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   663795篇
  免费   7390篇
  国内免费   1936篇
化学   335904篇
晶体学   10083篇
力学   31898篇
综合类   19篇
数学   90583篇
物理学   204634篇
  2021年   4962篇
  2020年   5517篇
  2019年   6076篇
  2018年   13360篇
  2017年   13697篇
  2016年   14233篇
  2015年   7871篇
  2014年   11472篇
  2013年   27584篇
  2012年   23752篇
  2011年   33896篇
  2010年   23006篇
  2009年   22877篇
  2008年   30589篇
  2007年   32683篇
  2006年   22004篇
  2005年   23986篇
  2004年   20397篇
  2003年   18300篇
  2002年   16872篇
  2001年   17942篇
  2000年   13776篇
  1999年   10725篇
  1998年   8988篇
  1997年   8718篇
  1996年   8464篇
  1995年   7634篇
  1994年   7539篇
  1993年   7426篇
  1992年   8113篇
  1991年   8223篇
  1990年   7834篇
  1989年   7684篇
  1988年   7742篇
  1987年   7534篇
  1986年   7105篇
  1985年   9318篇
  1984年   9660篇
  1983年   7692篇
  1982年   7914篇
  1981年   7857篇
  1980年   7385篇
  1979年   7885篇
  1978年   8124篇
  1977年   7925篇
  1976年   7939篇
  1975年   7303篇
  1974年   7291篇
  1973年   7340篇
  1972年   5201篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
82.
83.
We summarize the results of our recent work on Bäcklund transformations (BTs), particularly focusing on the relation between BTs and infinitesimal symmetries. We present a BT for an associated Degasperis–Procesi (aDP) equation and its superposition principle and investigate the solutions generated by applying this BT. Following our general methodology, we use the superposition principle of the BT to generate the infinitesimal symmetries of the aDP equation.  相似文献   
84.
Mesoscopic modeling at the pore scale offers great promise in exploring the underlying structure transport performance of flow through porous media. The present work studies the fluid flow subjected to capillarity-induced resonance in porous media characterized by different porous structure and wettability. The effects of porosity and wettability on the displacement behavior of the fluid flow through porous media are discussed. The results are presented in the form of temporal evolution of percentage saturation and displacement of the fluid front through porous media. The present study reveals that the vibration in the form of acoustic excitation could be significant in the mobilization of fluid through the porous media. The dependence of displacement of the fluid on physicochemical parameters like wettability of the surface, frequency along with the porosity is analyzed. It was observed that the mean displacement of the fluid is more in the case of invading fluid with wetting phase where the driving force strength is not so dominant.  相似文献   
85.
86.
87.
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.  相似文献   
88.
A new strategy for the synthesis of tetrahydroisoquinolines based on the Pd0‐catalyzed intramolecular α‐arylation of sulfones is reported. The combination of this Pd‐catalyzed reaction with intermolecular Michael and aza‐Michael reactions allows the development of two‐ and three‐step domino processes to synthesize diversely functionalized scaffolds from readily available starting materials.  相似文献   
89.
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