全文获取类型
收费全文 | 401686篇 |
免费 | 4055篇 |
国内免费 | 1434篇 |
专业分类
化学 | 219964篇 |
晶体学 | 6408篇 |
力学 | 16609篇 |
综合类 | 6篇 |
数学 | 43369篇 |
物理学 | 120819篇 |
出版年
2018年 | 3519篇 |
2017年 | 3406篇 |
2016年 | 5670篇 |
2015年 | 3990篇 |
2014年 | 5871篇 |
2013年 | 17621篇 |
2012年 | 12322篇 |
2011年 | 15341篇 |
2010年 | 10165篇 |
2009年 | 10075篇 |
2008年 | 13807篇 |
2007年 | 14000篇 |
2006年 | 13042篇 |
2005年 | 12096篇 |
2004年 | 10950篇 |
2003年 | 9773篇 |
2002年 | 9575篇 |
2001年 | 11714篇 |
2000年 | 8841篇 |
1999年 | 7006篇 |
1998年 | 5695篇 |
1997年 | 5738篇 |
1996年 | 5483篇 |
1995年 | 5116篇 |
1994年 | 4896篇 |
1993年 | 4855篇 |
1992年 | 5566篇 |
1991年 | 5415篇 |
1990年 | 5199篇 |
1989年 | 5224篇 |
1988年 | 5227篇 |
1987年 | 5140篇 |
1986年 | 4832篇 |
1985年 | 6603篇 |
1984年 | 6798篇 |
1983年 | 5574篇 |
1982年 | 6056篇 |
1981年 | 5905篇 |
1980年 | 5692篇 |
1979年 | 5881篇 |
1978年 | 6281篇 |
1977年 | 6023篇 |
1976年 | 6137篇 |
1975年 | 5624篇 |
1974年 | 5651篇 |
1973年 | 5956篇 |
1972年 | 3790篇 |
1971年 | 3030篇 |
1968年 | 3266篇 |
1967年 | 3223篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
We solve the de la Vallée Poussin problem for a functional-differential equation by the projection-iterative method. We construct an algorithm, establish conditions sufficient for the convergence of the method, and present a computational scheme. 相似文献
142.
D. Amutha Rani Y. Yamamoto S. Mohri M. Sivakumar Y. Tsujita H. Yoshimizu 《Journal of Polymer Science.Polymer Physics》2003,41(3):269-273
In this study, a novel stepwise extraction method has been examined. The guest molecules housed between the helices of the clathrate δ form of syndiotactic polystyrene can be removed completely with this method. A systematic study of the preparation of a solvent‐free mesophase (emptied clathrate) membrane, its helical and residual solvent contents, and its structural transformations has been performed. In this first attempt, an enhancement in the TTGG helical content has been observed in the extracted membrane, and a conceptual mechanism is proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 269–273, 2003 相似文献
143.
The title compound is a centrosymmetric dimer with each cadmium in a distorted CdS5 square pyramidal geometry. The Cd–S bond distances range from 2.5626(11) to 2.8459(11) Å. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
144.
145.
C. Mattea N. Fatkullin E. Fischer U. Beginn E. Anoardo M. Kroutieva R. Kimmich 《Applied magnetic resonance》2004,27(3-4):371-381
Linear polyethylene oxides with molecular weightsM w of 1665 and 10170 confined in pores with variable diameters in a solid methacrylate matrix were studied by proton field-cycling nuclear magnetic resonance relaxometry. The pore diameter was varied in the range of 9–57 nm. In all cases, the spin-lattice relaxation time shows a frequency dependence close toT 1∞ v3/4 in the range ofv=3·10?1-2·101 MHz as predicted by the tube-reptation model. This protonT 1 dispersion essentially reproduces that found in a previous deuteron study (R. Kimmich, R.-O. Seitter, U. Beginn, M. Möller, N. Fatkullin: Chem. Phys. Lett. 307, 147, 1999). As a feature particularly characteristic for reptation, this finding suggests that reptation is the dominating chain dynamics mechanism under pore confinement in the corresponding time range. The absolute values of the spin-lattice relaxation times indicate that the diameter of the effective tubes in which reptation occurs is much smaller than the pore diameters on the time scale of spin-lattice relaxation experimens. An estimation leads to a valued *~0.5 nm. The impenetrability of the solid pore walls, the uncrossability of polymer chains (“excluded volume”) and the low value of the compressibility in polymer melts create the “corset effect” which reduces the lateral motions of polymer chains to a microscopic scale of only a few tenths of a nanometer. 相似文献
146.
