全文获取类型
收费全文 | 19357篇 |
免费 | 2339篇 |
国内免费 | 2337篇 |
专业分类
化学 | 14398篇 |
晶体学 | 150篇 |
力学 | 1337篇 |
综合类 | 88篇 |
数学 | 599篇 |
物理学 | 7461篇 |
出版年
2024年 | 27篇 |
2023年 | 230篇 |
2022年 | 280篇 |
2021年 | 516篇 |
2020年 | 626篇 |
2019年 | 530篇 |
2018年 | 455篇 |
2017年 | 514篇 |
2016年 | 707篇 |
2015年 | 761篇 |
2014年 | 953篇 |
2013年 | 1483篇 |
2012年 | 1096篇 |
2011年 | 1464篇 |
2010年 | 1134篇 |
2009年 | 1286篇 |
2008年 | 1354篇 |
2007年 | 1409篇 |
2006年 | 1333篇 |
2005年 | 1072篇 |
2004年 | 978篇 |
2003年 | 778篇 |
2002年 | 782篇 |
2001年 | 526篇 |
2000年 | 495篇 |
1999年 | 445篇 |
1998年 | 399篇 |
1997年 | 346篇 |
1996年 | 274篇 |
1995年 | 275篇 |
1994年 | 230篇 |
1993年 | 206篇 |
1992年 | 177篇 |
1991年 | 149篇 |
1990年 | 121篇 |
1989年 | 83篇 |
1988年 | 88篇 |
1987年 | 65篇 |
1986年 | 60篇 |
1985年 | 56篇 |
1984年 | 50篇 |
1983年 | 20篇 |
1982年 | 45篇 |
1981年 | 28篇 |
1980年 | 30篇 |
1979年 | 19篇 |
1978年 | 16篇 |
1977年 | 14篇 |
1976年 | 17篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 843 毫秒
121.
122.
123.
124.
On the basis of the hydrodynamic equations for nonlinear elastic-gravity waves beneath a solid ice cover and their Hamiltonian representation, a three-wave kinetic equation for the time evolution of the wave spectrum is formulated. The properties of the kernel of the kinetic integral describing the nonlinear interactions between wave triplets are investigated. An algorithm for numerically calculating the kinetic integral is developed. The rate of nonlinear energy transfer over the wave spectrum is estimated quantitatively and its most important characteristics are found. 相似文献
125.
Shawn Shih 《中国化学会会志》2003,50(4):815-822
A large class of stereochemcial and related interactions in organic chemistry are repulsive and others are attractive, but the relative orientation of two methyl groups and the amount of energy required to twist one relative to the other (the hindered rotation energy barriers), or the alignment of such a group with respect to a conjugated ring to which it is attached (widely attributed to a mechanism called “hyperconjugation”) are estimated to be small in compared with the total energy of the molecule. We used theories of both isotropic and anisotropic proton hyperfine interactions in the π‐electron systems developed in the early sixties. They are approximated by the magnetic dipole nteractions between each proton and an electron spin magnetization that is distributed in 2s and 2p Slater atomic orbitals center on carbon atoms. We have extended these theories to the non‐planar olefinic cation radicals, which are very important in biochemistry as well as in petroleum catalysis. A three dimensional electron spin density equation has been developed in this paper to handle some Jahn‐Teller vibronic molecules. The new electron spin density equation related the observed proton hyperfine splittings to the non‐planar structures of the open‐chain alkene cation radicals generated by radiolysis and various chemical oxidation methods. The spin densities and the conformational calculations based on valence bond theory and symmetry principles are compared with some more elaborated molecular orbital calculations in the literature. The localized valence bond approaches are better in accord with our experimental results. The anomalous line‐width effect of the four methyl groups observed in the 2,3‐dimethyl‐2‐butene cation radicals also confirmed the positive sign of the electron‐proton hyperfine constant of hyper‐conjugation mechanism. A methyl substituent attached to a conjugated molecule often behaves as if it formed part of the region of conjugation; the charge appears to flow from the methyl group into the π electron system and it may also give rise to an appreciable dipole moment. Methylation also gives rise to an appreciable dipole moment, and the resultant red shift of electronic absorption bands is of some importance in the design of dye molecules. 相似文献
126.
D. Sasireka E. Palaniyandi K. Iyakutti 《International journal of quantum chemistry》2004,99(3):142-152
Local lattice relaxation of substitutional donors in silicon investigated using self‐consistent multiple scattering Xα (MSXα) method within the framework of the standard muffin‐tin potential approximation is extended to substitutional donors in germanium and substitutional acceptors in both silicon and germanium. Incorporating the effect of lattice relaxation surrounding the impurity makes the model suitable for both shallow and deep levels. Chemical trends of some aspects of impurity states, such as local lattice relaxation and charge transfer, of the impurities both in silicon and germanium are inferred. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 相似文献
127.
P. Schlagheck A. Buchleitner 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,22(3):401-415
We provide a detailed quantum treatment of the spectral characteristics and of the dynamics of nondispersive two-electron
wave packets along the periodically driven, collinear frozen planet configuration of helium. These highly correlated, long-lived
wave packets arise as a quantum manifestation of regular islands in a mixed classical phase space, which are induced by nonlinear
resonances between the external driving and the unperturbed dynamics of the frozen-planet configuration. Particular emphasis
is given to the dependence of the ionization rates of the wave packet states on the driving field parameters and on the quantum
mechanical phase space resolution, preceded by a comparison of 1D and 3D life times of the unperturbed frozen planet. Furthermore,
we study the effect of a superimposed static electric field component, which, on the grounds of classical considerations,
is expected to stabilize the real 3D dynamics against large (and possibly ionizing) deviations from collinearity.
Received 7 November 2002 / Received in final form 2 December 2002 Published online 28 January 2003 相似文献
128.
在电流及温度分布满足“不变性”原理的基础上,本文分析了等离子体中心热区的能量平衡,求出了电子热传导系数,根据实验数据拟合出HL-1装置电子热传导系数的定标关系为X_e=6.8(n_(eq1))~(-1.2)。此关系与其他托卡马克装置的结果类似。 相似文献
129.
Heat transfer in lattice BGK modeled fluid 总被引:1,自引:0,他引:1
The thermal lattice BGK model is a recently suggested numerical tool aiming at solving problems of thermohydrodynamics. The quality of the lattice BGK simulation is checked in this paper by calculating temperature profiles in the Couette flow under different Eckert and Mach numbers. A revised lower order model is proposed to improve the accuracy and the higher order model is proved to be advantageous in this respect, especially in the flow regime with a higher Mach number. 相似文献
130.
The current status of the R & D activities is presented on electron beam processing of polymers being carried out at TRCRE. Topics included are sterilization of medical products, graft polymers for selective separation or absorbents and curing of liquid prepolymer-monomer systems. 相似文献