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   460356篇
  免费   5301篇
  国内免费   1540篇
化学   236983篇
晶体学   7167篇
力学   21620篇
综合类   13篇
数学   54071篇
物理学   147343篇
  2021年   3866篇
  2020年   4189篇
  2019年   4656篇
  2018年   6061篇
  2017年   6302篇
  2016年   9138篇
  2015年   5578篇
  2014年   8899篇
  2013年   21839篇
  2012年   16949篇
  2011年   20701篇
  2010年   14636篇
  2009年   14557篇
  2008年   18660篇
  2007年   18680篇
  2006年   17297篇
  2005年   15399篇
  2004年   14219篇
  2003年   12452篇
  2002年   12401篇
  2001年   14534篇
  2000年   10976篇
  1999年   8524篇
  1998年   6896篇
  1997年   6834篇
  1996年   6481篇
  1995年   5662篇
  1994年   5492篇
  1993年   5307篇
  1992年   5933篇
  1991年   6106篇
  1990年   5686篇
  1989年   5589篇
  1988年   5406篇
  1987年   5440篇
  1986年   5140篇
  1985年   6895篇
  1984年   6903篇
  1983年   5686篇
  1982年   5958篇
  1981年   5620篇
  1980年   5492篇
  1979年   5701篇
  1978年   5776篇
  1977年   5764篇
  1976年   5658篇
  1975年   5447篇
  1974年   5308篇
  1973年   5399篇
  1972年   3640篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
202.
203.
We consider the large sparse symmetric linear systems of equations that arise in the solution of weak constraint four‐dimensional variational data assimilation, a method of high interest for numerical weather prediction. These systems can be written as saddle point systems with a 3 × 3 block structure but block eliminations can be performed to reduce them to saddle point systems with a 2 × 2 block structure, or further to symmetric positive definite systems. In this article, we analyse how sensitive the spectra of these matrices are to the number of observations of the underlying dynamical system. We also obtain bounds on the eigenvalues of the matrices. Numerical experiments are used to confirm the theoretical analysis and bounds.  相似文献   
204.
Li  C.  Zhang  D.  Cheng  G.  Zhu  Y. 《Experimental Mechanics》2020,60(3):329-343
Experimental Mechanics - There have been relatively few studies on mechanical properties of nanomaterials under high strain rates, mainly due to the lack of capable nanomechanical testing devices....  相似文献   
205.
The first immobilized catalyst for Ir‐catalyzed asymmetric allylic aminations is described. The catalyst is a cationic (π‐allyl)Ir complex bound by cation exchange to an anionic silica gel support. Preparation of the catalyst is facile, and the supported catalyst displayed considerably enhanced activity compared with the parent homogeneous catalyst. Up to 43 consecutive amination runs were possible in recycling experiments.  相似文献   
206.
207.
Radiophysics and Quantum Electronics - We propose and experimentally test a numerical method for correction of the influence of fluctuations in the distance to objects during noncontact probing in...  相似文献   
208.
The aim of the study presented in this paper was to characterize quantitatively the erosion of aluminum cathodes in high-current vacuum arcs. The experimental setup comprised two current generators. The first one, capable of generating a current of amplitude up to 350 kA, was used to produce a plasma jet, that is, the object to be investigated. The second generator was used to produce a source of probe radiation for imaging the object under investigation in soft x rays of energy ?ν ≈ 0.5–3 keV. The findings of the study are based on experimental data obtained by electrophysical and radiographic methods. It has been shown that the cathode erosion rate in a high-current vacuum arc is a function of the charge passed through the cathode.  相似文献   
209.
Nonlinear Dynamics - Introduction of stiffness nonlinearities to broaden the frequency bandwidth of vibratory energy harvesters has the adverse influence of complicating the response behavior of...  相似文献   
210.
We describe the synthesis and the physical properties of polyaromatic hydrocarbons (PAHs) containing a phosphorus atom at the edge. In particular, the impact of the successive addition of aromatic rings on the electronic properties was investigated by experimental (UV/Vis absorption, fluorescence, cyclic voltammetry) and theoretical studies (DFT). The physical properties recorded in solution and in the solid state showed that the P‐containing PAHs exhibit properties expected for an emitter in white organic light‐emitting diodes (WOLEDs).  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