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排序方式: 共有395条查询结果,搜索用时 46 毫秒
1.
Jorge Ferreira Raquel Zilz Igor S. Boeira Sabrina M. da Silva Adriana C.A. Casagrande Osvaldo L. Casagrande 《应用有机金属化学》2019,33(3)
A new set of Cr(III) complexes, {L}CrCl3(THF), based on thiophene–imine ( 2a , L = PhOC6H4(N═CH)‐2‐SC4H3; 2b , L = PhOC2H4(N═CH)‐2‐SC4H3; 2c , L = Ph(NH)C2H4(N═CH)‐2‐SC4H3; 2d , L = PhOC6H4(N═CH)‐2‐SC4H2‐5‐Ph; 2e , L = Ph(NH)C2H4(N═CH)‐2‐SC4H2‐5‐Ph) have been prepared and characterized using elemental analysis and infrared spectroscopy. Upon activation with methylaluminoxane, all the chromium complexes generated active systems affording a nonselective distribution of α‐olefins with turnover frequencies in the range 9500–93 500 (mol ethylene) (mol Cr)?1 h?1, and producing mostly oligomers (95.0–99.3 wt% of total products). Small amounts of polymer were produced in these oligomerization reactions (0.8–8.2 wt%). The catalytic activities were quite sensitive to the ligand environment. Moreover, the effects of oligomerization parameters (temperature, [Al]/[Cr] molar ratio, time) on the activity and on the product distribution were examined. 相似文献
2.
María Fernanda Manrique-de-la-Cuba Luis Leyva-Parra Diego Inostroza Prof. Badhin Gomez Dr. Alejandro Vásquez-Espinal Prof. Jorge Garza Dr. Osvaldo Yañez Prof. William Tiznado 《Chemphyschem》2021,22(10):906-910
We report the global minima structures of Li8Si8, Li10Si9, and Li12Si10 systems, in which silicon moieties maintain structural and chemical bonding characteristics similar to those of their building blocks: the aromatic clusters Td−Li4Si4 and C2v−Li6Si5. Electron counting rules, chemical bonding analysis, and magnetic response properties verify the silicon unit‘s aromaticity persistence. This study demonstrates the feasibility of assembling silicon-based nanostructures from aromatics clusters as building blocks. 相似文献
3.
A new method for determining viscosity of liquids is examined. The method employs the principles of vibration and measures the viscous damping due to the motion of a liquid placed in a cylindrical tube. The apparatus and the test liquid are treated as a dynamic system and the measured mechanical impedances are used to calculate energy dissipation due to the viscous damping. The newly designed apparatus is able to generate shear deformations in the liquid without using moving solid surfaces. A harmonic varying force with a frequency close to the resonance frequency of the system is applied through a piston and the resulting velocities of the oscillations generated in the system are measured. Liquids with higher viscosities result in lower velocities due to the higher damping. Analytical equations are provided to relate the viscous damping of the dynamic system to the viscosity of the liquids. The viscosities obtained from the proposed method are in good agreement with the ones obtained from standard rotational viscometry using a cone and plate geometry. 相似文献
4.
Meta-learning, or “learning to learn”, refers to techniques that infer an inductive bias from data corresponding to multiple related tasks with the goal of improving the sample efficiency for new, previously unobserved, tasks. A key performance measure for meta-learning is the meta-generalization gap, that is, the difference between the average loss measured on the meta-training data and on a new, randomly selected task. This paper presents novel information-theoretic upper bounds on the meta-generalization gap. Two broad classes of meta-learning algorithms are considered that use either separate within-task training and test sets, like model agnostic meta-learning (MAML), or joint within-task training and test sets, like reptile. Extending the existing work for conventional learning, an upper bound on the meta-generalization gap is derived for the former class that depends on the mutual information (MI) between the output of the meta-learning algorithm and its input meta-training data. For the latter, the derived bound includes an additional MI between the output of the per-task learning procedure and corresponding data set to capture within-task uncertainty. Tighter bounds are then developed for the two classes via novel individual task MI (ITMI) bounds. Applications of the derived bounds are finally discussed, including a broad class of noisy iterative algorithms for meta-learning. 相似文献
5.
Raquel da Silva Cabral Andre L. L. Aquino Alejandro C. Frery Osvaldo A. Rosso Jaime A. Ramírez 《Central European Journal of Physics》2013,11(12):1645-1652
Wireless sensor networks are an important technology for making distributed autonomous measures in hostile or inaccessible environments. Among the challenges they pose, the way data travel among them is a relevant issue since their structure is quite dynamic. The operational topology of such devices can often be described by complex networks. In this work, we assess the variation of measures commonly employed in the complex networks literature applied to wireless sensor networks. Four data communication strategies were considered: geometric, random, small-world, and scale-free models, along with the shortest path length measure. The sensitivity of this measure was analyzed with respect to the following perturbations: insertion and removal of nodes in the geometric strategy; and insertion, removal and rewiring of links in the other models. The assessment was performed using the normalized Kullback-Leibler divergence and Hellinger distance quantifiers, both deriving from the Information Theory framework. The results reveal that the shortest path length is sensitive to perturbations. 相似文献
6.
