全文获取类型
收费全文 | 417736篇 |
免费 | 2805篇 |
国内免费 | 716篇 |
专业分类
化学 | 197494篇 |
晶体学 | 6471篇 |
力学 | 23677篇 |
综合类 | 13篇 |
数学 | 58268篇 |
物理学 | 135334篇 |
出版年
2021年 | 4061篇 |
2020年 | 4411篇 |
2019年 | 5346篇 |
2018年 | 7786篇 |
2017年 | 7922篇 |
2016年 | 10255篇 |
2015年 | 4837篇 |
2014年 | 9111篇 |
2013年 | 17417篇 |
2012年 | 14171篇 |
2011年 | 16631篇 |
2010年 | 13566篇 |
2009年 | 13842篇 |
2008年 | 15936篇 |
2007年 | 15684篇 |
2006年 | 13977篇 |
2005年 | 12202篇 |
2004年 | 11740篇 |
2003年 | 10756篇 |
2002年 | 10891篇 |
2001年 | 11165篇 |
2000年 | 8654篇 |
1999年 | 6526篇 |
1998年 | 5946篇 |
1997年 | 5854篇 |
1996年 | 5369篇 |
1995年 | 4693篇 |
1994年 | 4851篇 |
1993年 | 4825篇 |
1992年 | 5034篇 |
1991年 | 5457篇 |
1990年 | 5328篇 |
1989年 | 5340篇 |
1988年 | 5104篇 |
1987年 | 5103篇 |
1986年 | 4786篇 |
1985年 | 5803篇 |
1984年 | 6070篇 |
1983年 | 5131篇 |
1982年 | 5331篇 |
1981年 | 5004篇 |
1980年 | 4629篇 |
1979年 | 5334篇 |
1978年 | 5355篇 |
1977年 | 5623篇 |
1976年 | 5672篇 |
1975年 | 5327篇 |
1974年 | 5161篇 |
1973年 | 5365篇 |
1972年 | 4377篇 |
排序方式: 共有10000条查询结果,搜索用时 937 毫秒
861.
V. Ballarini G. Barucca E. Bennici C. F. Pirri C. Ricciardi E. Tresso F. Giorgis 《Physica E: Low-dimensional Systems and Nanostructures》2003,16(3-4):591
All amorphous silicon-nitride planar optical microcavities operating in the visible range have been grown by plasma enhanced chemical vapor deposition. The luminescence intensity of the N-rich silicon-nitride layer from a microcavity with 6 period distributed Bragg reflectors (DBRs) is two order of magnitude higher than that of the luminescent layer without the cavity. Moreover, a strong directionality of the microcavities emission can be observed. Such results can be ascribed to the anisotropic optical density of states induced in the Fabry–Perot structure. The quality factors of the resonators are strictly correlated to the number of periods of the DBRs. 相似文献
862.
O. V. Sizova E. P. Sokolova V. I. Baranovskii D. A. Rozmanov O. A. Tomashenko 《Journal of Structural Chemistry》2004,45(5):762-770
Ab initio MP2 calculations with several basis sets proved the existence of a stable benzonitrile dimer with a planar structure and a short contact between the H atom of one molecule and the N atom of another. The structure is greatly stabilized by attraction between the neighboring oppositely directed dipoles and donor-acceptor interaction between the orbital of the lone electron pair of the N′ atom and the vacant antibonding orbital localized on the C-H bond.Original Russian Text Copyright © 2004 by O. V. Sizova, E. P. Sokolova, V. I. Baranovskii, D. A. Rozmanov, and O. A. Tomashenko__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 807–815, September–October, 2004. 相似文献
863.
Relationships between the electrodynamic (wave propagation constants) and quantum-mechanical parameters (probability of induced transitions) of interaction between energy quanta and particles of the medium are derived for dipole and paramagnetic interaction types. It is demonstrated that quantum-mechanical parameters are generally complex. 相似文献
864.
We prove that any irreducible faithful representation of an almost torsion-free Abelian group G of finite rank over a finitely generated field of characteristic zero is induced from an irreducible representation of a finitely generated subgroup of the group G. 相似文献
865.
The structure, harmonic frequencies, and nonresonance Raman intensities for porphin, tetraazaporphin (TAP), and three of its isotopomers are calculated by the density-functional theory of B3LYP/6-31G(d). Scaling of force constants for porphin in nonredundant natural coordinates is performed. The scaling factors obtained were used to predict the force field and normal modes of TAP and three of its isotopomers. Two alternative methods are used to carry out reliable assignment of the TAP frequencies: wavenumber-linear scaling method and frequency-shift method. There is good agreement between the frequencies predicted within the framework of the three methods used. The conservativeness of the out-of-plane B
2g
- and B
3g
-modes for porphin and TAP is examined. The Raman spectrum for TAP is simulated. A refinement of the assignment of the experimental frequencies for TAP of even symmetry types on the basis of the calculations performed is made. 相似文献
866.
A previously developed laser spallation technique has been modified to measure the tensile strength of thin film interfaces in-situ at temperatures up to 1100°C. Tensile strengths of Nb/A-plane sapphire, FeCrAl/A-plane sapphire and FeCrAlY/A-plane sapphire were measured up to 950°C. The measured strengths at high temperatures were substantially lower compared with their corresponding strengths at ambient temperature. For example, at 850°C, the interface tensile strength for the Nb/sapphire (151 ± 17 MPa), FeCrAl/sapphire (62 ± 8 MPa) and FeCrAlY/sapphire (82 ± 11 MPa) interface systems were lower by factors of approximately, 3, 5, and 8, respectively, over their corresponding ambient values. These results underscore the importance of using such in-situ measured values under operating conditions as the failure criterion in any life prediction or reliability models of such coated systems where local interface temperature excursions are expected. The results on alloy film interfaces also demonstrate that the presence of Y increases the strength of FeCrAl/Al2O3 interfaces. 相似文献
867.
868.
V. N. Popov 《Technical Physics》2003,48(5):541-545
The slip velocity of a rarefied gas nonuniform in temperature and mass velocity is calculated for gas slip over the surface
of a right circular cylinder. The calculation uses the two-moment boundary condition in an approximation linear in Knudsen
number. Corrections to the slip velocity that are due to the interface curvature, volume temperature stresses, and nonuniform
temperature distribution in the Knudsen layer are studied as func-tions of the accommodation coefficients in the first two
moments of the distribution function. The Bhatnagar-Gross-Krook model of the Boltzmann kinetic equation is employed as the
basic equation for the gas state. 相似文献
869.
The synthesis and characterization of a new homologous series of compounds, the 2-cyano-1,3-phenylene bis[4-(4-n-alkoxyphenyliminomethyl)benzoates] derived from 2-cyanoresorcinol is reported. All the compounds are enantiotropic mesogens and exhibit the fascinating B7 mesophase. The characterization of the mesophase was performed using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies. 相似文献
870.
V. Z. Kasimov O. V. Ushakova Yu. P. Khomenko 《Journal of Applied Mechanics and Technical Physics》2003,44(5):612-619
An improved procedure for modeling the operation of a light-gas gun is proposed. The motion of working bodies in both the firing chamber and the light-gas chamber is studied within the framework of the mechanics of heterogeneous media. The problem of barrel heating taking into account its melting and removal of thermal ablation products into the medium inside the bore is solved in a coupled formulation. Heat and mass transfer and friction on the barrel surface are calculated using empirical dependences. The deformable piston is considered compressible and elastoviscoplastic. Allowance is made for the presence of a clearance between the lateral surface of the piston and the barrel bore walls and the associated gas flow between the firing and the light-gas chamber. Calculation results are given. 相似文献