全文获取类型
收费全文 | 242139篇 |
免费 | 12538篇 |
国内免费 | 7962篇 |
专业分类
化学 | 119075篇 |
晶体学 | 2640篇 |
力学 | 9714篇 |
综合类 | 344篇 |
数学 | 21265篇 |
物理学 | 68231篇 |
无线电 | 41370篇 |
出版年
2023年 | 2121篇 |
2022年 | 3260篇 |
2021年 | 4117篇 |
2020年 | 4149篇 |
2019年 | 3896篇 |
2018年 | 3842篇 |
2017年 | 3908篇 |
2016年 | 5748篇 |
2015年 | 5086篇 |
2014年 | 6690篇 |
2013年 | 11706篇 |
2012年 | 11181篇 |
2011年 | 12482篇 |
2010年 | 8988篇 |
2009年 | 9136篇 |
2008年 | 10990篇 |
2007年 | 10623篇 |
2006年 | 9960篇 |
2005年 | 9063篇 |
2004年 | 7664篇 |
2003年 | 6546篇 |
2002年 | 6405篇 |
2001年 | 6532篇 |
2000年 | 5355篇 |
1999年 | 4803篇 |
1998年 | 4182篇 |
1997年 | 4027篇 |
1996年 | 3676篇 |
1995年 | 3448篇 |
1994年 | 3273篇 |
1993年 | 2936篇 |
1992年 | 3063篇 |
1991年 | 3096篇 |
1990年 | 2768篇 |
1989年 | 2651篇 |
1988年 | 2481篇 |
1987年 | 2117篇 |
1985年 | 2537篇 |
1984年 | 2592篇 |
1983年 | 2178篇 |
1982年 | 2291篇 |
1981年 | 2161篇 |
1980年 | 2074篇 |
1979年 | 2168篇 |
1978年 | 2313篇 |
1977年 | 2199篇 |
1976年 | 2244篇 |
1975年 | 2138篇 |
1974年 | 2170篇 |
1973年 | 2168篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
Generation of Technology-Independent Retargetable Analog Blocks 总被引:1,自引:0,他引:1
R. Castro-López F. V. Fernández F. Medeiro A. Rodríguez-Vázquez 《Analog Integrated Circuits and Signal Processing》2002,33(2):157-170
This paper introduces a complete methodology for retargeting of analog cells to different sets of specifications. This methodology is technology-independent, thus allowing the migration, from one technology to another, of the circuit under retargeting. Careful integration of the device sizing and layout generation tasks via the incorporation of layout constraints during the sizing process allows to generate fully functional designs in a few minutes. The methodology is illustrated via the retargeting of a fully-differential Miller-compensated two-stage operational amplifier for a new set of specifications and two different technological processes.An erratum to this article can be found at 相似文献
142.
This paper proposes a software pipelining framework, CALiBeR (ClusterAware Load Balancing Retiming Algorithm), suitable for compilers targetingclustered embedded VLIW processors. CALiBeR can be used by embedded systemdesigners to explore different code optimization alternatives, that is, high-qualitycustomized retiming solutions for desired throughput and program memory sizerequirements, while minimizing register pressure. An extensive set of experimentalresults is presented, demonstrating that our algorithm compares favorablywith one of the best state-of-the-art algorithms, achieving up to 50% improvementin performance and up to 47% improvement in register requirements. In orderto empirically assess the effectiveness of clustering for high ILP applications,additional experiments are presented contrasting the performance achievedby software pipelined kernels executing on clustered and on centralized machines. 相似文献
143.
