全文获取类型
收费全文 | 438409篇 |
免费 | 4201篇 |
国内免费 | 1476篇 |
专业分类
化学 | 202282篇 |
晶体学 | 6139篇 |
力学 | 16099篇 |
综合类 | 5篇 |
数学 | 44744篇 |
物理学 | 117367篇 |
无线电 | 57450篇 |
出版年
2020年 | 3304篇 |
2019年 | 3651篇 |
2018年 | 4501篇 |
2017年 | 4452篇 |
2016年 | 7013篇 |
2015年 | 4392篇 |
2014年 | 6990篇 |
2013年 | 18578篇 |
2012年 | 13396篇 |
2011年 | 16341篇 |
2010年 | 11557篇 |
2009年 | 12111篇 |
2008年 | 16107篇 |
2007年 | 16459篇 |
2006年 | 15580篇 |
2005年 | 14517篇 |
2004年 | 13358篇 |
2003年 | 12040篇 |
2002年 | 11881篇 |
2001年 | 13328篇 |
2000年 | 10590篇 |
1999年 | 8600篇 |
1998年 | 7446篇 |
1997年 | 7355篇 |
1996年 | 6919篇 |
1995年 | 6533篇 |
1994年 | 6372篇 |
1993年 | 6433篇 |
1992年 | 6788篇 |
1991年 | 6761篇 |
1990年 | 6413篇 |
1989年 | 6198篇 |
1988年 | 6134篇 |
1987年 | 5413篇 |
1986年 | 5128篇 |
1985年 | 6776篇 |
1984年 | 6962篇 |
1983年 | 5933篇 |
1982年 | 6277篇 |
1981年 | 5965篇 |
1980年 | 5817篇 |
1979年 | 5927篇 |
1978年 | 6178篇 |
1977年 | 5983篇 |
1976年 | 6197篇 |
1975年 | 5653篇 |
1974年 | 5756篇 |
1973年 | 6251篇 |
1972年 | 3992篇 |
1971年 | 3236篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
We show that, in dimensions equal or greater than three, classical strings do not join or split. This is part of a no-interaction theorem that is also proved. The quantum theory is considered. 相似文献
82.
Faulkner D.W. Payne D.B. Stern J.R. Ballance J.W. 《Lightwave Technology, Journal of》1989,7(11):1741-1751
Some of the options for optical technology within the local loop environment are examined. In particular, passive shared access networks have been considered in some detail. These networks show great promise for delivering existing telephone services to small to medium business customers (4-30 lines) economically by the early 1990s. Extending fiber to the home will also be possible by virtue of a similar passive network infrastructure for customers requiring new broadband services beyond the single telephone line. For one-line plain old telephone service (POTS) customers, an intermediate approach of terminating the fiber network at the final network distribution point, with copper retained for the final leg, may be used prior to the provision of broadband services. A key feature of the passive optical network architecture is the use of wavelength-division multiplexing (WDM) as an upgrade strategy, allowing graceful upgrading from telephone services to multichannel high-definition television (HDTV) on gigabit/second bearers and full two-way switched broadband services employing wavelength routing across the network 相似文献
83.
84.
D. N. Karimov E. A. Krivandina Z. I. Zhmurova B. P. Sobolev V. A. Bezhanov S. P. Chernov G. M. Shapochkin 《Crystallography Reports》2006,51(6):1009-1015
Crystalline materials that are transparent in the vacuum UV spectral region and currently used have been reviewed. Transmission of crystals of solid solutions with the fluorite structure Ca1?x R xF2+x (R = Sc, Y, La, Yb, Lu) in the UV and vacuum UV spectral regions has been investigated. It is shown that application of different methods of purification of fluorides from some impurities can significantly improve the optical quality of fluoride multicomponent crystals in the short-wavelength spectral region. 相似文献
85.
Sarto M.S. Voti R.L. Sarto F. Larciprete M.C. 《Electromagnetic Compatibility, IEEE Transactions on》2005,47(3):602-611
New nanolayered coatings are designed and deposited on flexible plastic substrate having the thickness of 100 /spl mu/m, in order to realize lightweight ultrathin transparent shielding foils. The structure of the coating is optimized considering three figures of merit: the average transmittance in the visible range for normal incidence, the normalized average transmittance for oblique incidence at 550 nm, and the transmittance quality factor. The nanotechnology exploited for the deposition of the transparent metals is the dual ion beam sputtering. Tests of durability, optical transmission, and shielding effectiveness demonstrate that the film has a high adhesion under mechanical solicitation, high resistance against aging, peak transmittance in the visible range higher than 70%, omnidirectional properties in the range 0/spl deg/-60/spl deg/, and shielding effectiveness of 40 dB up to 6 GHz. 相似文献
86.
R. Salomon 《Zeitschrift für Physik B Condensed Matter》1989,73(4):519-527
The diffusion constant and the diagonal conductivity for non-interacting electrons in a two-dimensional, disordered system are studied. A homogeneous magnetic field perpendicular to the electron system is assumed. For weak short-range random potentials and high fields the Landau quantum numbern can be used as expansion parameter. In the limit of high Landau levels the system shows metallic behaviour. Corrections for finiten decrease the conductivity and indicate localized states in the whole energy band. A breakdown of the expansion and stronger localization are observed only for the lowest Landau levels if the typical experimental length scale of the quantized Hall effect is used. 相似文献
87.
H E Cline W E Lorensen R J Herfkens G A Johnson G H Glover 《Magnetic resonance imaging》1989,7(1):45-54
A three-dimensional examination of blood vessels is provided using MR data from seven cases. The vascular surfaces are constructed with an algorithm that automatically follows the selected artery or vein and generates a projected three-dimensional gradient shaded image. Fast 3DFT pulse sequences were optimized to enhance the time-of-flight contrast of the intravascular region. By increasing the surface threshold value in a three-dimensional head study, the flesh of a patient's face was peeled away to demonstrate the superfacial temporal artery. Gated cardiac images show the great vessels and cardiac chambers. A three-dimensional view of the aorta shows an irregular surface in the vicinity of an adrenal tumor. 3D MR exams provide a non-invasive technique for assessing vascular morphology in a clinical setting. 相似文献
88.
Peak-to-average power reduction in space division multiplexing based OFDM systems through spatial shifting 总被引:2,自引:0,他引:2
A new method to reduce the peak-to-average power ratio (PAPR) in space division multiplexing systems applying orthogonal frequency division multiplexing is proposed. The method applies spatial shifting to partial transmit sequences to achieve a decreased PAPR on all transmit branches. 相似文献
89.
Alinur Büyükaksoy 《电信纪事》1985,40(7-8):402-410
An asymptotic solution to the problem of the diffraction of high-frequency waves by the edge of a cylindrically curved surface whose convex and concave faces present soft and hard boundary conditions is obtained. The results permit us to define new diffraction coefficients related to this kind of mixed boundary conditions. When the radius of the curvature becomes infinitely large, the edgediffraction coefficient reduces to this known for the softhard half-plane. 相似文献
90.