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   216848篇
  免费   1808篇
  国内免费   627篇
化学   119156篇
晶体学   3202篇
力学   8359篇
综合类   6篇
数学   23542篇
物理学   65018篇
  2020年   1984篇
  2019年   2277篇
  2018年   3214篇
  2017年   3122篇
  2016年   4296篇
  2015年   2413篇
  2014年   3860篇
  2013年   9381篇
  2012年   7035篇
  2011年   8594篇
  2010年   6144篇
  2009年   5970篇
  2008年   8286篇
  2007年   8353篇
  2006年   7792篇
  2005年   7075篇
  2004年   6274篇
  2003年   5729篇
  2002年   5559篇
  2001年   5998篇
  2000年   4747篇
  1999年   3495篇
  1998年   3017篇
  1997年   3052篇
  1996年   2851篇
  1995年   2341篇
  1994年   2438篇
  1993年   2463篇
  1992年   2616篇
  1991年   2669篇
  1990年   2568篇
  1989年   2540篇
  1988年   2474篇
  1987年   2436篇
  1986年   2365篇
  1985年   3054篇
  1984年   3201篇
  1983年   2566篇
  1982年   2841篇
  1981年   2709篇
  1980年   2514篇
  1979年   2728篇
  1978年   2837篇
  1977年   2863篇
  1976年   2835篇
  1975年   2691篇
  1974年   2642篇
  1973年   2788篇
  1972年   1856篇
  1967年   1832篇
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
91.
Kim  F. H.  Moylan  S. P.  Phan  T. Q.  Garboczi  E. J. 《Experimental Mechanics》2020,60(7):987-1004
Experimental Mechanics - Insufficient data are available to fully understand the effects of metal additive manufacturing (AM) defects for widespread adoption of the emerging technology....  相似文献   
92.
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   
93.
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   
94.
95.
96.
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   
97.
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   
98.
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   
99.
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   
100.
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   
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