全文获取类型
收费全文 | 3490篇 |
免费 | 112篇 |
国内免费 | 18篇 |
专业分类
化学 | 2157篇 |
晶体学 | 12篇 |
力学 | 96篇 |
数学 | 764篇 |
物理学 | 591篇 |
出版年
2023年 | 34篇 |
2022年 | 79篇 |
2021年 | 88篇 |
2020年 | 59篇 |
2019年 | 91篇 |
2018年 | 61篇 |
2017年 | 69篇 |
2016年 | 139篇 |
2015年 | 107篇 |
2014年 | 118篇 |
2013年 | 197篇 |
2012年 | 248篇 |
2011年 | 279篇 |
2010年 | 157篇 |
2009年 | 177篇 |
2008年 | 239篇 |
2007年 | 225篇 |
2006年 | 203篇 |
2005年 | 160篇 |
2004年 | 157篇 |
2003年 | 97篇 |
2002年 | 96篇 |
2001年 | 38篇 |
2000年 | 47篇 |
1999年 | 31篇 |
1998年 | 25篇 |
1997年 | 25篇 |
1996年 | 32篇 |
1995年 | 14篇 |
1994年 | 20篇 |
1993年 | 16篇 |
1992年 | 28篇 |
1991年 | 18篇 |
1990年 | 30篇 |
1989年 | 12篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 13篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 13篇 |
1982年 | 9篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1974年 | 8篇 |
1971年 | 6篇 |
1963年 | 5篇 |
排序方式: 共有3620条查询结果,搜索用时 24 毫秒
21.
In this work, the improvement of the dynamic range of a micro displacement sensor based on fiber specklegrams holographically stored in a photorefractive BSO (Bi12SiO20) crystal is reported. In our experimental setup, a plastic optical fiber (POF) was used to generate a subjective speckle pattern that was recorded in the crystal using a two-wave mixing arrangement. The speckle size was controlled by modifying the diameter of a pupil aperture adjacent to a lens producing the image of the speckle. Fringe patterns were obtained at the output of the system by producing micro displacements of the fiber output end. An increase in the visibility of the fringe patterns was appreciated when the pupil aperture diameter decreased even without controlling the average modulation of the intensity of the light pattern, i.e. when the speckle length increased and the average light modulation simultaneously decreased. This behavior allowed recovering the autocorrelation functions of fringe patterns associated with displacements that initially led to decorrelation, and therefore, significantly to improve the dynamic range of the metrological system. To the best of our knowledge this is the first report about the influence of speckle size on the dynamic range of fiber specklegrams sensors recorded on photorefractive materials. 相似文献
22.
M. D. Martín L. Via J. K. Son R. Ruf E. E. Mendez 《Physica E: Low-dimensional Systems and Nanostructures》2000,6(1-4)
We have used time-resolved photoluminescence spectroscopy to study the light emission dynamics in a semiconductor microcavity as a function of excitation density and exciton-cavity detuning. We paid special attention to polariton spin relaxation by using circularly polarized excitation. We have found a striking behavior of the photoluminescence degree of polarization, which reaches its maximum value at a finite time. As the excitation density is increased and the system enters the stimulated emission regime, this maximum is followed by a negative dip, whose depth strongly depends on exciton-cavity detuning. 相似文献
23.
In this paper we introduce and study the anisotropic local Hardy spaces
hAp(\mathbbRn)h_{A}^{p}(\mathbb{R}^{n}) 0<p≤1, associated with the expansive matrix A. We obtain an atomic characterization of the distributions in
hAp(\mathbbRn)h_{A}^{p}(\mathbb{R}^{n}). Also we describe the dual spaces of our local Hardy anisotropic spaces as anisotropic Campanato type spaces. 相似文献
24.
Hoyos-Montilla Ary A. Puertas F. Tobón Jorge I. 《Journal of Thermal Analysis and Calorimetry》2018,131(3):2395-2410
Journal of Thermal Analysis and Calorimetry - The objective of this research is to know, through the flow of heat released, the effect of the addition of calcium hydroxide and the temperature on... 相似文献
25.
