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31.
Fukunaga M Horovitz SG van Gelderen P de Zwart JA Jansma JM Ikonomidou VN Chu R Deckers RH Leopold DA Duyn JH 《Magnetic resonance imaging》2006,24(8):979-992
A number of recent studies of human brain activity using blood-oxygen-level-dependent (BOLD) fMRI and EEG have reported the presence of spatiotemporal patterns of correlated activity in the absence of external stimuli. Although these patterns have been hypothesized to contain important information about brain architecture, little is known about their origin or about their relationship to active cognitive processes such as conscious awareness and monitoring of the environment. In this study, we have investigated the amplitude and spatiotemporal characteristics of resting-state activity patterns and their dependence on the subjects' alertness. For this purpose, BOLD fMRI was performed at 3.0 T on 12 normal subjects using a visual stimulation protocol, followed by a 27 min rest period, during which subjects were allowed to fall asleep. In subjects who were asleep at the end of the scan, we found (a) a higher amplitude of BOLD signal fluctuation during rest compared with subjects who were awake at the end of the scan; (b) spatially independent patterns of correlated activity that involve all of gray matter, including deep brain nuclei; (c) many patterns that were consistent across subjects; (d) that average percentage levels of fluctuation in visual cortex (VC) and whole brain were higher in subjects who were asleep (up to 1.71% and 1.16%, respectively) than in those who were awake (up to 1.15% and 0.96%) at the end of the scan and were comparable with those levels evoked by intense visual stimulation (up to 1.85% and 0.76% for two subject groups); (e) no confirmation of correlation, positive or negative, between thalamus and VC found in earlier studies. These findings suggest that resting-state activity continues during sleep and does not require active cognitive processes or conscious awareness. 相似文献
32.
A high-efficiency waveguide-to-fiber grating coupler for silicon-on-insulator waveguides was designed. Perfectly vertical fiber coupling is achieved by using an asymmetric grating structure to suppress the second-order Bragg reflection from the grating. The ability to use a perfectly vertical positioned optical fiber simplifies the packaging of the photonic integrated circuit. A coupling efficiency of 80% at a wavelength of 1.55 microm is obtained. 相似文献
33.
Guuske F. Busscher Laurent Lefort Jozef G.O. Cremers Marco Mottinelli Roel W. Wiertz Ben de Lange Yutaka Okamura Yukinori Yusa Kazuhiko Matsumura Hideo Shimizu Johannes G. de Vries André H.M. de Vries 《Tetrahedron: Asymmetry》2010,21(13-14):1709-1714
Two routes for the preparation of an N-aryl β-amino acid, an important precursor for the cholesterol-lowering drug Ezetimibe, were investigated. The first pathway proceeds via an Rh- or Ir-catalyzed asymmetric hydrogenation of N-aryl enamine giving the desired product with up to 82% ee. The other pathway involves a direct asymmetric reductive amination (DARA) of the β-keto ester which yielded the β-amino ester in high yield and 97% ee. Subsequent copper-catalyzed N-arylation gave the target compound. 相似文献
34.
35.
Deepti S. Gholap Johannes T. van Elteren Roel Evens Colin Janssen Inge Lindemann Frank Vanhaecke 《Analytica chimica acta》2010,664(1):19-239
Visualization of elemental distributions in thin sections of biological tissue is gaining importance in many disciplines of biological and medical research. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and scanning micro-X-ray fluorescence spectrometry (micro-XRF) are two widely used microanalytical techniques for elemental mapping. This article compares the capabilities of the two techniques for imaging the distribution of selected elements in the model organism Daphnia magna in terms of detection power and spatial resolution. Sections with a thickness of 10 and 20 μm of the fresh water crustacean Daphnia magna were subjected to LA-ICP-MS and micro-XRF analysis. The elemental distributions obtained for Ca, P, S and Zn allow element-to-tissue correlation. LA-ICP-MS and micro-XRF offer similar limits of detection for the elements Ca and P and thus, allow a cross-validation of the imaging results. LA-ICP-MS was particularly sensitive for determining Zn (LOD 20 μg g−1, 15 μm spot size) in Daphnia magna, while the detection power of micro-XRF was insufficient in this context. However, LA-ICP-MS was inadequate for the measurement of the S distributions, which could be better visualized with micro-XRF (LOD 160 μg g−1, 5 s live time). Both techniques are thus complementary in providing an exhaustive chemical profiling of tissue samples. 相似文献
36.
