全文获取类型
收费全文 | 5494篇 |
免费 | 132篇 |
国内免费 | 19篇 |
专业分类
化学 | 3913篇 |
晶体学 | 17篇 |
力学 | 100篇 |
数学 | 767篇 |
物理学 | 848篇 |
出版年
2021年 | 44篇 |
2020年 | 48篇 |
2019年 | 48篇 |
2017年 | 29篇 |
2016年 | 89篇 |
2015年 | 88篇 |
2014年 | 103篇 |
2013年 | 208篇 |
2012年 | 180篇 |
2011年 | 199篇 |
2010年 | 137篇 |
2009年 | 119篇 |
2008年 | 184篇 |
2007年 | 188篇 |
2006年 | 215篇 |
2005年 | 201篇 |
2004年 | 162篇 |
2003年 | 176篇 |
2002年 | 182篇 |
2001年 | 126篇 |
2000年 | 100篇 |
1999年 | 92篇 |
1998年 | 88篇 |
1997年 | 76篇 |
1996年 | 83篇 |
1995年 | 80篇 |
1994年 | 80篇 |
1993年 | 85篇 |
1992年 | 98篇 |
1991年 | 84篇 |
1990年 | 71篇 |
1989年 | 88篇 |
1988年 | 88篇 |
1987年 | 94篇 |
1986年 | 89篇 |
1985年 | 116篇 |
1984年 | 114篇 |
1983年 | 79篇 |
1982年 | 115篇 |
1981年 | 95篇 |
1980年 | 104篇 |
1979年 | 107篇 |
1978年 | 87篇 |
1977年 | 96篇 |
1976年 | 78篇 |
1975年 | 72篇 |
1974年 | 93篇 |
1973年 | 61篇 |
1971年 | 50篇 |
1970年 | 30篇 |
排序方式: 共有5645条查询结果,搜索用时 14 毫秒
71.
Summary The question of the existence of good Markov [good stationary] policies is studied for a general class of Borel [stationary] dynamic programming models. It is shown, for example, that Markov [stationary] policies are uniformly adequate if every transition law is absolutely continuous with respect to a fixed measure [and the reward function is positive or the model satisfies certain compactness and continuity conditions].Research supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 72Research supported by National Science Foundation Grant MCS 8100789 相似文献
72.
73.
Minko S Müller M Motornov M Nitschke M Grundke K Stamm M 《Journal of the American Chemical Society》2003,125(13):3896-3900
We report a route to fabricate two-level structured self-adaptive surfaces (SAS) of polymer materials. The first level of structure is built by a rough polymer film that consists of needlelike structures of micrometer size. The second level of structure is formed by the nanoscopic self-assembled domains of a demixed polymer brush irreversibly grafted onto the needles. By exposing the surface to solvents that are selective to one of the components of the brush, we reversibly tune the surface properties. The large-scale surface structure amplifies the response and enables us to control wettability, adhesion, and chemical composition of the surface over a wide range. 相似文献
74.
75.
Brunner H Cattey H Meier W Mugnier Y Stückl AC Wachter J Wanninger R Zabel M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(16):3796-3802
The reaction of the cluster salts [Cp(2*) Nb(CO)(2)](n)[Co(11)Te(7)(CO)(10)] (Cp*=C(5)Me(5); n=1, 2) with excess PMe(2)Ph gave the neutral, dark brown clusters [Co(11)Te(7)(CO)(6)(PMe(2)Ph)(4)] (5) and [Co(11)Te(7)(CO)(5)(PMe(2)Ph)(5)] (6) with 147 metal valence electrons. The new compounds were characterized by IR spectroscopy, elemental analyses, and mass spectrometry. The molecular structure of 6 was determined by X-ray crystallography. Like its precursor anion, it consists of a pentagonal-prismatic [Co(11)Te(7)] core, but with a ligand sphere composed of five CO and five PMe(2)Ph ligands. Detailed electrochemical studies of both reactions reveal that a stepwise substitution of CO ligands in the initial cluster anions takes place leading to intermediate [Co(11)Te(7)(CO)(10-m)(PMe(2)Ph)(m)](n-) ions (m=1-5; n=1, 2). Each of these intermediates is distinguished by at least one oxidation and two reduction waves, giving rise to a total of 21 redox couples and 27 electroactive species. The electron sponge character of the new compounds is particularly pronounced in 5, which exhibits charges n between +1 and -4 corresponding to metal valence electron counts of between 146 and 151. 相似文献
76.
