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排序方式: 共有158条查询结果,搜索用时 46 毫秒
1.
A. R. M. Castro M. M. Serna R. N. Faria N. B. Lima 《Journal of magnetism and magnetic materials》2004,280(2-3):273-280
In this study, magnetic domains in Pr16Fe76B8 sintered magnets have been observed by Kerr effect and a histogram of the angular distribution of domain orientations has been used to determine the magnetic texture (cos Φ). The degree of easy-axis alignment of Pr2Fe14B matrix grains in these magnets has been also determined by X-ray pole figure analysis using the (0 0 4) reflection. The (0 0 4) pole figure measurements were carried out by the Schultz's reflection method. The (0 0 4) normalized intensity data has been fitted for a Gaussian distribution and the degree of crystal alignment, cos Θ, has been calculated using the Stoner–Wohlfarth model. Comparison of these methods has been carried out. It has been shown that in magnets with medium and high degrees of crystallographic alignment, the pole figure values are higher than that obtained by the Kerr effect method. Conversely, in magnets with low degrees of alignment, cos Θ is lower than cos Φ. 相似文献
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José Fernández Bertrán Boris La Serna Klaus Doerffel Klaus Dathe Gunther Kabish 《Journal of Molecular Structure》1982
The Fermi doublet V2?V3 + V4 of CH3CN in basic, inert and acidic solvents has been studied by IR and Raman spectroscopy. The values of W, the Fermi coupling coefficient, obtained from IR spectra varies with the nature of the solvent while W evaluated from Raman data remains constant at 12.5 ± 0.5 cm?1. The similar effects of Bronsted and Lewis acids on the band frequencies and intensities is evidence that the CN group complexes with acids via the N atom “n” electron pair and not the π bond. 相似文献
3.
The behavior of a rigid rotor perturbed by an electric field is studied. An hypervirial calculation, to obtain the perturbation series without calculation of the perturbed wavefunctions, is used to determine the Stark-shifted eigenvalues. A numerical estimation of the m equals; 0 → m = 1 transition for the lowest vibrational state (v = 0) and the first rotational state (J = 1) for the HF and DF molecules using Padé approximants is made. 相似文献
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J. L. Bernal M. J. Nozal L. Toribio M. L. Serna F. Borrull R. M. Marcé E. Pocurull 《Chromatographia》1997,46(5-6):295-300
Summary The eleven Environmental Protection Agency (EPA) priority phenolic compounds have been determined by solid-phase extraction
(SPE) coupled on-line to supercritical-fluid chromatography (SFC) with diodearray detection. The variables affecting chromatographic
separation were optimized and the analytes were separated at 40 °C in two diol columns connected in series; a gradient of
methanol, as modifier, and CO2 was used as mobile phase. Under these conditions, all the compounds studied were separated to baseline in less than 13 min.
PLRP-S and LiChrolut EN were tested as sorbents in a 10×3 mm i.d. laboratory-packed precolumn for solid-phase extraction.
An ion-pair reagent, tetrabutylammonium bromide (TBA), was used in the extraction process to increase break-through volumes.
The performance of the method was checked with tap and river waters and the pre-concentration of 20 mL of sample in a PLRP-S
pre-column enabled phenolic compounds to be determined at low μg L−1 levels with limits of detection ranging between 0.4 and 2 μg L−1. The repeatability and reproducibility between days (n=3) for real samples spiked at 10 μg L−1 were lower than 10%. 相似文献
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Dicle Yardimci Dr. Pedro Serna Prof. Bruce C. Gates 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(4):1235-1245
The preparation of dinuclear rhodium clusters and their use as catalysts is challenging because these clusters are unstable, evolving readily into species with higher nuclearities. We now present a novel synthetic route to generate rhodium dimers on the surface of MgO by a stoichiometrically simple surface‐mediated reaction involving [Rh(C2H4)2] species and H2. X‐ray absorption and IR spectra were used to characterize the changes in the nuclearity of the essentially molecular surface species as they formed, including the ligands on the rhodium and the metal‐support interactions. The support plays a key role in stabilizing the dinuclear rhodium species, allowing the incorporation of small ligands (ethyl, hydride, and/or CO) and enabling a characterization of the catalytic performance of the supported species for the hydrogenation of ethylene as a function of the metal nuclearity and ligand environment. A change in the nuclearity from one to two Rh atoms leads to a 58‐fold increase in the catalytic activity for ethylene hydrogenation, a reaction involving unsaturated, but stable, dimeric rhodium species. 相似文献
10.
采用TiO2上高度分散的金纳米粒子做催化剂,从脂肪族硝基化合物、醛和氢气可高选择性合成硝酮。与碳负载Pt催化剂相比,该催化体系上硝酮的选择性从50%增加到90%。其催化性能可有活性位结构、载体特性和反应条件精确调节。 相似文献