A simple and high‐resolution HPLC‐PDA method for simultaneous quantification of local anesthetics in in vitro buccal permeation enhancement studies 下载免费PDF全文
Renê O. Couto Camila Cubayachi Renata F. V. Lopez Cristiane M. de Gaitani Vinícius Pedrazzi Osvaldo de Freitas 《Biomedical chromatography : BMC》2016,30(6):857-866
A simple, isocratic, high‐resolution and prompt HPLC‐PDA method was developed and validated for the simultaneous quantification of prilocaine (PCL) and lidocaine (LCL) hydrochlorides in in vitro buccal iontophoresis‐driven permeation studies. A reversed‐phase C18 column (250 mm x 4.6 mm, 3μm, 110Å) was used for the chromatographic separation. The mobile phase contained acetonitrile: 0.1M sodium phosphate buffer, pH 7.0 (1:1, v/v), plus 0.05% (v/v) diethylamine. The isocratic flow rate was set at 1 mL/min and the detection wavelength was 203 nm. PCL and LCL eluted in 8.9 min and 13 min, respectively, and the system suitability parameters varied within an acceptable range. The method was selective, sensitive, precise, accurate and robust, producing a linear plot at the concentration range of 0.25 to 10 µg/mL. The application of this method was demonstrated by a significant enhancement of the permeation of PCL and LCL with the application of iontophoresis (1 mA/cm2 per 1 h) through isolated porcine esophageal epithelium. The amount of the drug retained in the epithelium also increased with the application of an electrical current. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
7.
High‐density polyethylene/expanded graphite nanocomposites produced by polymerization‐filling technique using an industrial heterogeneous catalyst 下载免费PDF全文
Kátia Z. C da Cruz Adriana C. A. Casagrande Osvaldo L. Casagrande Jr 《Journal of polymer science. Part A, Polymer chemistry》2017,55(7):1260-1267
High‐density polyethylene nanocomposites with different expanded graphite (EG) contents (0.34–1.80 wt %) were prepared by polymerization‐filling technique using an industrial heterogeneous catalyst ( cat K ), and characterized using a range techniques: melting flow index (MFI), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM). The MFI data showed that EG acts as a plasticizer decreasing melt viscosity in comparison to neat HDPE produced exclusively by cat K . DSC results showed that EG nucleated the HDPE crystallization as established by the increased crystallization temperature, and the degree of crystallinity. HDPE/EG nanocomposites displayed a significant improvement in the flexural (increased from 1458 to 1831 MPa), and storage modulus (increased from 122 to 1627 MPa) at only 1.80 wt % EG content. TEM images confirmed a homogeneous distribution of EG into the polymer matrix with the presence of dispersed, intercalated and aggregated EG nanofillers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 55, 1260–1267 相似文献
8.
9.
Giorgio Olivo Giorgio Capocasa Barbara Ticconi Osvaldo Lanzalunga Stefano Di Stefano Miquel Costas 《Angewandte Chemie (International ed. in English)》2020,59(31):12703-12708
Predictability is a key requirement to encompass late‐stage C?H functionalization in synthetic routes. However, prediction (and control) of reaction selectivity is usually challenging, especially for complex substrate structures and elusive transformations such as remote C(sp3)?H oxidation, as it requires distinguishing a specific C?H bond from many others with similar reactivity. Developed here is a strategy for predictable, remote C?H oxidation that entails substrate binding to a supramolecular Mn or Fe catalyst followed by elucidation of the conformation of the host‐guest adduct by NMR analysis. These analyses indicate which remote C?H bonds are suitably oriented for the oxidation before carrying out the reaction, enabling prediction of site selectivity. This strategy was applied to late‐stage C(sp3)?H oxidation of amino‐steroids at C15 (or C16) positions, with a selectivity tunable by modification of catalyst chirality and metal. 相似文献
10.
Jorge A. Vila Yelena A. Arnautova Osvaldo A. Martin Harold A. Scheraga 《Journal of computational chemistry》2014,35(4):309-312
The goal of this study is twofold. First, to investigate the relative influence of the main structural factors affecting the computation of the 13C′ shielding, namely, the conformation of the residue itself and the next nearest‐neighbor effects. Second, to determine whether calculation of the 13C′ shielding at the density functional level of theory (DFT), with an accuracy similar to that of the 13Cα shielding, is feasible with the existing computational resources. The DFT calculations, carried out for a large number of possible conformations of the tripeptide Ac‐G XY ‐NMe, with different combinations of X and Y residues, enable us to conclude that the accurate computation of the 13C′ shielding for a given residue X depends on the: (i) (?,ψ) backbone torsional angles of X ; (ii) side‐chain conformation of X ; (iii) (?,ψ) torsional angles of Y ; and (iv) identity of residue Y . Consequently, DFT‐based quantum mechanical calculations of the 13C′ shielding, with all these factors taken into account, are two orders of magnitude more CPU demanding than the computation, with similar accuracy, of the 13Cα shielding. Despite not considering the effect of the possible hydrogen bond interaction of the carbonyl oxygen, this work contributes to our general understanding of the main structural factors affecting the accurate computation of the 13C′ shielding in proteins and may spur significant progress in effort to develop new validation methods for protein structures. © 2013 Wiley Periodicals, Inc. 相似文献