全文获取类型
收费全文 | 718篇 |
免费 | 33篇 |
国内免费 | 2篇 |
专业分类
化学 | 475篇 |
晶体学 | 14篇 |
力学 | 11篇 |
数学 | 66篇 |
物理学 | 187篇 |
出版年
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 3篇 |
2018年 | 16篇 |
2017年 | 8篇 |
2016年 | 10篇 |
2015年 | 17篇 |
2014年 | 16篇 |
2013年 | 47篇 |
2012年 | 26篇 |
2011年 | 33篇 |
2010年 | 23篇 |
2009年 | 20篇 |
2008年 | 59篇 |
2007年 | 31篇 |
2006年 | 44篇 |
2005年 | 36篇 |
2004年 | 26篇 |
2003年 | 22篇 |
2002年 | 29篇 |
2001年 | 11篇 |
2000年 | 5篇 |
1999年 | 9篇 |
1998年 | 5篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 12篇 |
1992年 | 10篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 11篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 17篇 |
1976年 | 7篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1962年 | 2篇 |
排序方式: 共有753条查询结果,搜索用时 156 毫秒
171.
Oxidation of Cu3Au(1 1 0) using a hyperthermal O2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident energy dependence of the O-uptake curve, the precursor-mediated dissociative adsorption occurs, where the trapped O2 molecule can migrate and dissociate at the lower activation-barrier sites, dominantly at thermal O2 exposures. Dissociative adsorption of O2 on Cu3Au(1 1 0) is as effective at the thermal O2 exposure as on Cu(1 1 0). On the other hand, at the incident energies of HOMB where the direct dissociative adsorption is dominant, it was determined that the dissociative adsorption of O2 implies a higher activation barrier and therefore less reactivity due to the Au alloying in comparison with the HOMB oxidation of Cu(1 1 0). The dissociative adsorption progresses with the Cu segregation on Cu3Au(1 1 0) similarly as on Cu3Au(1 0 0). The growth of Cu2O for 2 eV HOMB suggests that the diffusion of Cu atoms also contribute to the oxidation process through the open face, which makes the difference from Cu3Au(1 0 0). 相似文献
172.
Synthesis of a 6/6/6 tricyclic ether system (3) corresponding to the ABC ring fragment of yessotoxin (1) has been achieved via coupling of a triflate and a 2-lithiofuran followed by intramolecular hetero-Michael addition. The IJ ring fragment (4) of 1 was readily synthesized via successive Sharpless epoxidation and 6-endo cyclization of the resulting vinyl epoxide. 相似文献
173.
Negishi Y Tsunoyama H Suzuki M Kawamura N Matsushita MM Maruyama K Sugawara T Yokoyama T Tsukuda T 《Journal of the American Chemical Society》2006,128(37):12034-12035
We report herein the X-ray magnetic circular dichroism (XMCD) at the Au L2,3 edges of a series of Au clusters protected by glutathione (GSH). The samples used here included AuN(SG)M with (N, M) = (10, 10), (15, 13), (18, 14), (22, 16), (25, 18), (29, 20), (39, 24) and a sodium gold(I) thiomalate (SGT) as a reference. Magnetic moments per cluster were found to be increased with size, whereas those per Au-S bond were nearly constant. This finding suggests that a localized hole created by Au-S bonding at the gold/glutathione interface, rather than the quantum size effect, is responsible for the spin polarization of gold clusters. 相似文献
174.
The structure of a protein molecule is considered to be primarily determined by the inter-amino-acid nonbonded interactions, such as hydrogen bonds. However, the conformational space of the polypeptide chain should be simultaneously restricted by the intrinsic conformational preferences of the individual amino acids. We present here precise single amino acid potential (SAAP) surfaces for glycine (For-Gly-NH(2)) and alanine (For-Ala-NH(2)) in water (epsilon = 78.39) and ether (epsilon = 4.335), which were calculated at the HF/6-31+G(d,p) level applying the self-consistent isodensity polarizable continuum model (SCIPCM) reaction field with geometry optimization in the corresponding solvents. The obtained Ramachandran potential surfaces in water showed distinct potential wells in the alpha- and beta-regions. The profiles were in almost perfect agreement with the Ramachandran plots of glycine and alanine residues in folded proteins, suggesting the Boltzmann distributions on the SAAP surfaces. Molecular simulations of polyalanines (For-Ala(n)-NH(2); n = 3-5) by using the SAAP force field equipped with the SCIPCM potentials revealed that the polyalanines readily form 3(10)-helical structures in water but not in vacuo. In ether (hydrophobic environments), the helical structures were relatively stable, but the most stable structure was assigned to a different one. These results indicated that the intrinsic conformational preferences of the individual amino acids (i.e., the SAAPs) in water are of significant importance not only for describing conformations of a polypeptide chain in the random coil state but also for understanding the folding to the secondary and tertiary structures. 相似文献
175.
Yasushi Homma 《Transactions of the American Mathematical Society》2006,358(1):87-114
Gradients are natural first order differential operators depending on Riemannian metrics. The principal symbols of them are related to the enveloping algebra and higher Casimir elements. We give formulas in the enveloping algebra that induce not only identities for higher Casimir elements but also all Bochner-Weitzenböck formulas for gradients. As applications, we give some vanishing theorems.
176.
