全文获取类型
收费全文 | 495277篇 |
免费 | 5420篇 |
国内免费 | 1419篇 |
专业分类
化学 | 233811篇 |
晶体学 | 6489篇 |
力学 | 18764篇 |
综合类 | 13篇 |
数学 | 49946篇 |
物理学 | 133177篇 |
无线电 | 59916篇 |
出版年
2021年 | 4132篇 |
2020年 | 4649篇 |
2019年 | 4985篇 |
2018年 | 6398篇 |
2017年 | 6213篇 |
2016年 | 9532篇 |
2015年 | 5878篇 |
2014年 | 9294篇 |
2013年 | 21495篇 |
2012年 | 16450篇 |
2011年 | 20262篇 |
2010年 | 14591篇 |
2009年 | 15179篇 |
2008年 | 19939篇 |
2007年 | 20293篇 |
2006年 | 18986篇 |
2005年 | 17290篇 |
2004年 | 16078篇 |
2003年 | 14379篇 |
2002年 | 14049篇 |
2001年 | 15776篇 |
2000年 | 12391篇 |
1999年 | 10034篇 |
1998年 | 8612篇 |
1997年 | 8280篇 |
1996年 | 7965篇 |
1995年 | 7214篇 |
1994年 | 7100篇 |
1993年 | 6741篇 |
1992年 | 7379篇 |
1991年 | 7476篇 |
1990年 | 7113篇 |
1989年 | 6789篇 |
1988年 | 6616篇 |
1987年 | 6002篇 |
1986年 | 5650篇 |
1985年 | 7188篇 |
1984年 | 7264篇 |
1983年 | 5814篇 |
1982年 | 5809篇 |
1981年 | 5796篇 |
1980年 | 5342篇 |
1979年 | 5659篇 |
1978年 | 5786篇 |
1977年 | 5679篇 |
1976年 | 5568篇 |
1975年 | 5141篇 |
1974年 | 5108篇 |
1973年 | 5116篇 |
1972年 | 3626篇 |
排序方式: 共有10000条查询结果,搜索用时 7 毫秒
961.
F. Devaux N. Souli A. Ougazzaden F. Huet M. Carre 《Photonics Technology Letters, IEEE》1996,8(2):218-220
We report on a tandem of electroabsorption modulators integrated with an amplifier by the identical active layer (IAL) approach. A 14 dB fiber-to-fiber is obtained by the use of constant As-P ratio MQW active layer and operation at 10 Gb/s is demonstrated. A record modulator saturation power of 35 mW is also reported. 相似文献
962.
In the first part of this work, a novel implementation of the well-known four-point bend test is described that determines the strength of thin beams and optical fibers by measuring the loading pin displacement, rather than the applied load. This paper extends the analysis of the nonlinear bending behavior to account for the stochastic nature of strength. A statistical analysis is presented that determines the effective tested length in bending and the tension to bending strength ratio. Results are given for both surface and volume flaws as well as for specimens of both circular and rectangular section. Strength measurements on a deliberately weakened silica optical fiber are consistent with the predictions of the analysis 相似文献
963.
Rafferty W. Slobin S.D. Stelzried C.T. Sue M.K. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1994,82(5):636-645
Ground antennas are the major visible components of NASA's Deep Space Network (DSN). The role, key characteristics, and performance of these antennas in deep-space telecommunications are described. The system analyses and tradeoffs to optimize the overall ground-to-spacecraft link and to define future missions are elaborated from an antenna perspective. Overall performance of receiving systems is compared using the widely accepted G/T figure-of-merit, i.e., net antenna gain divided by the operating system noise temperature. Performance of past, present, and future antennas and receiving systems is discussed, including the planned development of a world-wide network of 34-m diameter beam-waveguide antennas. The need for multifrequency operation, presently in the S- and X-bands, and in the future in the Ka-band, is discussed. The resulting requirements placed on antenna technology are highlighted. Beam-waveguide antenna performance to further improve performance and operational advantages is discussed 相似文献
964.
A capacitance-to-pulse duration converter, specifically designed for interfacing capacitive sensors, is presented. The operating principle is a double slope approach implemented using transconductor-based Miller integrators. The main strengths of the proposed circuit are: 1) intrinsically small sensitivity to temperature; 2) simplicity of trimming offset and gain to correct the sensor parameter spread; and 3) fast wake-up time. The circuit nonidealities are analyzed in order to identify the elements responsible for the residual temperature sensitivity and jitter on the pulse duration. The effectiveness of the method is demonstrated by measurements performed on a prototype, designed and fabricated using the 0.35 mum, 3.3 V Bipolar-CMOS-DMOS process BCD6 of STMicroelectronics. 相似文献
965.
966.
967.
A. V. Gorshkov V. V. Evreinov I. A. Tarasova M. V. Gorshkov 《Polymer Science Series B》2007,49(3-4):93-107
A peptide separation model based on the technique of liquid chromatography of macromolecules at the critical condition was proposed. In terms of this model, the array of experimental data on the separation of peptides is considered. The main phenomenological parameters of the model—effective adsorption energies of amino acid residues—were determined, thus allowing the influence of character of their alternation in the chain on retention times to be predicted. The model is applicable to investigation into the feasibility of separation in different chromatographic modes of not only peptides with the same amino acid composition and different sequences of units in the chain but also peptides containing amino acid isomers and mirror sequences with different terminal groups. 相似文献
968.
V. F. Balakirev S. Kh. Estemirova A. M. Yankin S. G. Titova 《Doklady Physical Chemistry》2007,416(1):240-242
969.
Electrochemical oxidation of thio/carbohydrazide and their hydrazone derivatives Benzaldehyde thiocarbohydrazone [BTCH] diacetylene
thiocarbohydrazone [DATCH] have been studied in Brit-ton Robinson buffer in aqueous and nonaqueous media at a glassy carbon
electrode. The effects of pH, sweep rate, concentration, temperature and surfactants have been studied. The complex bis (carbo/thiocabohydrazide) Zn(II) chloride was also subjected to voltammetric analysis in order to understand the reactivity
both in free and metal bound states. The reaction conditions were optimized for the determination of above compounds in micrograms
quantities by differential pulse voltammetry, analytical utility of this investigation is also highlighted. 相似文献
970.
Pichler M. Stelzer A. Gulden P. Seisenberger C. Vossiek M. 《IEEE transactions on circuits and systems. I, Regular papers》2007,54(5):1006-1017
The synthesis of linear frequency sweeps or chirps is required, among others, in frequency-modulated continuous-wave radar systems for object position estimation. Low phase and frequency errors in sweeps with high bandwidth are a prerequisite for good accuracy and resolution, but, in certain applications where high measurement rates are desired, the additional demand for short sweep cycles has to be met. Transient phenomena in dynamic synthesizers as well as nonlinear system behavior usually cause unknown phase errors in the system output. For the class of phase-locked-loop (PLL)-based frequency synthesizers, a novel output phase-measurement method and dedicated circuitry are proposed that allow significant reduction of phase errors by adaptive input predistortion. The measurement procedure is implemented within the PLL control circuitry and does not require external equipment. The application of this method to PLL system identification and linearization of extremely short frequency sweeps is shown 相似文献