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   467225篇
  免费   3678篇
  国内免费   1435篇
化学   249309篇
晶体学   7351篇
力学   20536篇
综合类   11篇
数学   54279篇
物理学   140852篇
  2020年   4028篇
  2019年   4461篇
  2018年   5172篇
  2017年   5069篇
  2016年   7825篇
  2015年   4887篇
  2014年   7529篇
  2013年   20079篇
  2012年   14801篇
  2011年   18191篇
  2010年   12540篇
  2009年   12195篇
  2008年   16649篇
  2007年   16921篇
  2006年   15645篇
  2005年   14388篇
  2004年   13031篇
  2003年   11696篇
  2002年   11643篇
  2001年   12928篇
  2000年   9882篇
  1999年   7728篇
  1998年   6675篇
  1997年   6646篇
  1996年   6258篇
  1995年   5816篇
  1994年   5813篇
  1993年   5628篇
  1992年   6259篇
  1991年   6257篇
  1990年   6025篇
  1989年   5926篇
  1988年   6064篇
  1987年   5873篇
  1986年   5605篇
  1985年   7509篇
  1984年   7962篇
  1983年   6617篇
  1982年   7005篇
  1981年   6875篇
  1980年   6745篇
  1979年   6750篇
  1978年   7164篇
  1977年   7036篇
  1976年   7239篇
  1975年   6606篇
  1974年   6753篇
  1973年   7196篇
  1972年   4772篇
  1971年   3863篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
83.
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   
84.
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   
85.
We have prepared new polyesters containing quadratic, nonlinear optical (NLO) active chromophores covalently incorporated into the main chain. In these polymers, the sequence of the chromophore units along the main chain is rigorously head to tail. All the polyesters are processable, both in the melt and in solution. For one polyester, a full second‐order NLO characterization has been performed. An out‐of‐resonance d33 coefficient of 21 pm/V at 1368 nm has been measured. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2719–2725, 2007  相似文献   
86.
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   
87.
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   
88.
89.
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   
90.
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