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排序方式: 共有329条查询结果,搜索用时 15 毫秒
61.
Zugates GT Anderson DG Little SR Lawhorn IE Langer R 《Journal of the American Chemical Society》2006,128(39):12726-12734
The safe and efficient delivery of DNA remains the major barrier to the clinical application of non-viral gene therapy. Here, we present novel, biodegradable polymers for gene delivery that are capable of simple graft modification and demonstrate the ability to respond to intracellular conditions. We synthesized poly(beta-amino ester)s using a new amine monomer, 2-(pyridyldithio)-ethylamine (PDA). These cationic, degradable polymers contain pyridyldithio functionalities in the side chains that react with high specificity toward thiol ligands. This reactivity is demonstrated using both mercaptoethylamine (MEA) and the thiol peptide RGDC, a ligand that binds with high affinity to certain integrin receptors. These two polymer derivatives displayed strong DNA binding as determined using electrophoresis and dye exclusion assays. In addition, the MEA-based polymer and plasmid DNA were shown to self-assemble into cationic complexes with effective diameters as low as 100 nm. Furthermore, this DNA binding ability was substantially reduced in response to intracellular glutathione concentrations, which may aid in DNA unpackaging inside the cell. These complexes also displayed low cellular toxicity and were able to mediate transfection at levels comparable to PEI in human hepatocellular carcinoma cells. These results suggest that PDA-based poly(beta-amino ester)s may serve as a modular platform for polymer-mediated gene delivery. 相似文献
62.
We report on the fabrication and performance of a gel microfluidic chip interfaced to laser desorption/ionization (LDI) mass spectrometry with a time-of-flight mass analyzer. The chip was fabricated from poly(methylmethacrylate) with a poly(dimethyl siloxane) cover. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was performed in the channel of the microfluidic chip. After electrophoresis, the cover was removed and either the PDMS chip or the PMMA cover was mounted in a modified MALDI ion source for analysis. Ions were formed by irradiating the channel with 2.95 microm radiation from a pulsed optical parametric oscillator (OPO), which is coincident with IR absorption by N-H and O-H stretch of the gel components. No matrix was added. The microfluidic chip design allowed a decrease in the volume of material required for analysis over conventional gel slabs, thus enabling improvement in the detection limit to a pmol level, a three orders of magnitude improvement over previous studies in which desorption was achieved from an excised section of a conventional gel. 相似文献
63.
High-lift airfoil trailing edge separation control using a single dielectric barrier discharge plasma actuator 总被引:3,自引:0,他引:3
Control of flow separation from the deflected flap of a high-lift airfoil up to Reynolds numbers of 240,000 (15 m/s) is explored
using a single dielectric barrier discharge (DBD) plasma actuator near the flap shoulder. Results show that the plasma discharge
can increase or reduce the size of the time-averaged separated region over the flap depending on the frequency of actuation.
High-frequency actuation, referred to here as quasi-steady forcing, slightly delays separation while lengthening and flattening
the separated region without drastically increasing the measured lift. The actuator is found to be most effective for increasing
lift when operated in an unsteady fashion at the natural oscillation frequency of the trailing edge flow field. Results indicate
that the primary control mechanism in this configuration is an enhancement of the natural vortex shedding that promotes further
momentum transfer between the freestream and separated region. Based on these results, different modulation waveforms for
creating unsteady DBD plasma-induced flows are investigated in an effort to improve control authority. Subsequent measurements
show that modulation using duty cycles of 50–70% generates stronger velocity perturbations than sinusoidal modulation in quiescent
conditions at the expense of an increased power requirement. Investigation of these modulation waveforms for trailing edge
separation control similarly shows that additional increases in lift can be obtained. The dependence of these results on the
actuator carrier and modulation frequencies is discussed in detail. 相似文献
64.
J. W. Bethel A. Maitland P. M. Beecham T. C. Bell S. Webb C. E. Little 《Optical and Quantum Electronics》1994,26(12):1079-1087
Laser oscillation on the 430.5 nm transition of Sr+, which replies on three-body electron-ion recombination for its excitation, has been achieved in a pulsed, selfheated, microwave-pumped discharge. 相似文献
65.
