全文获取类型
收费全文 | 4649篇 |
免费 | 157篇 |
国内免费 | 27篇 |
专业分类
化学 | 3343篇 |
晶体学 | 37篇 |
力学 | 98篇 |
数学 | 661篇 |
物理学 | 694篇 |
出版年
2023年 | 28篇 |
2022年 | 46篇 |
2021年 | 114篇 |
2020年 | 79篇 |
2019年 | 74篇 |
2018年 | 63篇 |
2017年 | 67篇 |
2016年 | 173篇 |
2015年 | 111篇 |
2014年 | 135篇 |
2013年 | 279篇 |
2012年 | 350篇 |
2011年 | 384篇 |
2010年 | 216篇 |
2009年 | 168篇 |
2008年 | 326篇 |
2007年 | 313篇 |
2006年 | 256篇 |
2005年 | 297篇 |
2004年 | 225篇 |
2003年 | 196篇 |
2002年 | 162篇 |
2001年 | 62篇 |
2000年 | 63篇 |
1999年 | 36篇 |
1998年 | 44篇 |
1997年 | 35篇 |
1996年 | 70篇 |
1995年 | 30篇 |
1994年 | 32篇 |
1993年 | 47篇 |
1992年 | 22篇 |
1991年 | 29篇 |
1990年 | 36篇 |
1989年 | 25篇 |
1988年 | 25篇 |
1987年 | 30篇 |
1986年 | 16篇 |
1985年 | 31篇 |
1984年 | 22篇 |
1983年 | 12篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 14篇 |
1979年 | 17篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1974年 | 6篇 |
1970年 | 2篇 |
排序方式: 共有4833条查询结果,搜索用时 15 毫秒
941.
Juan Sabuco Samuel Zambrano Miguel A.F. Sanjuán James A. Yorke 《Communications in Nonlinear Science & Numerical Simulation》2012,17(11):4274-4280
Many discrete-time dynamical systems have a region Q from which all or almost all trajectories leave, or at least they leave in the presence of perturbations that we call disturbances. We partially control systems so that despite disturbances the trajectories of a dynamical system stay in the region Q at least for some initial points in Q. The disturbances can be thought of as either noise or as purposeful, hostile efforts of an enemy to drive the trajectory out of the region. Our goal is to keep trajectories inside Q despite the disturbances and our partial control of chaos method succeeds.Surprisingly this goal can be achieved with a control whose maximum allowable size is smaller than the maximum allowed disturbance. A fundamental step towards this goal is to compute a set called the safe set that had, until now, been found only in certain very special situations.This paper provides a general algorithm for computing safe sets. The algorithm is able to compute the safe sets for a specified region in phase space, the maximum disturbance value, and the maximum allowed control. We call it the Sculpting Algorithm. Its operation is analogous to removing material while sculpting a statue. The algorithm sculpts the safe sets. Our Sculpting Algorithm is independent of the dimension and is fast for one- and two-dimensional dynamical systems. As examples, we apply the algorithm to two paradigmatic nonlinear dynamical systems, namely, the Hénon map and the Duffing oscillator. 相似文献
942.
943.
Miguel R. Visbal Raymond E. Gordnier Marshall C. Galbraith 《Experiments in fluids》2009,46(5):903-922
The present paper highlights results derived from the application of a high-fidelity simulation technique to the analysis
of low-Reynolds-number transitional flows over moving and flexible canonical configurations motivated by small natural and
man-made flyers. This effort addresses three separate fluid dynamic phenomena relevant to small fliers, including: laminar
separation and transition over a stationary airfoil, transition effects on the dynamic stall vortex generated by a plunging
airfoil, and the effect of flexibility on the flow structure above a membrane airfoil. The specific cases were also selected
to permit comparison with available experimental measurements. First, the process of transition on a stationary SD7003 airfoil
section over a range of Reynolds numbers and angles of attack is considered. Prior to stall, the flow exhibits a separated
shear layer which rolls up into spanwise vortices. These vortices subsequently undergo spanwise instabilities, and ultimately
breakdown into fine-scale turbulent structures as the boundary layer reattaches to the airfoil surface. In a time-averaged
sense, the flow displays a closed laminar separation bubble which moves upstream and contracts in size with increasing angle
of attack for a fixed Reynolds number. For a fixed angle of attack, as the Reynolds number decreases, the laminar separation
bubble grows in vertical extent producing a significant increase in drag. For the lowest Reynolds number considered (Re
c
= 104), transition does not occur over the airfoil at moderate angles of attack prior to stall. Next, the impact of a prescribed
high-frequency small-amplitude plunging motion on the transitional flow over the SD7003 airfoil is investigated. The motion-induced
high angle of attack results in unsteady separation in the leading edge and in the formation of dynamic-stall-like vortices
which convect downstream close to the airfoil. At the lowest value of Reynolds number (Re
c
= 104), transition effects are observed to be minor and the dynamic stall vortex system remains fairly coherent. For Re
c
= 4 × 104, the dynamic-stall vortex system is laminar at is inception, however shortly afterwards, it experiences an abrupt breakdown
associated with the onset of spanwise instability effects. The computed phased-averaged structures for both values of Reynolds
number are found to be in good agreement with the experimental data. Finally, the effect of structural compliance on the unsteady
flow past a membrane airfoil is investigated. The membrane deformation results in mean camber and large fluctuations which
improve aerodynamic performance. Larger values of lift and a delay in stall are achieved relative to a rigid airfoil configuration.
