全文获取类型
收费全文 | 1788篇 |
免费 | 45篇 |
国内免费 | 11篇 |
专业分类
化学 | 1176篇 |
晶体学 | 6篇 |
力学 | 81篇 |
数学 | 324篇 |
物理学 | 257篇 |
出版年
2022年 | 16篇 |
2021年 | 28篇 |
2020年 | 23篇 |
2019年 | 36篇 |
2018年 | 28篇 |
2017年 | 15篇 |
2016年 | 53篇 |
2015年 | 27篇 |
2014年 | 36篇 |
2013年 | 74篇 |
2012年 | 80篇 |
2011年 | 97篇 |
2010年 | 55篇 |
2009年 | 65篇 |
2008年 | 85篇 |
2007年 | 97篇 |
2006年 | 89篇 |
2005年 | 100篇 |
2004年 | 81篇 |
2003年 | 70篇 |
2002年 | 46篇 |
2001年 | 22篇 |
2000年 | 28篇 |
1999年 | 34篇 |
1998年 | 19篇 |
1997年 | 26篇 |
1996年 | 23篇 |
1995年 | 22篇 |
1994年 | 21篇 |
1993年 | 13篇 |
1992年 | 23篇 |
1991年 | 10篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1986年 | 14篇 |
1985年 | 24篇 |
1984年 | 21篇 |
1983年 | 14篇 |
1982年 | 14篇 |
1981年 | 11篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1930年 | 6篇 |
1888年 | 8篇 |
排序方式: 共有1844条查询结果,搜索用时 15 毫秒
931.
Csar Pascual García Vittorio Pellegrini Aron Pinczuk Massimo Rontani Guido Goldoni Elisa Molinari Brian S. Dennis Loren N. Pfeiffer Ken W. West 《Physica E: Low-dimensional Systems and Nanostructures》2006,34(1-2):304
We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock–Darwin levels. In QDs with very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified. 相似文献
932.
Consider the radially symmetric p-Laplacian for p?2 under zero Dirichlet boundary conditions. The main result of the present paper is that under appropriate conditions a solution of a perturbed (radially symmetric) p-Laplacian can be compared with the solution of the unperturbed one. As a consequence one obtains a sign preserving result for a system of p-Laplacians which are coupled in a nonquasimonotone way. 相似文献
933.
934.
Two new diterpenoids, junceellonoids A and B ( 1 and 2 , resp.), together with the seven known diterpenoids 3 – 9 , all possessing the briarane carbon skeleton, were isolated from the gorgonian Junceella fragilis Ridley , collected in the South Sea of China. The structures and relative configurations of the new metabolites were elucidated based on extensive spectral analysis and by comparison with known spectral data. 相似文献
935.
We prove a uniqueness result for BV solutions of scalar conservation laws with discontinuous flux in several space dimensions. The proof is based on the notion of kinetic solution and on a careful analysis of the entropy dissipation along the discontinuities of the flux. 相似文献
936.
Agnieszka Kucharska Adrian Covaci Guido Vanermen Stefan Voorspoels 《Analytical and bioanalytical chemistry》2014,406(26):6665-6675
Flame retardants (FRs), such as polybrominated diphenyl ethers (PBDEs) and phosphate flame retardants (PFRs), are a diverse group of compounds that are used to improve fire safety in many consumer products, such as furniture, textiles, electronics, etc. As these compounds are potentially harmful for human health, there is a need to better understand human exposure. Exposure to environmental contaminants can be monitored by the measurement of external sources of exposures and also by the determination of contaminant levels in human samples. For ethical and practical reasons, noninvasive matrices, such as hair, are preferred but, unfortunately, not widely used due to methodological limitations. A major challenge is sample availability: only small amounts can be sampled per individual. Multi-residue methods are therefore essential in order to determine multiple compounds in low sample amounts. In the framework of the FP7 project (INFLAME), an analytical method for the simultaneous determination of PBDEs and PFRs in human hair has been optimized and validated. Before extraction, hair samples (200 mg) were denaturated in nitric acid (HNO3) for 25 min at 25 °C. Consecutively, the samples were extracted using a mixture of hexane:dichloromethane, and extracts were further fractionated on Florisil. Fraction A which contained PBDEs was additionally cleaned on acidified silica gel and measured by gas chromatography coupled with electron capture negative ionization mass spectrometry (GC-ECNI-MS), while fraction B containing PFRs was directly analyzed by liquid chromatography tandem mass spectrometry (LC-MS/MS). This approach resulted in recoveries between 81–120 % for PBDEs and 75–113 % for PFRs (relative standard deviation (RSD)?16 %, n?=?9). The optimized multi-residue method has been applied to 20 human hair samples. The obtained results indicated that the levels of PBDEs in hair samples were very low (0.2–12 ng/g) in relation to PBDE levels in human hair samples from other studies and most of the time below the method limit of quantification (LOQm). On the contrary, the PFR levels were relatively high as they were in the range of the levels previously found in dust samples (2–5,032 ng/g hair). We would like to highlight that the contribution of air and dust cannot be neglected (especially in the case of PFRs); therefore, we suggest that hair might be a good indicator of retrospective and integral exposure (which includes atmospheric deposition as well as endogenous mechanisms). Moreover, the aim of our study is focused on exposure assessment and levels detected in hair (independently of whether they come from internal or external exposure) and will significantly contribute to the exposure assessment. Graphical abstract
Hair - an efficient way for sampling of personal cloud of exposure. Copyright ©2014 iStockphoto LP 相似文献
937.
