全文获取类型
收费全文 | 14986篇 |
免费 | 638篇 |
国内免费 | 43篇 |
专业分类
化学 | 10195篇 |
晶体学 | 235篇 |
力学 | 482篇 |
综合类 | 1篇 |
数学 | 1168篇 |
物理学 | 3586篇 |
出版年
2024年 | 49篇 |
2023年 | 168篇 |
2022年 | 381篇 |
2021年 | 434篇 |
2020年 | 478篇 |
2019年 | 536篇 |
2018年 | 500篇 |
2017年 | 475篇 |
2016年 | 692篇 |
2015年 | 486篇 |
2014年 | 776篇 |
2013年 | 1293篇 |
2012年 | 1132篇 |
2011年 | 1189篇 |
2010年 | 775篇 |
2009年 | 606篇 |
2008年 | 751篇 |
2007年 | 746篇 |
2006年 | 593篇 |
2005年 | 512篇 |
2004年 | 402篇 |
2003年 | 324篇 |
2002年 | 256篇 |
2001年 | 158篇 |
2000年 | 134篇 |
1999年 | 101篇 |
1998年 | 72篇 |
1997年 | 101篇 |
1996年 | 100篇 |
1995年 | 79篇 |
1994年 | 69篇 |
1993年 | 96篇 |
1992年 | 100篇 |
1991年 | 80篇 |
1990年 | 73篇 |
1989年 | 72篇 |
1988年 | 49篇 |
1987年 | 46篇 |
1986年 | 43篇 |
1985年 | 64篇 |
1984年 | 62篇 |
1983年 | 53篇 |
1982年 | 47篇 |
1981年 | 40篇 |
1980年 | 45篇 |
1979年 | 62篇 |
1978年 | 48篇 |
1977年 | 54篇 |
1976年 | 42篇 |
1975年 | 34篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
151.
A new Schiff base has been synthesized from 4-aminoantipyrine and 3-formylsalicylic acid. The ligand has a dianionic tetradentate compartmental OONO donor system. The cobalt(II), nickel(II), copper(II) and dioxouranium(VI) complexes exist in phenolato-bridged dinuclear species, while palladium(II) gives a mononuclear complex with free –COOH groups. The complexes have been characterized by elemental analyses, i.r., u.v.-vis, thermal and magnetic measurements. 相似文献
152.
Summary New dioxomolybdenum(VI) complexes MoO2L · H2O (LH2=Schiff base) derived from Salicylhydrazide and salicylaldehyde, 5-chloro-, 5-bromo-, 5-nitro-, 3-methoxy, 3-ethoxy-, 3,5-dichlorosalicylaldehyde and 2-hydroxy-1-naphthaldehyde have been synthesized and characterized, together with the MoO2L · THF adducts. The complexes are monomers, nonelectrolytes and diamagnetic, and contain acis O=Mo=O 相似文献
153.
Chloride1), borohydrides2) and hydrides3) of dicyclopentadienyl thallium (III) and diindenyl thallium (III) have already been reported. The present communication deals with a study on the preparation and characterization of difluorenyl thallium (III) chloride, -hydride and -borohydride. 相似文献
154.
An efficient route for the regio- and stereoselective ring opening of N-tosylaziridines with zinc dihalides (ZnX2, X = Cl, Br, I) is described. Depending on the solvent and Zn(II) halide, β-halo amines or imidazolines are obtained selectively in good to excellent yields. 相似文献
155.
Akhilesh K. Verma Rupesh Kumar Amit Saxena Subho Mozumdar 《Tetrahedron letters》2005,46(31):5229-5232
A novel method for effecting the aza-Michael reactions of N-alkyl- and N-arylpiperazines with acrylonitrile using Cu-nanoparticles is described. The method features the use of 10 mol % Cu (14-17 nm) nanoparticles under mild reaction conditions to afford the addition products in good to excellent yields. The Cu-nanoparticles selectively catalysed the aza-Michael reaction of N-alkyl- and N-arylpiperazines in the presence of aromatic amino or aliphatic hydroxy groups. 相似文献
156.
J.S. Yadav B.V. Subba Reddy K.V. Raghavendra Rao G.G.K.S. Narayana Kumar 《Tetrahedron letters》2007,48(31):5573-5576
Indoles undergo smooth alkylation with propargylic alcohols in the presence of 10 mol % of scandium triflate under mild conditions to produce 3-propargylated indoles in excellent yields with high selectivity. 相似文献
157.
An efficient and mild method for the reduction of aromatic nitro and azido groups on solid support using Al/NiCl2·6H2O and Al/NH4Cl is described. This solid phase reduction technique has been applied towards the synthesis of DNA binding pyrrolo[2,1-c][1,4]benzodiazepine antitumour antibiotics. 相似文献
158.
Because Nitrosomonas europaea contains ammonia-oxidizing enzyme, nitrite reductase, and nitrous oxide reductase, the conversion of ammonia to dinitrogen
was tried with different reaction conditions. In aerobic reaction conditions, ammonium was converted to nitrite (NO
2
−
), while under oxygen-limiting or oxygen-free conditions, NO
2
−
-N formed from ammonia oxidation by N. europaea was reduced to N2O and dinitrogen with 22% conversion. During denitrification, optimal pH for the production of N2O and dinitrogen was found to be 7.0–8.0. Dinitrogen was not produced in acidic pH<7.0. A low partial oxygen pressure as well
as oxygen-free conditions are favorable for high production of dinitrogen. 相似文献
159.
Bjelopavlic M Singh PK El-Shall H Moudgil BM 《Journal of colloid and interface science》2000,226(1):159-165
Hydrogen bonding is generally thought to be an ubiquitous adsorption mechanism, which often foils selective adsorption schemes. Through investigation of hydrogen bonding energy and its dependence on surface molecular architecture, it may be possible to develop new methodologies to control the adsorption of surfactants and polymeric flocculants, depressants, and dispersants used in particulate processing industries. A model system using St?ber silica spheres and polyethylene oxide, a polymer known for its ability to form hydrogen bonds, was examined. The effect of two different surface treatments of the silica particles, calcination and rehydroxylation, upon the adsorption of two polymer molecular weights was studied. The adsorption behavior was then linked to the respective surface structures via characterization of the surfaces using FTIR, NMR, and Raman techniques. In this paper role of hydrogen bonding sites and surface architecture on adsorption is discussed. Copyright 2000 Academic Press. 相似文献
160.
A simple and accurate method has been developed for the determination of organoisothiocyanates and organoisocyanates based on their quantitative reaction with n-butylamine in dimethylformamide medium to form substituted thioureas and ureas, respectively. The solution is mixed with 6 to 7 times its volume of acetonitrile and an excess of carbon disulphide, which converts the residual amine quantitatively into the corresponding n-butylammonium n-butyl dithiocarbamate. The determination is completed by titrating the resulting dithiocarbamate with iodine monobromide solution in acetonitrile. The end point is detected visually by the yellow color imparted to the solution by the first drop of oxidant solution in excess. The dithiocarbamates are quantitatively oxidized to the corresponding thiuram disulphides. 相似文献