全文获取类型
收费全文 | 506篇 |
免费 | 14篇 |
国内免费 | 21篇 |
专业分类
化学 | 338篇 |
力学 | 11篇 |
综合类 | 1篇 |
数学 | 14篇 |
物理学 | 177篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 5篇 |
2021年 | 4篇 |
2019年 | 1篇 |
2018年 | 7篇 |
2017年 | 5篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 4篇 |
2013年 | 2篇 |
2012年 | 42篇 |
2011年 | 69篇 |
2010年 | 9篇 |
2009年 | 5篇 |
2008年 | 31篇 |
2007年 | 46篇 |
2006年 | 46篇 |
2005年 | 53篇 |
2004年 | 37篇 |
2003年 | 21篇 |
2002年 | 23篇 |
2001年 | 10篇 |
2000年 | 15篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 6篇 |
1995年 | 12篇 |
1994年 | 10篇 |
1993年 | 18篇 |
1992年 | 9篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1982年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有541条查询结果,搜索用时 15 毫秒
41.
In a previous work [Heinz, Castelijns, and Suter, J. Am. Chem. Soc. 115, 9500 (2003)], we developed an accurate force field and simulated the phase transitions in C18-mica (octadecyltrimethylammonium-mica) as well as the absence of such transitions in 2C18-mica (dioctadecyldimethylammonium-mica) between room temperature and 100 degrees C. Here we analyze (i) average z coordinates of the carbon atoms and interdigitation of the hydrocarbon bilayers, (ii) density profiles, and (iii) pressure profiles of the structures along all Cartesian axes. In C18-mica, the standard deviation in the z coordinate for the chain atoms is high and more than doubles in the disordered phase. The order-disorder transition is accompanied by a change in the orientation of the ammonium head group, as well as decreasing tensile and shear stress in the disordered phase. In 2C18-mica, the standard deviation in the z coordinate for the chain atoms is low and does not increase remarkably on heating. The backbones display a highly regular structure, which is slightly obscured by rotations in the C18 backbones and minor head group displacements at 100 degrees C. Close contacts between the bulky head groups with sidearms cause significant local pressure which is in part not relieved at 100 degrees C. An increase of the basal-plane spacing at higher temperature is found in both systems due to larger separation between the two hydrocarbon layers and an increased z spacing between adjacent chain atoms (=decreased tilt of the chains relative to the surface normal), and, in C18-mica only, a stronger upward orientation of the C18 chain at the ammonium head group. The likelihood for chain interdigitation between the two hydrocarbon layers is 24%-30% for C18-mica, and 65%-26% for 2C18-mica (for 20-100 degrees C). 相似文献
42.
Dinu LC Muller HG Kazamias S Mullot G Augé F Balcou P Paul PM Kovacev M Breger P Agostini P 《Physical review letters》2003,91(6):063901
The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur 190+/-20 as after each pump field maxima (twice per optical cycle), in agreement with the "short" quantum path of the quasiclassical model of harmonic generation. 相似文献
43.
Paul D Melin F Hirtz C Wytko J Ochsenbein P Bonin M Schenk K Maltese P Weiss J 《Inorganic chemistry》2003,42(12):3779-3787
The respective affinities of various imidazole derivatives, imidazole (ImH), 2-methylimidazole (2-MeImH), 2-phenylimidazole (2-PhImH), N-methylimidazole (N-MeIm), 2-methylbenzimidazole (2-MeBzImH), and 4,5-dimethylbenzimidazole (4,5-Me(2)BzImH), for two phenanthroline (Phen) strapped zinc(II) porphyrin receptors porphen-Zn 1-Zn and 2-Zn have been studied. The formation of a supplementary H-bond considerably enhances the affinity of the zinc(II)-porphen receptor for imidazoles unsubstituted on the pyrrolic nitrogen (ImH) versus N-substituted imidazoles such as N-MeIm. The ImHs subset porphen-Zn complexes are formed with association constants up to 4 orders of magnitude superior to those measured either for N-MeIm as substrate or TPP-Zn as receptor. Distal or proximal binding of the substrates was determined by (1)H NMR measurements and titration. In two cases, the very high stability of the inclusion complex enabled the use of 2D NMR techniques. Excellent correlation between solution and solid-state structures has been obtained. A total of six X-ray structures are detailed in this article showing that the evolution of the shape of the zinc(II) receptor is mostly dependent on the steric constraints induced by the substitution on the imidazole. Hindered guests also progressively induce considerable mobility restrictions and severe distortions on the receptor, especially in the case of 2-MeBzImH and 2-PhImH. 相似文献
44.
