全文获取类型
收费全文 | 586875篇 |
免费 | 5364篇 |
国内免费 | 1448篇 |
专业分类
化学 | 268469篇 |
晶体学 | 7645篇 |
力学 | 25989篇 |
综合类 | 17篇 |
数学 | 67780篇 |
物理学 | 163531篇 |
无线电 | 60256篇 |
出版年
2021年 | 5454篇 |
2020年 | 6047篇 |
2019年 | 6899篇 |
2018年 | 9128篇 |
2017年 | 9228篇 |
2016年 | 12511篇 |
2015年 | 6523篇 |
2014年 | 11292篇 |
2013年 | 24938篇 |
2012年 | 19338篇 |
2011年 | 22960篇 |
2010年 | 17647篇 |
2009年 | 18189篇 |
2008年 | 22973篇 |
2007年 | 23244篇 |
2006年 | 21294篇 |
2005年 | 19240篇 |
2004年 | 18019篇 |
2003年 | 16325篇 |
2002年 | 16194篇 |
2001年 | 16900篇 |
2000年 | 13586篇 |
1999年 | 10774篇 |
1998年 | 9581篇 |
1997年 | 9330篇 |
1996年 | 8550篇 |
1995年 | 7805篇 |
1994年 | 7878篇 |
1993年 | 7577篇 |
1992年 | 7822篇 |
1991年 | 8275篇 |
1990年 | 7904篇 |
1989年 | 7679篇 |
1988年 | 7389篇 |
1987年 | 6769篇 |
1986年 | 6459篇 |
1985年 | 8012篇 |
1984年 | 8306篇 |
1983年 | 7021篇 |
1982年 | 7186篇 |
1981年 | 6718篇 |
1980年 | 6390篇 |
1979年 | 6823篇 |
1978年 | 7064篇 |
1977年 | 6938篇 |
1976年 | 6856篇 |
1975年 | 6557篇 |
1974年 | 6394篇 |
1973年 | 6699篇 |
1972年 | 4891篇 |
排序方式: 共有10000条查询结果,搜索用时 536 毫秒
941.
Uncooled DBR laser directly Modulated at 3.125 gb/s as athermal transmitter for low-cost WDM systems
Yiran Liu Davies A.R. Ingham J.D. Penty R.V. White I.H. 《Photonics Technology Letters, IEEE》2005,17(10):2026-2028
An uncooled three-section tunable distributed Bragg reflector laser is demonstrated as an athermal transmitter for low-cost uncooled wavelength-division-multiplexing (WDM) systems with tight channel spacing. A /spl plusmn/0.02-nm thermal wavelength drift is achieved under continuous-wave operation up to 70/spl deg/C. Dynamic sidemode suppression ratio of greater than 35 dB is consistently obtained under 3.125-Gb/s direct modulation over a 20/spl deg/C-70/spl deg/C temperature range, with wavelength variation of as low as /spl plusmn/0.2 nm. This indicates that more than an order of magnitude reduction in coarse WDM channel spacing is possible using this source. 相似文献
942.
A.J.?Lpez-Moreno 《Acta Mathematica Hungarica》2005,106(4):343-354
Summary We obtain preservation inequalities for Lipschitz constants of higher order in simultaneous approximation processes by Bernstein type operators. From such inequalities we derive the preservation of the corresponding Lipschitz spaces. 相似文献
943.
The effect of the amount of desorbed water on the mechanical properties of composites based on low-density polyethylene and linen yarn production waste (LW) is analyzed by statistical methods. It is shown that the amount of absorbed water decreases during the desorption process at room temperature both for specimens modified and unmodified with diphenylmethane diisocyanate (DIC.) The most sensitive to the action of water is the elastic modulus, which decreases considerably under the effect of water and is fully restored in the desorption process. The tensile strain also increases with the amount of absorbed water. It is found that the elastic modulus of the unmodified composite correlates linearly with the amount of desorbed water. Between the amount of desorbed water and the tensile strain, as well the specific work of deformation, a negative linear correlation is revealed. After water desorption, all strength and deformation characteristics of both the modified and unmodified composites are fully recovered.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 4, pp. 515–524, July–August, 2005. 相似文献
944.
A.F. Mesquita A.O. Porto G.M. de Lima J.M. Delgado F.S. Lameiras 《Solid State Communications》2005,136(3):133-137
Hexagonal and cubic cadmium selenide were prepared from a chemical route by using cadmium chloride and potassium selenium hydride obtained from reaction of selenium powder and potassium boron hydride. The product obtained was thermally treated under argon flux at 300, 500 and 600 °C for 2 h and characterized by X-ray photoelectron spectroscopy and X-ray diffraction. The X-ray diffraction data were refined by Rietveld method and the structural parameters were determined for the phases of each annealed samples. At 300 °C five phases were identified: Cubic and hexagonal cadmium selenides and the contaminants: Potassium chloride, boron oxide and cadmium boron oxide. At 500 and 600 °C only the hexagonal cadmium selenide phase was identified besides the other above mentioned contaminant. 相似文献
945.
A. Nikakhtar A. Nasehzadeh G. A. Mansoori 《Journal of the Iranian Chemical Society》2006,3(2):161-167
A thermodynamic cycle for the nanoclusteration between DNA and dendronized polymer is proposed and electrostatic free energies of a series of DNA- dendronized polymer nanoclusteration processes are calculated. The free energies for assembling fixed charges and mobile ions, and the bending energies of the DNA chain wrapped around the dendronized polymer are taken into consideration. The free energies of nanoclusteration are calculated for a number of nanoclusters with different conformations at constant temperature and constant ionic strength. The effects of temperature and ionic strength on the free energy of nanoclusteration and stability of their conformations are also investigated. 相似文献
946.
947.
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
948.
949.
A. Schukoffsky 《Fresenius' Journal of Analytical Chemistry》1873,12(1):237-238
Ohne Zusammenfassung 相似文献
950.
I. M. Kolthoff S. R. Cooper V. J. Tulane F. Reimers H. A. J. Pieters K. Höppner V. H. Matula C. B. Macek W. N. Skworzow J. S. Schepelewa F. Čuta K. Kámen S. Cohen R. E. Oesper D. S. Narayanamurthi T. R. Seshadri H. M. State A. W. Kirssanow W. M. Tscherkassow D. M. Mukherjee T. Akiyama Y. Mine S. Yabe Y. Volmar M. Déribéré 《Analytical and bioanalytical chemistry》1937,111(5-6):192-197