For Re
c
= 4.85 × 104, it is shown that correct prediction of the transitional process is critical to capturing the proper membrane structural
response. 相似文献
944.
Bulent Kozanoglu Fabio Ferrero Miguel Muñoz Josep Arnaldos Joaquim Casal 《Heat and Mass Transfer》2008,44(12):1549-1555
A mathematical model is developed to simulate the thin layer boilover phenomenon. This model takes into account convective
currents as well as conduction and radiation absorption through the fuel layer and is resolved numerically employing a scheme
of Runge–Kutta, combined with the numerical method of lines. Solutions of the model showed a good agreement with the experimental
data, both from this work and by other authors, demonstrating the importance of the convective currents. The model provided
velocities of these currents, of the same order of magnitude as the values reported in the technical literature. Thickness
of the remaining fuel and the interface temperature are correctly calculated by the model, allowing the prediction of the
time required for the boilover to start. 相似文献
945.
Ramirez AG Miller AR Gallion K San Miguel de Majors S Chalela P García Arámburo S 《Community genetics》2008,11(4):215-223
946.
Mustapha Aitali Larbi El Firdoussi Abdellah Karim Alejandro F. Barrero Miguel Quirs 《Acta Crystallographica. Section C, Structural Chemistry》2000,56(9):1088-1089
The title compound, [(S)‐2‐(anilinomethyl)pyrrolidine‐N,N′]‐chloro(η6‐para‐cymene)ruthenium(II) chloride, [RuCl‐(C10H14)(C11H16N2)]Cl, has been synthesized by the reaction of [RuCl2(p‐cymene)]2 (p‐cymene is para‐isopropyltoluene) with (S)‐2‐(anilinomethyl)pyrrolidine in triethylamine/2‐propanol. The Ru atom is in a pseudo‐tetrahedral environment coordinated by a chloride ligand, the aromatic hydrocarbon is linked in a η6 manner and the amine is linked via its two N atoms. The chloride anion is involved in hydrogen bonding with the diamine moieties through N—H?Cl interactions, with N?Cl distances of 3.273 (4) and 3.352 (4) Å. 相似文献
947.
0.5–3 wt% nanosilica was added to an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) and cured at 25, 40 or 60 °C using isophoronediamine (IPDA) as hardener. Aggregates of nanosilica were properly dispersed into the DGEBA-IPDA resin and agglomerates formation was avoided. Addition of nanosilica increased the storage modulus E′ and the area and height of the tan δ curve of DGEBA-IPDA resin cured at 25 °C, but no significant differences were found by curing at higher temperature. Gel time measurements and the results obtained by applying the Kamal model to isotherm DSC curing of DGEBA-IPDA-nanosilica revealed that nanosilica catalysed the curing reaction between DGEBA and IPDA, in less extent by increasing the curing temperature. 相似文献
948.
Microwave tomographic imaging is showing significant promise as a new technique for the early detection of breast cancer. Its physical basis is the contrast between the dielectric properties of the healthy breast tissue and the malignant tumors at microwave frequencies. We propose and analyze a novel shape-reconstruction technique for the early detection of breast cancer from microwave data which is based on a level-set technique. The advantages of this method compared to more traditional pixel-based approaches are well-defined boundaries and the incorporation of an intrinsic regularization in form of a-priori assumptions about the general anatomical structure of breast that reduces the dimensionality of the inverse problem and thereby stabilizes the reconstruction. Our goal is not only to detect the tumors but to simultaneously determine their approximate locations, sizes and permittivity values. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
949.
Ignacio Baena Pedro Prez-Fernndez Manuela Rodríguez-Gallardo Jos Miguel Arias 《Entropy (Basel, Switzerland)》2022,24(1)
A quantum phase transition (QPT) in a simple model that describes the coexistence of atoms and diatomic molecules is studied. The model, which is briefly discussed, presents a second-order ground state phase transition in the thermodynamic (or large particle number) limit, changing from a molecular condensate in one phase to an equilibrium of diatomic molecules–atoms in coexistence in the other one. The usual markers for this phase transition are the ground state energy and the expected value of the number of atoms (alternatively, the number of molecules) in the ground state. In this work, other markers for the QPT, such as the inverse participation ratio (IPR), and particularly, the Rényi entropy, are analyzed and proposed as QPT markers. Both magnitudes present abrupt changes at the critical point of the QPT. 相似文献
950.
Arturo Echeverría-Enríquez Miguel C. Muoz-Lecanda Narciso Romn-Roy 《Fortschritte der Physik》1996,44(3):235-280
We construct a lagrangian geometric formulation for first-order field theories using the canonical structures of first-order jet bundles, which are taken as the phase spaces of the systems in consideration. First of all, we construct all the geometric structures associated with a first-order jet bundle and, using them, we develop the lagrangian formalism, defining the canonical forms associated with a lagrangian density and the density of lagrangian energy, obtaining the Euler-Lagrange equations in two equivalent ways: as the result of a variational problem and developing the jet field formalism (which is a formulation more similar to the case of mechanical systems). A statement and proof of Noether's theorem is also given, using the latter formalism. 相似文献