Lafon R Chaloupka JL Sheehy B Paul PM Agostini P Kulander KC DiMauro LF 《Physical review letters》2001,86(13):2762-2765
The double ionization of helium in the strong-field limit has been studied using an electron-ion coincidence technique. The observed double ionization electron energy spectra differ significantly from the single ionization distributions. This gives new support to the rescattering model of double ionization and explicitly reveals the role of backward electron emission following the e-2e ionizing collision. 相似文献
45.
Oxidative dehydrogenation of propane under steady-state and transient regimes over alumina-supported catalysts prepared from mixed V_2W_4O_(19)~(4-) hexametalate precursors 下载免费PDF全文
Sébastien Paul Elise Berrier Marcia Cristina Kaezer Frana Jean-Guillaume Eon 《天然气化学杂志》2010,19(2):123-133
An integrated approach combining the development of an innovative catalyst and the research of a set of adequate operating conditions for the propane oxidative dehydrogenation (ODH) is described. The experimental set-up, specially designed for steady-state and transient studies is presented. The preparation method, the characterization and the performances in steady-state and transient regimes of catalysts based on V_2W_4O_(19)~(4-) Lindqvist isopolyanion used as a precursor and supported on alumina are reported. The influence of the preparation method of the catalyst and the role of water in the feed gas are more particularly discussed. 相似文献
46.
Chen Z Concepcion JJ Luo H Hull JF Paul A Meyer TJ 《Journal of the American Chemical Society》2010,132(50):17670-17673
The complex [Ru(Mebimpy)(bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2'-bipyridine] and its 4,4'-(PO(3)H(2)CH(2))(2)bpy derivative on oxide electrodes are water oxidation catalysts in propylene carbonate and 2,2,2-trifluoroethanol (TFE) to which water has been added as a limiting reagent. The rate of water oxidation is greatly enhanced relative to that with water as the solvent and occurs by a pathway that is first-order in H(2)O; an additional pathway that is first-order in acetate appears when TFE is used as the solvent. 相似文献
47.
Mithu VS Paul S Kurur ND Madhu PK 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,209(2):359-363
We compare in this communication several heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance experiments under a magic-angle spinning frequency of 60 kHz. The decoupling radiofrequency field amplitudes considered are 190 and 10 kHz. No substantial difference was found among the sequences considered here in performance barring the difference in the optimisation protocol of the various schemes, an aspect that favours the use of swept-frequency two pulse phase modulation (SW(f)-TPPM). 相似文献
48.
Two-component bottle-brush polymers, where flexible side chains containing N = 20, 35 and 50 effective monomers are grafted alternatingly to a rigid backbone, are studied by Molecular Dynamics simulations,
varying the grafting density s \sigma and the solvent quality. Whereas for poor solvents and large enough s \sigma the molecular brush is a cylindrical object with monomers of different type occupying locally the two different halves of
the cylinder, for intermediate values of s \sigma an axially inhomogeneous structure of “pearl-necklace” type is formed, where microphase separation between monomers of different
type within a cluster takes place. These “pearls” have a strongly non-spherical ellipsoidal shape, due to the fact that several
side chains cluster together in one “pearl”. We discuss the resulting structures in detail and we present a comparison with
the single-component bottle-brush case. 相似文献
49.
Alcorta M Rehm KE Back BB Bedoor S Bertone PF Deibel CM DiGiovine B Esbensen H Greene JP Hoffmann CR Jiang CL Lighthall JC Marley ST Pardo RC Paul M Rogers AM Ugalde C Wuosmaa AH 《Physical review letters》2011,106(17):172701
The structure of (15)C, with an s(1/2) neutron weakly bound to a closed-neutron shell nucleus (14)C, makes it a prime candidate for a one-neutron halo nucleus. We have for the first time studied the cross section for the fusion-fission reaction (15)C+(232)Th at energies in the vicinity of the Coulomb barrier and compared it to the yield of the neighboring (14)C+(232)Th system measured in the same experiment. At sub-barrier energies, an enhancement of the fusion yield by factors of 2-5 was observed for (15)C, while the cross sections for (14)C match the trends measured for (12,13)C. 相似文献
50.
We present molecular dynamics simulations of a realistic model of an ultrathin film of BaTiO3 sandwiched between short-circuited electrodes to determine and understand effects of film thickness, epitaxial strain, and the nature of electrodes on its ferroelectric phase transitions as a function of temperature. We determine a full epitaxial strain-temperature phase diagram in the presence of perfect electrodes. Even with the vanishing depolarization field, we find that ferroelectric phase transitions to states with in-plane and out-of-plane components of polarization exhibit dependence on thickness; it arises from the interactions of local dipoles with their electrostatic images in the presence of electrodes. Secondly, in the presence of relatively bad metal electrodes which only partly compensate the surface charges and depolarization field, a qualitatively different phase with stripelike domains is stabilized at low temperature. 相似文献