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   436078篇
  免费   4848篇
  国内免费   1231篇
化学   233782篇
晶体学   6489篇
力学   18764篇
综合类   13篇
数学   49941篇
物理学   133168篇
  2021年   3768篇
  2020年   4185篇
  2019年   4519篇
  2018年   5853篇
  2017年   5744篇
  2016年   8832篇
  2015年   5562篇
  2014年   8543篇
  2013年   20185篇
  2012年   15648篇
  2011年   19261篇
  2010年   13498篇
  2009年   13350篇
  2008年   17647篇
  2007年   17543篇
  2006年   16252篇
  2005年   14551篇
  2004年   13524篇
  2003年   11893篇
  2002年   11728篇
  2001年   13429篇
  2000年   10085篇
  1999年   7917篇
  1998年   6570篇
  1997年   6314篇
  1996年   6082篇
  1995年   5471篇
  1994年   5394篇
  1993年   5197篇
  1992年   5829篇
  1991年   5924篇
  1990年   5665篇
  1989年   5472篇
  1988年   5493篇
  1987年   5376篇
  1986年   5091篇
  1985年   6562篇
  1984年   6694篇
  1983年   5302篇
  1982年   5387篇
  1981年   5368篇
  1980年   4955篇
  1979年   5353篇
  1978年   5491篇
  1977年   5385篇
  1976年   5328篇
  1975年   4914篇
  1974年   4885篇
  1973年   4881篇
  1972年   3455篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
973.
Summary A gas Chromatographic method for the determination of estrone, estradiol and estriol in plasma of normal females is described. Purification is done by means of TLC of the free compounds and the acetates and quantitation is achieved by electron capture-gas chromatography of the estrogen heptafluorobutyrates. Experiments on the validation of the method are described along with some examples of its application.
Zusammenfassung Eine gaschromatographische Methode zur Bestimmung von Östron, Östradiol und Östriol im normalen weiblichen Plasma wurde beschrieben. Die Reinigung erfolgte durch Dünnschichtchromatographie der freien Verbindungen und ihrer Acetate. Die quantitative Bestimmung wurde gaschromatographisch mit Hilfe eines Elektroneneinfangdetektors an Hand der Östrogenheptafluorobutyrate bewerkstelligt. Versuche zur Bewertung dieser Methode sowie Beispiele für ihre Anwendung wurden beschrieben.


Supported by USPHS Grants AM 09908 and RCDA 5 K 3-AM-31, 321 (S. K.) and a grant from the Ford Foundation.  相似文献   
974.
975.
The synthesis of 2,3,5,6-tetrahydropyrrolo[3,2-c]pyrid-6-one was accomplished by rearrangement of 8H,1-cyano-8-dimethylaminomethylene-2,5,6,7-tetrahydropyrrolo[1, 2-c]pyrimidine. Pyrrolo[3,2-c]pyrimidine, 1,6-naphthyridine, and pyrimido[4,3-b]-azepine derivatives were synthesized on the basis of enamino dinitriles. The hydrolysis of 8H,1-cyano-8-dimethylaminomethylene-2,5,6,7-tetrahydropyrrolo[1,2-c]-pyrimidine in 50% CH3COOH leads to a pyrrolo[1,2-c]pyrido[4,3-d]pyrimidine derivative. A similar dipyrido[4,3-d-1,2-c]pyrimidine derivative was obtained from 1-cyano-9-dimethylaminomethylene-2,5,6,7,8,9-hexahydropyrido[1,2-c]pyrimidine under these conditions, and 3,4-dioxo-3,4,7,8,9,10-hexahydropyrido[1,2-c]pyrano[4,3-d]-pyrimidine was synthesized bytreatment with a 1 N solution of HCl.See [1] for Communication 34.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 518–522, April, 1982.  相似文献   
976.
In this work, an analysis of the parametric sensitivity of the overshoot in the concentration of the adsorbate in the adsorbed phase, which occurs under certain conditions during an ion-exchange adsorption process, is presented and used to suggest practical implications of the concentration overshoot phenomenon on operational policies and configurations of chromatographic columns and finite bath adsorption systems. The results presented in this work demonstrate and explain how the development of an overshoot in the concentration of the adsorbate in the adsorbed phase could be enhanced or suppressed by (i) varying the diffusion coefficient, D3, of the adsorbate relative to the diffusion coefficients, D1 and D2, of the cations and anions, respectively, of the background/buffer electrolyte, (ii) altering the initial surface charge density, delta0, of the charged adsorbent particles, (iii) varying the Debye length, lambda, and (iv) changing the initial concentration, Cd3(0), of the adsorbate in the bulk liquid of the finite bath. The influence of the pH and ionic strength, Iinfinity, of the liquid solution on the development of an overshoot in the concentration of the adsorbate in the adsorbed phase is also presented and discussed through the relationships of these parameters to delta0 and lambda, respectively. Furthermore, a detailed explanation of the effects of each parameter on the interplay between the diffusive and electrophoretic molar fluxes, as well as on the structure and functioning of the electrical double layer, which are responsible for the concentration overshoot phenomenon, is presented.  相似文献   
977.
Zhou C  Birney DM 《Organic letters》2002,4(19):3279-3282
[structure: see text] Two sequential transition states are calculated at the B3LYP/6-31G(d,p) level for the deazetization of 3b. The intrinsic reaction coordinate for loss of nitrogen passes through a transition state and then a valley-ridge inflection point and ultimately leads to the transition state for the Cope rearrangement of semibullvalene 4b. The energetic and geometrical consequences are discussed.  相似文献   
978.
Conclusions A fundamentally new scheme was found for the synthesis of (±)-muscarine, in which the required arrangement of all the substituents is achieved in a single step, involving the hydroboration of 5-dimethylaminomethyl-2-methyl-4,5-dihydrofuran.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1383–1386, June, 1989.  相似文献   
979.
Institute of Chemistry of Plant Substances, Uzbekistan Academy of Sciences, Tashkent. Botanical Garden of Moscow State University. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 301–303, March–April, 1993.  相似文献   
980.
Radionuclide X-ray fluorescence analysis was used for the determination of Cr, Fe, Cu, Zn and Pb in industrial wastewaters (from surface treatment of metals and glasses) after precipitation of determined elements by 8-hydroxyquinoline.  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