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111.
Two coexisting adsorption states of molecularly adsorbed acetylene on the Si(001)-(2 x 1) surface have been identified by a combined study based on the high-resolution electron energy loss spectroscopy and density functional computations. Seven possible adsorbate-substrate structures are considered theoretically including their full vibrational analysis. Based on a significantly enhanced experimental resolution, the assignment of 15 C2H2- and C2D2-derived vibrational modes identifies a dominant di-sigma bonded molecule adsorbed on top of a single Si-Si dimer. Additionally there is clear evidence for a second minority species which is di-sigma bonded between two Si-Si dimers within the same dimer row (end-bridge geometry). The possible symmetries of the adsorbate complexes are discussed based on the specular and off-specular vibrational measurements. They suggest lower than ideal C(2v) and C(s) symmetries for on-top and end-bridge species, respectively. At low coverages the symmetry reductions might be lifted. 相似文献
112.
Xu X Rosi NL Wang Y Huo F Mirkin CA 《Journal of the American Chemical Society》2006,128(29):9286-9287
Gold nanoparticles (AuNPs) were anisotropically functionalized with two different oligonucleotide sequences using magnetic microparticles as geometric restriction templates for site-selective enzymatic extension of particle-bound oligonucleotides. The divalent linking capability of the resulting AuNPs allowed for the design and programmable assembly of discrete nanoparticle heterostructures. 相似文献
113.
Haick H Hurley PT Hochbaum AI Yang P Lewis NS 《Journal of the American Chemical Society》2006,128(28):8990-8991
Silicon nanowires (Si NWs) modified by covalent Si-CH3 functionality, with no intervening oxide, show atmospheric stability, high conductance values, low surface defect levels, and allow for the formation of air-stable Si NW Field-Effect Transistors (FETs) having on-off ratios in excess of 105 over a relatively small gate voltage swing (+/-2 V). 相似文献
114.
The bicyclic core of octosyl acid A has been prepared using a diastereoselective acetylide addition and 6-endo selenoetherification as key steps. A detailed study of the selenoetherification reaction and difficulties encountered in the conversion of a phenyl group to a carboxylic acid will be discussed. 相似文献
115.
Kerber RN Kermagoret A Callens E Florian P Massiot D Lesage A Copéret C Delbecq F Rozanska X Sautet P 《Journal of the American Chemical Society》2012,134(15):6767-6775
The determination of the nature and structure of surface sites after chemical modification of large surface area oxides such as silica is a key point for many applications and challenging from a spectroscopic point of view. This has been, for instance, a long-standing problem for silica reacted with alkylaluminum compounds, a system typically studied as a model for a supported methylaluminoxane and aluminum cocatalyst. While (27)Al solid-state NMR spectroscopy would be a method of choice, it has been difficult to apply this technique because of large quadrupolar broadenings. Here, from a combined use of the highest stable field NMR instruments (17.6, 20.0, and 23.5 T) and ultrafast magic angle spinning (>60 kHz), high-quality spectra were obtained, allowing isotropic chemical shifts, quadrupolar couplings, and asymmetric parameters to be extracted. Combined with first-principles calculations, these NMR signatures were then assigned to actual structures of surface aluminum sites. For silica (here SBA-15) reacted with triethylaluminum, the surface sites are in fact mainly dinuclear Al species, grafted on the silica surface via either two terminal or two bridging siloxy ligands. Tetrahedral sites, resulting from the incorporation of Al inside the silica matrix, are also seen as minor species. No evidence for putative tri-coordinated Al atoms has been found. 相似文献
116.
Fu Y Kasinath V Moorman VR Nucci NV Hilser VJ Wand AJ 《Journal of the American Chemical Society》2012,134(20):8543-8550
The cooperative nature of protein substructure and internal motion is a critical aspect of their functional competence about which little is known experimentally. NMR relaxation is used here to monitor the effects of high pressure on fast internal motion in the protein ubiquitin. In contrast to the main chain, the motions of the methyl-bearing side chains have a large and variable pressure dependence. Within the core, this pressure sensitivity correlates with the magnitude of motion at ambient pressure. Spatial clustering of the dynamic response to applied hydrostatic pressure is also seen, indicating localized cooperativity of motion on the sub-nanosecond time scale and suggesting regions of variable compressibility. These and other features indicate that the native ensemble contains a significant fraction of members with characteristics ascribed to the recently postulated "dry molten globule". The accompanying variable side-chain conformational entropy helps complete our view of the thermodynamic architecture underlying protein stability, folding, and function. 相似文献
117.
Ultrafast photocurrent measurements are performed on individual carbon nanotube p-i-n photodiodes. The photocurrent response to subpicosecond pulses separated by a variable time delay Δt shows strong photocurrent suppression when two pulses overlap (Δt=0). The picosecond-scale decay time of photocurrent suppression scales inversely with the applied bias V(SD), and is twice as long for photon energy above the second subband E22 as compared to lower energy. The observed photocurrent behavior is well described by an escape time model that accounts for carrier effective mass. 相似文献
118.
Dan Archdeacon C. Paul Bonnington Nathaniel Dean Nora Hartsfield Katherine Scott 《Journal of Graph Theory》2001,38(1):42-64
A graph is outer‐cylindrical if it embeds in the sphere so that there are two distinct faces whose boundaries together contain all the vertices. The class of outer‐cylindrical graphs is closed under minors. We give the complete set of 38 minor‐minimal non‐outer‐cylindrical graphs, or equivalently, an excluded minor characterization of outer‐cylindrical graphs. We also give the obstruction sets under the related topological ordering and Y Δ‐ordering. © 2001 John Wiley & Sons, Inc. J Graph Theory 38: 42–64, 2001 相似文献
119.
All possible isomers of C60O
x
have been investigated for x=1, 2, and 3. The method used is a modified extended Hückel method. There is a single C60O isomer, 8 unique C60O2 isomers, and 47 unique C60O3 isomers. It is found that a single C60O2 isomer is much more stable than the remaining 7 and that 3 C60O3 isomers are more stable than the remaining 44. A qualitative reason for these results is proposed. The results indicate that the three lowest-energy C60O3 isomers should exist in equilibrium at room temperature. 相似文献
120.
Baldwin Rusty O. Davis IV Nathaniel J. Kobza John E. Midkiff Scott F. 《Queueing Systems》2000,35(1-4):1-21
Real-time scheduling, or scheduling with respect to a deadline, is critical in many application areas such as telecommunications,
control systems, and manufacturing. This paper presents a novel approach to real-time scheduling based on a queueing theory
model. Using real-time queueing theory (RTQT), one can analytically determine the distribution of the lead-time profile (i.e.,
the time until the deadline is reached) of customers waiting for service. Emphasis is placed on the development of the equations
used to determine the lead-time profile distribution. The development of the GI/G/1 case is presented and confirmed using
simulation. Simulation results confirm prior research for the M/M/1 and GI/M/1 case. As a practical application, RTQT is used
to implement a packet admission control algorithm for a telecommunications network. Using this algorithm, packet lateness
was reduced by up to 31%.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献