全文获取类型
收费全文 | 416829篇 |
免费 | 4476篇 |
国内免费 | 2082篇 |
专业分类
化学 | 219757篇 |
晶体学 | 6626篇 |
力学 | 18442篇 |
综合类 | 61篇 |
数学 | 48198篇 |
物理学 | 130303篇 |
出版年
2020年 | 3333篇 |
2019年 | 3678篇 |
2018年 | 4858篇 |
2017年 | 4805篇 |
2016年 | 7045篇 |
2015年 | 4503篇 |
2014年 | 6962篇 |
2013年 | 17690篇 |
2012年 | 14001篇 |
2011年 | 17218篇 |
2010年 | 12213篇 |
2009年 | 12062篇 |
2008年 | 15992篇 |
2007年 | 16104篇 |
2006年 | 15217篇 |
2005年 | 13821篇 |
2004年 | 12557篇 |
2003年 | 11069篇 |
2002年 | 11007篇 |
2001年 | 12368篇 |
2000年 | 9459篇 |
1999年 | 7164篇 |
1998年 | 6092篇 |
1997年 | 5969篇 |
1996年 | 5830篇 |
1995年 | 5191篇 |
1994年 | 5202篇 |
1993年 | 5161篇 |
1992年 | 5505篇 |
1991年 | 5653篇 |
1990年 | 5325篇 |
1989年 | 5186篇 |
1988年 | 5142篇 |
1987年 | 5006篇 |
1986年 | 4849篇 |
1985年 | 6531篇 |
1984年 | 6740篇 |
1983年 | 5606篇 |
1982年 | 5920篇 |
1981年 | 5702篇 |
1980年 | 5341篇 |
1979年 | 5670篇 |
1978年 | 5916篇 |
1977年 | 5876篇 |
1976年 | 5941篇 |
1975年 | 5424篇 |
1974年 | 5583篇 |
1973年 | 5663篇 |
1972年 | 4177篇 |
1971年 | 3408篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Approximate method of calculating stresses in a rectangular plate locally loaded by a harmonic force
From double trigonometric series for bending moments the slowly converging components are picked out. When the loading facets are small compared with the width of the plate, they are approximately summed by the analytical method. This simplifies substantially the calculation of bending moments and stresses in the zone of small loading facets compared with the usual expansion with respect to the natural vibration forms.Translated from Teoreticheskaya i Prikladnaya Mekhanika, No. 20, pp. 84–88, 1989. 相似文献
993.
We give a review and a comparison of recent methods of analyzing circular and noncircular optical waveguides. Comparison among competing methodologies is made as follows: Galerkin's method is used with Laguerre-Gauss basis functions in circular geometry to examine the modal solution in a step index fiber, and comparison with the exact solution is made. A W-fiber, which has no exact solution, is then examined. Rectangular geometry is considered, and discussion centers on the use of Galerkin's method using trigonometric basis functions and Hermite-Gauss basis functions. Re difficulty arising from the use of basis functions that do not decay exponentially for large argument (trigonometric functions) is illustrated. Finally, a square step index waveguide is used to illustrate a comparison between a variational method that uses the Gaussian approximation as the starting point, and Galerkin's method using Hermite-Gauss basis functions. We conclude that the variational method does well in predicting the propagation constant β but does not do well in predicting the modal field. 相似文献
994.
Tunnelling in periodically driven bistable symmetric potential wells is investigated in an analytical approximation in a domain where the driving frequency is large compared to the tunnelling frequency and only the four lowest lying levels contribute significantly. The influence of finite level widths is taken into account, and a smooth variation of the amplitude of the driving field is allowed for. 相似文献
995.
996.
The Borel-Weil (BW) construction for unitary irreps of a compact Lie group is extended to a construction of all unitary irreps of the quantum groupU
q(n). Thisq-BW construction uses a recursion procedure forU
q(n) in which the fiber of the bundle carries an irrep ofU
q(n–1)×U
q(1) with sections that are holomorphic functions in the homogeneous spaceU
q(n)/U
q(n–1)×U
q(1). Explicit results are obtained for theU
q(n) irreps and for the related isomorphism of quantum group algebras.Supported in part by the National Science Foundation, No. PHY-9008007 相似文献
997.
P. Recht 《Mathematical Methods of Operations Research》1992,36(3):201-210
In this paper we present a concept of the construction of generalized gradients by considering a development of directional derivatives into spherical harmonics. This leads to a derivation system as a system of generalized partial derivatives. Necessary conditions for local extrema for a broad class of not necessarily differentiable function can be given and a characterization of points of differentiability can be proved by using generalized gradients. 相似文献
998.
999.
The rheological behavior of a mixture of two liquid-crystal polymers, hydroxypropyl cellulose and ethyl cellulose, in acetone solution is studied. The total polymer concentration in the solvent is held constant (40%) as the ratio of the two polymers is varied. The mixtures are anisotropic, isotropic, or biphasic (isotropic/anisotropic), depending on the concentration. Curves of viscosity vs shear rate for all the mixtures studied show three regions of viscosity, as described by Onogi and Asada for liquid-crystal polymers. The viscosity as a function of the weight ratio of the two polymers at constant shear rate exhibits deviations from additivity of viscosities of the two components at all concentrations. In mixtures of two polymers in the melt, deviations are also observed; the negative ones are attributed to phase separation and the positive ones to homogenous mixing (comparison with the phase diagram). All the mixtures studied (anisotropic, isotropic, or biphasic), show ranges of shear rates where the first normal-stress difference is negative, as is generally observed for anisotropic liquid-crystal polymers. It is concluded that the isotropic solutions become anisotropic under shear, as they are not far from the critical concentration. © 1994 John Wiley & Sons, Inc. 相似文献
1000.
Michael S. Kent Matthew Tirrell Timothy P. Lodge 《Journal of Polymer Science.Polymer Physics》1994,32(11):1927-1941
Solution properties for random and diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) have been measured by dynamic and total intensity light scattering in solvents of differing quality. The results are compared with the corresponding properties for PS and PMMA homopolymers of similar molecular weight, in order to determine if interactions between unlike monomers are significant. The hydrodynamic radius (Rh) and diffusion second virial coefficient (kd) for the random copolymer are found to be larger than the corresponding values for the homopolymers in a solvent which is near-theta for the two homopolymers, whereas no such effect is observed for the block copolymer. This suggests that most intrachain interactions occur a relatively short distance along the chain backbone. In a mutual good solvent Rh and kd of the random copolymer are comparable to the average of the values for the homopolymers, indicating that in a good solvent monomer/solvent interactions dominate over monomer/monomer interactions. For an isolated diblock copolymer in a mutual good solvent, there is no evidence that interactions between unlike monomers lead to additional expansion of the entire molecule, as measured by Rh, nor expansion of the individual blocks as probed by light scattering with one block optically masked. However, at low but finite concentration there is evidence (the coefficients of the binary interaction terms in the viscosity and the mutual diffusion coefficient, and the second and third virial coefficients) that a weak ordering effect may exist in block copolymer solutions, far from the conditions where microphase separation occurs. Finally, measurements of ternary polymer-polymer-solvent solutions show no dependence on monomer composition or monomer distribution for the tracer diffusion of probe PS-PMMA copolymers in a PMMA/toluene matrix. This indicate that the frictional interaction is largely unaffected by interactions between unlike monomers. However, there is evidence that the thermodynamic interaction is more unfavorable between a random copolymer and the homopolymer matrix than between a diblock and the matrix. © 1994 John Wiley & Sons, Inc. 相似文献