全文获取类型
收费全文 | 638篇 |
免费 | 34篇 |
国内免费 | 6篇 |
专业分类
化学 | 305篇 |
力学 | 37篇 |
数学 | 205篇 |
物理学 | 94篇 |
无线电 | 37篇 |
出版年
2022年 | 4篇 |
2021年 | 9篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 16篇 |
2017年 | 14篇 |
2016年 | 18篇 |
2015年 | 22篇 |
2014年 | 38篇 |
2013年 | 46篇 |
2012年 | 50篇 |
2011年 | 53篇 |
2010年 | 32篇 |
2009年 | 34篇 |
2008年 | 22篇 |
2007年 | 48篇 |
2006年 | 40篇 |
2005年 | 28篇 |
2004年 | 43篇 |
2003年 | 25篇 |
2002年 | 21篇 |
2001年 | 7篇 |
2000年 | 8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 | 4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 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1932年 | 1篇 |
排序方式: 共有678条查询结果,搜索用时 656 毫秒
31.
Jaume Llibre Marco Antonio Teixeira Joan Torregrosa 《Mathematical Physics, Analysis and Geometry》2007,10(3):237-249
The goal of this paper is double. First, we illustrate a method for studying the bifurcation of limit cycles from the continuum
periodic orbits of a k-dimensional isochronous center contained in ℝ
n
with n ⩾ k, when we perturb it in a class of differential systems. The method is based in the averaging theory. Second, we consider a particular polynomial differential
system in the plane having a center and a non-rational first integral. Then we study the bifurcation of limit cycles from
the periodic orbits of this center when we perturb it in the class of all polynomial differential systems of a given degree.
As far as we know this is one of the first examples that this study can be made for a polynomial differential system having
a center and a non-rational first integral.
The first and third authors are partially supported by a MCYT/FEDER grant MTM2005-06098-C01, and by a CIRIT grant number 2005SGR-00550.
The second author is partially supported by a FAPESP–BRAZIL grant 10246-2. The first two authors are also supported by the
joint project CAPES–MECD grant HBP2003-0017. 相似文献
32.
HOMO Stabilisation in π‐Extended Dibenzotetrathiafulvalene Derivatives for Their Application in Organic Field‐Effect Transistors 下载免费PDF全文
Dr. Yan Geng Dr. Raphael Pfattner Antonio Campos Prof. Wei Wang Dr. Olivier Jeannin Dr. Jürg Hauser Dr. Joaquim Puigdollers Prof. Stefan T. Bromley Prof. Silvio Decurtins Prof. Jaume Veciana Prof. Concepció Rovira Dr. Marta Mas‐Torrent Dr. Shi‐Xia Liu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(50):16672-16679
Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB‐TTF) central core and a 2,1,3‐chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB‐TTF, have been synthesised as active materials for organic field‐effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron‐withdrawing 2,1,3‐chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution‐processed single‐crystal transistors exhibit high performance with a hole mobility up to 0.04 cm2 V?1 s?1 as well as good ambient stability. 相似文献
33.
Zghal Imen Farjas Jordi Camps Jaume Sánchez-Rodríguez D. Dammak Mohamed Roura-Grabulosa Pere 《Journal of Thermal Analysis and Calorimetry》2020,140(5):2087-2096
Journal of Thermal Analysis and Calorimetry - Thermal energy storage (TES) has been identified as a breakthrough concept in development of renewable technologies. However, the main challenges are... 相似文献
34.
35.
This paper consists of two parts. In the first part we study the relationship between conic centers (all orbits near a singular point of center type are conics) and isochronous centers of polynomial systems. In the second part we study the number of limit cycles that bifurcate from the periodic orbits of cubic reversible isochronous centers having all their orbits formed by conics, when we perturb such systems inside the class of all polynomial systems of degree n. 相似文献
36.
Jaume Giné 《Journal of Differential Equations》2004,197(1):147-161
We consider the class of polynomial differential equations , where Pn and Qn are homogeneous polynomials of degree n. These systems have a focus at the origin if λ≠0, and have either a center or a focus if λ=0. Inside this class we identify a new subclass of Darbouxian integrable systems having either a focus or a center at the origin. Additionally, under generic conditions such Darbouxian integrable systems can have at most one limit cycle, and when it exists is algebraic. For the case n=2 and 3, we present new classes of Darbouxian integrable systems having a focus. 相似文献
37.
Domínguez M Alvarez R Martras S Farrés J Parés X de Lera AR 《Organic & biomolecular chemistry》2004,2(22):3368-3373
Ring-oxidized retinoids have been synthesized stereoselectively using the Stille cross-coupling reaction. Kinetic constants of mouse class I alcohol dehydrogenase (ADH1) with these retinoids were determined. 相似文献
38.
39.
Ribas X Dias JC Morgado J Wurst K Santos IC Almeida M Vidal-Gancedo J Veciana J Rovira C 《Inorganic chemistry》2004,43(12):3631-3641
The deprotonated form of the pyrazine-2,3-diselenol (pds) ligand, pds(2-), reacts with Ni(II) inorganic salts to form the nickel compounds [Ni(II)(pds)(2)](nBu(4)N)(2) (1), [Ni(II)(pds)(2)]Na(2).2H(2)O (2), and [Ni(III)(pds)(2)](2)Na(2).4H(2)O (3), depending on the reaction conditions. They are characterized by NMR, EPR, UV-vis, and IR spectroscopies, elemental analysis, cyclic voltammetry, and X-ray crystallography. The crystal structure of compound 3 shows the formation of segregated stacks of Ni(pds)(2-) units, with a strong dimerization along the stacks. The stacked fashion of the crystal packing was expected since the supramolecular forces of the alkaline side coordination to the pyrazine moieties dominate, as happens in the recently reported analogous copper system [Cu(III)(pds)(2)]Na.2H(2)O. The structure of 2 further emphasizes the alkaline coordination as the dominating supramolecular event, and an orthogonal array of 2D layers is observed. The absence of alkaline cations in complex 1 is reflected in a crystal packing with isolated complex Ni(pds)(2)(2-) units. The dimerization found in the paramagnetic Ni(III) complex 3 promotes a very strong antiferromagnetic interaction, leading to a singlet ground state. 相似文献
40.
[reaction: see text] Highly stereoselective titanium-mediated aldol reactions based on lactate-derived ketones are reported. The stereochemical outcome of the process depends on the protecting group (PMB or Bn) and the Lewis acid (i-PrOTiCl(3) or TiCl(4)) used in the enolization step, the corresponding anti-syn or syn-syn aldols being prepared in high yields and with diastereomeric ratios up to 99:1. 相似文献