全文获取类型
收费全文 | 611篇 |
免费 | 12篇 |
国内免费 | 12篇 |
专业分类
化学 | 333篇 |
力学 | 4篇 |
数学 | 212篇 |
物理学 | 86篇 |
出版年
2021年 | 6篇 |
2020年 | 7篇 |
2019年 | 8篇 |
2018年 | 5篇 |
2017年 | 7篇 |
2016年 | 6篇 |
2015年 | 16篇 |
2014年 | 18篇 |
2013年 | 22篇 |
2012年 | 17篇 |
2011年 | 35篇 |
2010年 | 13篇 |
2009年 | 11篇 |
2008年 | 26篇 |
2007年 | 23篇 |
2006年 | 23篇 |
2005年 | 16篇 |
2004年 | 20篇 |
2003年 | 25篇 |
2002年 | 28篇 |
2001年 | 9篇 |
2000年 | 13篇 |
1999年 | 19篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 15篇 |
1984年 | 6篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1981年 | 10篇 |
1980年 | 16篇 |
1979年 | 9篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 14篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1971年 | 5篇 |
1969年 | 5篇 |
排序方式: 共有635条查询结果,搜索用时 0 毫秒
51.
Winfried Just 《Journal of Mathematical Analysis and Applications》2008,348(2):620-636
It is demonstrated that a Kolmogorov-type competition model featuring species allocation and gain functions can possess multiple coexistence states. Two examples are constructed: one in which the two competing species possess rectangular allocation functions but distinct gain functions, and the other in which one species has a rectangular allocation function, the second species has a bi-rectangular allocation function, and the two species share a common gain function. In both examples, it is shown that the species nullclines may intersect multiple times within the interior of the first quadrant, thus creating both locally stable and unstable equilibrium points. These results have important applications in the study of plant competition for sunlight, in which the allocation functions describe the vertical placement of leaves for two competing species, and the gain functions represent rates of photosynthesis performed by leaves at different heights when shaded by overlying leaves belonging to either species. 相似文献
52.
Christoph Rameshan Werner Stadlmayr Christian Weilach Simon Penner Harald Lorenz Michael Hvecker Raoul Blume Tulio Rocha Detre Teschner Axel Knop‐Gericke Robert Schlgl Norbert Memmel Dmitry Zemlyanov Günther Rupprechter Bernhard Kltzer 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2010,122(18):3292-3296
53.
Continuous electrochemical sensing is often carried out in order to track the growth of cells as an alternative to optical monitoring. Planar sensors and multi-sensor chips are applied in case of adhered growing cells, and usually introduced into lab-on-chip systems. Repeated recalibration is necessary with most chemosensors so far and this limits the operational lifetime of such lab-on-chip systems to a few days. An assembly is presented here that eliminates this disadvantage and enables the monitoring of a long-term cultivation of tissue. Cell cultures and sensor are arranged such that they can be separated or contacted at variable times without having an impact on the growth of the cells. The use of a biocompatible nano-porous membrane is especially important. A multi-well system is described where each well is supplied with a planar multi-sensor chip below the adhered cells to determine (a) pH, (b) glucose, (c) oxygen, and—optionally—impedance, for example during the cultivation of cartilage tissue. 相似文献
54.
Hans Postma Luc Amkreutz Alessandro Borella Martijn Clarijs Hans Kamermans Winfried Kockelmann Anna Paradowska Peter Schillebeeckx Dirk Visser 《Journal of Radioanalytical and Nuclear Chemistry》2010,283(3):641-652
Two neutron based techniques, neutron resonance capture analysis (NRCA) and time-of-flight neutron-diffraction (TOF-ND) have
been used to determine the elemental composition and structure of a precious and very well preserved all-metal sword from
the Bronze Age. This Buggenum sword was on loan from the National Museum of Antiquities (NMA) in Leiden (NL). NRCA and TOF-ND
experiments have been carried out at a number of more or less identical positions of the sword. The tin-bronze ratio and the
relative amounts of some minor elements (Sb, As, Ag, In) have been determined. The results of neutron diffraction measurements
showed considerable tin-segregation, and clear indications of hardening on the edges of the blade. In addition, radiographs
using Bremsstrahlung revealed the construction of the hilt–blade connection. The work was carried out at the EC Joint Research
Centre IRMM in Geel (B) and at the ISIS facility of the Rutherford Appleton Laboratory (UK). 相似文献
55.
56.
We investigate optimal linear approximations (approximation numbers) in the context of periodic Sobolev spaces Hs(Td) of fractional smoothness s>0 for various equivalent norms including the classical one. The error is always measured in L2(Td). Particular emphasis is given to the dependence of all constants on the dimension d. We capture the exact decay rate in n and the exact decay order of the constants with respect to d, which is in fact polynomial. As a consequence we observe that none of our considered approximation problems suffers from the curse of dimensionality. Surprisingly, the square integrability of all weak derivatives up to order three (classical Sobolev norm) guarantees weak tractability of the associated multivariate approximation problem. 相似文献
57.
Maciej Garbarczyk Farida Grinberg Nikolaus Nestle Winfried Kuhn 《Journal of Polymer Science.Polymer Physics》2001,39(18):2207-2216
We present a novel NMR approach to the determination of crosslink densities in rubber materials. The method is based on the dipolar correlation effect (DCE) on the stimulated echo examined in a series of rubber samples and linear polyisoprene. The parameter evaluated from the echo attenuation curves is the mean‐squared dipolar fluctuation associated with anisotropic reorientations of macromolecular backbones. The contributions to the DCE of the constraints due to excluded volume effects and chemical crosslinks are estimated. A strong dependence of the mean‐squared dipolar fluctuation on the crosslink density of rubber combined with the simplicity of performing the measurements with inexpensive low‐field instruments suggests that the DCE is a useful tool for routine applications. The potential and problems of performing DCE measurements in low‐magnetic‐field conditions are discussed in detail. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2207–2216, 2001 相似文献
58.
Winfried Teschner 《基于设计的结构力学与机械力学》2013,41(4):491-500
The application of optimal control theory to minimum weight design of continuous one-dimensional structural elements subject to eigenvalue constraints is discussed. If not only the value of an eigenvalue is prescribed but also its position in the sequence of the ordered eigenvalues—for example, the critical buckling load of a column—the corresponding optimal control problem is shown to include necessarily all eigenvalues. Considering the unspecified eigenvalues as free parameters, necessary conditions for minimum weight design are derived. These conditions are compared with those obtained by use of variational methods. Attention is focused on the special case of multimodal solutions. 相似文献
59.
Schünemann V. Jung C. Lendzian F. Barra A.-L. Teschner T. Trautwein A. X. 《Hyperfine Interactions》2004,156(1-4):247-256
Hyperfine Interactions - In this communication we present a complimentary Mössbauer- and EPR-study of the time dependance of the reaction of substrate free P450cam with peracetic acid within a... 相似文献
60.
We present a MATLAB package for boundary value problems in ordinary differential equations. Our aim is the efficient numerical solution of systems of ODEs with a singularity of the first kind, but the solver can also be used for regular problems. The basic solution is computed using collocation methods and a new, efficient estimate of the global error is used for adaptive mesh selection. Here, we analyze some of the numerical aspects relevant for the implementation, describe measures to increase the efficiency of the code and compare its performance with the performance of established standard codes for boundary value problems. 相似文献