全文获取类型
收费全文 | 15225篇 |
免费 | 1813篇 |
国内免费 | 1900篇 |
专业分类
化学 | 5512篇 |
晶体学 | 111篇 |
力学 | 2137篇 |
综合类 | 256篇 |
数学 | 8305篇 |
物理学 | 2617篇 |
出版年
2024年 | 46篇 |
2023年 | 174篇 |
2022年 | 233篇 |
2021年 | 415篇 |
2020年 | 526篇 |
2019年 | 470篇 |
2018年 | 468篇 |
2017年 | 572篇 |
2016年 | 676篇 |
2015年 | 429篇 |
2014年 | 709篇 |
2013年 | 1382篇 |
2012年 | 845篇 |
2011年 | 843篇 |
2010年 | 711篇 |
2009年 | 915篇 |
2008年 | 957篇 |
2007年 | 1010篇 |
2006年 | 862篇 |
2005年 | 732篇 |
2004年 | 684篇 |
2003年 | 618篇 |
2002年 | 620篇 |
2001年 | 510篇 |
2000年 | 492篇 |
1999年 | 481篇 |
1998年 | 387篇 |
1997年 | 321篇 |
1996年 | 269篇 |
1995年 | 254篇 |
1994年 | 220篇 |
1993年 | 189篇 |
1992年 | 124篇 |
1991年 | 132篇 |
1990年 | 93篇 |
1989年 | 74篇 |
1988年 | 61篇 |
1987年 | 57篇 |
1986年 | 46篇 |
1985年 | 64篇 |
1984年 | 54篇 |
1983年 | 26篇 |
1982年 | 38篇 |
1981年 | 32篇 |
1980年 | 27篇 |
1979年 | 26篇 |
1978年 | 13篇 |
1977年 | 21篇 |
1976年 | 11篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
941.
942.
Björn Birnir 《Journal of statistical physics》2007,128(1-2):535-568
A system of ordinary differential equations (ODEs) is derived from a discrete system of Vicsek, Czirók et al. [Phys. Rev. Lett.
75(6):1226–1229, 1995], describing the motion of a school of fish. Classes of linear and stationary solutions of the ODEs are
found and their stability explored using equivariant bifurcation theory. The existence of periodic and toroidal solutions
is also proven under deterministic perturbations and structurally stable heteroclinic connections are found. Applications
of the model to the migration of the capelin, a pelagic fish that undertakes an extensive migration in the North Atlantic,
are discussed and simulation of the ODEs presented. 相似文献
943.
D.H. Delphenich 《Annalen der Physik》2007,16(9):615-639
An alternative to the representation of complex relativity by self‐dual complex 2‐forms on the spacetime manifold is presented by assuming that the bundle of real 2‐forms is given an almost‐complex structure. From this, one can define a complex orthogonal structure on the bundle of 2‐forms, which results in a more direct representation of the complex orthogonal group in three complex dimensions. The geometrical foundations of general relativity are then presented in terms of the bundle of oriented complex orthogonal 3‐frames on the bundle of 2‐forms in a manner that essentially parallels their construction in terms of self‐dual complex 2‐forms. It is shown that one can still discuss the Debever‐Penrose classification of the Riemannian curvature tensor in terms of the representation presented here. 相似文献
944.
Viscous potential flow analysis of Kelvin-Helmholtz instability with heat and mass transfer has been studied. A dispersion relation has been obtained. Stability criterion is given by a critical value of relative velocity. It has been found that heat and mass transfer has destabilizing effect on relative velocity when lower fluid viscosity is low while it has stabilizing effect when lower fluid viscosity is high. Various graphs have been plotted for relative velocity and growth rate. In statically unstable situation viscosity has stabilizing effect while in statically stable situation it has destabilizing effect. 相似文献
945.
A new kind of generalized synchronization of two chaotic systems with uncertain parameters is proposed. Based on a pragmatical asymptotical stability theorem and an assumption of equal probability for ergodic initial conditions, an adaptive control law is derived so that it can be proved strictly that the common null solution of error dynamics and of parameter dynamics is actually asymptotically stable, i.e. these two identical systems are in generalized synchronization and the estimated parameters approach the uncertain values. It is called pragmatical generalized synchronization. Finally, two numerical examples are studied for two Quantum-CNN oscillator chaotic systems to show the effectiveness of the proposed generalized synchronization strategy with a double Duffing chaotic system as a goal system. 相似文献
946.
This paper is concerned with the analysis problem for the exponential stability of a class of Cohen-Grossberg neural networks with variable and distributed delays. Some sufficient conditions ensuring the existence, uniqueness and exponential stability of the equilibrium point are obtained by employing Brouwer’s fixed-point theorem and by applying the inequality technique. In the results, we do not assume that the activation function satisfies the boundedness and the Lipschitz condition. Three numerical examples are given to show the effectiveness of the obtained results. 相似文献
947.
The aim of this paper is to investigate the effects of external optical injection taking account of polarization and electron spin properties in vertical-cavity surface-emitting lasers (VCSELs). Using external polarized injection we seek the locked phases and amplitudes of specific polarized fields in terms of injection level and frequency detuning, taking account of two kinds of distinguishable carrier density (spin-up and spin-down). For the conventional form of optical injection without taking account of spin-polarized fields there are three fundamental equations describing the carrier density, field amplitude and phase. However, by using the spin flip model (SFM), the combined effect of polarized fields along two perpendicular crystal axes and electron spin properties results in six equations. We analyse the conditions for stable locking and also the influence of birefringence effects on the stability map of detuning versus optical injection for both cases of injection polarized parallel and perpendicular to the lasing mode of the solitary VCSEL. For given values of pumping and spin relaxation rate there is a minimum birefringence rate for orthogonal injection. Above this value three regions of elliptical polarization are found in the stability map, namely “quasi-stability” (QS), “coupled limit cycle” (CLC) and “coupled chaos” (CC). The three regions of linear polarization, namely chaos, limit cycle and stability, are reduced in area compared to the case of parallel injection. For orthogonal injection it is found that increased birefringence or reduced spin relaxation rate causes the stable locking region to begin at higher injected power and frequency detuning. 相似文献
948.
CHANG Da-Wei 《理论物理通讯》2007,47(6):1143-1144
In this note, we would like to point out that (i) of Corollary 1 given by Zhang et al. (cf Commun. Theor. Phys. 39 (2003) 381) is incorrect in general. 相似文献
949.
Chadwick C. Rasmussen Sulabh K. Dhanuka James F. Driscoll 《Proceedings of the Combustion Institute》2007,31(2):2505-2512
Laser-induced fluorescence of OH and CH2O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH2O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal. 相似文献
950.
在LiNbO3中掺进0.015mass%MnCO3和 0.1mass%CeO2,以Czchralski法生长Li/Nb比为1.38的近化学计量比Ce∶Mn∶LiNbO3(Ce∶Mn∶SLN)晶体和Li/Nb比为0.946的同成分Ce∶Mn∶LiNbO3(Ce∶Mn∶CLN)晶体.测试了晶体的红外光谱和紫外可见吸收光谱,讨论了Ce∶Mn∶SLN晶体OH-吸收峰和吸收边移动机理.利用二波耦合光路测试了晶体的指数增益系数,Ce∶Mn∶SLN晶体指数增益系数Γ达到27 cm2-.同时,研究了Ce∶Mn∶SLN指数增益系数提高的机理. 相似文献