L. T. Little S. R. Davies 《International Journal of Infrared and Millimeter Waves》1994,15(6):935-942
The construction of SIS heterodyne imaging arrays for submillimetre wave-lengths requires multiple coupling of the local oscillator signal. The quasi-optical analogue of a multiple cross-guide coupler, employing 45° beam-splitters successively stacked along the local oscillator beam, allows for individual adjustment of local oscillator power to each channel. We analyse the coupling as a function of the focal ratio of the incident beams and the number of beam widths off-axis through which the coupler is extended, and describe a simple construction method to realise a compact and effective design. 相似文献
66.
It is shown that no graph contains a maximal 3-ring of bonds. 相似文献
67.
David J. Rousell Sucharita M. Dutta Mark W. Little Kermit K. Murray 《Journal of mass spectrometry : JMS》2004,39(10):1182-1189
Infrared soft laser desorption/ionization was performed using a 2.94 µm Er : YAG laser and a commercial reflectron time-of-flight mass spectrometer. The instrument was modified so that a 337 nm nitrogen laser could be used concurrently with the IR laser to interrogate samples. Matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization and desorption/ionization on silicon with UV and IR lasers were compared. Various target materials were tested for IR soft desorption ionization, including stainless steel, aluminum, copper, silicon, porous silicon and polyethylene. Silicon surfaces gave the best performance in terms of signal level and low-mass interference. The internal energy resultant of the desorption/ionization was assessed using the easily fragmented vitamin B12 molecule. IR ionization produced more analyte fragmentation than UV-MALDI analysis. Fragmentation from matrix-free IR desorption from silicon was comparable to that from IR-MALDI. The results are interpreted as soft laser desorption and ionization resulting from the absorption of the IR laser energy by the analyte and associated solvent molecules. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
68.
Let D = (V 1, V 2; A) be a directed bipartite graph with |V 1| = |V 2| = n ≥ 2. Suppose that d D (x) + d D (y) ≥ 3n for all x ∈ V 1 and y ∈ V 2. Then, with one exception, D contains two vertex-disjoint directed cycles of lengths 2n 1 and 2n 2, respectively, for any positive integer partition n = n 1 + n 2. This proves a conjecture proposed in [9]. 相似文献
69.
70.
James R. Rabinowitz Stephen B. Little 《International journal of quantum chemistry》1994,52(3):681-691
In computational studies to understand the interaction of polycyclic aromatic hydrocarbons (PAHs) with biomolecular systems, the semiempirical method AM 1 has been used previously to determine the geometry of the PAH and its metabolites and relevant intermediates. A number of studies have shown that AM 1 provides geometries for parent PAHs that are acceptably close to experimentally determined structures. However, many of the properties that determine the manner by which PAHs interact with biological nucleophiles depend on the structure of metabolites and reactive intermediates where less experimental information is available. In a previous study, we used AM 1 to obtain the molecular geometries of reactive intermediates of cyclopenta-PAHs (cPAHs) and then used single-point Hartree-Fock calculations, with the gaussian 3-21g basis set, to obtain molecular energies and charge distributions, in order to predict the direction of epoxide ring opening. Recent advances in the availability of computational hardware and software have provided other, more rigorous, methods for approaching this problem. In this study, we used hartree-fock methods in the gaussian series of programs employing the 3-21g and 6-31g basis sets and the local density functional method Dmol to obtain molecular geometries, energies, and charge distributions of the epoxides and the two potential hydroxycarbocations that could result from protonated ring opening, for a series of cPAHs. We have also performed the same calculations with AMSOL /SM 2, a semiempirical method that adds the effect of the aqueous environment to the AM 1 Hamiltonian. The division of the cPAHs into classes is not altered by these more rigorous calculations. The inclusion of water in the Hamiltonian has a greater effect on the results than using the ab initio methods to obtain the structure. © 1994 John Wiley & Sons, Inc. 相似文献