全文获取类型
收费全文 | 7247篇 |
免费 | 329篇 |
国内免费 | 58篇 |
专业分类
化学 | 5450篇 |
晶体学 | 62篇 |
力学 | 131篇 |
数学 | 661篇 |
物理学 | 1330篇 |
出版年
2023年 | 35篇 |
2022年 | 110篇 |
2021年 | 141篇 |
2020年 | 112篇 |
2019年 | 136篇 |
2018年 | 107篇 |
2017年 | 111篇 |
2016年 | 222篇 |
2015年 | 215篇 |
2014年 | 248篇 |
2013年 | 436篇 |
2012年 | 516篇 |
2011年 | 598篇 |
2010年 | 352篇 |
2009年 | 282篇 |
2008年 | 467篇 |
2007年 | 419篇 |
2006年 | 449篇 |
2005年 | 380篇 |
2004年 | 334篇 |
2003年 | 299篇 |
2002年 | 261篇 |
2001年 | 137篇 |
2000年 | 116篇 |
1999年 | 85篇 |
1998年 | 61篇 |
1997年 | 57篇 |
1996年 | 72篇 |
1995年 | 56篇 |
1994年 | 55篇 |
1993年 | 40篇 |
1992年 | 36篇 |
1991年 | 30篇 |
1990年 | 40篇 |
1989年 | 38篇 |
1988年 | 30篇 |
1987年 | 26篇 |
1986年 | 20篇 |
1985年 | 43篇 |
1984年 | 33篇 |
1983年 | 25篇 |
1982年 | 28篇 |
1981年 | 28篇 |
1980年 | 21篇 |
1979年 | 32篇 |
1978年 | 21篇 |
1977年 | 27篇 |
1976年 | 23篇 |
1975年 | 19篇 |
1973年 | 28篇 |
排序方式: 共有7634条查询结果,搜索用时 0 毫秒
41.
42.
43.
Inferring statistical complexity 总被引:1,自引:0,他引:1
44.
William W. Paudler Richard A. Vandahm Young N. Park 《Journal of heterocyclic chemistry》1972,9(1):81-85
The diazaanalog of “cycl[3,2,2]azine”, “1,4-diazacycl[3,2,2]azine” (1,4,7b-triazacyclopent-[cd]indene) and its 2-methyl derivative were prepared. These compounds are subject to facile acid-catalyzed hydrolysis affording substituted imidazo[1,2-a]pyridines. 相似文献
45.
Applied Biochemistry and Biotechnology - The US chemical industry is facing maturing markets, variability in raw materials supply, and stiff competition from emerging chemical industries in other... 相似文献
46.
Three new Ni(BDC)/2,2'-bipy compounds, Ni2(BDC)(HBDC)2(2,2'-bipy)2 (2), Ni3(BDC)3(2,2'-bipy)2 (3), and Ni(BDC)(2,2-bipy)2.2H2O (5), in addition to the previously reported Ni(BDC)(2,2'-bipy).0.75H2BDC (1) and Ni(BDC)(2,2'-bipy)(H2O) (4) [BDC = 1,4-benzenedicarboxylate; 2,2'-bipy = 2,2'-bipyridine], have been synthesized by hydrothermal reactions. A systematic investigation of the effect of the reaction temperature and pH resulted in a series of compounds with different compositions and dimensionality. The diverse product slate illustrates the marked sensitivity of the structural chemistry of polycarboxylate aromatic ligands to synthesis conditions. Compound 1, which has a channel structure containing guest H2BDC molecules, is formed at the lowest pH. The guest H2BDC molecules are connected by hydrogen bonds and form extended chains. At a slightly higher pH, a dimeric molecular compound 2 is formed with a lower number of protonated carboxylate groups per nickel atom and per BDC ligand. Reactions at higher temperature and the same pH lead to the transformation of 1 and 2 into the two-dimensional, layered trinuclear compound 3. As the pH is increased, a one-dimensional polymer 4 is formed with a water molecule coordinated to Ni2+. Bis-monodentate and bischelating BDC ligands alternate along the chain to give a crankshaft rather than a regular zigzag arrangement. A further increase of the pH leads to the one-dimensional chain compound 5, which has two chelating 2,2'-bipy ligands. Crystal data: 2, triclinic, space group P, a = 7.4896(9) angstroms, b = 9.912(1) angstroms, c = 13.508(2) angstroms, alpha = 86.390(2) degrees , beta = 75.825(2) degrees, gamma = 79.612(2) degrees, Z = 2; 3, orthorhombic, space group Pbca, a = 9.626(2) angstroms, b = 17.980(3) angstroms, c = 25.131(5) angstroms, Z = 4; 5, orthorhombic, space group Pbcn, a = 14.266(2) angstroms, b = 10.692(2) angstroms, c = 17.171(2) angstroms, Z = 8. 相似文献
47.
Viorica Lopez-Avila Richard Young Werner F. Beckert 《Journal of separation science》1997,20(9):487-492
This paper describes the extraction of 49 organophosphorus pesti-cides (OPPs) from water samples using solid-phase microextraction (SPME). Three fibers, including a 15-μm XAD-coated fiber, a 85-μm polyacrylate-coated fiber, and a 30-μm polydimethylsilox-ane-coated fiber (PDMS), were evaluated here. The effects of stirring and the addition of NaCl to the sample were examined for the polyacrylate-coated fiber. The precision of the technique was examined for all three fibers and the extraction kinetics were investigated using the XAD- and polyacrylate-coated fibers. With some exceptions, the XAD- and polyacrylate-coated fibers performed better than the PDMS-coated fiber. The superiority of the XAD-nd polyacrylate-coated fiber. The superiority of the XAD- and polyacrylate-coated fibers over the PDMS-coated fibers can be attribuibuted to the aromatic functionalities of the XAD and the polar functionalities in the polyacrylate. The relatively high percent RSDs indicate that the SPME technique needs to be further refined before it can be used for anything other than screening. A more effective form of agitation than mechanical stirring may be neccessary to reduce variability and achieve a faster equilibrium between the sample and the SPME fiber. 相似文献
48.
The second-order rate constants for the nucleophilic replacement of fluorine by isopropoxide in 5,6,7,8-tetrafluoro- and 6,7,-difluoro-1,2,3,4-tetrahydro-9-alkenyl-1,4-methanonaphthalenes and related 9-alkyl systems have been measured. A factor of 6 - 7 separates the most reactive compounds [the 9- (4′-trifluoromethylbenzylidene) derivatives] from the least reactive compounds [the syn-9-isopropyl derivatives] in both the tetrafluoro- and difluoro-series. It is concluded for these small reactivity differences. 相似文献
49.
Ovshinsky SR Young RT Allred DD DeMaggio G Van der Leeden GA 《Physical review letters》1987,58(24):2579-2581
50.