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   220919篇
  免费   2698篇
  国内免费   1324篇
化学   122727篇
晶体学   3228篇
力学   8888篇
综合类   61篇
数学   24365篇
物理学   65672篇
  2020年   1754篇
  2019年   1896篇
  2018年   2483篇
  2017年   2531篇
  2016年   3645篇
  2015年   2424篇
  2014年   3537篇
  2013年   8872篇
  2012年   7686篇
  2011年   9395篇
  2010年   6649篇
  2009年   6451篇
  2008年   8792篇
  2007年   8900篇
  2006年   8384篇
  2005年   7705篇
  2004年   6870篇
  2003年   6053篇
  2002年   5985篇
  2001年   6703篇
  2000年   5067篇
  1999年   3669篇
  1998年   3038篇
  1997年   3008篇
  1996年   3011篇
  1995年   2618篇
  1994年   2718篇
  1993年   2581篇
  1992年   2829篇
  1991年   2894篇
  1990年   2703篇
  1989年   2616篇
  1988年   2557篇
  1987年   2497篇
  1986年   2520篇
  1985年   3300篇
  1984年   3359篇
  1983年   2804篇
  1982年   3046篇
  1981年   2811篇
  1980年   2607篇
  1979年   2790篇
  1978年   2996篇
  1977年   3032篇
  1976年   3059篇
  1975年   2793篇
  1974年   2906篇
  1973年   2956篇
  1972年   2318篇
  1971年   1845篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
TheSU(2) rotation matricesD (j), specified in terms of axis and angle of rotation, are expressed as linear combination of normalized irreducible tensorial matricces (NITM) of rankl = 0 to 2j rotated to the polar angles of the axis. The rotated NITM are constructed from spherical harmonics of the same rank. Since this formulation requires no matrix products, it may be computationally more efficient than Euler angle formulas, particularly for largej. Rotated NITM and formulas for theD (j) withj = 1/2 andj = 1 are written out explicitly. A formula for the structure constants of the products of conformable NITM is also given in terms of 3-j and 6-j symbols.  相似文献   
42.
Silver nitrate-acetonitrile and π iodine-benzene complexes in thermotropic liquid crystals have been studied by 1H, 2H, and 13C NMR spectroscopy and by optical microscopy. Evidence for at least two silver complexes in each liquid crystal is presented.  相似文献   
43.
We study a generalization of the notion of the chromatic number of a graph in which the colors assigned to adjacent vertices are required to be, in a certain sense, far apart. © 1993 John Wiley & Sons, Inc.  相似文献   
44.
45.
Experimental data demonstrate that a study of accelerated aging can enable prognostication of the working capacity and storage life of polymeric formulations used in aircraft components on the basis of changes in thermogravimetric constants of these polymeric formulations.  相似文献   
46.
The thermal and dielectric behaviors of poly(vinylidene fluoride-trifluoroethylene) copolymers near the ferroelectric-to-paraelectric phase transition are investigated for samples with 20, 25, 30, and 40 mol% trifluoroethylene (TrFE). The data suggest that the transition becomes continuous for a particular composition near 50 mol% TrFE. Experimental data are sensitive to thermal history (kinetics of crystallization, and kinetics and cycling over the structural transition). It is found that several anomalies are present at the structural change, and in particular the 30 mol% TrFE sample shows the most marked anomalies. These phenomena can be attributed to defects, but another possibility would be the existence of an intemediate supplementary phase. Both hypotheses are discussed.  相似文献   
47.
48.
49.
50.
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   
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