全文获取类型
收费全文 | 20666篇 |
免费 | 3102篇 |
国内免费 | 2130篇 |
专业分类
化学 | 11624篇 |
晶体学 | 194篇 |
力学 | 840篇 |
综合类 | 72篇 |
数学 | 1550篇 |
物理学 | 5748篇 |
无线电 | 5870篇 |
出版年
2024年 | 107篇 |
2023年 | 529篇 |
2022年 | 647篇 |
2021年 | 793篇 |
2020年 | 779篇 |
2019年 | 782篇 |
2018年 | 703篇 |
2017年 | 654篇 |
2016年 | 892篇 |
2015年 | 955篇 |
2014年 | 1209篇 |
2013年 | 1494篇 |
2012年 | 1692篇 |
2011年 | 1801篇 |
2010年 | 1303篇 |
2009年 | 1208篇 |
2008年 | 1389篇 |
2007年 | 1275篇 |
2006年 | 1121篇 |
2005年 | 976篇 |
2004年 | 713篇 |
2003年 | 630篇 |
2002年 | 572篇 |
2001年 | 485篇 |
2000年 | 437篇 |
1999年 | 381篇 |
1998年 | 324篇 |
1997年 | 289篇 |
1996年 | 247篇 |
1995年 | 223篇 |
1994年 | 253篇 |
1993年 | 179篇 |
1992年 | 160篇 |
1991年 | 130篇 |
1990年 | 116篇 |
1989年 | 95篇 |
1988年 | 64篇 |
1987年 | 42篇 |
1986年 | 43篇 |
1985年 | 38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 0 毫秒
91.
92.
93.
Young-Hwan You Won-Gi Jeon Jong-Ho Paik Dae-Ki Hong Hyoung-Kyu Song 《Broadcasting, IEEE Transactions on》2003,49(4):354-361
In this paper, an OFDM-CDMA system employing various diversity schemes is considered for a possible candidate of broadband wireless access networks and broadcasting applications. With an emphasis on a preamble design for multi-channel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in OFDM-based multiple-antenna transmission systems. By properly designing each preamble for multiple antennas to be orthogonal in the time domain, the channel estimation can be applied to the HIPERLAN/2 and IEEE 802.11a standards in the case of more than two transmit antennas. Also, the effect of diversity techniques on the performance of OFDM-CDMA based broadband wireless access networks is investigated and the maximum achievable diversity gain for a two-path Rayleigh fading environment is evaluated. Simulation results show that the OFDM-CDMA system applying a space-time-frequency diversity with a full-rate full diversity code can give the diversity of D=4 and D=8 for both multi-user cases of maximum user and half user capacities, respectively. 相似文献
94.
M.H. Song B. Park S. Nishimura T. Toyooka I.J. Chung Y. Takanishi K. Ishikawa H. Takezoe 《Advanced functional materials》2006,16(14):1793-1798
A liquid crystal (LC) photonic device with an anisotropic optical heterojunction structure has been fabricated. The device has a phase‐retarding nematic LC (NLC) layer sandwiched between two polymer cholesteric LC films with right‐handed helices of different pitches. Electrotunable non‐reciprocal light transmittance and unidirectional circularly polarized (CP) lasing emission have been successfully demonstrated for this device structure. Two left CP (LCP) lasing emission peaks are observed at the edges of the overlapping region between the two photonic bands in the structure and are shifted upon the application of a voltage. In contrast, a non‐reciprocal right CP (RCP) lasing emission peak emerges at one of the band edges and diminishes upon the application of a voltage. These phenomena are interpreted based on the selective reflection of RCP light and the reorientation of the NLC molecules by the application of a voltage. 相似文献
95.
96.
97.
K- and Q-bands CMOS frequency sources with X-band quadrature VCO 总被引:1,自引:0,他引:1
Sangsoo Ko Jeong-Geun Kim Taeksang Song Euisik Yoon Songcheol Hong 《Microwave Theory and Techniques》2005,53(9):2789-2800
Fully integrated 10-, 20-, and 40-GHz frequency sources are presented, which are implemented with a 0.18-/spl mu/m CMOS process. A 10-GHz quadrature voltage-controlled oscillator (QVCO) is designed to have output with a low dc level, which can be effectively followed by a frequency multiplier. The proposed multipliers generate signals of 20 and 40 GHz using the harmonics of the QVCO. To have more harmonic power, a frequency doubler with pinchoff clipping is used without any buffers or dc-level shifters. The QVCO has a low phase noise of -118.67 dBc/Hz at a 1-MHz offset frequency with a 1.8-V power supply. The transistor size effect on phase noise is investigated. The frequency doubler has a low phase noise of -111.67 dBc/Hz at a 1-MHz offset frequency is measured, which is 7 dB higher than a phase noise of the QVCO. The doubler can be tuned between 19.8-22 GHz and the output is -6.83 dBm. A fourth-order frequency multiplier, which is used to obtain 40-GHz outputs, shows a phase noise of -102.0 dBc/Hz at 1-MHz offset frequency with the output power of -18.0 dBm. A large tuning range of 39.3-43.67 GHz (10%) is observed. 相似文献
98.
中国西南武陵山区跳蛛新种记述(蜘蛛目:跳蛛科) 总被引:2,自引:0,他引:2
本文记述跳蛛科13个新种,采自西南武陵山地区。暗跳蛛Asemonea、列锡蛛Lycidas和法老蛛Pharacocerus 3属为我国新记录属。 相似文献
99.
100.
Xuefeng Yue Xinliang Lu Yongyuan Song Zongshu Shao Daliang Sun Quanzhong Jiang Huanchu Chen 《Applied physics. B, Lasers and optics》1991,53(5-6):319-322
The use of Cu-doped (K0.5Na0.5)0.2(Sr0.61Ba0.39)0.9Nb2O6 as self-pumped phase conjugators using internal reflection is reported. Reflectivities as high as 63% at 5145.5 nm and 60% at 632.8 nm were realized. It was found that the photorefractive response changes significantly at different doping concentration. The responses of the crystals to the laser wavelength, incident angle and dopant concentration are studied. 相似文献