全文获取类型
收费全文 | 386478篇 |
免费 | 3112篇 |
国内免费 | 1105篇 |
专业分类
化学 | 184628篇 |
晶体学 | 5288篇 |
力学 | 13883篇 |
综合类 | 9篇 |
数学 | 35695篇 |
物理学 | 102323篇 |
无线电 | 48869篇 |
出版年
2021年 | 2873篇 |
2020年 | 3261篇 |
2019年 | 3644篇 |
2018年 | 4824篇 |
2017年 | 4726篇 |
2016年 | 6781篇 |
2015年 | 3921篇 |
2014年 | 6266篇 |
2013年 | 15285篇 |
2012年 | 11518篇 |
2011年 | 14087篇 |
2010年 | 10343篇 |
2009年 | 10830篇 |
2008年 | 14451篇 |
2007年 | 14841篇 |
2006年 | 14112篇 |
2005年 | 13001篇 |
2004年 | 11806篇 |
2003年 | 10946篇 |
2002年 | 10660篇 |
2001年 | 12410篇 |
2000年 | 9948篇 |
1999年 | 7776篇 |
1998年 | 6483篇 |
1997年 | 6423篇 |
1996年 | 6049篇 |
1995年 | 5416篇 |
1994年 | 5528篇 |
1993年 | 5268篇 |
1992年 | 5936篇 |
1991年 | 6021篇 |
1990年 | 5710篇 |
1989年 | 5510篇 |
1988年 | 5256篇 |
1987年 | 4773篇 |
1986年 | 4484篇 |
1985年 | 5658篇 |
1984年 | 5768篇 |
1983年 | 4664篇 |
1982年 | 4860篇 |
1981年 | 4543篇 |
1980年 | 4352篇 |
1979年 | 4734篇 |
1978年 | 4919篇 |
1977年 | 4842篇 |
1976年 | 4781篇 |
1975年 | 4508篇 |
1974年 | 4492篇 |
1973年 | 4610篇 |
1972年 | 3159篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
21.
Douay M. Xie W.X. Taunay T. Bernage P. Niay P. Cordier P. Poumellec B. Dong L. Bayon J.F. Poignant H. Delevaque E. 《Lightwave Technology, Journal of》1997,15(8):1329-1342
A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models 相似文献
22.
Sugawara F. Aoki K. Yamaguchi H. Sasaki K. Sasaki T. Fujisaki H. 《Electron Device Letters, IEEE》1997,18(10):483-485
A new lateral MOS-gated thyristor, called the Base-Current-Controlled Thyristor, is described. This device is designed so that most holes at the on-stage reach the P base through the floating P+ region adjacent to the P base and the on-state MOSFET. At the turn-off stage, the interruption of the hole current to the P base due to switching off the above MOSFET occurs simultaneously with the conventional turn-off operation. The concept of this device is verified experimentally by using the fabricated lateral device with the external MOSFET. This device exhibits a better trade-off relation between the on-state voltage and the turn-off time compared uith the conventional MOS-gated thyristor 相似文献
23.
The capacitive idling converters derived from the Cuk, SEPIC, Zeta, and flyback topologies allow soft commutation of power switches without the need for additional circuitry, making it possible to increase the switching frequency while maintaining high efficiency 相似文献
24.
Fractal error for detecting man-made features in aerial images 总被引:3,自引:0,他引:3
A technique is proposed for aiding photointerpreters in detecting man-made features in aerial reconnaissance images. The technique, which uses a metric called fractal error, is based on the observed propensity of natural image features to fit a fractional Brownian motion model. Man-made features usually do not fit this model well, and consequently the fractal error metric may be used as a discriminant function for detecting man-made scene features 相似文献
25.
26.
A narrow linewidth has been obtained in a new 1.5 ?m distributed feedback laser integrated monolithically with a tunable external cavity. The linewidth of 18 MHz has been achieved by tuning the current in the external cavity, and the FM response was flat from 100 kHz to 500 MHz. 相似文献
27.
A novel typed of hybrid-mode feed horn is presented. It comprises a hollow conical dielectric waveguide whose outer surface is metallized, and whose inner surface is coated with circumferentially oriented conducting strips. It may be designed to have mimimal cross polarization at two arbitrarily separated frequencies, as for a dual depth corrugated horn. Compared to the corrugated horn, it has the potential of lower weight and lower price. Small horns will have some cross polarization due to undesired radiation from the fields transmitted within the dielectric. 相似文献
28.
Hayashi M. Tanaka H. Ohara K. Otani T. Suzuki M. 《Lightwave Technology, Journal of》2002,20(2):236-242
An all-optical multiplexing technique using wavelength division multiplexing (WDM)-time division multiplexing (TDM) conversion with an electroabsorption wavelength converter has been proposed and demonstrated. The effectiveness of this WDM-TDM conversion technique for various pulsewidth settings was experimentally investigated. The fluctuation of the signal performance, which was inevitably caused by the coherent crosstalk between adjacent pulses in the conventional optical time division multiplexing (OTDM) technique, were successfully suppressed, even in the case of wide pulse duration. High Q-factor performance has been maintained for a wide range of duty ration from 36% to 74%. By introducing this technique to the optical time division multiplexer, a highly stable and high-quality 40-Gb/s optical signal can be effectively produced without generating the short pulse or setting two tributaries at orthogonal polarization states, and without introducing high-speed electronics for signal multiplexing. The WDM-TDM conversion with an electroabsorption wavelength converter was extended to 60-Gb/s operation by using three 20-Gb/s tributaries. A clear eye opening was confirmed for a waveform after the WDM-TDM conversion of the 60-Gb/s signal 相似文献
29.
30.
An examination is presented of three techniques used for the efficient computation of fields diffracted by a subreflector that has been shaped by geometrical optics synthesis. It is found that these techniques, which are based on the geometrical theory of diffraction (GTD), produce errors in the computed fields that are specific to shaped reflectors. These errors are examined for a reflector system shaped to produce maximum gain from a tapered feed illumination. The discrepancies are directly related to the caustic being located near an observation point of the GTD calculations. The errors found are localized, and they increase in magnitude as the caustic approaches the main reflector. In a general offset geometry, the location of the caustic may be located arbitrarily close to the main reflector given a prescribed output aperture distribution. For the specific case considered here-the common situation of shaping to produce maximum gain-the caustic is located near the edge of the main reflector and on the reflection shadow boundary. A local correction is derived which creates a uniform solution through the caustic and across the reflection shadow boundary. Away from this point the calculation recedes to the standard GTD solution 相似文献