全文获取类型
收费全文 | 7578篇 |
免费 | 386篇 |
国内免费 | 496篇 |
专业分类
化学 | 2891篇 |
晶体学 | 245篇 |
力学 | 84篇 |
综合类 | 59篇 |
数学 | 2058篇 |
物理学 | 1034篇 |
无线电 | 2089篇 |
出版年
2024年 | 14篇 |
2023年 | 83篇 |
2022年 | 97篇 |
2021年 | 144篇 |
2020年 | 129篇 |
2019年 | 146篇 |
2018年 | 112篇 |
2017年 | 151篇 |
2016年 | 181篇 |
2015年 | 213篇 |
2014年 | 323篇 |
2013年 | 496篇 |
2012年 | 374篇 |
2011年 | 568篇 |
2010年 | 508篇 |
2009年 | 562篇 |
2008年 | 533篇 |
2007年 | 509篇 |
2006年 | 440篇 |
2005年 | 365篇 |
2004年 | 332篇 |
2003年 | 264篇 |
2002年 | 404篇 |
2001年 | 172篇 |
2000年 | 165篇 |
1999年 | 144篇 |
1998年 | 122篇 |
1997年 | 87篇 |
1996年 | 100篇 |
1995年 | 83篇 |
1994年 | 69篇 |
1993年 | 51篇 |
1992年 | 53篇 |
1991年 | 32篇 |
1990年 | 17篇 |
1988年 | 12篇 |
1985年 | 38篇 |
1984年 | 19篇 |
1983年 | 23篇 |
1982年 | 23篇 |
1981年 | 44篇 |
1980年 | 30篇 |
1979年 | 14篇 |
1978年 | 24篇 |
1977年 | 30篇 |
1976年 | 26篇 |
1975年 | 23篇 |
1974年 | 28篇 |
1973年 | 22篇 |
1971年 | 16篇 |
排序方式: 共有8460条查询结果,搜索用时 78 毫秒
141.
M. A. Natsheh 《Periodica Mathematica Hungarica》1993,26(1):7-14
Leth be a cyclic action of periodn onM, whereM is eitherS
1×K, K is the Klein bottle or on
, the twisted Klein bottle bundle overS
1, such that there is a fiberingq:MS
1 with fiber a Klein bottleK or a torusT with respect to which the action is fiber preserving. We classify all such actions and show that they might be distinguished by their fixed points or by their orbit spaces. 相似文献
142.
143.
Isolation of lipase producer and its performance in enantioselective hydrolysis of glycidyl butyrate
A racemic glycidyl butyrate resolving strain, preliminarily identified as a Rhizopus sp., had been isolated from soil. Its extracellular lipase was found to enantioselectively hydrolyze the (S)-enantiomer of
the chiral ester, with optimal activities at pH 5.3 and 42°C. Higher en antioselectivity of theenzyme was observed at lower
temperatures, while the best anantioselectivity was obtained at pH 5.5–6.0, with an, E value (enantiomeric ratio) of 57. 相似文献
144.
Sousa Pedrares A Teng W Ruhlandt-Senge K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(9):2019-2024
Novel magnesium pyridine-2-thiolates were prepared by using alkane elimination chemistry. The resulting complexes display a metal coordination environment composed of sulfur/nitrogen bonding from the intramolecularly stabilized mercaptopyridine ligand, in addition to coordination by the oxygen centers from two THF donors. The compounds are well-suited model compounds for the magnesium centers in Photosystem I, in which magnesium, situated in the central chlorophyll ligand, is bound to sulfur from a nearby methionine residue. All compounds were characterized by (1)H, (13)C NMR, and IR spectroscopy, in addition to Xray crystallography. 相似文献
145.
