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   2318篇
  免费   113篇
  国内免费   12篇
化学   2003篇
晶体学   9篇
力学   16篇
数学   242篇
物理学   173篇
  2023年   15篇
  2022年   27篇
  2021年   61篇
  2020年   62篇
  2019年   55篇
  2018年   29篇
  2017年   43篇
  2016年   89篇
  2015年   75篇
  2014年   84篇
  2013年   125篇
  2012年   161篇
  2011年   178篇
  2010年   100篇
  2009年   101篇
  2008年   143篇
  2007年   136篇
  2006年   149篇
  2005年   120篇
  2004年   103篇
  2003年   95篇
  2002年   84篇
  2001年   31篇
  2000年   20篇
  1999年   21篇
  1998年   18篇
  1997年   12篇
  1996年   23篇
  1995年   21篇
  1994年   13篇
  1993年   9篇
  1992年   11篇
  1991年   13篇
  1990年   9篇
  1989年   8篇
  1988年   8篇
  1986年   3篇
  1985年   4篇
  1984年   6篇
  1983年   13篇
  1982年   7篇
  1981年   7篇
  1980年   7篇
  1979年   9篇
  1978年   3篇
  1976年   5篇
  1973年   3篇
  1958年   55篇
  1957年   20篇
  1934年   4篇
排序方式: 共有2443条查询结果,搜索用时 156 毫秒
181.
We have demonstrated the preparation of a series of photoaddressable supramolecular block copolymers by mixing a carboxy‐terminated azobenzene derivative, 6‐[4‐(4′‐cyanophenylazo)phenyloxy]hexanoic acid (AZO), and two polystyrene‐b‐poly(4‐vinylpiridine) (PS‐b‐P4VP) block copolymers. AZO can be selectively attached to the P4VP block of PS‐b‐P4VP through hydrogen bonding interactions. The assembly of AZO with vinylpyridine group‐containing polymers was initially investigated on a model system composed of P4VP homopolymer and AZO. Homogeneous liquid crystalline materials were obtained for ratios of AZO to vinylpyridine repeating unit, x, lower or equal to 0.50. Mixtures with higher x resulted in heterogeneous materials showing clear macrophase separation. Accordingly, a series of hydrogen‐bonded complexes of PS‐b‐P4VP and AZO, PS‐b‐P4VP(AZO)x, with x = 0.25 and x = 0.50 were prepared. Lamellar and spherical morphologies were observed for the complexes based on PS24‐b‐P4VP9.5 (Mn,PS = 24,000, Mn,P4VP = 9500) and PS24‐b‐P4VP1.9 (Mn,PS = 24,000, Mn,P4VP = 1900), respectively. Photoinduced orientation of the azobenzene units was obtained in films of P4VP(AZO)x and PS‐b‐P4VP(AZO)x with x = 0.25 and 0.50 by using 488 nm linearly polarized light and characterized through birefringence and dichroism measurements. This investigation shows a versatile and less laborious approach to azobenzene‐containing polymer materials with low chromophore content, of interest in optical application. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013  相似文献   
182.
Summary.  Lipoyl imidazolide reacts with aza-15-crown-5 (1,4,7,10-tetraoxa-13-azacyclopentadecane) or aza-18-crown-6 (1,4,7,10,13-pentaoxa-16-azacyclooctadecane) to afford new N-lipoylated azacrown compounds in good yields. These compounds can be transformed into 1,3-dithiols and amines by reduction with complex hydrides of the disulfide and/or amide group of the lipoyl chain. The new pendant-arm macrocycles react as heteroditopic ligands by forming dithiolate and disulfide complexes with the ‘soft’ metal ions Ni2+ and Pd2+, respectively, and an amine complex with the ‘hard’ Li+ ion. Semiempirical and DFT calculations on the complexation of a lithium ion give a most favourable structure in which the azacrown and two solvent molecules are in contact with the metal but not the pendant arm. Received January 29, 2002; accepted (revised) March 25, 2002  相似文献   
183.
In this paper we establish lower bounds for the weakly convergent sequence coefficient WCS(X) of a Banach space X, in terms of some well known moduli and coefficients. By mean of these bounds we identify several properties, of geometrical nature, which imply normal structure. We show that these properties are strictly more general than other previously known sufficient conditions for normal structure.  相似文献   
184.
185.
In this short note we give a complete characterization of a certain class of compact corank one Poisson manifolds, those equipped with a closed one-form defining the symplectic foliation and a closed two-form extending the symplectic form on each leaf. If such a manifold has a compact leaf, then all the leaves are compact, and furthermore the manifold is a mapping torus of a compact leaf. These manifolds and their regular Poisson structures admit an extension as the critical hypersurface of a b-Poisson manifold as we will see in [9].  相似文献   
186.
187.
188.
189.
A sugar-based phosphite–phosphoroamidite and diphosphoroamidite ligand library L1L5ag was tested in the asymmetric Cu-catalyzed 1,4-conjugate addition reactions of β-substituted and β,β′-disubstituted enones. Our results indicated that the selectivity was strongly dependent on the ligand parameters and on the substrate structure. Moderate-to-good enantioselectivities (ees up to 84%) were obtained in the 1,4-addition of several types of β-substituted cyclic and linear substrates. Of particular note is the high enantioselectivity (ees up to 90%) obtained for the more challenging β,β′-disubstituted 3-methyl-cyclohexenone.  相似文献   
190.
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