In disaster operations management, a challenging task for rescue organizations occurs when they have to assign and schedule their rescue units to emerging incidents under time pressure in order to reduce the overall resulting harm. Of particular importance in practical scenarios is the need to consider collaboration of rescue units. This task has hardly been addressed in the literature. We contribute to both modeling and solving this problem by (1) conceptualizing the situation as a type of scheduling problem, (2) modeling it as a binary linear minimization problem, (3) suggesting a branch-and-price algorithm, which can serve as both an exact and heuristic solution procedure, and (4) conducting computational experiments – including a sensitivity analysis of the effects of exogenous model parameters on execution times and objective value improvements over a heuristic suggested in the literature – for different practical disaster scenarios. The results of our computational experiments show that most problem instances of practically feasible size can be solved to optimality within ten minutes. Furthermore, even when our algorithm is terminated once the first feasible solution has been found, this solution is in almost all cases competitive to the optimal solution and substantially better than the solution obtained by the best known algorithm from the literature. This performance of our branch-and-price algorithm enables rescue organizations to apply our procedure in practice, even when the time for decision making is limited to a few minutes. By addressing a very general type of scheduling problem, our approach applies to various scheduling situations. 相似文献
938.
Emanuela Guido Jenny Alongi Claudio Colleoni Alessandro Di Blasio Federico Carosio Marc Verelst Giulio Malucelli Giuseppe Rosace 《Polymer Degradation and Stability》2013
Polyester fabrics have been treated with hybrid organic-inorganic sols containing aluminium hydroxide, namely a synthetic boehmite. The treated fabrics have been thoroughly investigated by infrared spectroscopy, scanning electron microscopy coupled to elemental analysis and thermogravimetry. Furthermore, the flammability of the coated fabrics has been assessed and compared with that achieved by applying the two components separately. More specifically, flammability tests have shown that the sol–gel treatment in the presence of boehmite nanoparticles is able to suppress the dripping phenomenon, which represents the main issue to fulfil for polyester fabrics. 相似文献
939.
Francesco Saliu Guido Galliani Marco Orlandi Bruno Rindone Eeva-Liisa Tolppa Ricardo Suarez-Bertoa 《合成通讯》2013,43(7):956-962
The reaction between aliphatic amines and propylene carbonate can be performed in solventless conditions under microwave irradiation, becoming nearly complete within 15 min of irradiation. Oxidation of the formed mixture of 2-hydroxyethylcarbamates gives 3,5-methylalkyl-oxazolidine-2,4-diones. These compounds can react further with aliphatic primary amines to give N-lactoylureas.
940.
Matthias Hardy Niklas Struch Julian J. Holstein Gregor Schnakenburg Norbert Wagner Marianne Engeser Johannes Beck Guido H. Clever Arne Lützen 《Angewandte Chemie (International ed. in English)》2020,59(8):3195-3200
Two new heterobimetallic cages, a trigonal‐bipyramidal and a cubic one, were assembled from the same mononuclear metalloligand by adopting the molecular library approach, using iron(II) and palladium(II) building blocks. The ligand system was designed to readily assemble through subcomponent self‐assembly. It allowed the introduction of steric strain at the iron(II) centres, which stabilizes its paramagnetic high‐spin state. This steric strain was utilized to drive dynamic complex‐to‐complex transformations with both the metalloligand and heterobimetallic cages. Addition of sterically less crowded subcomponents as a chemical stimulus transformed all complexes to their previously reported low‐spin analogues. The metalloligand and bipyramid incorporated the new building block more readily than the cubic cage, probably because the geometric structure of the sterically crowded metalloligand favours the cube formation. Furthermore it was possible to provoke structural transformations upon addition of more favourable chelating ligands, converting the cubic structures into bipyramidal ones. 相似文献