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   2636篇
  免费   88篇
  国内免费   524篇
化学   1618篇
晶体学   2篇
力学   181篇
综合类   17篇
数学   488篇
物理学   942篇
  2024年   1篇
  2022年   1篇
  2021年   3篇
  2020年   2篇
  2019年   1篇
  2016年   10篇
  2015年   23篇
  2014年   12篇
  2013年   28篇
  2012年   29篇
  2011年   529篇
  2010年   281篇
  2009年   220篇
  2008年   36篇
  2007年   41篇
  2006年   58篇
  2005年   160篇
  2004年   480篇
  2003年   231篇
  2002年   215篇
  2001年   252篇
  2000年   54篇
  1999年   51篇
  1998年   66篇
  1997年   49篇
  1996年   39篇
  1995年   31篇
  1994年   31篇
  1993年   100篇
  1992年   81篇
  1991年   53篇
  1990年   23篇
  1989年   30篇
  1988年   8篇
  1987年   2篇
  1986年   5篇
  1985年   1篇
  1984年   1篇
  1982年   3篇
  1979年   1篇
  1975年   1篇
  1964年   2篇
  1955年   1篇
  1933年   2篇
排序方式: 共有3248条查询结果,搜索用时 10 毫秒
51.
Thin PVA/cobalt acetate composite fibers were prepared by using sol-gel processing and electrospinning technique.After calcination of the above precursor fibers,Co3O4 nanofibers with a diameter of 50-150 nm could be successfully obtained.The fibers were characterized by SEM,FT-IR,WAXD,respectively.  相似文献   
52.
The oxidation of styrene with molecular oxygen catalyzed by PdCl2 CuCl2 has been investigated in supercritical CO2 with a batch reactor. The oxidative system of styrene contains four components at the beginning and seven components during the reaction. The critical temperature, critical pressure, and critical density at different conversions are determined by using a high-pressure view cell. The effect of phase behavior on the conversion and selectivity were studied. Experimental results showed that the critical parameters of the reaction mixture at fixed initial molar ratio changed with the conversion of reactant. The conversion of styrene reached maximum near the critical density of the reaction mixture. Product selectivity also varied with density of reaction mixture and could be tuned to some degree.  相似文献   
53.
The increasing demand to produce enantio-pure pharmaceuticals, flavors, and other fine chemicals has advanced the field of asymmetry catalytic technologies. Among all asymmetry catalytic methods, catalytic enantioselective hydrogenation of unsaturated bonds such as olefins, ketones, and imines, which employs dihydrogen and small amounts of transition-metal complexes modified intrinsically by chiral ligands, is now recognized as being the most promising strategy for the synthesis of large amount…  相似文献   
54.
Quinoline derivatives were efficiently prepared through acid-catalyzed Friedlander reaction in ionic liquid ([bmim] [BF4]). It is shown that the proposed method is operationally simple and environmentally benign in that the reaction media and the catalyst can be recovered and be reused effectively for at least four times.  相似文献   
55.
A SAPO-11 silicoaluminophosphate molecular sieve with stable crystal structure was synthesized for the first time. After removing template by calcination, its crystal space group still retains Icm2 which the as-synthesized has. The catalyst deriving from the present SAPO-I 1 materials shows higher isomerization selectivity and higher paraffin hydroisomerization yield than those reported elsewhere.  相似文献   
56.
57.
IntroductionItiswellknownthatyttriumcanbeusedinmanyfields ,suchasmetallurgy ,ceramics ,lasersandelectron ics ,especiallyinfluorescentmaterialswhoseneedforhighpurityyttriumoxideisincreasing .InChina ,there sourceofyttriumisrich ,andthehighpureyttriumisbe ingo…  相似文献   
58.
The present paper employed density function theory to investigate two reaction pathways for isomerization of enol ester proposed by Yang(path a) and the present authors(path a), respectively. The base catalytic effects of solvent triethylamine on these two reactions were also evaluated. It is demonstrated that path B is more preferable than path a due to low barrier height for the rate-determining step.  相似文献   
59.
A new abietane diterpene 12-hydroxy-7-oxo-5, 8, 11, 13-tetraene-18, 6-abietanolide, together with a new natural sterol stigmasta-7, 22-diene-3β, 5α, 6α-triol have been isolated from the fruiting body of the fungus Phellinus igniarius. Their structures were elucidated by spectroscopic methods including 2D NMR techniques.  相似文献   
60.
赵生敏  王梅祥 《中国化学》2002,20(11):1291-1299
IntroductionBiotransformationsofnitriles ,eitherthroughanitri lase catalyzeddirectconversiontothecarboxylicacidsorviaanitrilehydratase catalyzedhydrationtotheamidesfollowedbythehydrolysistotheacidsmediatedbyanamidase ,haveprovidedausefulandenvironmental…  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