Günter Haufe Manfred Mühlstädt Dr. Jürgen Graefe 《Monatshefte für Chemie / Chemical Monthly》1977,108(4):803-811
In addition totrans-2-bromocyclooctanol andtrans-1,2-dibromocyclooctane,cis-4-bromocyclooctanol,cis-1,4-dibromocyclooctane,trans-1,4-dibromocyclooctane, and (Z)-5-bromocyclooctene are obtained, when (Z)-cyclooctene is treated with N-bromosuccinimide in the presence of water. Similarly the methoxybromination of (Z)-cyclooctene gives transanular products. 相似文献
77.
Determination of Molecular Weights by Gel Chromatography of Oligoimidoamides of Phosphoric Acid. Oligoimidoamides of phosphoric acid were eluted with nonionic water on a Sephadex LH-20 column. The measured distribution coefficient of the oligoimidoamides of phosphoric acid was independent of sample concentration and pH of the eluent. Andrews equation was obeyed: a linear relation exist between elution volume and logarithm of molecular weight both cyclic and acyclic oligoimidoamides of phosphoric acid. 相似文献
78.
Breher F Rüegger H Mlakar M Rudolph M Deblon S Schönberg H Boulmaâz S Thomaier J Grützmacher H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(3):641-653
The formation of adducts of the square-planar 16-electron complexes trans-[M(tropp(ph))(2)](+) and cis-[M(tropp(ph))(2)](+) (M=Rh, Ir; tropp(Ph)=5-diphenylphosphanyldibenzo[a,d]cycloheptene) with acetonitrile (acn) and Cl(-), and the redox chemistry of these complexes was investigated by various physical methods (NMR and UV-visible spectroscopy, square-wave voltammetry), in order to obtain some fundamental thermodynamic and kinetic data for these systems. A trans/cis isomerization cannot be detected for [M(tropp(ph))(2)](+) in non-coordinating solvents. However, both isomers are connected through equilibria of the type trans-[M(tropp(ph))(2)](+)+L<==>[ML(tropp(ph))(2)](n)<==>cis-[M(tropp(ph))(2)](+)+L, involving five-coordinate intermediates [ML(tropp(ph))(2)](n) (L=acn, n=+1; L=Cl(-), n=0). Values for K(d) (K(f)), that is, the dissociation (formation) equilibrium constant, and k(d) (k(f)), that is, the dissociation (formation) rate constant, were obtained. The formation reactions are fast, especially with the trans isomers (k(f)>1x10(5) m(-1) s(-1)). The reaction with the sterically more hindered cis isomers is at least one order of magnitude slower. The stability of the five-coordinate complexes [ML(tropp(ph))(2)](n) increases with Ir>Rh and Cl(-)>acn. The dissociation reaction has a pronounced influence on the square-wave (SW) voltammograms of trans/cis-[Ir(tropp(ph))(2)](+). With the help of the thermodynamic and kinetic data independently determined by other physical means, these reactions could be simulated and allowed the setting up of a reaction sequence. Examination of the data obtained showed that the trans/cis isomerization is a process with a low activation barrier for the four-coordinate 17-electron complexes [M(tropp(ph))(2)](0) and especially that a disproportionation reaction 2 trans/cis-[M(tropp(ph))(2)](0)-->[M(tropp(ph))(2)](+)+[M(tropp(ph))(2)](-) may be sufficiently fast to mask the true reactivity of the paramagnetic species, which are probably less reactive than their diamagnetic equilibrium partners. 相似文献
79.
In the past few years millimeter and submillimeter wave spectroscopy has been the backbone for modern molecular line astronomy. The detection of unstable and high-temperature molecular species in the interstellar environment has stimulated the development of methods of detecting molecular ions, radicals and unstable molecules in the laboratory by means of millimeter wave technology. Some of the recent experimental advances as well as the developments which have occurred in the analysis of high resolution rotational and vibrational spectra with regard to understanding molecular dynamics will be discussed. 相似文献
80.
M. G. O. Santana-Marques A. J. V. Ferrer-Correia K. A. Caldwell M. L. Gross 《Journal of the American Society for Mass Spectrometry》1993,4(10):819-827
Aromatic β-hydroxyoximes undergo unusual fragmentation reactions as protonated or cationized species, as radical cations, or as (M - H)? ions, As protonated species, they expel OH ’ from the oxime functionality in violation of the even electron rule. Parallel eliminations of alkyl radicals follow OH’ loss when the aromatic ring is substituted with an alkyl chain. Alkyl radical losses appear to be characteristic of radical cations that can isomerize to ions in which the alkyl chain bears a radical site and the charged site is the conjugate acid of a basic functionality (e.g., oxime or imine). Evidence for the mechanisms was found in the ion chemistry of oxime and imine radical cations. The imine reference compounds were conveniently generated by fast atom bombardment-induced reduction of oximes, removing the requirement for using conventional chemical synthesis. Protonated imines and the (M - H)? ions of oximes fragment extensively via charge-remote processes to eliminate the elements of alkanes. This chemistry is not shared by the protonated oximes. 相似文献