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   5484篇
  免费   310篇
  国内免费   57篇
化学   4111篇
晶体学   22篇
力学   99篇
综合类   1篇
数学   918篇
物理学   700篇
  2023年   49篇
  2022年   90篇
  2021年   105篇
  2020年   148篇
  2019年   123篇
  2018年   104篇
  2017年   85篇
  2016年   183篇
  2015年   155篇
  2014年   200篇
  2013年   276篇
  2012年   448篇
  2011年   475篇
  2010年   230篇
  2009年   170篇
  2008年   399篇
  2007年   373篇
  2006年   375篇
  2005年   339篇
  2004年   311篇
  2003年   233篇
  2002年   206篇
  2001年   81篇
  2000年   39篇
  1999年   39篇
  1998年   38篇
  1997年   46篇
  1996年   45篇
  1995年   33篇
  1994年   35篇
  1993年   32篇
  1992年   31篇
  1991年   34篇
  1990年   21篇
  1989年   16篇
  1988年   19篇
  1987年   23篇
  1986年   24篇
  1985年   25篇
  1984年   15篇
  1983年   19篇
  1982年   15篇
  1981年   19篇
  1980年   19篇
  1979年   17篇
  1978年   14篇
  1976年   11篇
  1975年   11篇
  1974年   11篇
  1973年   8篇
排序方式: 共有5851条查询结果,搜索用时 15 毫秒
181.
182.
183.
The synthesis of four 3,4-di(alkylsulfanyl)benzyl alcohol derivatives is described, in five steps from methyl 3,4-di(hydroxy)benzoate via a Newman–Kwart rearrangement. Incubation of these derivatives in formic acid affords 2,3,7,8,12,13-hexakis(alkylsulfanyl)-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene products, which are hexa-sulfanyl analogues of the well-known supramolecular cavitand host, cyclotriveratrylene (CTV). The yield of this cyclization depends strongly on the alkylsulfanyl substituents present, in the order SMe > SEt ≈ SiPr ? SBn. A crystal structure determination of one of the cyclotrimers shows a mode of self-association that is commonly exhibited by CTV itself.  相似文献   
184.
185.
186.
In recent work, it was reported that changes in solvent composition, precisely the addition of water, significantly inhibits the catalytic activity of Au/TiO2 catalyst in the aerobic oxidation of 1,4‐butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidation in methanol is generally valid, the solvent effect on the aerobic catalytic oxidation of 1,3‐propanediol and its two methyl‐substituted homologues, 2‐methyl‐1,3‐propanediol and 2,2‐dimethyl‐1,3‐propanediol, over a Au/TiO2 catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed‐field gradient nuclear magnetic resonance (PFG‐NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixture. A reactivity trend within the group of diols was also observed. Combined NMR diffusion and relaxation time measurements suggest that molecular diffusion and, in particular, the relative strength of diol adsorption, are important factors in determining the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non‐invasive characterisation tools for catalytic materials, which complement conventional reaction data.  相似文献   
187.
188.
Covalently tethering photosensitizers to catalytically active 1T-MoS2 surfaces holds great promise for the solar-driven hydrogen evolution reaction (HER). Herein, we report the preparation of two new RuII-complex-functionalized MoS2 hybrids [RuII(bpy)2(phen)]-MoS2 and [RuII(bpy)2(py)Cl]-MoS2. The influence of covalent functionalization of chemically exfoliated 1T-MoS2 with coordinating ligands and RuII complexes on the HER activity and photo-electrochemical performance of this dye-sensitized system was studied systematically. We find that the photo-electrochemical performance of this RuII-complex-sensitized MoS2 system is highly dependent on the surface extent of photosensitizers and the catalytic activity of functionalized MoS2. The latter was strongly affected by the number and the kind of functional groups. Our results underline the tunability of the photovoltage generation in this dye-sensitized MoS2 system by manipulation of the surface functionalities, which provides a practical guidance for smart design of future dye-sensitized MoS2 hydrogen production devices towards improved the photofuel conversion efficiency.  相似文献   
189.
β-Ketonitriles bearing a quaternary carbon at the 2-position were prepared through Rh-catalyzed addition of aryl boronic acids to 2,2-disubstituted malononitriles. In contrast to the previously described transnitrilative cyanation of aryl boronic acids with dialkylmalononitriles, the present reaction avoids retro-Thorpe collapse of the intermediate addition product through the use of a milder base. The reaction was amenable to a variety of aryl boronic acids and disubstituted malononitriles, providing a diverse array of β-ketonitriles. The products could be further derivatized to valuable chiral α,α-disubstituted-β-aminonitriles through addition reactions to the corresponding N-tert-butanesulfinyl imines.  相似文献   
190.
The existence and importance of the proton radius puzzle, observed via a Lamb shift measurement in muonic atoms, is discussed. Possible resolutions of the puzzle are discussed. Then the broader question of the meaning of the proton radius is addressed and examples of correctly defined charge densities are presented.  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