De Mondt R Baert K Geuens I Van Vaeck L Hubin A 《Langmuir : the ACS journal of surfaces and colloids》2006,22(26):11360-11368
Detection of the interactions between low molecular weight organic compounds and metals in the form of sols on a nanoscale is analytically challenging. This study aims to provide experimental evidence using a combination of UV-Vis absorption spectrometry, surface-enhanced Raman spectrometry (SERS), and static secondary ion mass spectrometry (S-SIMS). The field of application is thermography where silver images are formed via heat-catalyzed reactions. Several organic compounds called tone modifiers and stabilizers are used in thermographic materials for the optimization of the image quality. With exploitation of the strengths of each of the above-mentioned methods, an affinity ranking of several tone modifiers and a stabilizer was established on the basis of competitive adsorption experiments using different model systems. Specifically, silver sols, SERS probes, and sputter-coated silver substrates were exposed to systems with one or two additives. The UV-Vis results provided insight on the aggregation of silver nanoparticles in a hydrosol, which was necessary for the interpretation of the SERS data. Both SERS and S-SIMS measurements led to a similar ranking of the relative affinity of the additives in two components, which was largely consistent with empirical knowledge derived from macroscopic behavior. 相似文献
37.
The effect of a nonspherical particle shape and shape polydispersity on the structure of densely packed hard colloidal particles was studied in real space by confocal microscopy. We show that the first layer at the wall of concentrated size-monodisperse but shape-polydisperse polyhedral colloids exhibits significant deviations from a hexagonal lattice. These deviations are identified as bond-orientational fluctuations which lead to percolating "mismatch lines." While the shape-induced geometrical frustration of the hexagonal symmetry suppresses translational order, bond-orientational order is clearly retained, indicating a hexaticlike structure of the polyhedral colloids. 相似文献
38.
We present the first real-space analysis on a single-particle level of the dipolar chains and branched clusters self-assembling in magnetic fluids in zero field. Spatial correlations and chain-length distributions directly obtained from tracked particle positions in vitrified films of synthetic magnetic (Fe3O4) dispersions provide a quantitative test for simulations and theory of dipolar fluids. A pertinent example is the cluster-size distribution that can be analyzed with a one-dimensional aggregation model to yield a dipolar attraction energy that agrees well with the dipole moment found from independent magnetization measurements. 相似文献
39.
Bus E Miller JT Kropf AJ Prins R van Bokhoven JA 《Physical chemistry chemical physics : PCCP》2006,8(27):3248-3258
X-Ray absorption spectra of supported Pt catalysts with various Pt cluster sizes were collected between 77 and 673 K, in inert and hydrogen atmospheres. When analyzing these spectra with the standard EXAFS equation, a Pt-Pt bond contraction and a large increase in the inner potential correction were observed with increasing temperature. These errors are up to 0.08 A and 10 eV for clusters of 1 nm diameter. They were corrected by including the third and fourth cumulants as fit parameters. Fit guidelines were developed to analyze EXAFS data of supported metal catalysts collected at elevated temperatures, allowing for asymmetry and broadening or sharpening of the pair distribution function. These comprise fixing fit parameters, using different k-weightings and identifying trends in a series of experiments. By fitting the EXAFS spectra using these guidelines, it was determined that in small Pt clusters the Pt-Pt bond is 0.10 A shorter than in bulk Pt. These contracted bonds relax to distances near that of bulk Pt upon hydrogen chemisorption. 相似文献
40.
We show that the number of unit-area triangles determined by a set of n points in the plane is O(n 9/4+ε ), for any ε>0, improving the recent bound O(n 44/19) of Dumitrescu et al. 相似文献