全文获取类型
收费全文 | 598295篇 |
免费 | 6003篇 |
国内免费 | 2050篇 |
专业分类
化学 | 262119篇 |
晶体学 | 7485篇 |
力学 | 25191篇 |
综合类 | 10篇 |
数学 | 83148篇 |
物理学 | 148688篇 |
无线电 | 79707篇 |
出版年
2020年 | 3498篇 |
2019年 | 3626篇 |
2018年 | 15253篇 |
2017年 | 14892篇 |
2016年 | 13687篇 |
2015年 | 5941篇 |
2014年 | 7949篇 |
2013年 | 21704篇 |
2012年 | 19736篇 |
2011年 | 30212篇 |
2010年 | 19623篇 |
2009年 | 20287篇 |
2008年 | 26281篇 |
2007年 | 29226篇 |
2006年 | 19854篇 |
2005年 | 19623篇 |
2004年 | 18347篇 |
2003年 | 17064篇 |
2002年 | 15777篇 |
2001年 | 16645篇 |
2000年 | 13408篇 |
1999年 | 10842篇 |
1998年 | 9343篇 |
1997年 | 8959篇 |
1996年 | 8879篇 |
1995年 | 8240篇 |
1994年 | 7884篇 |
1993年 | 7712篇 |
1992年 | 8286篇 |
1991年 | 8109篇 |
1990年 | 7601篇 |
1989年 | 7204篇 |
1988年 | 7332篇 |
1987年 | 6307篇 |
1986年 | 6015篇 |
1985年 | 8216篇 |
1984年 | 8356篇 |
1983年 | 7025篇 |
1982年 | 7380篇 |
1981年 | 7194篇 |
1980年 | 6961篇 |
1979年 | 7078篇 |
1978年 | 7171篇 |
1977年 | 7045篇 |
1976年 | 6985篇 |
1975年 | 6793篇 |
1974年 | 6643篇 |
1973年 | 6949篇 |
1972年 | 4297篇 |
1971年 | 3354篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
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
92.
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
93.
It has been demonstrated recently that it is possible to guide broadband Terahertz pulses utilising a novel coaxial waveguide. This technique is hindered by the difficulty in exciting the radially polarised fundamental mode of the waveguide. To optimise mode matching, a novel radially symmetric photoconductive Terahertz antenna is proposed. Using finite element method simulations and analytical calculations, it is shown that this antenna can generate the desired radial polarisation. 相似文献
94.
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
95.
96.
97.
98.
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
99.
Seo K. Heiblum M. Knoedler C.M. Oh J.E. Pamulapati J. Bhattacharya P. 《Electron Device Letters, IEEE》1989,10(2):73-75
A high-gain ballistic hot-electron device is described. The GaAs-AlGaAs heterostructure device, with a 21-mm-thick pseudomorphic In 0.12Ga0.88As base, had a current gain of 27 at 77 K and 41 at 4.2 K. As characteristically seen in ballistic devices, transfer into the L valley limited the maximum gain. The Γ-L valley separation in the strained In0.12Ga0.88As was estimated to be about 380 meV 相似文献
100.
Andrew Barton 《The Journal of the Operational Research Society》1987,38(10):965-974
This paper describes some work carried out in the Scientific Research and Development Branch (SRDB) of the Home Office, intended to contribute to an overall aim of building up internal expertise in the field of expert systems. This was done by carrying out a number of pilot and demonstrator projects, two of which are described in some detail. The plan, which proved successful, was to build as far as possible on the relevant skills already possessed by an OR group in the branch. The lessons learned from these are summarized in the hope that they will be of use to other groups who wish to become involved in this important area. 相似文献