全文获取类型
收费全文 | 780465篇 |
免费 | 9941篇 |
国内免费 | 2539篇 |
专业分类
化学 | 368752篇 |
晶体学 | 9596篇 |
力学 | 29065篇 |
综合类 | 20篇 |
数学 | 81866篇 |
物理学 | 198760篇 |
无线电 | 104886篇 |
出版年
2021年 | 6149篇 |
2020年 | 6733篇 |
2019年 | 7069篇 |
2018年 | 8398篇 |
2017年 | 8107篇 |
2016年 | 13398篇 |
2015年 | 9061篇 |
2014年 | 13586篇 |
2013年 | 33684篇 |
2012年 | 26065篇 |
2011年 | 31973篇 |
2010年 | 22151篇 |
2009年 | 23000篇 |
2008年 | 31066篇 |
2007年 | 31917篇 |
2006年 | 30146篇 |
2005年 | 27788篇 |
2004年 | 25535篇 |
2003年 | 22879篇 |
2002年 | 22337篇 |
2001年 | 23856篇 |
2000年 | 19097篇 |
1999年 | 15591篇 |
1998年 | 13402篇 |
1997年 | 13020篇 |
1996年 | 12779篇 |
1995年 | 11605篇 |
1994年 | 11333篇 |
1993年 | 10901篇 |
1992年 | 11804篇 |
1991年 | 11697篇 |
1990年 | 10956篇 |
1989年 | 10492篇 |
1988年 | 10398篇 |
1987年 | 9208篇 |
1986年 | 8774篇 |
1985年 | 11538篇 |
1984年 | 11650篇 |
1983年 | 9512篇 |
1982年 | 9823篇 |
1981年 | 9752篇 |
1980年 | 9190篇 |
1979年 | 9508篇 |
1978年 | 9651篇 |
1977年 | 9603篇 |
1976年 | 9398篇 |
1975年 | 8919篇 |
1974年 | 8766篇 |
1973年 | 8926篇 |
1972年 | 5953篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
91.
Analytical expressions involving both system parameters and step-size are proposed to represent the local simulation error for the symmetrized split-step Fourier (SSSF) simulation method. This analytical expression can be used for a step-size selection rule to achieve comparable local simulation accuracy for SSSF simulations. This can lead to computational savings since there is no waste of computation in each simulation step. Furthermore, based on the local error expression, scaling rules are derived to achieve comparable global simulation accuracy for wide ranges of key system parameter values. This is significant in enhancing the computational efficiency in optical fiber communication system design and optimization. Extensive validation tests were performed to explore the application range of the proposed step-size selection and scaling rules. The desired global accuracy can be achieved with the use of our local error expression and scaling rules by only a couple of test trial simulation runs for a variety of practical applications. 相似文献
92.
A homologous series of azomesogens, 2″-[4-(4′-n-alkoxybenzoyloxy)-2-chlorophenylazo] naphthalenes, with lateral chloro groups was synthesised. All the homologues synthesized
exhibit enantiotropic nematic mesophase. The mesomorphic properties of the present series are compared with other structurally
related series to evaluate the effect of lateral chloro group and its position on mesomorphism
This paper was presented at the 10th National Conference on Liquid Crystals held at Bangalore, India during 9–11 October 2003. 相似文献
93.
Yu. M. Dyukarev 《Mathematical Notes》2004,75(1-2):66-82
In this paper, we obtain criteria for the indeterminacy of the Stieltjes matrix moment problem. We obtain explicit formulas for Stieltjes parameters and study the multiplicative structure of the resolvent matrix. In the indeterminate case, we study the analytic properties of the resolvent matrix of the moment problem. We describe the set of all matrix functions associated with the indeterminate Stieltjes moment problem in terms of linear fractional transformations over Stieltjes pairs. 相似文献
94.
J. Krause und F. Umland 《Fresenius' Journal of Analytical Chemistry》1989,335(7):791-795
Summary This paper presents some applications of cyclic voltammetry at the interface of two immiscible electrolyte solutions by means of an experimental set-up described in a former publication. When using special sample preparation good results are obtained for the determination of iodide and quinine in pharmaceutical products as well as for the determination of quinine in tonic water and dodecylsulfate in a tooth-salt.
Herrn Prof. Dr. Günther Tölg zum 60. Geburtstag gewidmet 相似文献
Herrn Prof. Dr. Günther Tölg zum 60. Geburtstag gewidmet 相似文献
95.
X Y Liu C J Li 《Science in China. Series B, Chemistry, life sciences & earth sciences》1989,32(10):1215-1223
To further evaluate the causality between endogenous pyrogen (EP)-induced fever and cyclic adenosine-3',5'-monophosphate (cyclic AMP) level, the effects of sodium salicylate (SS) on the febrile response and increased levels of cyclic AMP in both cerebrospinal fluid (c.s.f.) and plasma during EP-induced fever in rabbits were observed. The results suggest that cyclic AMP is probably involved in the central mediation of EP-induced fever and that increased concentration of cyclic AMP in c.s.f. associated with EP-induced fever is not the result of temperature elevation but appears to be caused by the increased synthesis in the central nervous system. In addition it is confirmed that blood is impossibly a contributory source of increased cyclic AMP in c.s.f. during EP fever, and that SS may act subsequent to the increase in cyclic AMP. 相似文献
96.
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
97.
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
98.
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
99.
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
100.
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