H.L. Xu A. Persson S. Svanberg 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,23(2):233-236
New radiative lifetime measurements based on time-resolved laser-induced fluorescence techniques are reported for 18 even-parity
levels belonging to the 4f5d26p and 4f
25d
2 configurations of Ce I and 6 even-parity levels belonging to the 5d26s, 4f5d6p, and 4f6s6p configurations of Ce II. Free neutral and singly ionized cerium atoms were produced by laser ablation. The Ce I and Ce II
levels range in energy from 26 545 to 29 102 cm-1, and 42 573 to 48 152 cm-1, respectively.
Received 25 September 2002 Published online 4 March 2003 相似文献
147.
Pascher W. Den Besten J. H. Caprioli D. Leijtens X. Smit M. van Dijk R. 《Optical and Quantum Electronics》2003,35(4-5):453-464
Based on a rigorous vectorial analysis, a fast travelling-wave Mach–Zehnder modulator is modelled and designed. The cross-section of the semiconductor layer stack and the lossy electrodes are carefully modelled using the method of lines in order to investigate propagation characteristics, velocity and losses. This yields an accurate microwave and optical field distribution to explain the behaviour of the component. In order to enhance the modulation efficiency, design curves are derived and the cross-sectional dimensions for minimum microwave loss are determined. The loss of the optimized modulator agrees very well with small-signal measurements up to 40 GHz and HFSS simulations. The layerstack of the fabricated device is suitable for integration with InP multi-wavelength lasers. 相似文献
148.
R. Pizzoferrato L. Lagonigro T. Ziller A. Di Carlo R. Paolesse F. Mandoj A. Ricci C. Lo Sterzo 《Chemical physics》2004,300(1-3):217-225
We study the infrared emission at 1.54 μm of an organolanthanide complex, Er(III)-tetraphenylporphyrin [Er(TPP)acac], both as a result of direct optical excitation and via energy transfer from host π-conjugate polymers of type poly(arylene–ethynylene) [PAE]. In the first case, the emission of the neat complex is characterized in inert transparent materials and a value of the quantum yield at 1.54 μm φIR=4×10−4 is measured. Then, fluorescence resonance transfer is investigated in blends of Er(TPP)acac with PAEs by monitoring the quenching of the polymer fluorescence along with the enhancement of both the visible emission of the ligand and the near-infrared band of Er3+. These different procedures allow a detailed analysis of the transfer efficiency within a specific implementation of the Förster model for polymeric donors. The experimental values of the critical radius R0, ranging from 1.3 to 2.5 nm for the different blends, are in good agreement with theory for a wide interval of the physical and spectroscopic parameters. This suggests that other mechanisms for excitation transfer do not play a significant role in these materials. 相似文献
149.
R. Hefelmann P. Mann A. Aignan F. Filsinger und H. Amsel 《Fresenius' Journal of Analytical Chemistry》1902,41(1):66-68
Ohne Zusammenfassung 相似文献
150.
A parallel method for time discretization of parabolic equations based on Laplace transformation and quadrature 总被引:5,自引:0,他引:5
We consider the discretization in time of an inhomogeneous parabolicequation in a Banach space setting, using a representation ofthe solution as an integral along a smooth curve in the complexleft half-plane which, after transformation to a finite interval,is then evaluated to high accuracy by a quadrature rule. Thisreduces the problem to a finite set of elliptic equations withcomplex coefficients, which may be solved in parallel. The paperis a further development of earlier work by the authors, wherewe treated the homogeneous equation in a Hilbert space framework.Special attention is given here to the treatment of the forcingterm. The method is combined with finite-element discretizationin spatial variables. 相似文献