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   214949篇
  免费   1797篇
  国内免费   628篇
化学   117768篇
晶体学   3043篇
力学   8356篇
综合类   6篇
数学   23362篇
物理学   64839篇
  2020年   1975篇
  2019年   2266篇
  2018年   3207篇
  2017年   3117篇
  2016年   4285篇
  2015年   2389篇
  2014年   3828篇
  2013年   9285篇
  2012年   6918篇
  2011年   8463篇
  2010年   6093篇
  2009年   5936篇
  2008年   8152篇
  2007年   8240篇
  2006年   7647篇
  2005年   6961篇
  2004年   6183篇
  2003年   5653篇
  2002年   5506篇
  2001年   5948篇
  2000年   4693篇
  1999年   3480篇
  1998年   3012篇
  1997年   3035篇
  1996年   2828篇
  1995年   2323篇
  1994年   2424篇
  1993年   2453篇
  1992年   2598篇
  1991年   2667篇
  1990年   2561篇
  1989年   2533篇
  1988年   2463篇
  1987年   2435篇
  1986年   2360篇
  1985年   3040篇
  1984年   3195篇
  1983年   2560篇
  1982年   2828篇
  1981年   2661篇
  1980年   2468篇
  1979年   2688篇
  1978年   2798篇
  1977年   2838篇
  1976年   2800篇
  1975年   2661篇
  1974年   2640篇
  1973年   2786篇
  1972年   1850篇
  1967年   1830篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Journal of Analytical Chemistry - A new liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) method has been developed and validated for alfuzosin quantification in...  相似文献   
92.
π-Extended molecules are key components for the development of materials science. In fact, polyaromatic structures are fundamental for the scientific and technological progress of fields such as organic electronics and bio-applications. Beneficial properties of π-extended structures are absorption in the visible region, often luminescence, high electron mobilities and stability. Common approaches to adjust the properties of polyaromatic structures to functional setups involve changes in shape and size at the molecular level. Recently, incorporating hetero-elements emerged as successful approach. In this regard, organophosphorus conjugated molecules are new materials holding great promise for potential applications. In this review, we comprehensively discuss the design/development of polyaromatic phosphorus materials and their applicability. We establish structure/property/applicability relationships to provide key guidelines for the engineering of newer, future applications. This article thus provides a source of information for the further development of this rapidly evolving field of research.  相似文献   
93.
94.
95.
Optics and Spectroscopy - The population kinetics of a two-level system has been theoretically investigated taking into account the transfer of spontaneous radiation under continuous optical...  相似文献   
96.
A combination of pentafluorophenylboronic acid and oxalic acid catalyses the dehydrative substitution of benzylic alcohols with a second alcohol to form new C−O bonds. This method has been applied to the intermolecular substitution of benzylic alcohols to form symmetrical ethers, intramolecular cyclisations of diols to form aryl-substituted tetrahydrofuran and tetrahydropyran derivatives, and intermolecular crossed-etherification reactions between two different alcohols. Mechanistic control experiments have identified a potential catalytic intermediate formed between the aryl boronic acid and oxalic acid.  相似文献   
97.
The practical application of Shilov-type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ-hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state-of-the-art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2-fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non-selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented.  相似文献   
98.
Gurbatov  S. N.  Pelinovsky  E. N. 《Doklady Physics》2020,65(8):269-272
Doklady Physics - In this paper, we study the statistical characteristics of the Riemann wave and the integral of it. Such problems arise in the physics of fronts’ propagation, in particular,...  相似文献   
99.
The preparation of new ring opening metathesis polymerization (ROMP) monomers using a 1,3‐dipolar cycloaddition between aryl azides and norbornadiene is described. Various norbornenetriazolines, obtained through a solvent‐and catalyst‐free reaction, can subsequently be incorporated into polymer backbones through ROMP reactions. Furthermore, thermal decomposition of the triazoline moiety can allow for further polymer functionalization. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2357–2362  相似文献   
100.
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