全文获取类型
收费全文 | 619篇 |
免费 | 27篇 |
国内免费 | 6篇 |
专业分类
化学 | 313篇 |
力学 | 40篇 |
数学 | 205篇 |
物理学 | 94篇 |
出版年
2022年 | 5篇 |
2021年 | 8篇 |
2020年 | 7篇 |
2019年 | 12篇 |
2018年 | 12篇 |
2017年 | 12篇 |
2016年 | 17篇 |
2015年 | 21篇 |
2014年 | 30篇 |
2013年 | 46篇 |
2012年 | 47篇 |
2011年 | 54篇 |
2010年 | 30篇 |
2009年 | 31篇 |
2008年 | 22篇 |
2007年 | 44篇 |
2006年 | 40篇 |
2005年 | 28篇 |
2004年 | 41篇 |
2003年 | 23篇 |
2002年 | 21篇 |
2001年 | 9篇 |
2000年 | 11篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 11篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1932年 | 1篇 |
排序方式: 共有652条查询结果,搜索用时 15 毫秒
561.
Núria Crivillers Marta Mas-Torrent Stefan T Bromley Klaus Wurst Jaume Veciana Concepció Rovira 《Chemphyschem》2007,8(10):1565-1571
We study the assemblies that tetra(carbomethoxy)tetrathiafulvalene (TCM-TTF) forms in solution and when deposited on a surface depending on intermolecular interactions and on the interactions with the substrate and the solvent. Its organization on graphite and mica substrates was studied by atomic force microscopy, and different molecular assemblies were observed depending on the prevailing interactions. The promotion of molecule-molecule interactions gave rise to the formation of molecular fibers. The investigation of the influence of the solvent-molecule interactions on TCM-TTF molecular organizations was carried out by UV/Vis spectroscopy, and a new TCM-TTF polymorph was obtained by changing the nature of the solvent. Finally, an explanation for all these phenomena, supported by computational modeling, is put forward. 相似文献
562.
Eva Rentschler Marco Affronte Carlo Massobrio Pierre Rabu Jaume Veciana 《Solid State Sciences》2009,11(5):970-971
A critical outlook of the field of molecular magnetic materials is presented. This article is inspired by an international symposium devoted to the “Design, Characterization and Modelling of Molecule-Based Magnetic Materials (DCM4-II)” that took place at Strasbourg (France), from May 28th to June 1st, within the E-MRS 2007 Spring Meeting (Symposium R) organized by the European Materials Research Society in collaboration with the European Science Foundation. A series of papers linked to this symposium are published in this issue and in the previous issue (Volume 11, Issue 4) of Solid State Sciences. 相似文献
563.
Jaume Llibre 《Journal of Differential Equations》2009,246(6):2192-189
In this paper we classify the centers, the cyclicity of its Hopf bifurcation and their isochronicity for the polynomial differential systems in R2 of arbitrary degree d?3 odd that in complex notation z=x+iy can be written as
564.
One of the main open problems in secret sharing is the characterization of the access structures of ideal secret sharing schemes.
Brickell and Davenport proved that every one of these ideal access structures is related in a certain way to a unique matroid.
Specifically, they are matroid ports. In addition to the search of general results, this difficult open problem has been studied
in previous works for several families of access structures. In this paper we do the same for access structures with rank
3, that is, structures whose minimal qualified subsets have at most three participants. We completely characterize and classify
the rank-3 access structures that are matroid ports. We prove that all access structures with rank three that are ports of
matroids greater than 3 are ideal. After the results in this paper, the only open problem in the characterization of the ideal
access structures with rank three is to characterize the rank-3 matroids that can be represented by an ideal secret sharing
scheme.
A previous version of this paper appeared in Fifth Conference on Security and Cryptography for Networks, SCN 2006, Lecture Notes in Computer Science
4116 (2006) 201–215. 相似文献
565.
Let X be a homogeneous polynomial vector field of degree 2 on $
\mathbb{S}^2
$
\mathbb{S}^2
. We show that if X has at least a non-hyperbolic singularity, then it has no limit cycles. We give necessary and sufficient conditions for determining
if a singularity of X on $
\mathbb{S}^2
$
\mathbb{S}^2
is a center and we characterize the global phase portrait of X modulo limit cycles. We also study the Hopf bifurcation of X and we reduce the 16
th
Hilbert’s problem restricted to this class of polynomial vector fields to the study of two particular families. Moreover,
we present two criteria for studying the nonexistence of periodic orbits for homogeneous polynomial vector fields on $
\mathbb{S}^2
$
\mathbb{S}^2
of degree n. 相似文献
566.
