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   3879篇
  免费   209篇
  国内免费   174篇
化学   3151篇
晶体学   2篇
力学   49篇
综合类   12篇
数学   422篇
物理学   626篇
  2024年   10篇
  2023年   70篇
  2022年   74篇
  2021年   83篇
  2020年   113篇
  2019年   141篇
  2018年   123篇
  2017年   112篇
  2016年   116篇
  2015年   108篇
  2014年   221篇
  2013年   282篇
  2012年   225篇
  2011年   254篇
  2010年   205篇
  2009年   206篇
  2008年   207篇
  2007年   201篇
  2006年   170篇
  2005年   165篇
  2004年   166篇
  2003年   123篇
  2002年   105篇
  2001年   63篇
  2000年   63篇
  1999年   91篇
  1998年   84篇
  1997年   44篇
  1996年   65篇
  1995年   42篇
  1994年   30篇
  1993年   32篇
  1992年   28篇
  1991年   27篇
  1990年   17篇
  1989年   17篇
  1988年   26篇
  1987年   14篇
  1986年   23篇
  1985年   27篇
  1984年   22篇
  1983年   8篇
  1982年   18篇
  1981年   16篇
  1980年   11篇
  1979年   5篇
  1978年   2篇
  1976年   1篇
  1974年   3篇
  1973年   2篇
排序方式: 共有4262条查询结果,搜索用时 15 毫秒
81.
Chromatography has been an important tool in marine laboratories. Since the 1960s, marine laboratories have been involved in the analysis of polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs), and brominated flame retardants (BFRs). Column chromatography and liquid chromatography (LC) techniques have been used, mainly in the clean-up phase, while gas chromatography (GC) has been used extensively in the final determination of these contaminants. Developments have been observed from the use of packed GC columns, via capillary columns to the use of heart-cut multi-dimensional GC and comprehensive multi-dimensional GC. The progress made in interlaboratory studies and the availability of certified reference materials are discussed.  相似文献   
82.
By using the relationship equation of Partition coefficients with water solubilities for polycyclic aromatic hydrocarbons (PAHs), we have estimated the solubilities of PAHs. In addition the high-performance liquid chromatograph (HPLC) method has been used for the indirect determination of solubilities of PAHs. t3ased on the di-region theory, We have studied the relationship of the solubilities of PAHs with carcinogenicity and introduced the solubility parameter into the di-region theory equation. The calculated carcinogenicity of PAHs from the obtained equation shows little difference from that calculated from Dai Qianhuan's equation.  相似文献   
83.
1. INTRODUCTION When investigating contaminated sites it is often essential to determine specific pollutants in a large number of ambient air samples. Amongst these sites, polycylic aromatic hydrocarbons (PAHs) are a major class of compounds that hav…  相似文献   
84.
Summary Using the example of perhydrophenanthrene, perhydroanthracene and cyclopentanodecalin isomers a possibility is shown to use packed capillary columns containing graphitized thermal carbon black for a complete separation of high-boiling mixtures of polycyclic saturated hydrocarbon isomers in accordance with the geometric structure of their molecules.  相似文献   
85.
SPORES AND POLLEN IN CRUDE OILS AND PETROLEUM SOURCE OF TARIM BASIN   总被引:1,自引:0,他引:1  
One hundred and forty-seven species of spores and pollen referred to 77 genera found in the crudeoil samples taken from four petroliferous provinces of the Tarim Basin of Xinjiang Provincehave been studied in this paper. By means of these spore and pollen fossils, the geological ages anddistribution of the petroleum source rocks of the basin are judged. Also, the principles and methodsto judge the petroleum source of a petroleum province by spore/pollen and other microfossils in crudeoils are explained.  相似文献   
86.
在填充柱气相色谱法的基础上,使用12m×0.22mmi.d.的SE-30短毛细管柱(WCOT),分析了乙烯生产副产物——未加氢裂解汽油(55℃~220℃馏分)中共轭二烯烃的含量,分析一次需22min,内标物正十六烷得到了很好的分离。  相似文献   
87.
A new method for the calculation of bond orders in alternant hydrocarbons is presented. The method requires a summation over the contributions of various superposition diagrams. Quantitatively, the method is almost as reliable as PPP, and due to its simplicity it can be used for fast and relatively accurate calculation of bond orders. In addition some simple rules are derived, which in many cases can be used to predict the signs of bond orders between nonbonded atoms.Dedicated to Professor J. Koutecký on the occasion of his 65th birthdayResearch supported by the Robert A. Welch Foundation of Houston, Texas  相似文献   
88.
89.
Summary The present research studies the possibility of using the correlation dependence between molecular parameters of unsubstituted polyaromatic hydrocarbons (PAH) and their retention in reversed-phase liquid chromatography to optimize the conditions for the separation and identification of unknown peaks on the chromatograms of multicomponent mixtures. A linear correlation equation, that takes the number and environment of the carbon atom in the PAH molecule into account as well as the differences in the specific interactions of isomeric molecules with polar eluent, has been proposed. The adequacy of the proposed PAH retention model was verified by comparing the calculated retention values with the experimental data. The possibility of identifying unsubstituted PAH according to the number of carbon atoms of various types and according to the values of the molecules lengths (calculated on the basis of the retention of these substances under different eluent compositions) was exemplified by various chromatographic systems (reversed phase-eluent-PAH molecules).  相似文献   
90.
Asymmetric synthesis of four possible stereoisomers of (9,10)-epoxyheptadecan-4,6-diyn-3-one was accomplished, and the absolute configuration of the naturally occurring (9R,10S)-epoxyheptadecan-4,6-diyn-3-one (1) was elucidated.  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