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   462665篇
  免费   5731篇
  国内免费   1534篇
化学   244112篇
晶体学   6731篇
力学   20106篇
综合类   25篇
数学   59117篇
物理学   139839篇
  2021年   3814篇
  2020年   4242篇
  2019年   4579篇
  2018年   7038篇
  2017年   7159篇
  2016年   9584篇
  2015年   6156篇
  2014年   9074篇
  2013年   21000篇
  2012年   18532篇
  2011年   21480篇
  2010年   14953篇
  2009年   14619篇
  2008年   18271篇
  2007年   18245篇
  2006年   16908篇
  2005年   18705篇
  2004年   17138篇
  2003年   14050篇
  2002年   12224篇
  2001年   13611篇
  2000年   10178篇
  1999年   8036篇
  1998年   6660篇
  1997年   6386篇
  1996年   6137篇
  1995年   5514篇
  1994年   5437篇
  1993年   5228篇
  1992年   5951篇
  1991年   6038篇
  1990年   5768篇
  1989年   5556篇
  1988年   5561篇
  1987年   5421篇
  1986年   5122篇
  1985年   6587篇
  1984年   6724篇
  1983年   5327篇
  1982年   5407篇
  1981年   5384篇
  1980年   4972篇
  1979年   5380篇
  1978年   5516篇
  1977年   5401篇
  1976年   5384篇
  1975年   4942篇
  1974年   4922篇
  1973年   4921篇
  1972年   3489篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
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   
62.
63.
This study was aimed at the development of a conductometric biosensor based on acetylcholinesterase considering the feasibility of its application for the inhibitory analysis of various toxicants. In this paper, the optimum conditions for enzyme immobilization on the transducer surface are selected as well as the optimum concentration of substrate for inhibitory analysis. Sensitivity of the developed biosensor to different classes of toxic compounds (organophosphorus pesticides, heavy metal ions, surfactants, aflatoxin, glycoalkaloids) was tested. It is shown that the developed biosensor can be successfully used for the analysis of pesticides and mycotoxins, as well as for determination of total toxicity of the samples. A new method of biosensor analysis of toxic substances of different classes in complex multicomponent aqueous samples is proposed.  相似文献   
64.
65.
Experimental Techniques - The analysis of residual stress in glass is usually carried out by means of photoelastic methods. This article considers the automation of the white light photoelastic...  相似文献   
66.
67.
Russian Journal of General Chemistry - 8-Phenyltetrazolo[5,1-b][1,3,4]thiadiazepine was obtained as a result of intermolecular cyclization of 1-amino-1H-tetrazole-5-thiol and 3-phenyl-2-propynal....  相似文献   
68.
69.
70.
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