全文获取类型
收费全文 | 470683篇 |
免费 | 4363篇 |
国内免费 | 1571篇 |
专业分类
化学 | 212486篇 |
晶体学 | 6345篇 |
力学 | 17700篇 |
综合类 | 7篇 |
数学 | 47141篇 |
物理学 | 126843篇 |
无线电 | 66095篇 |
出版年
2021年 | 3300篇 |
2020年 | 3669篇 |
2019年 | 4180篇 |
2018年 | 5240篇 |
2017年 | 5058篇 |
2016年 | 7643篇 |
2015年 | 4738篇 |
2014年 | 7551篇 |
2013年 | 19805篇 |
2012年 | 14646篇 |
2011年 | 17842篇 |
2010年 | 12501篇 |
2009年 | 13005篇 |
2008年 | 17445篇 |
2007年 | 18146篇 |
2006年 | 17286篇 |
2005年 | 15958篇 |
2004年 | 14679篇 |
2003年 | 13215篇 |
2002年 | 12865篇 |
2001年 | 14441篇 |
2000年 | 11628篇 |
1999年 | 9435篇 |
1998年 | 8057篇 |
1997年 | 8159篇 |
1996年 | 7525篇 |
1995年 | 7078篇 |
1994年 | 6917篇 |
1993年 | 6806篇 |
1992年 | 7229篇 |
1991年 | 7216篇 |
1990年 | 6907篇 |
1989年 | 6557篇 |
1988年 | 6437篇 |
1987年 | 5768篇 |
1986年 | 5473篇 |
1985年 | 7054篇 |
1984年 | 7390篇 |
1983年 | 6180篇 |
1982年 | 6541篇 |
1981年 | 6368篇 |
1980年 | 6144篇 |
1979年 | 6171篇 |
1978年 | 6462篇 |
1977年 | 6260篇 |
1976年 | 6486篇 |
1975年 | 5951篇 |
1974年 | 6000篇 |
1973年 | 6408篇 |
1972年 | 4112篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
P. R. Latthe P. S. Shinge Bharati V. Badami P. B. Patil S. N. Holihosur 《Journal of Chemical Sciences》2006,118(3):249-256
3-[4-(Azidocarbonyl)]phenylsydnone (2) obtained from 3-(4-hydrazinocarbonyl) phenylsydnone (1) on Curtius rearrangement with alcohols, water and amines afforded the corresponding carbamates (3a-h), 4,4′-(sydnone-3-yl) diphenyl urea (4) and 4-(heterocyclyl)phenyl ureas (5a-l). Compounds (5a-l) on one-pot ring conversion yielded the 1,3,4-oxadiazolin-2-one derivatives (6a-l), which on reaction with N2H4 gave the 4-amino-1,2,4-triazolin-3-ones (7a-l). All these compounds exhibited moderate antimicrobial activity against the few microbes tested. The carbamates have been
found to be more toxic against fourth instar larvae ofAedes aegypti, in particular, then-butyl derivative (3e). 相似文献
72.
Troya A. Maharatna K. Krstic M. Grass E. Jagdhold U. Kraemer R. 《IEEE transactions on circuits and systems. I, Regular papers》2008,55(2):672-686
In this paper, we propose low-power designs for the synchronizer and channel estimator units of the Inner Receiver in wireless local area network systems. The objective of the work is the optimization, with respect to power, area, and latency, of both the signal processing algorithms themselves and their implementation. Novel circuit design strategies have been employed to realize optimal hardware and power efficient architectures for the fast Fourier transform, arc tangent computation unit, numerically controlled oscillator, and the decimation filters. The use of multiple clock domains and clock gating reduces the power consumption further. These blocks have been integrated into an experimental digital baseband processor for the IEEE 802.11a standard implemented in the 0.25mum- 5-metal layer BiCMOS technology from Institute for High Performance Microelectronics. 相似文献
73.
Energy-efficient error control for IEEE 802.15.1 (Bluetooth) video communication is proposed. The scheme is based on block-oriented incremental redundancy provided by rateless coding and receiver feedback. Results are presented for time-sensitive video streaming applications under a Markovian channel model. When the proposed algorithm is compared to variations of Bluetooth forward error control, there are improvements of around 3 dB in received video quality and of over 10% in energy efficiency. 相似文献
74.
75.
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
76.
77.
Summary Using a saturated non-boiling hydrocarbon mixture, the influence of two parameters on the results of field ionization mass spectrometry (FI-MS) measurements was studied: (a) the potential difference between the FI emitter and the counterelectrode; (b) the emitter temperature.Variation of the potential difference had only a minor effect on the average molecular mass measured and had no evident effect on the relative ring number distribution in the sample. In contrast, when the emitter temperature was increased, higher average molecular masses were recorded. Moreover, the average molecular masses shifted to higher ring numbers. In order to control the relationship between the described influences during mixture analysis, measuring instructions have been developed that enable the quantitative analysis of unknown saturated samples. However, average molecular mass of the mixture must be known.
Entwicklung einer quantitativen FI-MS-Methode zur Charakterisierung von gesättigten hoch- und nichtsiedenden Kohlenwasserstoffgemischen相似文献
78.
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
79.
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
80.
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