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   27740篇
  免费   2616篇
  国内免费   2230篇
化学   12888篇
晶体学   120篇
力学   923篇
综合类   294篇
数学   13137篇
物理学   5224篇
  2024年   36篇
  2023年   445篇
  2022年   359篇
  2021年   501篇
  2020年   768篇
  2019年   770篇
  2018年   753篇
  2017年   679篇
  2016年   963篇
  2015年   896篇
  2014年   1386篇
  2013年   2399篇
  2012年   1355篇
  2011年   1863篇
  2010年   1610篇
  2009年   1941篇
  2008年   1988篇
  2007年   1895篇
  2006年   1671篇
  2005年   1304篇
  2004年   1077篇
  2003年   1029篇
  2002年   1028篇
  2001年   778篇
  2000年   687篇
  1999年   602篇
  1998年   567篇
  1997年   399篇
  1996年   410篇
  1995年   388篇
  1994年   313篇
  1993年   234篇
  1992年   230篇
  1991年   162篇
  1990年   114篇
  1989年   112篇
  1988年   97篇
  1987年   62篇
  1986年   67篇
  1985年   92篇
  1984年   77篇
  1983年   34篇
  1982年   71篇
  1981年   64篇
  1980年   51篇
  1979年   58篇
  1978年   51篇
  1977年   41篇
  1976年   37篇
  1973年   27篇
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
991.
992.
Some boundedness properties of function spaces (considered as topo- logical groups) are studied.  相似文献   
993.
This review article provides an overview of the most recent and exciting developments in palladium‐catalyzed C–H activation and mechanistic aspects of these catalytic reactions as the fast‐growing field for the synthesis of phenanthridine derivatives.  相似文献   
994.
Fu and Lu et al. 7 showed that the commutator $\mathcal {H}_{\beta ,b}$ generated by the fractional Hardy operator and a locally integrable function b is bounded on the homogenous Herz spaces if and only if b is a central bounded mean oscillation function. We show that their result is optimal by giving a counterexample.  相似文献   
995.
Synthesis of bis(4‐pyridylamino)triazine stabilized on silica‐coated nano‐Fe3O4 particles, and their feasibility as a reusable heterogeneous basic catalyst are reported. The catalytic performance of this novel material was studied for the green synthesis of highly functionalized 4H ‐pyran, 4H ‐thiopyran and 1,4‐dihydropyridine derivatives via one‐pot multicomponent reactions. Eco‐friendly method, high yield and purity of desired products, short reaction time along with ease of workup procedure outline the advantages of these new methodologies over earlier ones. Surface and magnetic properties of the core–shell hybrid nanoparticles were characterized via transmission and scanning electron microscopies, X‐ray diffraction, energy‐dispersive X‐ray and Fourier transform infrared spectroscopies and vibrating sample magnetometry. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
996.
A Liouville–Green (or WKB) asymptotic approximation theory is developed for a class of almost-diagonal (‘asymptotically diagonal’) linear second-order matrix difference equations. Rigorous and explicitly computable bounds for the error terms are obtained, the asymptotics being made with respect to both, the index and some parameter affecting the equation. The case of the associated inhomogeneous equations is also considered in detail. Some examples and a number of applications are presented for the purpose of illustration.  相似文献   
997.
Graphene oxide ‐ Fe3O4 ‐ NH3+H2PW12O40 magnetic nanocomposite (GO/Fe3O4/HPW) was prepared by linking amino ‐ functionalized Fe3O4 nanoparticles (Fe3O4 ‐ NH2) on the graphene oxide (GO), and then grafting 12 ‐ tungstophosphoric acid (H3PW12O40) on the graphene oxide ‐ magnetite hybrid (GO ‐ Fe3O4 ‐ NH2). The obtained GO/Fe3O4/HPW nanocomposite was well characterized with different techniques such as FT ‐ IR, TEM, SEM, XRD, EDX, TGA ‐ DTA, AGFM, ICP and BET measurements. The used techniques showed that the graphene oxide layers were well prepared and the various stages of preparation of the GO/Fe3O4/HPW nanocomposites successfully completed. This new nanocomposite displayed excellent performance as a heterogeneous catalyst in the oxidation of alcohols with H2O2. The as ‐ prepared GO/Fe3O4/HPW catalyst was more stable and recyclable at least five times without significantly reducing its catalytic activity.  相似文献   
998.
999.
The paper deals with second order nonlinear evolution inclusions and their applications. We study evolution inclusions involving a Volterra-type integral operator, which are considered within the framework of an evolution triple of spaces. First, we deliver a result on the unique solvability of the Cauchy problem for the inclusion by combining a surjectivity result for multivalued pseudomonotone operators and the Banach contraction principle. Next, we provide a theorem on the continuous dependence of the solution to the inclusion with respect to the operators involved in the problem. Finally, we consider a dynamic frictional contact problem of viscoelasticity for materials with long memory and indicate how the result on evolution inclusion is applicable to the model of the contact problem.  相似文献   
1000.
Hesperidin is flavonoid molecule found in citrus fruits (Citrus reticulata), especially difficult to extract, classify and characterize. Present work is to study the unresolved relative configuration of Hesperidin through the dihedral angle, coupling constant and different NMR techniques. The Karplus equation and its modifications have been originated from the valence bond theory and associated with dihedral angle and coupling constant. The result data set of calculated dihedral angle can probe significant method to assign the virtual configuration of natural products and also resolved stereochemistry of Hesperidin at C‐2 position in. Copyright © 2016 John Wiley & Sons, Ltd.  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