The purpose of this study was to examine the potential of diffusion-weighted (DW) three-dimensional (3D) MP-RAGE MRI for diffusion-tensor mapping of the rat brain in vivo. A DW-3D-MP-RAGE (3D-DWI) sequence was implemented at 2.0 T using six gradient orientations and a b value of 1000 s/mm2. In this sequence, the preparation sequence with a "90 degrees RF-motion proving gradient (MPG): MPG-180 degrees RF-MPG-90 degrees RF" pulse train (DW driven equilibrium Fourier transform) was used to sensitize the magnetization to diffusion. A centric k-space acquisition order was necessary to minimize saturation effects (T1 contamination) from tissues with short relaxation time. The image matrix was 128x128x128 (interpolated from 64x64x64 acquisitions), which resulted in small isotropic DW image data (voxel size: 0.273x0.273x0.273 mm3). Moreover, 3D-DWI-derived maps of the fractional anisotropy (FA), relative anisotropy (RA) and main-diffusion direction were completely free of susceptibility-induced signal losses and geometric distortions. Two well-known commissural fibers, the corpus callosum and anterior commissure, were indicated and shown to be in agreement with the locations of these known stereotaxic atlases. The experiment took 45 min, and shorter times should be possible in clinical application. The 3D-DWI sequence allows for in vivo 3D diffusion-tensor mapping of the rat brain without motion artifacts and susceptibility to distortion. 相似文献
177.
Yamada M Minowa M Sato S Kako M Slanina Z Mizorogi N Tsuchiya T Maeda Y Nagase S Akasaka T 《Journal of the American Chemical Society》2010,132(50):17953-17960
Thermal carbosilylation of endohedral dimetallofullerene La(2)@I(h)-C(80) with silirane (silacyclopropane) is reported herein for the first time. Two diastereomers of the carbosilylated La(2)@I(h)-C(80) have been isolated and characterized. The fascinating molecular structure of one diastereomer of the carbosilylated derivatives has been determined unambiguously using X-ray crystallographic analysis. Detailed characteristics of the molecular structures including their metal atom movements have also been revealed using NMR spectroscopic studies and computational calculations. Results revealed that two La atoms move dynamically inside the carbon sphere. Furthermore, electrochemical study has demonstrated that carbosilylation is effective to fine-tune the La(2)@I(h)-C(80) electronic properties. 相似文献
178.
Sadakane M Tsukuma D Dickman MH Bassil B Kortz U Higashijima M Ueda W 《Dalton transactions (Cambridge, England : 2003)》2006,(35):4271-4276
We have synthesized the mono-ruthenium substituted Keggin-type silicotungstate [SiW(11)O(39)Ru(III)(H(2)O)](5-) (1a) by reaction of the mono-lacunary silicotungstate precursor [SiW(11)O(39)](8-) with Ru(acac)(3) under hydrothermal conditions and isolated as the caesium salt Cs(5)[SiW(11)O(39)Ru(III)(H(2)O)] (1). The DMSO-coordinated complex [SiW(11)O(39)Ru(III)(DMSO)](5-) (2a) was prepared by reaction of 1a with DMSO in aqueous solution at 353 K and isolated as the caesium-potassium mixed salt Cs(4.9)K(0.1)[SiW(11)O(39)Ru(III)(DMSO)] (2). Both compounds 1 and 2 were characterized by single-crystal X-ray structure analysis, powder X-ray structure analysis, UV-Vis spectroscopy, cyclic voltammetry, IR-spectroscopy and elemental analysis. 1 crystallized in the tetragonal space group P4(2)/ncm with a = 20.9299(4), c = 10.3603(4) Angstrom, Z = 4. The ruthenium atom in the Keggin unit could not be distinguished from the tungsten due to disorder. The structural analysis of 2 (monoclinic, P2(1)/c, a = 13.5850(4), b = 20.2764(7), c = 18.1326(4) Angstrom, beta = 90.8730(10) degrees , Z = 4) successfully revealed that the incorporated ruthenium atom is coordinated by DMSO through a Ru-S bond. Polyanion 2a represents the first mono-substituted Keggin ion in which the ruthenium center is not crystallographically disordered. UV-Vis spectroscopy combined with controlled potential electrolysis confirmed that the incorporated rutheniums in 1 and 2 have a valence state of +3. The IR spectra of both 1 and 2 were very similar. All these data indicate that 1 synthesized by reaction of the mono-lacunary silicotungstate K(8)[SiW(11)O(39)] with Ru(acac)(3) under hydrothermal conditions is truly the mono-ruthenium substituted Keggin-type silicotungstate. 相似文献
179.
Izawa K Yamada T Unal U Ida S Altuntasoglu O Koinuma M Matsumoto Y 《The journal of physical chemistry. B》2006,110(10):4645-4650
Photoelectrochemical oxidation of alcohol on various nanosheet electrodes such as Nb6O17, Ca2Nb3O10, Ti(0.91)O2, Ti4O9, and MnO2 system host layers were measured to evaluate the photocatalysis of water photolysis with alcohol as a sacrificial agent. The nanosheet electrodes were prepared by the layer-by-layer (LBL) method, using electrostatic principles. The highest photooxidation current density was observed in methanol solution for Nb6O17 and Ca2Nb3O10 nanosheets, while the density was lower for Ti(0.91)O2, Ti4O9, and MnO2 nanosheets in decreasing order. The rank in the photocurrent density was in agreement with that in the photocatalytic activity, which means that the degree of photooxidation of the alcohol determines the activity of the alcohol in the water photolysis process. The photocurrent was independent of the number of nanosheet layers on the electrode, indicating that only the mono-nanosheet layer attached directly on a substrate acts as a photoelectrocatalyst and that the interlayer space is not important. Consequently, higher photooxidation current on the Nb6O17 mono-nanosheet layer means that the charge separation of electron and hole under illumination is very large and that the hole-capturing process by CH3OH is very quick compared with the surface recombination on the Nb6O17 nanosheet. The adsorption of a transition metal cation on the nanosheet acted as the surface recombination center, because the photocurrent decreased after the adsorption. The photocatalytic mechanism has been discussed in detail in terms of various photoelectrochemical behaviors. 相似文献
180.