全文获取类型
收费全文 | 1288篇 |
免费 | 35篇 |
国内免费 | 9篇 |
专业分类
化学 | 819篇 |
晶体学 | 7篇 |
力学 | 88篇 |
数学 | 147篇 |
物理学 | 271篇 |
出版年
2022年 | 26篇 |
2021年 | 19篇 |
2020年 | 7篇 |
2019年 | 16篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 25篇 |
2015年 | 15篇 |
2014年 | 33篇 |
2013年 | 54篇 |
2012年 | 74篇 |
2011年 | 95篇 |
2010年 | 41篇 |
2009年 | 62篇 |
2008年 | 79篇 |
2007年 | 62篇 |
2006年 | 70篇 |
2005年 | 67篇 |
2004年 | 35篇 |
2003年 | 44篇 |
2002年 | 39篇 |
2001年 | 27篇 |
2000年 | 29篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 18篇 |
1996年 | 33篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 23篇 |
1992年 | 21篇 |
1991年 | 23篇 |
1990年 | 15篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1978年 | 7篇 |
1977年 | 24篇 |
1976年 | 11篇 |
1975年 | 5篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1972年 | 5篇 |
排序方式: 共有1332条查询结果,搜索用时 15 毫秒
21.
22.
An explicit expression of the Wigner operator is derived, such that the Wigner function of a quantum state is equal to the expectation value of this operator with respect to the same state. This Wigner operator leads to a representation-independent procedure for establishing the correspondence between the inhomogeneous symplectic group applicable to linear canonical transformations in classical mechanics and the Weyl-metaplectic group governing the symmetry of unitary transformations in quantum mechanics. 相似文献
23.
24.
C. S. Yeh N. Urimindi Jin Liu G. A. Evans J. K. Butler 《International Journal of Infrared and Millimeter Waves》1993,14(4):885-892
This paper concerns the experimental characteristics of metal coated dielectric waveguides with a rectangular surface corrugation. Waveguide are designed to operate at a second Bragg frequency of 90 GHz. The period, height and the duty cycle of a rectangular grating were calculated using the chosen frequency. A metallic layer of aluminum is sputtered on one side of the slab waveguide. The purpose of the metallic layer is to simulate a layer of high density plasma on the surface of the waveguide similar to that obtained by optical excitation of semiconductor structures. Experiments were performed to examine the far field radiation pattern, attenuation constant and the dispersion relation. Due to the presence of the plasma layer there will be an angular shift in the far field radiation pattern. We have observed angular shifts of about 20 in the radiation pattern of the waveguide before and after coating. Measurements are made in the frequency range of 88–95 GHz. This waveguide structure can be used to design an electronically steerable antenna and an electronic phase shifter operating in the millimeter-wave frequency band.Supported in part by the Army Research Office. 相似文献
25.
26.
27.
Bogert D Burnstein R Fisk R Fuess S Morfin J Ohska Stutte L Walker JK Bofill J Busza W Eldridge T Friedman JI Goodman MC Kendall HW Kostoulas IG Lyons T Magahiz R Mattison T Mukherjee A Osborne L Pitt R Rosenson L Sandacz A Tartaglia M Taylor FE Verdier R Whitaker S Yeh GP Abolins M Brock R Cohen A Ernwein J Owen D Slate J Weerts H 《Physical review letters》1985,55(6):574-577
28.
Bright (maximum 10034 cd m(-2), 455 cd m(-2) at 20 mA cm(-2)) and efficient (maximum 2.4% at 4 mA cm(-2)) red (lambda(max)el 634-636 nm) organic light-emitting diodes employ arylamino-substituted fumaronitrile as the novel host emitter, which is readily prepared and easily purified. 相似文献
29.
We report a ruthenium-catalyzed reaction for various 3-benzyl but-1-ynyl ethers with suitable functionalities. Treatment of these substrates with TpRu(PPh3)(CH3CN)2PF6 (8.0 mol %) catalyst in 1,2-dichloroethane (80 degrees C, 12 h) afforded functionalized 1,3-dienes and benzyl aldehyde in good yields. This process is considered to be a tandem dealkoxylation and transfer hydrogenation. Deuterium-labeling experiments reveal that the migration of different hydrogen atoms proceeds regiospecifically. A plausible mechanism is proposed on the basis of the results of isotope experiment. 相似文献
30.
The boundary effect on the drag on two identical, nonuniformly structured flocs moving along the axis of a cylindrical tube
filled with a Newtonian fluid is investigated at a small to medium larger Reynolds number. A two-layer model is adopted to
simulate various possible structures of a floc, and the flow field inside is described by Darcy–Brinkman model. The results
of numerical simulation reveal that a convective flow is present in the rear region of a floc when Reynolds number is on the
order of 40. The presence of the tube wall and/or the porous structure of a floc has the effect of reducing that convective
flow. For a fixed level of the volume-average permeability of a floc, the influence of the tube wall on the drag depends upon
floc structure; the influence on a nonuniformly structured floc is more significant than that on a uniformly structured floc.
The more nonuniform the floc structure, the more appreciable the deviation of the drag coefficient–Reynolds number curve from
a Stokes’-law-like relation becomes. The smaller the volume-average permeability of a floc and/or the smaller the separation
distance between the two flocs, the greater is the deviation, but the presence of the tube wall has the effect of reducing
that deviation. 相似文献