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   10598篇
  免费   1541篇
  国内免费   1245篇
化学   7566篇
晶体学   118篇
力学   670篇
综合类   67篇
数学   1287篇
物理学   3676篇
  2024年   45篇
  2023年   252篇
  2022年   392篇
  2021年   374篇
  2020年   423篇
  2019年   444篇
  2018年   380篇
  2017年   363篇
  2016年   509篇
  2015年   501篇
  2014年   575篇
  2013年   784篇
  2012年   868篇
  2011年   939篇
  2010年   656篇
  2009年   590篇
  2008年   726篇
  2007年   606篇
  2006年   551篇
  2005年   414篇
  2004年   370篇
  2003年   279篇
  2002年   312篇
  2001年   212篇
  2000年   202篇
  1999年   259篇
  1998年   187篇
  1997年   134篇
  1996年   145篇
  1995年   140篇
  1994年   101篇
  1993年   110篇
  1992年   91篇
  1991年   91篇
  1990年   86篇
  1989年   58篇
  1988年   44篇
  1987年   37篇
  1986年   22篇
  1985年   33篇
  1984年   21篇
  1983年   14篇
  1982年   12篇
  1981年   5篇
  1980年   4篇
  1979年   3篇
  1978年   5篇
  1976年   3篇
  1975年   3篇
  1957年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
The lowest-energy geometries and electronic-structure properties have been obtained for AlnHn (n=1-10) clusters within the density-functional theory using the generalized gradient approximation for the exchange correlation potential. The resulting geometries show that the hydrogen atoms tend to occupy outside positions and no hollow positions are found. The subunit Aln of AlnHn (n=1-5) have little distortion, in comparison with corresponding pure Aln cluster, whereas the subunit Aln have large distortion from n=6. The stability has been investigated by analyzing the binding energy per atom and the second difference in energy, indicating that Al8H8 exhibit higher stability than others. The bonding property has been analyzed by calculating the Mulliken charges and Al–H distances. The calculated energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO), the vertical ionization potential, and the vertical electron affinity also confirm that Al8H8 is a stable cluster. The density of states (DOS) shows that AlnHn exhibit changes from molecular-like (Al1H1) to band-like structure (Al10H10) as n increases.  相似文献   
942.
In this work, the multiple-quantum-well InGaAsN laser structures with indirect-GaAsP and direct-GaAsN barriers are investigated by using LASTIP simulation program. We vary the quantum-well number, from 1 to 5, to find appropriate barrier material for InGaAsN laser structures. The simulation results show that InGaAsN laser structure has higher characteristic temperature regardless of what quantum-well number is if the indirect-GaAsP barrier is utilized. Furthermore, for InGaAsN laser structure, the usage of indirect-GaAsP barrier is beneficial for reducing the threshold current when the quantum-well number is from 1 to 2 and the usage of direct-GaAsN barrier is beneficial for reducing the threshold current when the quantum-well number is from 3 to 5.  相似文献   
943.
Why is it that drops do not slip down inclined thin fibers or spider silks? The possible explanation is based on the existence of fiber size, which causes a sustaining force that pins these drops. Following this explanation, the drop remains in equilibrium until a critical value of the sustaining force is reached. We suggest an alternative analyses, from the point of view of the inclined fiber at which the drop slips down is predicted. This result does not depend explicitly on silk surface roughness, but only on the drop size and surface tensions.  相似文献   
944.
Bangben Yao 《Tetrahedron letters》2008,49(37):5385-5388
In the presence of triflic anhydride, sulfonylation of arenes with sulfonamides proceeded smoothly in Cl2CHCHCl2 at 80-140 °C, which gave rise to the desired products in good to excellent yields.  相似文献   
945.
This work presents a theoretical analysis for the Black–Scholes equation. Given a terminal condition, the analytical solution of the Black–Scholes equation is obtained by using the Adomian approximate decomposition technique. The mathematical technique employed in this work also has significance in studying some other problems in finance theory.  相似文献   
946.
This is a continuation of the article(Comm.Partial Differential Equations 26(2001)965).In this article,the authors consider the one-dimensional compressible isentropic Navier-Stokes equations with gravitational force,fixed boundary condition,a general pressure and the density-dependent viscosity coefficient when the viscous gas connects to vacuum state with a jump in density.Precisely,the viscosity coefficient μ is proportional to ρ^θ and 0〈θ〈1/2,where ρ is the density,and the pressure P=P(ρ)is a general pressure.The global existence and the uniqueness of weak solution are proved.  相似文献   
947.
The purpose of this paper is to study the convergence problem of an iterative method for nonexpansive mappings in Banach spaces under some new control conditions on parameters.  相似文献   
948.
定性和定量测定甲基氯化锡,文献上报道的方法较多。有薄层色谱,气相色谱,极谱法等。但测定方法麻烦,重现性较差。我们采用核磁共振波谱的方法,利用溶剂化效应对化学位移的影响,分别定性定量测定了水溶液中的SnCl_4,(CH_3)_2SnCl_2和CH_3SnCl_3。并结合薄层分离和原子吸收分光光度法进行了比较,结果较为满意。  相似文献   
949.
基于平面曲线的二次微商,导出了二重点的判别条件,结合参数曲线的局部凸性条件,得到了参数闭曲线的充要条件。给出了参数曲线的拐点判别条件,从而得到了参数曲线局部凸的充要条件。  相似文献   
950.
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