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   412837篇
  免费   4035篇
  国内免费   984篇
化学   222319篇
晶体学   6165篇
力学   17759篇
综合类   12篇
数学   46793篇
物理学   124808篇
  2020年   3384篇
  2019年   3559篇
  2018年   4715篇
  2017年   4720篇
  2016年   7247篇
  2015年   4566篇
  2014年   6870篇
  2013年   17229篇
  2012年   13075篇
  2011年   16059篇
  2010年   11171篇
  2009年   10896篇
  2008年   15232篇
  2007年   15156篇
  2006年   14295篇
  2005年   12895篇
  2004年   11701篇
  2003年   10681篇
  2002年   10470篇
  2001年   12230篇
  2000年   9297篇
  1999年   7157篇
  1998年   5773篇
  1997年   5834篇
  1996年   5620篇
  1995年   5191篇
  1994年   5377篇
  1993年   5010篇
  1992年   5814篇
  1991年   5757篇
  1990年   5593篇
  1989年   5399篇
  1988年   5490篇
  1987年   5360篇
  1986年   5023篇
  1985年   6661篇
  1984年   6873篇
  1983年   5693篇
  1982年   5815篇
  1981年   5650篇
  1980年   5563篇
  1979年   5757篇
  1978年   6182篇
  1977年   6042篇
  1976年   5985篇
  1975年   5555篇
  1974年   5613篇
  1973年   5783篇
  1972年   3970篇
  1971年   3288篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The article referenced above was first published online on 30 August 2007 with incorrect pagination; the pagination has now been corrected online and in print. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
72.
We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux.  相似文献   
73.
X.B. Liu  J.G. Li 《Journal of Non》2004,333(1):95-100
The microstructure evolution of decagonal quasicrystals in Al72Ni12Co16 alloy was investigated by the electromagnetic melting and cyclic superheating method. Single-phase decagonal quasicrystals have been obtained when the undercoolings were larger than 60 K. The decagonal quasicrystals formed at various undercoolings show different microstructural morphologies. Furthermore, grain refinement was found near the undercooling of 120 K. Based on current thermodynamic and dendrite growth theories, a dimensionless superheating parameter was adopted to explain the effect of processing conditions on the microstructure of Al72Ni12Co16 alloy. The result indicate that the fine equiaxied microstructure of decagonal quasicrystal (D-phase) formed near on undercooling of 120 K originates from the break-up of dendrites.  相似文献   
74.
Palladium(II) complexes with N,N‐bis(diphenylphosphino)aniline ligands catalyse the Heck reaction between styrene and aryl bromides, affording stilbenes in good yield. The structures of two of the complexes used as pre‐catalysts have been determined by single‐crystal X‐ray diffraction. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
75.
76.
Pentacyclic isoxazolines were obtained by the cycloaddition of benzonitrile oxide to norbornene-azetidinone-fused 3,1-oxazines. The constitutions of two of the isomers obtained, and the configurations and conformations of all products, were determined by means of 1H and 13C NMR spectroscopy and DNOE experiments.  相似文献   
77.
78.
We theoretically investigated the mass dependence of the sympathetic cooling rate of gas-phase ions trapped in a linear radio-frequency-quadrupole ion trap. Using an a priori molecular dynamical calculation, tracing numerically with Newtonian equations of motion, we found that ions with a mass greater than 0.54±0.04 times that of the laser-cooled ions are sympathetically cooled; otherwise, they are heated. To understand the mass dependence obtained using the molecular-dynamical calculation, we made a heat-exchange model of sympathetic cooling, which shows that the factor of 0.54±0.04 is a consequence of absence of micro-motion along the axis of the linear ion trap. Received: 10 December 2001 / Revised version: 28 January 2002 / Published online: 14 March 2002  相似文献   
79.
80.
The results obtained in V. P. Maslov’s article [1] can automatically be carried over to arbitrary semiotic systems if the string of signs is sufficiently long. We shall show in this article, by using arguments based on the ideas of Kolmogorov complexity and by applying the theorem of V. P. Maslov from [2], that one can obtain similar formulas for the distribution of various semiotic objects. In order to do this, we must introduce notions generalizing the occurrence frequency of words in corpora of texts. We also interpret a formula due to Maslov (appearing in [3]) from the point of view of the general semiotic notions introduced here.  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