We study the bifurcation of limit cycles from the periodic orbits of a linear differential system in R4 in resonance 1:n perturbed inside a class of piecewise linear differential systems, which appear in a natural way in control theory. Our main result shows that at most 1 limit cycle can bifurcate using expansion of the displacement function up to first order with respect to a small parameter. This upper bound is reached. For proving this result we use the averaging theory in a form where the differentiability of the system is not needed. 相似文献
567.
Gonidec M Biagi R Corradini V Moro F De Renzi V del Pennino U Summa D Muccioli L Zannoni C Amabilino DB Veciana J 《Journal of the American Chemical Society》2011,133(17):6603-6612
The two-dimensional self-assembly of a terbium(III) double-decker phthalocyanine on highly oriented pyrolitic graphite (HOPG) was studied by atomic force microscopy (AFM), and it was shown that it forms highly regular rectangular two-dimensional nanocrystals on the surface, that are aligned with the graphite symmetry axes, in which the molecules are organized in a rectangular lattice as shown by scanning tunneling microscopy. Molecular dynamics simulations were run in order to model the behavior of a collection of the double-decker complexes on HOPG. The results were in excellent agreement with the experiment, showing that-after diffusion on the graphite surface-the molecules self-assemble into nanoscopic islands which align preferentially along the three main graphite axes. These low dimension assemblies of independent magnetic centers are only one molecule thick (as shown by AFM) and are therefore very interesting nanoscopic magnetic objects, in which all of the molecules are in interaction with the graphite substrate and might therefore be affected by it. The magnetic properties of these self-assembled bar-shaped islands on HOPG were studied by X-ray magnetic circular dichroism, confirming that the compounds maintain their properties as single-molecule magnets when they are in close interaction with the graphite surface. 相似文献
568.
The literature has shown numerous contributions on the synthesis and physicochemical properties of persistent organic radicals but there are a lesser number of reports about their use as building blocks for obtaining molecular magnetic materials exhibiting an additional and useful physical property or function. These materials show promise for applications in spintronics as well as bistable memory devices and sensing materials. This critical review provides an up-to-date survey to this new generation of multifunctional magnetic materials. For this, a detailed revision of the most common families of persistent organic radicals-nitroxide, triphenylmethyl, verdazyl, phenalenyl, and dithiadiazolyl-so far reported will be presented, classified into three different sections: materials with magnetic, conducting and optical properties. An additional section reporting switchable materials based on these radicals is presented (257 references). 相似文献
569.
Carretero-Genevrier A Gazquez J Magén C Varela M Ferain E Puig T Mestres N Obradors X 《Chemical communications (Cambridge, England)》2012,48(50):6223-6225
We report a chemical solution based method using nanoporous track-etched polymer templates for producing long and oriented LaSr-2 × 4 manganese oxide molecular sieve nanowires. Scanning transmission electron microscopy and electron energy loss spectroscopy analyses show that the nanowires are ferromagnetic at room temperature, single crystalline, epitaxially grown and self-aligned. 相似文献
570.
Strahil Berkov Francesc Viladomat Carles Codina Sergio Suárez Angel Ravelo Jaume Bastida 《Journal of mass spectrometry : JMS》2012,47(8):1065-1073
Galanthamine‐type alkaloids produced by plants of the Amaryllidaceae family are potent acetylcholinesterase inhibitors. One of them, galanthamine, has been marketed as a hydrobromide salt for the treatment of Alzheimer's disease. In the present work, gas chromatography with electron impact mass spectrometry (GC‐EIMS) fragmentation of 12 reference compounds isolated from various amaryllidaceous plants and identified by spectroscopic methods (1D and 2D nuclear magnetic resonance, circular dichroism, high‐resolution MS (HRMS) and EIMS) was studied by tandem mass spectrometry (GC‐MS/MS) and accurate mass measurements (GC‐HRMS). The studied compounds showed good peak shape and efficient GC separation with a GC‐MS fragmentation pattern similar to that obtained by direct insertion probe. With the exception of galanthamine‐N‐oxide and N‐formylnorgalanthamine, the galanthamine‐type compounds showed abundant [M]+. and [M‐H]+ ions. A typical fragmentation pattern was also observed, depending on the substituents of the skeleton. Based on the fragmentation pathways of reference compounds, three other galanthamine‐type alkaloids, including 3‐O‐(2′‐butenoyl)sanguinine, which possesses a previously unelucidated structure, were identified in Leucojum aestivum ssp. pulchelum, a species endemic to the Balearic islands. GC‐MS can be successfully applied to Amaryllidaceae plant samples in the routine screening for potentially new or known bioactive molecules, chemotaxonomy, biodiversity and identification of impurities in pharmaceutical substances. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献