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   523篇
  免费   23篇
  国内免费   6篇
化学   448篇
晶体学   4篇
力学   2篇
数学   18篇
物理学   80篇
  2023年   6篇
  2022年   8篇
  2021年   16篇
  2020年   28篇
  2019年   28篇
  2018年   7篇
  2017年   10篇
  2016年   17篇
  2015年   16篇
  2014年   19篇
  2013年   29篇
  2012年   42篇
  2011年   40篇
  2010年   16篇
  2009年   16篇
  2008年   46篇
  2007年   28篇
  2006年   22篇
  2005年   31篇
  2004年   22篇
  2003年   18篇
  2002年   18篇
  2001年   4篇
  2000年   4篇
  1999年   1篇
  1998年   3篇
  1997年   5篇
  1996年   4篇
  1995年   2篇
  1994年   3篇
  1993年   3篇
  1992年   2篇
  1991年   4篇
  1990年   1篇
  1989年   2篇
  1987年   1篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1980年   5篇
  1979年   3篇
  1978年   4篇
  1977年   3篇
  1976年   1篇
  1975年   5篇
  1972年   1篇
排序方式: 共有552条查询结果,搜索用时 15 毫秒
41.
42.
The extraction constant and the two-phase stability constant (KD,Mβ3) of tris(2-thenoyltrifluoroacetonato)europium(III) between 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]) as an ionic liquid and an aqueous phase were determined by considering the extraction equilibria including anionic tetrakis(2-thenoyltrifluoroacetonato)europate(III). Specific solute-solvent interactions between the neutral Eu(III) chelate and [C4mim][Tf2N] molecules were revealed from the relationships between the distribution constant of the enol form of 2-thenoyltrifluoroacetone (Htta) as a proton chelate and the distribution constant (KD,M) of the neutral Eu(III) chelate because the [C4mim][Tf2N] system gave the high KD,Mβ3 value compared with those in conventional molecular solvents such as benzene and 1,2-dichloroethane. The coordination environment of Eu3+ in the neutral Eu(III) chelate in [C4mim][Tf2N] was investigated by time-resolved laser-induced fluorescence spectroscopy and infrared absorption spectroscopy. Both methods consistently indicated that not only the Eu(III) chelate extracted but also Eu(tta)3(H2O)3 synthesized as a solid crystal were almost completely dehydrated in [C4mim][Tf2N] saturated with water. Consequently, the higher KD,M or extractability of the neutral Eu(III) chelate in the [C4mim][Tf2N] system can be ascribed to the dehydration of the Eu(III) chelate, which is caused by the specific solvation with [C4mim][Tf2N] molecules.  相似文献   
43.
44.
It was shown that dichloramine‐T ( 1 ) reacted with cyclohexene in acetonitrile to give N1‐(2‐chlorocyclohexyl) amidine 2a and N‐(2‐chlorocyclohexyl)acetamide ( 3 ) via the competitive addition of acetonitrile and N‐chloro‐N‐tosylamino anion to cyclohexenechloronium ion. This reaction can be catalyzed by Cu(OAc)2, primarily affording 2a . Furthermore, the resulting 2a can be cyclized to benzimidazol 14a in good yield by treating with KOH in dioxane.  相似文献   
45.
Photoinduced depolymerization of poly(olefin sulfone)s possessing photobase generators in the side‐chain was investigated. Irradiation with UV light generated base on the side‐chains and induced depolymerization based on proton abstraction on the main‐chain. The effect of the length of the spacer chain, which connects the photobase‐generating moiety to the polymer main chain on the photoinduced depolymerization, also was investigated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3873–3880  相似文献   
46.
47.
Melt rheological behaviors of hyperbranched‐polystyrene (PS) copolymerized by dendric macromonomer technique are presented. The time–temperature superposition principle was applicable to the hyperbranched‐PS. The branched‐PS showed slightly lower zero‐shear viscosity in comparison with linear PS regardless of a presence of a number of branches expected from the dendric macromonomer technique. Although the influence of use of multimethacryloyl macromonomer in the polymerization process was marginal for linear viscoelastic regime, nonlinear shear and uniaxial elongational flows showed distinct differences between linear and branched‐PS. The strain dependence of the damping function became weak as increase of macromonomer content. The branched‐PS exhibited the growing elongational viscosity function comparing with linear PS. This prominent effect on the elongational flow behavior can be explained by the molecular architecture of the branched‐PS. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2226–2237, 2009  相似文献   
48.
Cepaic acid was isolated as a novel xanthylium yellow pigment from the dried outer scales of the yellow onion Allium cepa Linne. Its structure was elucidated on the basis of ESI-MS and 2D NMR spectroscopy as a 9-carboxy-1,3,6,8-tetrahydroxyxanthylium, which suggests that cepaic acid and other yellow pigments in the dried outer skin of onion were formed by the nucleophilic reaction of phloroglucinol derived from quercetin, a flavonol in onion scales, by autoxidation to glyoxylic acid. To our knowledge, this is the first report of such a pigment in yellow onion.  相似文献   
49.
A hybrid binary subtracted joint transform correlator was constructed using a Bi12SiO12 (BSO) spatial light modulator and a laser scanner. The usefulness of this system was confirmed experimentally using a large number of binary and halL-tone images as the reference patterns.  相似文献   
50.
疋田巧 《化学教育》2002,23(7):92-94
日本的教育体制大体上类似于其他国家,即小学6年,初中3年,高中3年,大学本科4年。大学本科毕业后另有2年硕士学位,再加3年博士学位的教育。当前在校学习的人数逐年增加,人均在校学习的时间也逐渐延长。到2000年已有将近一半的18岁同龄人进入大学学习。为适应上述教育情况的变化,日本政府目前开始了一个重整大学教育体制的计划。该计划将所有原来模式相同的649所大学划分为两大类,一类是研究和教学型的,另一类是教学型的。目前已有9所大学已被认定为第一类型的大学。本文介绍日本教育体制的变化以及上述重整计划的长远目标。  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