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   668篇
  免费   25篇
  国内免费   2篇
化学   467篇
晶体学   3篇
力学   13篇
数学   92篇
物理学   120篇
  2023年   7篇
  2022年   8篇
  2021年   13篇
  2020年   19篇
  2019年   13篇
  2018年   14篇
  2017年   16篇
  2016年   25篇
  2015年   25篇
  2014年   23篇
  2013年   45篇
  2012年   46篇
  2011年   55篇
  2010年   27篇
  2009年   30篇
  2008年   34篇
  2007年   26篇
  2006年   36篇
  2005年   47篇
  2004年   30篇
  2003年   18篇
  2002年   24篇
  2001年   14篇
  2000年   7篇
  1999年   6篇
  1998年   7篇
  1997年   5篇
  1996年   7篇
  1995年   5篇
  1993年   4篇
  1992年   2篇
  1991年   2篇
  1990年   2篇
  1989年   4篇
  1988年   8篇
  1987年   3篇
  1984年   4篇
  1982年   3篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1977年   3篇
  1976年   5篇
  1975年   4篇
  1974年   2篇
  1972年   1篇
  1966年   2篇
  1963年   2篇
  1958年   2篇
  1874年   1篇
排序方式: 共有695条查询结果,搜索用时 46 毫秒
641.
The crystal structures of the title compound and that of its hemihydrate have been determined. The anhydrous compound is roughly planar due to an intramolecular hydrogen bond (IMHB). The orange color can be related to its planarity and to the greater degree of overlapping between the perimidine and the phenyl group than in the hemihydrate. The yellow hemihydrate is not planar and the water molecule joins through O-HN hydrogen bonds two molecules related by a crystallographic twofold axis. In solution the compound behaves like the anhydrous form.  相似文献   
642.
643.
Tosylation and acylation of cellulose were performed under mild reaction conditions using imidazolium based ionic liquids (ILs) as solvents. The non-degradative nature, lower viscosity, as well as higher solubility of cellulose in [amim]Cl encouraged us to carry out the reactions in this media. The reactions described here were optimised for this particular solvent in order to obtain different cellulose derivatives with high yields, homogeneity and degree of substitution (DS). Two reagents employed for the in situ activation of carboxylic acids were N,N′-carbonyldiimidazole (CDI) and 1-ethyl-3-(3′-dimethylaminopropyl)carbodiimide hydrochloride (EDCI). Final products were characterised by solution and solid-state NMR techniques. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   
644.
The Structure and Dynamics of Networks   总被引:2,自引:0,他引:2  
  相似文献   
645.
The covalent attachment of [60]fullerene (C60) to two poly(vinyl chloride) (PVC) samples with different isotactic content is achieved by direct reaction in o‐dichlorobenzene (o‐DCB) solution in the presence of AIBN. The extent of fullerenation is controlled by varying the C60 feed ratio. The pendant C60‐chemically modified PVC polymers are soluble in tetrahydrofuran (THF) and have been characterized by UV–vis, NMR, FTIR, DSC, TGA, cyclic voltammetry, and SEM. The quantitative microstructural analysis after covalent attachment of the bulky C60 moiety to the PVC has been followed by 13C NMR spectroscopy. From the results it can be concluded that the modification of PVC by graft reaction through free radical reaction proceeds by a stereoselective mechanism. This conclusion has been confirmed on the basis of the increase of the glass‐transition temperature (Tg) and the thermal stability of the C60‐chemical modified PVC samples. The fullerenated PVCs obtained show good electron acceptor properties, as evidenced by electrochemical investigations. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5408–5419, 2007  相似文献   
646.
The covalent attachment of [60] fullerene (C60) to isotactic polypropylene (i‐PP) is achieved by direct reaction in 1,2,4‐trichlorobenzene (TCB) solution in the presence of dicumyl peroxide (DCP). The chemically modified pendant C60/i‐PP polymers are soluble in chlorinated solvents and have been characterized by ultraviolet–visible and fourier transform infrared spectroscopy, scanning electron microscopy, X‐ray diffraction, differential scanning calorimetry, cyclic voltametry, and thermogravimetric analysis. From the results it can be concluded that the modification of i‐PP by grafting via a free‐radical reaction competes with the possibility of chain scission of i‐PP due to the presence of DCP. The functionalized polymers crystallize in the monoclinic crystal modification, and have high crystallinity. The incorporation of C60 significantly enhances the thermal stability of the i‐PP. Electrochemical measurements demonstrate good electron acceptor properties of the fullerenated i‐PP samples. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6722–6733, 2008  相似文献   
647.
Two new analogs of the human lethal toxin polycavernoside A, polycavernoside C and C2 (0.1-0.2 mg for each), were isolated from the red alga, Gracilaria edulis. The relative stereostructure of polycavernoside C and the absolute structure of polycavernoside C2 were determined by spectroscopic analysis and synthesis of the model of their aglycon.  相似文献   
648.
Chitosan was modified with lauroyl chloride to prepare a solid support for the application of matrix solid-phase dispersion (MSPD) for the extraction of flavonoids from the medicinal plant Aleurites moluccana. The laurylchitosan prepared by the homogenous and heterogeneous method, was characterized using analytical methods such as potentiometric titration and infrared spectroscopy. The hydrophobic properties were studied by naphthalene incorporation. The MSPD performance was evaluated by extraction of swertisin and 2″-O-rhamnosylswertisin from the leaves of A. moluccana.  相似文献   
649.
 We deal with complete k-partite hypergraphs and we show that for all k≥2 and n≠2,6 its hyperedges can be labeled by consecutive integers 1,2,…,n k such that the sum of labels of the hyperedges incident to (k−1) particular vertices is the same for all (k−1)-tuples of vertices from (k−1) independent sets. Received: December 8, 1997 Final version received: July 26, 1999  相似文献   
650.
Cryptococcosis is an opportunistic disease with a worldwide distribution. This disease is caused by fungi of the genus Cryptococcus, and its treatment is limited to several antifungals. In this study, the antifungal, cytotoxic and mutagenic properties of ethanol extracts from the bark and leaves of Annona coriacea were evaluated against the standard Cryptococcus species and clinical yeast specimens. Both extracts of A. coriacea showed inhibitory activity of 1.5 mg/mL for all of the yeasts tested. The number of viable cells at the lowest tested concentration was 0.187 mg/mL. The extracts that were tested showed inhibitory activity and reduced the fungal growth of the Cryptococcus gattii species and Cryptococcus neoformans species complexes, suggesting that this plant may be an effective alternative treatment for cryptococcosis.  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