F. Sauter J. Fröhlich A. Z. M. S. Chowdhury C. Hametner 《Monatshefte für Chemie / Chemical Monthly》1997,128(5):503-508
Summary A variety of partly novel tri- and tetracyclic hetero systems were obtained by reaction of heteroaromatic 2-aminoesters with 2-(methylthio)-2-thiazoline, yielding double-annelation of a thiazolo[2,3-b]pyrimido moiety in a one-pot process.On leave from University of Chittagong, Bangladesh 相似文献
146.
In the enantioselective esterification of racemic naproxen with 4(2-hydroxyethyl) morpholine by Lipase MY in organic solvents,
a productivity improvement of the desired (S)-naproxen ester from 0.42 to 0.72 mM at the reaction time of 130 h was observed,
when the surfactantbis (2-ethylhexyl) sodium sulfosuccinate (AOT) was added in the reaction mixture. The presence of a small amount of exogenously
added water dramatically activated the enzyme in AOT/cyclohexane-reversed micelles. Desorption of the surfactant molecule
from the enzyme mass and solubilization of the enzyme into reversed micelles were used to elucidate an existing maximum of
the initial rate of (S)-naproxen synthesis with the water content. Moreover, the effects of alcohol and surfactant concentration
on the enzyme activity are reported. 相似文献
147.
In a reaction system consisting of FeCl2,tetrathiometallate and cycloalkylthiolate,two Fe4S4 cubane-like cluster compounds were obtained with the following crystallographic data:(PhCH2NMe32[Fe4S4 (SC5H9)4](Ⅰ),monoclinic space group P21/c,a=1.6327(4),6=1.1229(3),c=2.802 5( 10) nm,β=94.63(2)°,Z=4,and R=0.074; (Et4N)2[Fe4S4(SC6H11)4](Ⅱ),tetragonal space group I42m,a=1.16705(9),b=1.167 06(2),c=2.063 26(5) nm,Z=2,Dabs=1.28 g/cm3,and R=0.078 The participation of cy-cloalkylthiolate ligand does not obviously arouse the change of the Fe4S4 core structure.Meanwhile,the influence of the cation on the structural symmetry of the Fe4S4 cluster dianion is also discussed. 相似文献
148.
Summary Free boundary value problems, too complicated for formulation as a variational inequality, are broken up into two problems on overlapping regions. On one region the problem is treated as an ordinary boundary value problem; on the second region, the free boundary part of the problem is reduced to a variational inequality. By solving the two problems successively it is shown that under certain conditions the successive solutions converge to a single function that gives a solution of the original problem. Application to a filtration problem is given. 相似文献
149.
在Pd(PPh3)4催化下,将单体(S)-6,6′-二溴-2,2′-二正丁氧基-1,1′-联萘[(S)-M-1]和(R)-6,6′-二溴-2,2′-二正丁氧基-1,1′-联萘[(R)-M-1]分别与2,5-二(4-三正丁基锡基苯)-1,3,4-噁二唑(M-2)通过Stille交叉耦合反应合成了手性高分子P-1与P-2,并用1HNMR、13CNMR、FTIR、UV、热分析、荧光光谱、GPC和CD等分析方法进行了表征.手性高分子P-1和P-2都能发射较强的蓝色荧光;在高分子侧链上引入丁氧基后使得手性高分子的溶解性能增强,并具有良好的成膜性能;在高分子主链引入亲电子的噁二唑生色团能使其特别适合于作为空穴电子传输层,对氧和热特别稳定,是一类潜在的光电高分子材料. 相似文献
150.
HuilingFan ChunhuLi HanxianGuo KechangXie 《天然气化学杂志》2003,12(1):43-48
The microkinetics os H2S removeal by ZnO desulfurization in H2O-CO2-N2,H2O-CO-N2 and H2O-O2-N2 gas mixtures was studied by thermogravimetric andlysis.Experiments were carried out with 100-120 mesh ZnO powder at temperatures from 473K to 563K.The results show that the kinetic behaviors of desulfurization could all be described by an improved shrinking-core model.The activation energies of the reaction and the diffusion in different gas atmospheres were estimated. 相似文献