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   252680篇
  免费   3703篇
  国内免费   1584篇
化学   144648篇
晶体学   3482篇
力学   9531篇
综合类   62篇
数学   30153篇
物理学   70091篇
  2020年   2309篇
  2019年   2438篇
  2018年   2860篇
  2017年   2889篇
  2016年   4521篇
  2015年   3199篇
  2014年   4461篇
  2013年   10451篇
  2012年   9658篇
  2011年   11595篇
  2010年   7865篇
  2009年   7538篇
  2008年   10805篇
  2007年   10774篇
  2006年   10239篇
  2005年   9451篇
  2004年   8368篇
  2003年   7300篇
  2002年   7173篇
  2001年   7149篇
  2000年   5513篇
  1999年   4050篇
  1998年   3396篇
  1997年   3391篇
  1996年   3413篇
  1995年   2918篇
  1994年   3046篇
  1993年   2893篇
  1992年   3112篇
  1991年   3146篇
  1990年   2932篇
  1989年   2817篇
  1988年   2755篇
  1987年   2701篇
  1986年   2688篇
  1985年   3645篇
  1984年   3675篇
  1983年   3062篇
  1982年   3355篇
  1981年   3071篇
  1980年   2876篇
  1979年   3014篇
  1978年   3205篇
  1977年   3240篇
  1976年   3257篇
  1975年   2964篇
  1974年   3061篇
  1973年   3068篇
  1972年   2394篇
  1971年   1901篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Although the most important use of nuclear magnetic resonance imaging (MRI) continues to be for diagnostic medicine, recognition is being gained for many nonmedical applications. Examples include the following areas: petrogeology, food, agriculture, polymers and polymer-composites, and pharmaceuticals. These areas all involve studies of species that have short spin-spin relaxation times, and consequently need far fast gradient switching. These technical details are discussed and typical applications given.  相似文献   
42.
In this paper we construct a primitive, non-symmetric 3-class association scheme with parameters v = 36, v1 = 7, p111 = 0 and p211 = 4 and show that such a scheme is determined by its parameters.  相似文献   
43.
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   
44.
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   
45.
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   
46.
47.
In this work, we propose that retardation in vinyl acetate polymerization rate in the presence of toluene is due to degradative chain transfer. The transfer constant to toluene (Ctrs) determined using the Mayo method is equal to 3.8 × 10?3, which is remarkably similar to the value calculated from the rate data, assuming degradative chain transfer (2.7 × 10?3). Simulations, including chain‐length‐dependent termination, were carried out to compare our degradative chain transfer model with experimental results. The conversion–time profiles showed excellent agreement between experiment and simulation. Good agreement was found for the Mn data as a function of conversion. The experimental and simulation data strongly support the postulate that degradative chain transfer is the dominant kinetic mechanism. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3620–3625, 2007  相似文献   
48.
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   
49.
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   
50.
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