全文获取类型
收费全文 | 687篇 |
免费 | 7篇 |
国内免费 | 1篇 |
专业分类
化学 | 417篇 |
晶体学 | 9篇 |
力学 | 4篇 |
数学 | 9篇 |
物理学 | 256篇 |
出版年
2022年 | 7篇 |
2021年 | 4篇 |
2020年 | 7篇 |
2018年 | 8篇 |
2017年 | 4篇 |
2016年 | 5篇 |
2015年 | 8篇 |
2014年 | 5篇 |
2013年 | 33篇 |
2012年 | 33篇 |
2011年 | 38篇 |
2010年 | 22篇 |
2009年 | 20篇 |
2008年 | 29篇 |
2007年 | 35篇 |
2006年 | 30篇 |
2005年 | 36篇 |
2004年 | 30篇 |
2003年 | 29篇 |
2002年 | 25篇 |
2001年 | 14篇 |
2000年 | 9篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1996年 | 11篇 |
1995年 | 5篇 |
1994年 | 15篇 |
1993年 | 11篇 |
1992年 | 16篇 |
1991年 | 9篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1987年 | 4篇 |
1986年 | 9篇 |
1985年 | 17篇 |
1984年 | 8篇 |
1983年 | 13篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1974年 | 6篇 |
1973年 | 10篇 |
1971年 | 3篇 |
1969年 | 3篇 |
1967年 | 3篇 |
排序方式: 共有695条查询结果,搜索用时 0 毫秒
51.
Scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) were used to examine the structural transitions and interface dynamics of octanethiol (OT) self-assembled monolayers (SAMs) caused by long-term storage or annealing at an elevated temperature. We found that the structural transitions of OT SAMs from the c(4 x 2) superlattice to the (6 x square root 3) superlattice resulting from long-term storage were caused by both the dynamic movement of the adsorbed sulfur atoms on several adsorption sites of the Au(111) surface and the change of molecular orientation in the ordered layer. Moreover, it was found that the chemical structure of the sulfur headgroups does not change from monomer to dimer by the temporal change of SAMs at room temperature. Contrary to the results of the long-term-stored SAMs, it was found that the annealing process did not modify either the interfacial or chemical structures of the sulfur headgroups or the two-dimensional c(4 x 2) domain structure. Our results will be very useful for a better understanding of the interface dynamics and stability of sulfur atoms in alkanethiol SAMs on Au(111) surfaces. 相似文献
52.
To explore the role of competing forward and reversed chain reactions in the growth of a one-dimensional (1D) molecular line on the Si(100)-(2 x 1)-H surface, controlled experiments were performed with various alkene molecules by scanning tunneling microscopy (STM) at various temperatures. It was observed that the end dangling bond (DB) of a styrene line, fabricated by a chain reaction on the Si(100)-(2 x 1)-H surface at 300 K, initiated a reverse chain reaction at 400 K, leading to the complete disappearance of the styrene line with zero-order desorption kinetics (rate constant k = 1.17 x 10-2 s-1 at 400 K). In the case of 2,4-dimethylstyrene, the reversed chain reaction was observed even at 300 K. These results suggest that the appearance of a molecular line in an STM image is determined by the rates of competing forward and reversed chain reactions at a given temperature. As predicted, 1D lines formed by the DB-initiated chain reaction of 1-hexene and 1-heptene on Si(100)-(2 x 1)-H were observed at 180 K because of the reduced desorption rate, despite the fact that those molecules showed no line growth at 300 K. These results indicate that the scope of forming 1D molecular lines on the Si(100)-(2 x 1)-H surface with various alkenes is much wider than anticipated in previous studies. 相似文献
53.
Isobe M Onoda M Shizuya M Tanaka M Takayama-Muromachi E 《Journal of the American Chemical Society》2007,129(47):14585-14596
The incommensurate modulated crystal structure of the new misfit-layer calcium cobalt oxide (Ca0.85OH)2alphaCoO2 was investigated using a superspace-group formalism with synchrotron X-ray diffraction data. The compound is a kind of composite crystal that consists of two interpenetrating subsystems, [CoO2]infinity layers containing triangular lattices formed by edge-sharing CoO6 octahedra, separated from each other by [2Ca0.85OH]infinity double-layered rock-salt-type slabs. Both the subsystems are monoclinic lattices with the unit cell parameters, a1 = 2.8180(4) A, b = 4.8938(6) A, c = 8.810(1) A, alpha0 = 95.75(3) degrees , and alpha(=|q|=a1/a2) = 0.57822(8), viz., a2 = 4.8736 A, with Z = 2. A possible superspace group is C2/m(alpha10)s0-C21/m(alpha(-1)10) for the respective subsystems. The atomic positions deviate from the average positions of the fundamental structure due to the incommensurable periodic interaction between the subsystems. A significant structural modulation was found in the [2Ca0.85OH] subsystem, whereas the modulation in the [CoO2] subsystem is less than in [2Ca0.85OH], due to the tight bonding of the close-packed CoO6 octahedra. The degree of modulation in the CoO2 layers, i.e., the potential modulation, is almost the same as those of other compounds of the misfit-layer cobalt oxides. Flattened CoO6 octahedra indicate hole doping into the CoO2 layers. The [2Ca0.85OH] blocks act as the charge reservoir layers, and the defect Ca ions are presumably the source of the holes. 相似文献
54.
