全文获取类型
收费全文 | 3382篇 |
免费 | 98篇 |
国内免费 | 11篇 |
专业分类
化学 | 2289篇 |
晶体学 | 24篇 |
力学 | 54篇 |
数学 | 295篇 |
物理学 | 829篇 |
出版年
2022年 | 18篇 |
2021年 | 44篇 |
2020年 | 47篇 |
2019年 | 37篇 |
2018年 | 32篇 |
2017年 | 32篇 |
2016年 | 53篇 |
2015年 | 55篇 |
2014年 | 59篇 |
2013年 | 164篇 |
2012年 | 136篇 |
2011年 | 166篇 |
2010年 | 81篇 |
2009年 | 62篇 |
2008年 | 129篇 |
2007年 | 152篇 |
2006年 | 164篇 |
2005年 | 126篇 |
2004年 | 135篇 |
2003年 | 130篇 |
2002年 | 113篇 |
2001年 | 72篇 |
2000年 | 93篇 |
1999年 | 50篇 |
1998年 | 46篇 |
1997年 | 41篇 |
1996年 | 53篇 |
1995年 | 58篇 |
1994年 | 44篇 |
1993年 | 53篇 |
1992年 | 48篇 |
1991年 | 41篇 |
1990年 | 44篇 |
1989年 | 27篇 |
1988年 | 27篇 |
1987年 | 41篇 |
1986年 | 43篇 |
1985年 | 52篇 |
1984年 | 57篇 |
1983年 | 31篇 |
1982年 | 37篇 |
1981年 | 47篇 |
1980年 | 48篇 |
1979年 | 50篇 |
1978年 | 40篇 |
1977年 | 48篇 |
1976年 | 49篇 |
1975年 | 35篇 |
1974年 | 34篇 |
1973年 | 46篇 |
排序方式: 共有3491条查询结果,搜索用时 15 毫秒
111.
A 14 MeV neutron activation analysis system is described that is being used primarily to determine trace levels of oxygen
in metals. No sample container is required for relatively inert solid metal samples thus increasing the sensitivity of the
method by a factor of five. Under these conditions the sensitivity and precision foroxygen is about 6±3.3 μg in a single determination,
resulting from collecting 1.216N counts per μg of oxygen. A method is described for calibrating the system for the analyses of large metal samples. The preparation
and use of sample containers for the analyses of particulate and/or reactive samples is also presented.
Work was performed at the Ames Laboratory of the U.S. Atomic Energy Commission. Contribution No. 2567. 相似文献
112.
Budarin VL Clark JH Tavener SJ Wilson K 《Chemical communications (Cambridge, England)》2004,(23):2736-2737
The reaction of localised C[double bond, length as m-dash]C bonds on the surface of activated carbons has been shown to be an effective method of chemical modification especially using microwave-assisted reactions. 相似文献
113.
The oxidation of polyethylene, polypropylene, and polystyrene by exposure to plasmas excited in pure oxygen and helium–oxygen mixtures at low power levels has been studied. A detailed curve resolution procedure is outlined, and the rate of oxidation is shown to be a strong function of the polymer structure for pure oxygen plasmas, as is the composition of the oxidized layer; this is not the case, however, for oxidation effected by helium–oxygen mixtures. It seems likely, from a consideration of the available data, that the oxidation is confined to the outermost monolayer and is initiated by a crosslinking mechanism that involves oxygen-containing functionalities. 相似文献
114.
The propagating species for 1-hexene, propene, and ethene polymerization as catalyzed by [rac-(C2H4(1-indenyl)2)Zr(CH3)][CH3B(C6F5)3] has been intercepted at T < -40 degrees C and characterized by NMR methods. Observation of the propagating species permits direct monitoring of initiation, propagation, and termination processes by NMR. Detailed examination of alkene incorporation into Zr-polymeryl species using 13C-labeled 1-alkenes supports a continuous insertion mechanism whereby the anion re-coordinates after each alkene insertion. 相似文献
115.
The enantioselective monoreduction of 2-alkyl 1,3-diketones by dynamic kinetic resolution using optically active ruthenium catalysts allowed the preparation of the C14-C25 fragment of bafilomycin A1. 相似文献
116.
117.
W. Clark Still Lawrence J. Macpherson Toshiro Harada James F. Callahan Arnold L. Rheingold 《Tetrahedron》1984,40(12):2275-2281
The conformations of macrocyclic intermediates provide a useful medium through which distant chiral centers may control chemical reactions. In this paper, we show that macrocycles made by cyclization of simple acyclic starting materials with an auxiliary spacer may be used to prepare stereochemically complex acyclic products. 相似文献
118.
The use of methanol or ethanol as the injection solvent for the gas chromatographic-mass spectral (GC-MS) analysis of low molecular weight amine drugs of abuse results in the formation of additional components in the sample. Primary amines, such as amphetamine, 3,4-methylenedioxyamphetamine, and phenethylamine, yield imines upon injection as methanol or ethanol solutions. In methanol, the imine formed has a mass that is 12 mass units higher than the parent compound. In ethanol, the products formed have 26 additional mass units. Secondary amines appear to undergo methylation under similar conditions with methanol as the injection solvent. These products are absent from the analysis of equivalent amine samples dissolved in chloroform. 相似文献
119.
Changes in the surface composition and structure of a number of non-aromatic polymers subjected to ultraviolet irradiation in oxygen (~ 1 atm., 20°C) have been studied. No changes in surface composition were detected after photo-oxidation of poly(vinylidene fluoride) (12 h of irradiation), low density polyethylene (7·5 h) or high density polyethylene (22·3 h). This reflects the absence in these polymers of an efficient ultraviolet absorber to initiate the photo-oxidation. The surfaces of poly(methyl methacrylate) and nylon 66 show an increase in oxygen content following irradiation. In these polymers this added oxygen is present principally as carboxyl groups and, in the case of poly(methyl methacrylate), as carbonyl groups. In nylon, there is no evidence for changes in the amide group and no oxidation of the nitrogen is detected. 相似文献
120.
trans-Pd(NO3)H(PCy3)2 (I) when treated with Et4NCl forms the hydride trans-PdClH(PCy3)2 which in turn reacts with AgPF6 in acetonitrile to give trans-[PdH(CH3CN)(PCy3)2]PF6 (III). Both I and III react smoothly with acetylenes containing one electron-withdrawing group to give alkenyl products. The geometry of the resulting alkenyl ligand implies that cis addition has occurred and that the hydridic hydrogen adds to the acetylenic carbon containing the electron-withdrawing group.Acetylenes containing two electron-withdrawing groups give mixtures from which both alkenyl and zerovalent acetylene compounds can sometimes be isolated. In the presence of proton sponge, monosubstituted acetylenes still give alkenyl products while those substituted with two electron-withdrawing groups give the zerovalent products in good yield. The relevance of these results to an understanding of the nature of the migratory insertion reaction is discussed. 相似文献