Guangbin Yang 《Reliability, IEEE Transactions on》2002,51(3):288-293
ESS (environmental stress screening) has been extensively used to reduce infant mortality by precipitating defects. The existing ESS plans precipitate defects by stressing all products for specified durations. The plans usually require long screen durations to allow nearly all defective items to fail, and thus generate excessive aging effects on good items. For some products, failures are defined in terms of performance characteristics exceeding their critical values. This paper describes the principles of using degradation measurements of performance characteristics to screen the products. In an ESS, the performance characteristics of defective items degrade considerably faster than those of good ones, yielding a bimodal distribution of the characteristics. As screen duration increases, the 2 modes of the distribution shift apart. It is possible to find a tightened critical value to weed out the defective items before they fail. This paper, based on these principles, further designs the optimal 2-level screen plans which minimize a segment of life-cycle cost and which simultaneously meet the reliability requirement. The minimum cost is achieved by choosing optimal part-level and unit-level screen durations and tightened critical values of parts. A numerical example is followed by discussion. Because the ESS regime of this paper allows defective parts to be screened out before they fail, the developed optimal ESS plans can reduce life-cycle cost, shorten part-level screen duration, and alleviate aging effects on good products 相似文献
144.
145.
A new technique for mixing optical waves to generate microwave-frequency signals using an injection laser diode and a fibre-optic interferometer is reported. A beat signal is produced by interfering light from the laser with light from the same laser emitted earlier at a different frequency. The interferometer consists of an evanescent-field fibre coupler joined to a length of single-mode optical fibre. The laser is tuned by pulsed or bipolar current waveforms superimposed on a DC bias current. Feedback from an external cavity stabilises the laser frequency and reduces its linewidth. Efficient mixing at difference frequencies from 250 MHz to 2 GHz is obtained, with spectral widths of less than 1% of the centre frequency. 相似文献
146.
A. R. Volkov B. V. Shul'gin T. I. Polupanova V. N. Lebedev A. A. Nagornyi V. L. Petrov Yu. F. Kargin 《Journal of Applied Spectroscopy》1991,54(6):585-590
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 970–975, June, 1991. 相似文献
147.
In a twisted nematic liquid crystal cell we report and study the self-transparency effect induced by the creation of an isotropic droplet. A simple theoretical model is presented and good agreement with experimental data is found. In a two beams configuration transistor-like behavior is observed and theoretically discussed. 相似文献
148.
T. Kumita Y. Kamiya M. Babzien I. Ben-Zvi K. Kusche I. V. Pavlishin I. V. Pogorelsky D. P. Siddons V. Yakimenko T. Hirose T. Omori J. Urakawa K. Yokoya D. Cline F. Zhou 《Laser Physics》2006,16(2):267-271
Thomson scattering of high-power laser and electron beams is a good test of electrodynamics in the high-field region. We demonstrated production of high-intensity X-rays in the head-on collision of a CO2 laser and 60-MeV electron beams at Brookhaven National Laboratory, Accelerator Test Facility. The energy of an X-ray photon was limited at 6.5 keV in the linear (lowest order) Thomson scattering, but the nonlinear (higher order) process produces higher energy X-rays. We measured the angular distribution of the high-energy X-rays and confirmed that it agrees with theoretical predictions. 相似文献
149.
M.F. Al-Kuhaili 《Journal of luminescence》2006,117(2):209-216
Thin films of sodium salicylate were deposited by spin coating from a solution prepared by dissolving sodium salicylate in methanol. The films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, UV-visible spectroscopy and spectrofluorometry. The films were crystalline with a strong blue fluorescence peak at an emission wavelength of 419.3 nm. The influence of solution concentration, spin speed and annealing temperature on the fluorescence intensity was studied. Optimum results were obtained with a solution whose concentration was 2.5 M. It was found that lower spin speed resulted in higher fluorescence intensity. Moreover, the fluorescence intensity decreased as the annealing temperature was increased. The durability of the films over a period of 30 days was also investigated, and films annealed at higher temperatures were found to be less degraded with time. 相似文献
150.
Evaluation of the Planck-mean absorption coefficients from HITRAN and HITEMP databases 总被引:1,自引:0,他引:1
Hongmei ZhangMichael F. Modest 《Journal of Quantitative Spectroscopy & Radiative Transfer》2002,73(6):649-653
New values of Planck-mean absorption coefficients are presented for various gases, based on line-by-line calculations from the high-resolution databases, HITRAN and HITEMP. The new values are compared with previously published Planck-mean absorption coefficients. 相似文献