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   23719篇
  免费   2819篇
  国内免费   7385篇
化学   28034篇
晶体学   366篇
力学   400篇
综合类   188篇
数学   71篇
物理学   4864篇
  2024年   77篇
  2023年   382篇
  2022年   773篇
  2021年   1054篇
  2020年   1500篇
  2019年   1188篇
  2018年   948篇
  2017年   1075篇
  2016年   1171篇
  2015年   1130篇
  2014年   1405篇
  2013年   2108篇
  2012年   1546篇
  2011年   1642篇
  2010年   1239篇
  2009年   1395篇
  2008年   1415篇
  2007年   1507篇
  2006年   1413篇
  2005年   1291篇
  2004年   1262篇
  2003年   1135篇
  2002年   946篇
  2001年   762篇
  2000年   761篇
  1999年   639篇
  1998年   595篇
  1997年   563篇
  1996年   479篇
  1995年   469篇
  1994年   402篇
  1993年   409篇
  1992年   319篇
  1991年   187篇
  1990年   158篇
  1989年   118篇
  1988年   108篇
  1987年   83篇
  1986年   46篇
  1985年   46篇
  1984年   29篇
  1983年   18篇
  1982年   26篇
  1981年   22篇
  1980年   23篇
  1979年   14篇
  1978年   8篇
  1977年   7篇
  1976年   9篇
  1973年   7篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
981.
A ligand,(N,N′-dibenzylethane-1,2-diamine)(L),and its complex with copper acetate was synthesized and characterized by some spectral analyses.The copper ! ion is six-coordinated and exhibites octahedral coordination geometry,the coordination atoms are four nitrogen atoms from two(L) ligands and two carboxyl oxygen atoms from two acetic acid groups,respectively.After studying the interaction of the complex with calf thymus DNA through UV and fluorescence spectra,we can find that there is a strong binding and a large affinity berween the complex and calf thymus DNA.CCDC: 649416.  相似文献   
982.
CuBr2-catalyzed three-component coupling of N-benzylallylamine, ethyl glyoxalate, and terminal alkynes afforded glycinate-tethered 1,6-enynes, which were further transformed into polycyclic pyrrole-2-carboxylates via iridium-catalyzed cycloisomerization/Diels-Alder cycloaddition/dehydrogenation sequence under conventional and microwave heating conditions. The corresponding β-amino acid analogues were obtained from the Ir-catalyzed reaction of enynes prepared from lithium amides of allylamines with methyl non-2-en-4-ynoate. The Cu-catalyzed Mannich-type condensation was further extended to the synthesis of the glycinate-tethered dienyne and cyclopropylenyne, which were subjected to Rh-catalyzed cycloisomerizations to furnish bicyclic amino acids.  相似文献   
983.
Two new macrocyclic crown ethers containing one or two selenium donor atoms have been prepared. Diselena-18-crown-6 (2) was found to transport silver ions through a methylene chloride bulk membrane at about the same rate as the analogous dithia- (3) and diaza-18-crown-6 (4) compounds and transported lead ions about the same as dithia-18-crown-6 but better than diaza-18-crown-6.  相似文献   
984.
考察氧化物担体对Fe-MnO催化剂反应性能影响的结果表明,担体的类型对Fe-MnO催化剂CO加氢反应性能影响很大,其低碳烯烃选择性相差悬殊,Al_2O_3、SiO_2和MgO担载的Fe-MnO催化剂都不利于低碳烯烃的生成,而TiO_2担载的Fe-MnO催化剂则具有较高的烯烃选择性和催化活性。从担体-金属相互作用本质的差异,研究了担体对金属活性组分化学状态的影响及催化活性相与F-T合成烯烃的关系,发现担体-金属间的电子效应有利于催化剂活性和选择性的提高,其它物理化学效应引起的相互作用则不利于改善催化剂性能;还表明Fe_(?)C是催化活性相,Fe~(2+)物种的存在不利于提高烯烃的选择性。  相似文献   
985.
The following report gives an overview on work done in the Catalysis Laboratory of the Department of Chemistry, National University of Singapore over the last 15 years (1989–2004). Much of this work can be described as “characterization of catalytically active surfaces through test reactions”. The methods, systems studied and the reactions that we evaluated will be described. The review will mostly concentrate on work from the authors’ laboratory, but other relevant work will also be cited.  相似文献   
986.
The initial oxidation of magnesium at oxygen partial pressures between 1.3 × 10?8 and 1.3 × 10?5 Pa and at temperatures ranging from 273 to 550 K has been investigated in situ with X‐ray photoelectron spectroscopy (XPS), ellipsometry and high resolution elastic recoil detection analysis (HERDA). Quantitative analysis of the XPS spectra showed a clear oxygen deficiency with respect to MgO for the initial oxide. HERDA measurements confirmed this relatively low oxygen content in the thin oxide layers formed. Ellipsometry measurements showed that the electronic structure of the initially formed oxide differs significantly from that of bulk MgO. The band gap values at room temperature for the oxide layers investigated are clearly smaller than the value for bulk MgO. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
987.
《Mendeleev Communications》2020,30(1):126-128
  1. Download : Download high-res image (65KB)
  2. Download : Download full-size image
  相似文献   
988.
王晓梅  吴越 《应用化学》1992,9(1):98-99
甲烷氧化偶联反应在不同催化剂体系上动力学的研究各不相同,我们已报道了La-Ba-O系复合氧化物催化剂对此反应的优良催化性能。本文则是对该催化剂上的甲烷氧化偶联反应的动力学进行初步探讨。  相似文献   
989.
Five new macrocyclic ligands each containing two sulfonamide groups have been prepared. Three of these compounds contain one or two chloro substituents and the other two have one or two pyridine subcyclic units. A seventeen-membered ring ligand (4) was found to be an excellent transport agent for all alkali metal cations in a water-methylene chloride-water bulk liquid membrane system when the pH of the source phase was 13 or higher. The chlorine-substituted analog (5) was a poor transport agent for the alkali metal cations possibly because the chlorine atom blocked entry to the macrocycle cavity. An open-chain analog containing two sulfonamide groups was particularly effective in transporting cesium ions.  相似文献   
990.
The mixed-ligand p-hydroxybenzoate complexes of Ni(II), Cu(II) and Zn(II) with nicotinamide and N,N-diethylnicotinamide were synthesized and characterized by elemental analysis, magnetic susceptibility measurements and mass spectrometry. The thermal behavior of the complexes was studied by simultaneous TG, DTG and DTA methods in static air atmosphere. The infrared spectral characteristics of the complexes are also discussed. The complexes contain two water molecules, two p-hydroxybenzoato (p-hba) and two nicotinamide (na) (or diethylnicotinamide (dena)) ligands per formula unit. In these complexes, all ligands are coordinated to the metal ion as monodendate ligands. In Zn(II)-na and Cu(II)-dena complexes, thep-hydroxybenzoate behaves as bidentate chelating ligand through carboxylic oxygen atoms. The decomposition pathways and the stability of the complexes are interpreted in the terms of the structural data. The final decomposition products were found to be the respective metal oxides. This revised version was published online in July 2006 with corrections to the Cover Date.  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