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   298836篇
  免费   3113篇
  国内免费   1746篇
化学   147722篇
晶体学   4024篇
力学   15534篇
综合类   61篇
数学   55779篇
物理学   80575篇
  2020年   1788篇
  2019年   1938篇
  2018年   12653篇
  2017年   12555篇
  2016年   9585篇
  2015年   3279篇
  2014年   3865篇
  2013年   9312篇
  2012年   11407篇
  2011年   19647篇
  2010年   12125篇
  2009年   12331篇
  2008年   15257篇
  2007年   17450篇
  2006年   8717篇
  2005年   9076篇
  2004年   8469篇
  2003年   8066篇
  2002年   7060篇
  2001年   6954篇
  2000年   5362篇
  1999年   3826篇
  1998年   3241篇
  1997年   3169篇
  1996年   3244篇
  1995年   2773篇
  1994年   2834篇
  1993年   2710篇
  1992年   2901篇
  1991年   2978篇
  1990年   2783篇
  1989年   2699篇
  1988年   2653篇
  1987年   2593篇
  1986年   2610篇
  1985年   3407篇
  1984年   3445篇
  1983年   2876篇
  1982年   3146篇
  1981年   2897篇
  1980年   2696篇
  1979年   2870篇
  1978年   3062篇
  1977年   3087篇
  1976年   3096篇
  1975年   2830篇
  1974年   2944篇
  1973年   3005篇
  1972年   2345篇
  1971年   1864篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
In the past 6 years changes have occurred in GCE A-levels. Inparticular, there have been several major changes in A-levelMathematics courses. As engineering students are usually requiredto have studied A-level Mathematics, or its equivalent, thesechanges have had an effect on their prior mathematical knowledge.Moreover, engineering students traditionally obtained a goodgrounding in mechanics as part of their A-level Mathematicsqualification. However, mechanics, which was once included inthe core syllabus, is now optional. This article investigatesthe current availability and uptake of mechanics modules withinA-level Mathematics courses in schools. Comparisons are drawnbetween these results and results of a survey of schools in2004 and surveys of first year engineering students conductedin 2004 and 2005. It is found that there is a decline in theavailability of mechanics modules and the uptake of more thanone mechanics module has also decreased. The implications ofthese findings for engineering educators are discussed.  相似文献   
53.
The known plant growth regulator, Al'den [1-(3,7-dimethyloctyl)-1-allylpiperidinium bromide], and also 1-(3,7-dimethyloctyl)-1-methylpiperidinium iodide were synthesized by treatment with allyl bromide and methyl iodide, respectively, of exhaustively hydrogenated 1-nerylpiperidine which was obtained by anionic telomerization of isoprene with piperidine. Various quaternary ammonium salts having a terpene sub- stituent with unnatural dimethyloctane skeleton, which effectively stimulate florification of Aster Chinensis L., were prepared by the action of alkyl halides on telomerization products derived from isoprene and piperidine or morpholine in the presence of palladium complexes.  相似文献   
54.
Ascidia malaca gametes before fertilization incubated in 10?5 or 10?7 M solutions of tributyltin(IV) chloride, TBTCl, for 3 h appear highly damaged under transmission electron microscopy observation. Also, the fertilization process is affected by the compound: the damaged spermatozoa are present in the vitelline coat and the egg does not cleave. An increase of microbodies, structurally similar to peroxisomes, have been detected in the egg peripheral cytoplasm, probably in relation to their role in alleviating damage to some cellular components. The results have shown that the reproduction of ascidians under unfavourable environmental conditions is prevented. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
55.
A new approach for developing a cloud-point extraction-flame atomic absorption spectrometric method has been described and used for the determination of cobalt. In this approach, water was removed from the final diluted surfactant rich phase obtained in cloud-point extraction procedure. The results indicated that removing water from this phase increased the enhancement factor by 4-fold. 1-(2-Pyridylazo)-2-naphthol (PAN) and octylphenoxypolyethoxyethanol (Triton X-114) were used as a hydrophobic ligand and a nonionic surfactant, respectively. The chemical variables affecting the preconcentration step were optimized. The effect of the water concentration in the final diluted methanolic surfactant solution on the analytical signal was investigated. The results showed that the analytical signal decreased by 30% and 52% in 15% and 25% water concentrations in methanol, respectively. An enhancement factor of 115 was obtained for cobalt extracted from only 10 ml of a sample. The detection limit obtained under the optimal condition was 0.38 microg l(-1). The proposed procedure was applied to the determination of cobalt in urine samples.  相似文献   
56.
Optics and Spectroscopy - The fluorescence of the nematic liquid crystal n-butyl-n′-methoxyazoxybenzene (BMAOB) in the form of a layer and in porous glasses with pores of different diameter...  相似文献   
57.
Preface     
Journal of Statistical Physics -  相似文献   
58.
59.
    
  相似文献   
60.
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