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   680篇
  免费   7篇
  国内免费   2篇
化学   504篇
晶体学   3篇
力学   8篇
数学   20篇
物理学   154篇
  2021年   6篇
  2020年   5篇
  2019年   6篇
  2018年   7篇
  2015年   5篇
  2014年   10篇
  2013年   16篇
  2012年   23篇
  2011年   23篇
  2010年   25篇
  2009年   20篇
  2008年   26篇
  2007年   29篇
  2006年   36篇
  2005年   38篇
  2004年   31篇
  2003年   12篇
  2002年   17篇
  2001年   20篇
  2000年   23篇
  1999年   15篇
  1998年   6篇
  1997年   5篇
  1996年   15篇
  1995年   10篇
  1994年   8篇
  1993年   6篇
  1992年   17篇
  1991年   14篇
  1990年   18篇
  1989年   6篇
  1988年   11篇
  1987年   7篇
  1986年   9篇
  1985年   6篇
  1984年   11篇
  1983年   6篇
  1982年   10篇
  1981年   8篇
  1980年   6篇
  1979年   9篇
  1978年   9篇
  1977年   8篇
  1976年   4篇
  1975年   7篇
  1974年   14篇
  1973年   17篇
  1972年   7篇
  1970年   7篇
  1968年   4篇
排序方式: 共有689条查询结果,搜索用时 0 毫秒
1.
Arsenic speciation analysis in marine samples was performed using ion chromatography (IC) with inductively coupled plasma mass spectrometry (ICP‐MS) detection. The separation of eight arsenic species, viz. arsenite, monomethyl arsonic acid, dimethylarsinic acid, arsenate, arsenobetaine, tetramethylarsine oxide, arsenocholine and tetramethylarsonium ion was achieved on a Dionex AS4A (weaker anion exchange column) by using a nitric acid pH gradient eluent (pH 3.3 to 1.3). The entire separation was accomplished in 12 min. The detection limits for the eight arsenic species by IC–ICP‐MS were in the range 0.03–1.6 µ g l?1, based on 3σ of the blank response (n = 6). The repeatability and day‐to‐day reproducibility were calculated to be less than 10% (residual standard deviation) for all eight species. The method was validated by analyzing a certified reference material (DORM‐2, dogfish muscle) and then successfully applied to several marine samples, e.g. oyster, fish muscle, shrimp and marine algae. The low power microwave digestion was employed for the extraction of arsenic from seafood products. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   
2.
Six alkaloids have been isolated from the fruits of the plant Daphniphyllaceae. Two of them are new alkaloids, namely methyl homodaphniphyllate (1) and daphnilactone-B (2). The structure of the former was deduced by chemical transformation from daphniphylline (3). The structure of daphnilactone-B was estimated by the exhaustive spectral analysis as well as by chemical evidences, and finally determined by an X-ray crystallographic analysis of the free base.  相似文献   
3.
From a biogenetic point of view, a great variety of related alkaloids isolated from the plant Daphniphyllaceae are related to one another by bond formation or fission. Thus, daphnialcohol acetate (6), a derivative of the degradation products of daphniphylline (1), was subjected to von Braun degradation followed by acid-catalyzed recyclization to give an isomer (8) of daphnialcohol, which has a new type of nitrogen heterocyclic skeleton. Furthermore, daphnilactone-B (3) was converted into a daphniphylline-type compound (19) via a plausible intermediate (21).  相似文献   
4.
5.
6.
Copolymerizations of N-vinylcarbazole with both isobutyl vinyl ether and N-vinyl-pyrrolidone initiated by some organic electron acceptors have been investigated for the purpose of elucidating the propagation mechanism in the charge-transfer polymerization. Copolymerizations of the same system catalyzed by authentic cationic catalysts have also been made for comparison. The results indicate that the propagation mechanism of the charge-transfer polymerization studied is catio ie.  相似文献   
7.
8.
We analyze the production of H dihyperson (J π=0+,S=?2) via the (K ?, K+) reaction by means of the non-relativistic quark model. First, the H mass and mass spectrum of single baryons are calculated. When the single baryon spectrum is well reproduced, the H dihyperson has the binding energies about 20 MeV or 60 MeV corresponding to the choice of the strength of the confinement potential. Using this model and parameters, cross sections for H production are estimated. A new effect, contributions from color-octetQ 3?Q 3 components of H dihyperson, is taken into account. The cross sections for H production are enhanced about ten times by these contributions. TheK +-neutron coincidence cross section for H production is found to be 99–115 nb/sr2 at theK + forward direction forM H=2212 MeV and theK ? beam momentum 1.8 GeV/c.  相似文献   
9.
10.
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