全文获取类型
收费全文 | 455684篇 |
免费 | 3637篇 |
国内免费 | 1159篇 |
专业分类
化学 | 226442篇 |
晶体学 | 7026篇 |
力学 | 23028篇 |
综合类 | 14篇 |
数学 | 56372篇 |
物理学 | 147598篇 |
出版年
2021年 | 4211篇 |
2020年 | 4442篇 |
2019年 | 5148篇 |
2018年 | 7243篇 |
2017年 | 7532篇 |
2016年 | 9900篇 |
2015年 | 5370篇 |
2014年 | 9331篇 |
2013年 | 20749篇 |
2012年 | 16038篇 |
2011年 | 19165篇 |
2010年 | 14640篇 |
2009年 | 14767篇 |
2008年 | 17579篇 |
2007年 | 17423篇 |
2006年 | 15968篇 |
2005年 | 13928篇 |
2004年 | 13145篇 |
2003年 | 11737篇 |
2002年 | 11847篇 |
2001年 | 13822篇 |
2000年 | 10255篇 |
1999年 | 7970篇 |
1998年 | 6797篇 |
1997年 | 6570篇 |
1996年 | 6101篇 |
1995年 | 5406篇 |
1994年 | 5312篇 |
1993年 | 5201篇 |
1992年 | 5589篇 |
1991年 | 6006篇 |
1990年 | 5755篇 |
1989年 | 5718篇 |
1988年 | 5397篇 |
1987年 | 5584篇 |
1986年 | 5231篇 |
1985年 | 6614篇 |
1984年 | 6664篇 |
1983年 | 5620篇 |
1982年 | 5714篇 |
1981年 | 5417篇 |
1980年 | 5221篇 |
1979年 | 5686篇 |
1978年 | 5817篇 |
1977年 | 5867篇 |
1976年 | 5936篇 |
1975年 | 5564篇 |
1974年 | 5498篇 |
1973年 | 5668篇 |
1972年 | 4231篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
A formulation of an implicit characteristic-flux-averaging method for the unsteady Euler equations with real gas effects is presented. Incorporation of a real gas into a general equation of state is achieved by considering the pressure as a function of density and specific internal energy. The Ricmann solver as well as the flux-split algorithm are modified by introducing the pressure derivatives with respect to density and internal energy. Expressions for calculating the values of the flow variables for a real gas at the cell faces are derived. The Jacobian matrices and the eigenvectors are defined for a general equation of state. The solution of the system of equations is obtained by using a mesh-sequencing method for acceleration of the convergence. Finally, a test case for a simple form of equation of state displays the differences from the corresponding solution for an ideal gas. 相似文献
993.
E. S. Moskalenko M. Larsson K. F. Karlsson P. O. Holtz B. Monemar W. V. Schoenfeld P. M. Petroff 《Physics of the Solid State》2007,49(10):1995-1998
We report on low-temperature microphotoluminescence (μ-PL) measurements of InAs/GaAs quantum dots (QDs) exposed to a lateral
external electric field. It is demonstrated that the QDs’ PL signal could be increased severalfold by altering the external
and/or the internal electric field, which could be changed by an additional infrared laser. A model which accounts for a substantially
faster lateral transport of the photoexcited carriers achieved in an external electric field is employed to explain the observed
effects. The results obtained suggest that the lateral electric fields play a major role for the dot luminescence intensity
measured in our experiment—a finding which could be used to tailor the properties of QD-based optoelectronic applications.
The text was submitted by the authors in English. 相似文献
994.
We investigate two families of Lorenz-like three-dimensional nonlinear dynamical systems (i) the generalized Lorenz system and (ii) the Burke–Shaw system. Analytical investigation of the former system is possible under the assumption (I) which in fact concerns four different systems corresponding to = ±1, m = 0, 1. The fixed points and stability characteristics of the Lorenz system under the assumption (I) are also classified. Parametric and temporal (t → ∞) asymptotes are also studied in connection to the memory of both the systems. We calculate the Lyapunov exponents and Lyapunov dimension for the chaotic attractors in order to study the influence of the parameters of the Lorenz system on the attractors obtained not only when the assumption (I) is satisfied but also for other values of the parameters σ, r, b, ω and m. 相似文献
995.
V. S. Zuev 《Optics and Spectroscopy》2007,102(5):742-753
The results of experiments, in which the probability of spontaneous Raman scattering from a molecule adsorbed at the surface of a silver nanoparticle was found to grow by 14 and more orders of magnitude, are interpreted. The molecule occurs in the field of a surface plasmon, which accounts for an increase in the local field strength and the density of states by 12–13 orders of magnitude. An additional increase by 1–2 orders of magnitude is explained by the antenna effect of a pair of nanoparticles, one being ultimately small and the other, sufficiently large to serve a receiving-transmitting antenna. 相似文献
996.
The theoretical investigation of the image states in front of an ultrathin iron film grown on copper has been performed by means of the embedding method and a recently developed procedure for the inclusion of the image potential tail in a first principle calculation. From the electronic response to an applied electric field, the image plane position has been evaluated. This also allows one to obtain useful information about the spin dependent screening properties of the system. Exchange splitting, effective mass, and lifetime of such surface states result in good agreement with recently performed two-photon photoemission experiments [see A.B. Schmidt, M. Pickel, M. Wiemhöfer, M. Donath, M. Weinelt, Phys. Rev. Lett. 95 (2005) 107402]. 相似文献
997.
T. I. Khabakhpasheva 《Fluid Dynamics》2003,38(4):592-600
The problem of surface wave diffraction on a floating elastic plate is considered. The relation between the parameters of the elastic vibrations of the plate and the transmitted and reflected wave amplitudes is investigated. It is shown that the maximum amplitudes of the plate stresses and deflections depend nonmonotonically on the incident wave frequency and are reached simultaneously with the maxima of both the transmission coefficient and the length of the wave penetrating into the plate. This makes it possible to use the transmission coefficient as a parameter for investigating the maximum and minimum amplitudes of the hydroelastic vibrations of the plate. As an example, this criterion is used to minimize the vibrations of a plate whose leading edge is elastically connected to the bottom. 相似文献
998.
999.
Mary E. Neubert DAVID G. ABDALLAH Jr Sandra S. Keast Julie M. Kim Soonnam Lee Ryan M. Stayshich Margaret E. Walsh Rolfe G. Petschek Shin-Tson Wu 《Liquid crystals》2003,30(6):711-731
New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1-4; and X = C n H 2n + 1 , F, CF 3 or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF 3 when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices.
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
1000.
S. M. Natanzon 《Proceedings of the Steklov Institute of Mathematics》2007,259(1):137-148
We prove that the quaternionic miniversal deformations of an A
n singularity have the structure of a noncommutative Frobenius manifold in the sense of the extended cohomological field theory. 相似文献