Imran Hussain Syed Anastasia Braer Frank Fleck Jorge Lacayo-Pineda 《Soft Materials》2018,16(4):275-288
In the present study, a specific type of reinforcing resin behavior is investigated using various mechanical approaches as well as microscopic techniques such as transmission electron microscopy and atomic force microscopy. Based on these observations, it could be concluded that the reinforcing resin introduces a synergistic effect with carbon black in order to strengthen the system. This is implied since the percolation threshold is significantly reduced and the morphology of the filler aggregates changes toward higher compactness, i.e., an increase of volume-to-size ratio with the addition of resin. 相似文献
26.
Jorge Krause 《International Journal of Theoretical Physics》1974,11(4):273-284
A flat space-time time-like curve is considered from the point of view of an instantaneous comoving inertial observer. In the context of the ‘vierbein’ formalism a projection operator is introduced, able to project 3-vectors, belonging to the 3-space of the comoving observer, out of space-time 4-vectors. The motion of an accelerated particle relative to the comoving inertial frame is briefly reviewed by means of the projection technique, and the three space-like components of the Frenet-Serret tetrad are thus projected, and their motion relative to the comoving observer neatly stated. Finally, the physical identification of the normals and the curvatures obtains in terms of three-dimensional kinematics as seen by the instantaneous comoving observer. 相似文献
27.
José Fernando Gonçalves Mauricio G. C. Resende Jorge J. M. Mendes 《Journal of Heuristics》2011,17(5):467-486
This paper presents a biased random-key genetic algorithm for the resource constrained project scheduling problem. The chromosome
representation of the problem is based on random keys. Active schedules are constructed using a priority-rule heuristic in
which the priorities of the activities are defined by the genetic algorithm. A forward-backward improvement procedure is applied
to all solutions. The chromosomes supplied by the genetic algorithm are adjusted to reflect the solutions obtained by the
improvement procedure. The heuristic is tested on a set of standard problems taken from the literature and compared with other
approaches. The computational results validate the effectiveness of the proposed algorithm. 相似文献
28.
In cardiac tissue, the propagation of electrical excitation waves is dependent on the active properties of the cell membrane (ionic channels) and the passive electrical properties of cardiac tissue (passive membrane properties, distribution of gap junctions, and cell shapes). Initiation of cardiac arrhythmias is usually associated with heterogeneities in the active and/or passive properties of cardiac tissue. However, as a result of the effect of wave front geometry (curvature) on propagation of cardiac waves, inexcitable anatomical obstacles, like veins and arteries, may cause the formation of self-sustained vortices and uncontrolled high-frequency excitation in normal homogeneous myocardium. (c) 1998 American Institute of Physics. 相似文献
29.
Jorge Antezana Gabriel Larotonda Alejandro Varela 《Communications in Mathematical Physics》2014,328(2):481-497
Given a positive and unitarily invariant Lagrangian ${\mathcal{L}}$ defined in the algebra of matrices, and a fixed time interval ${[0,t_0]\subset\mathbb R}$ , we study the action defined in the Lie group of ${n\times n}$ unitary matrices ${\mathcal{U}(n)}$ by $$\mathcal{S}(\alpha)=\int_0^{t_0} \mathcal{L}(\dot\alpha(t))\,dt, $$ where ${\alpha:[0,t_0]\to\mathcal{U}(n)}$ is a rectifiable curve. We prove that the one-parameter subgroups of ${\mathcal{U}(n)}$ are the optimal paths, provided the spectrum of the exponent is bounded by π. Moreover, if ${\mathcal{L}}$ is strictly convex, we prove that one-parameter subgroups are the unique optimal curves joining given endpoints. Finally, we also study the connection of these results with unitarily invariant metrics in ${\mathcal{U}(n)}$ as well as angular metrics in the Grassmann manifold. 相似文献
30.
A 1000 consonant–vowel–consonant structure logatoms corpus (CVC-structure), grouped in 10 phonetically equally balanced lists of 100 words each, was developed to satisfy the need of subjective assessment of speech intelligibility in American Spanish speaking environments. This corpus was tested and correlated with the Speech Transmission Index (STI) measurements to compare its articulation intelligibility score with other lists’ scores.Through the development of this work it was determined that in two different acoustically poor rooms that have the same STI (with STI < 0.50), the intelligibility score is lower when the articulation test is performed in a quiet room with high reverberation time than when it is performed in a very noisy room with low reverberation time. The final correlation curve of the American Spanish CVC-structure corpus was around 10% points higher than the CVCEQB curve obtained by Steeneken and Houtgast in 2002. 相似文献