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   429418篇
  免费   2773篇
  国内免费   799篇
化学   205137篇
晶体学   6851篇
力学   22660篇
综合类   6篇
数学   58779篇
物理学   139557篇
  2021年   3933篇
  2020年   4368篇
  2019年   5223篇
  2018年   7437篇
  2017年   7711篇
  2016年   10148篇
  2015年   4985篇
  2014年   9058篇
  2013年   18210篇
  2012年   14553篇
  2011年   17056篇
  2010年   13610篇
  2009年   13835篇
  2008年   16339篇
  2007年   16045篇
  2006年   14626篇
  2005年   12811篇
  2004年   12198篇
  2003年   11229篇
  2002年   11284篇
  2001年   11704篇
  2000年   9134篇
  1999年   6874篇
  1998年   6198篇
  1997年   5988篇
  1996年   5504篇
  1995年   5046篇
  1994年   5044篇
  1993年   4929篇
  1992年   5203篇
  1991年   5523篇
  1990年   5471篇
  1989年   5391篇
  1988年   5168篇
  1987年   5247篇
  1986年   4863篇
  1985年   5923篇
  1984年   6207篇
  1983年   5412篇
  1982年   5631篇
  1981年   5224篇
  1980年   4940篇
  1979年   5525篇
  1978年   5629篇
  1977年   5709篇
  1976年   5864篇
  1975年   5526篇
  1974年   5371篇
  1973年   5698篇
  1972年   4436篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
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   
972.
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   
973.
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   
974.
(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   
975.
976.
Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2154–2155, September, 1989.  相似文献   
977.
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   
978.
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   
979.
980.
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