全文获取类型
收费全文 | 20854篇 |
免费 | 3121篇 |
国内免费 | 2150篇 |
专业分类
化学 | 11705篇 |
晶体学 | 196篇 |
力学 | 865篇 |
综合类 | 73篇 |
数学 | 1564篇 |
物理学 | 5807篇 |
无线电 | 5915篇 |
出版年
2024年 | 106篇 |
2023年 | 535篇 |
2022年 | 668篇 |
2021年 | 814篇 |
2020年 | 793篇 |
2019年 | 785篇 |
2018年 | 711篇 |
2017年 | 659篇 |
2016年 | 899篇 |
2015年 | 963篇 |
2014年 | 1218篇 |
2013年 | 1500篇 |
2012年 | 1696篇 |
2011年 | 1815篇 |
2010年 | 1311篇 |
2009年 | 1220篇 |
2008年 | 1399篇 |
2007年 | 1275篇 |
2006年 | 1128篇 |
2005年 | 980篇 |
2004年 | 717篇 |
2003年 | 633篇 |
2002年 | 584篇 |
2001年 | 489篇 |
2000年 | 437篇 |
1999年 | 386篇 |
1998年 | 329篇 |
1997年 | 290篇 |
1996年 | 250篇 |
1995年 | 226篇 |
1994年 | 261篇 |
1993年 | 180篇 |
1992年 | 162篇 |
1991年 | 132篇 |
1990年 | 120篇 |
1989年 | 95篇 |
1988年 | 64篇 |
1987年 | 45篇 |
1986年 | 43篇 |
1985年 | 39篇 |
1984年 | 30篇 |
1983年 | 18篇 |
1982年 | 20篇 |
1981年 | 9篇 |
1980年 | 18篇 |
1979年 | 10篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The results calculated by computer for the double drift region IMPATT diode oscillator on the 8 mm waveband are reported in
this paper. A comparison between single and double drift devices concerning the power output and efficiency is given. The
effect of doping profile, current density and RF voltage on the performances of these devices have also been investigated.
The theoretical data of these double drift IMPATT oscillator and amplifier are provided. 相似文献
132.
133.
Jian Song Chongcheng Fan 《Lightwave Technology, Journal of》1995,13(3):546-550
A simplified formula for the dispersion limit of single-mode-fiber intensity modulation/direct detection (IM/DD) systems is derived for arbitrary given normalized pulse width U, eye opening penalty X and source linewidth enhancement factor α. From the comparison with published theoretical analyses, computer simulation and experimental data, its validity and the limits of existing formulae are shown. Using this simple and general formula, one can easily obtain the dispersion limit of various fiber types under different working conditions in IM/DD systems 相似文献
134.
Semilinear Differential Inclusions in Separable Banach SpacesXueXingmei(薛星美)andSongGuozhu(宋国柱)(DepartmentofMathematics,Nanjin... 相似文献
135.
T. S. Lee S. B. Lee J. M. Kim J. S. Kim S. H. Suh J. H. Song I. H. Park S. U. Kim M. J. Park 《Journal of Electronic Materials》1995,24(9):1057-1059
In order to improve the Zn homogeneity along the axial direction of CdZnTe boule, we have employed a modified Bridgman technique
using a (Cd, Zn) alloy source in communication with the melt, whose temperature has been gradually changed from 800 to 840°C
during growth. Electron probe microanalysis (EPMA) measurements of Zn composition in the boule shows an excellent homogeneity
of Zn along the axis of the CdZnTe boule compared with results in a boule grown by using a fixed source temperature. We have
performed a numerical simulation to obtain the approximate temperatures of additional heating and cooling needed to improve
the radial Zn homogeneity. CdZnTe boule has been grown by seeded vertical Bridgman furnace with two zones of heater and cooler.
Ultraviolet/visible spectroscopic measurements of Zn composition over the length of the boule indicate that the radial distribution
of Zn composition is very homogeneous in the body region of the boule, where the radial variation of Zn composition is ±0.0005. 相似文献
136.
Song J.-I. Palmstrom C.J. Van der Gaag B.P. Hong W.-P. Hayes J.R. Chough K.B. 《Electronics letters》1993,29(8):666-667
InP/In/sub 0.53/Ga/sub 0.47/As heterojunction bipolar transistors (HBTs) using a highly carbon-doped base are reported. High carbon doping has been achieved by chemical beam epitaxy (CBE). The resulting hole concentration in the carbon-doped base is as high as 7*10/sup 19//cm/sup 3/. To the authors' knowledge, this is the highest doping level reported using carbon. HBTs with a 20 AA spacer layer exhibited nearly ideal I-V characteristics with collector and base current ideality factor of 1.018 and 1.037, respectively. Current gain and breakdown voltage BV/sub CEO/ were 7 and 6 V, respectively.<> 相似文献
137.
Disturbances of signals on a coplanar waveguide (CPW) induced by the presence of LiTaO/sub 3/ and GaAs electrooptic probes in external electrooptic (EO) sampling have been simulated and compared quantitatively. The finite-difference-time-domain method is used to simulate the full wave field around a coplanar waveguide on a GaAs substrate in an external EO sampling configuration. The results indicate that the induced signal disturbance, or invasiveness, of a LiTaO/sub 3/ probe is almost ten times that of a GaAs probe in terms of the magnitude of S/sub 11/, but that LiTaO/sub 3/ yields about two times the EO response for a given S/sub 11/ and optical probing wavelength. The transparency of LiTaO/sub 3/ to shorter wavelengths, however, allows an even higher sensitivity for this material relative to GaAs. The results suggest that these probes do not exhibit significant invasiveness (magnitude of S/sub 11/ smaller than -40 dB), if they are removed from contact by the distance of CPWs center conductor width.<> 相似文献
138.
139.
140.