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   468948篇
  免费   4009篇
  国内免费   1306篇
化学   236049篇
晶体学   7473篇
力学   23621篇
综合类   17篇
数学   56781篇
物理学   150322篇
  2021年   4301篇
  2020年   4562篇
  2019年   5300篇
  2018年   7403篇
  2017年   7591篇
  2016年   10327篇
  2015年   5665篇
  2014年   9731篇
  2013年   21701篇
  2012年   16841篇
  2011年   20127篇
  2010年   15226篇
  2009年   15315篇
  2008年   18407篇
  2007年   18275篇
  2006年   16651篇
  2005年   14570篇
  2004年   13649篇
  2003年   12174篇
  2002年   12193篇
  2001年   14155篇
  2000年   10482篇
  1999年   8095篇
  1998年   6825篇
  1997年   6731篇
  1996年   6229篇
  1995年   5455篇
  1994年   5503篇
  1993年   5281篇
  1992年   5898篇
  1991年   6189篇
  1990年   5836篇
  1989年   5852篇
  1988年   5557篇
  1987年   5691篇
  1986年   5345篇
  1985年   6794篇
  1984年   6951篇
  1983年   5770篇
  1982年   5806篇
  1981年   5586篇
  1980年   5344篇
  1979年   5887篇
  1978年   5833篇
  1977年   6015篇
  1976年   5990篇
  1975年   5654篇
  1974年   5545篇
  1973年   5659篇
  1972年   4142篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
991.
Institute of the Chemistry of Plant Substances, Uzbek SSR Academy of Sciences, Tashkent. Abu Ali ibn Sina [Avicenna] Tashkent State Medical Institute, Dushanbe. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 279–281, March–April, 1990.  相似文献   
992.
The possibility has been studied of obtaining ecdysteroids — ecdysterone and turkesterone — with the aid of a culture of the tissues and cells of the plantAjuga turkestanica. Conditions have been selected under which the yield of ecdysteroids in the culture of tissues and cells is comparable with amounts of the same substances in the organs of the whole plant.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 51–52, January–February, 1990.  相似文献   
993.
A series of new adducts of fluorinated REE -diketonates with triphenylphosphine oxide was synthesized. The paths of their fragmentation under electron impact were investigated. It was shown that in contrast to the complex with 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione, the complexes of europium with 1-trifluoromethoxy-1,1-difluoro-5,5-dimethylhexane-2,4-dione and 1, 1,1-trifluoro-5,5-dimethylhexane-2,4-dione form dimeric associates in the gaseous phase. The reasons for this difference in the behavior of the complexes and the possible structure of one of the dimers are discussed.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. 2, March–April, 1990.  相似文献   
994.
(2-Quinoxalylthio)acetic acids have been found to cyclize on treatment with acetic anhydride to give the mesoionic thiazolo[3,2-a]quinoxalinium oxides. Some reactions of these compounds have been studied, and the relationship of the color of the compounds to their chemical structure examined.For Communication 18, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1680–1684, December, 1989.  相似文献   
995.
996.
Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2154–2155, September, 1989.  相似文献   
997.
998.
Mössbauer spectroscopy was used to study iron reduction in mineral originated from Aswan area in Egypt. The Mössbauer spectra of samples heat treated at 1000 °C in hydrogen atmosphere show a gradually reduction process of hematite. The main phase of sample aged for 1 h is magnetite, while alpha-iron as well as some silicates and wustite type oxide can be detected in sample aged for 3 h. Further aging of sample results in dissolution of alloying elements /Si, Al/, being present in the samples. This process starts already in samples aged at 1000 °C for 4 h, but it is more expressive in minerals heat treated at 1200 °C or 1300 °C, when all paramagnetic phases disappeared.  相似文献   
999.
The reaction of -cyano--aminocrotonic ester with DMFA diethylacetal gives 1-ethoxycarbonyl-1-cyano-2-(N-dimethylaminomethylene)-amino-4-dimethylaminobutadiene, the reaction of which with ammonium acetate and amines leads to 4-methylenepyrimidine derivatives. Condensation of -cyano--aminocrotonic ester with dimethylacetamide dieethylacetal gave 2-(ethoxycarbonylcyano)methylene-4-dimethylamino-6-methylpyridine. It was found that in an alkaline medium, 1-benzyl-4-(ethoxycarbonylcyano) methylenepyrimidine recyclizes into 1-benzyl-3-cyano-4-amino-2-pyridone.For Communication 52, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1109–1114, August, 1988.  相似文献   
1000.
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