O-Glycosylations using thiomethyl glycosides as donors were compared under both frozen and unfrozen conditions. In the presence of MeOTf as a promoter, enormous rate acceleration was observed when the glycosylation was conducted in p-xylene below its freezing point. 相似文献
55.
Periodic density functional theory calculations revealed strong enhancement of chemical reactivity by defects located at the oxide-metal interface for water dissociation on ultrathin MgO films deposited on Ag(100) substrate. Accumulation of charge density at the oxide-metal interface due to irregular interface defects influences the chemical reactivity of MgO films by changing the charge distribution at the oxide surface. Our results reveal the importance of buried interface defects in controlling chemical reactions on an ultrathin oxide film supported by a metal substrate. 相似文献
56.
57.
The aromatic thioketone xanthione has been investigated by means of the optically detected magnetic resonance (ODMR) technique in a n-hexane matrix at ≈ 1.1 K. It was established that the short-lived red emission, which is characteristic for many thione molecules, is phosphorescence. At high temperatures (77 K) this phosphorescence originates mainly (>80%) from the T1z (n, π*) sublevel (kz(r) >kx(r), ky(r). At low temperature (1.1 K) the intersystem crossing following S2 (π, π*) ← S0 excitation is a highly spin-sublevel selective process which populates predominantly the T1x and T1y, levels. Hence, the slow spin—lattice relaxation phosphorescence at low temperature originates from these sublevels. A value of 0.0611 cm?1 was obtained for the zero-field parameter |E|/hc. A lower limit of 0.66 cm?1 has been found for the zero-field parameter |D|/hc. This value is considerably larger than those observed for ketones, and it is shown that spin—orbit coupling contributes strongly to the zero-field splitting. 相似文献
58.
IR spectroscopic and volumetric study under reaction conditions of the mechanism of photocatalytic hydrogen production from gaseous methanol and water revealed that CO2 and H2 were produced by reaction between adsorbed CH3O(ad) and H2O. Another reaction path for H2 production, CH3 OH(ad) → H2 + HCHO, was suggested which is dominant in the absence of water. 相似文献
59.
A. Hinz J. S. Wells A. G. Maki 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1987,5(4):351-358
Heterodyne frequency measurements are reported for absorption transitions of N2O in the frequency range from 1257 to 1335 cm?1. The measurements use a CO laser as a transfer oscillator whose frequency is measured directly against combinations of frequencies of two stabilized CO2 lasers whose frequencies are well known. A tunable diode laser is locked to the N2O absorption feature and the frequency difference is measured between the diode laser and the CO laser. Thev 3 fundamental bands of the15N14N16O and14N15N16O isotopes are reported. Measurements are also given for the 0002–0001, 0201–0200, and 0221–0220 vibrational transitions of N2O. A table of frequencies is given for the 0002–0000 band near 2560 cm?1 based on these and earlier measurements. 相似文献
60.
Rigid-rod aromatic polyimdies having polydimethylsiloxane side chains were prepared for the purpose of dispersing rigid-rod molecules in silicone matrices for molecular reinforcement. The polyimides were obtained by imidizing the polyamide-acids bearing the side chains either thermally or chemically, which were synthesized by reacting 4,4′-diaminobiphenyl-terminated polysiloxane macromonomers with pyromellitic dianhydride in THF. The polyamide-acid films obtained by removing the solvent were soluble in THF, but the polyimides were insoluble in any common solvent. The polyimides showed no melting transition below 350°C on DSC analysis, at which temperature the side chain started decomposing. Although all the polyimides were anisotropic as observed by a polarizing microscope, x-ray diffraction analysis suggested the presence of only limited chain organization. Blends with linear polydimethylsiloxanes were prepared by casting the THF solution containing the polyamide-acid and the polydimethylsiloxane, followed by solvent evaporation and thermal imidization. With relatively longer side chains, the polyimides were proved to be compatibilized with polydimethylsiloxanes. © 1994 John Wiley & Sons, Inc. 相似文献