silsesquioxanes (POSS), which contain either polyethylene glycol (PEG) or octa(tetramethylammonium) (OctaTMA) as functional groups. The POSS‐coated MNPs exhibit superparamagnetic behavior with saturation magnetic moments (51–53 emu g?1) comparable to silica‐coated MNPs. They also provide good colloidal stability at different pH and salt concentrations, and low cytotoxicity to MCF‐7 human breast epithelial cells. The relaxivity data and magnetic resonance (MR) phantom images demonstrate the potential application of these MNPs in bioimaging.  相似文献   
49.
50.
The results of investigation of optical image detectors designed for the largest problem, near-VUV, range of the spectrum are presented. The possibility of using a dual-stage image detection system to appreciably lower the sensitivity threshold and make computer data processing feasible is considered. The integration of a UV module into a wideband image detector is studied.  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