全文获取类型
收费全文 | 7837篇 |
免费 | 525篇 |
国内免费 | 540篇 |
专业分类
电工技术 | 367篇 |
综合类 | 437篇 |
化学工业 | 791篇 |
金属工艺 | 662篇 |
机械仪表 | 446篇 |
建筑科学 | 505篇 |
矿业工程 | 144篇 |
能源动力 | 145篇 |
轻工业 | 520篇 |
水利工程 | 121篇 |
石油天然气 | 117篇 |
武器工业 | 59篇 |
无线电 | 743篇 |
一般工业技术 | 875篇 |
冶金工业 | 2138篇 |
原子能技术 | 119篇 |
自动化技术 | 713篇 |
出版年
2023年 | 77篇 |
2022年 | 176篇 |
2021年 | 246篇 |
2020年 | 164篇 |
2019年 | 155篇 |
2018年 | 155篇 |
2017年 | 158篇 |
2016年 | 155篇 |
2015年 | 189篇 |
2014年 | 253篇 |
2013年 | 348篇 |
2012年 | 356篇 |
2011年 | 374篇 |
2010年 | 368篇 |
2009年 | 387篇 |
2008年 | 363篇 |
2007年 | 392篇 |
2006年 | 343篇 |
2005年 | 283篇 |
2004年 | 190篇 |
2003年 | 174篇 |
2002年 | 173篇 |
2001年 | 138篇 |
2000年 | 137篇 |
1999年 | 139篇 |
1998年 | 685篇 |
1997年 | 405篇 |
1996年 | 259篇 |
1995年 | 156篇 |
1994年 | 151篇 |
1993年 | 172篇 |
1992年 | 57篇 |
1991年 | 68篇 |
1990年 | 60篇 |
1989年 | 58篇 |
1988年 | 58篇 |
1987年 | 61篇 |
1986年 | 54篇 |
1985年 | 42篇 |
1984年 | 39篇 |
1983年 | 34篇 |
1982年 | 34篇 |
1981年 | 37篇 |
1980年 | 51篇 |
1979年 | 33篇 |
1978年 | 29篇 |
1977年 | 70篇 |
1976年 | 136篇 |
1975年 | 37篇 |
1973年 | 35篇 |
排序方式: 共有8902条查询结果,搜索用时 15 毫秒
71.
平滑相位的π/4DQPSK调制及其在移动通信系统中的应用 总被引:3,自引:0,他引:3
本文详细介绍了一种新型π/ 4DQPSK调制解调方法 ,该方法将π/ 4DQPSK调制中的相位跳变改为平滑变化。仿真实验表明 ,与普通的π/ 4DQPSK相比 ,该方法使高次谐波分量大幅下降 ,可以有效降低功耗 ,压缩频带宽度。 相似文献
72.
73.
74.
75.
为改善协作分集系统的频谱效率,提出一种改进的跨层设计方案,对协作通信系统物理层的自适应调制编码(AMC)技术和链路层的混合自动重传(HARQ)协议进行联合优化设计。该方案利用少量比特的反馈,使得仅当目的节点通过直接信道不能正确译码分组时或者当直接信道处于深度衰落时触发中继节点转发协作伙伴数据。给出了所提方案基于瑞利衰落信道、解码转发(DF)条件下频谱效率的表达式,然后搜索在任意信噪比条件下使频谱效率最大化的调制与编码方案。通过计算机仿真对理论分析进行了验证。理论分析和仿真表明该跨层设计在中低信噪比(SNR)可进一步提升协作通信系统的频谱效率。 相似文献
76.
Alessandra Di Gaspare Eva Arianna Aurelia Pogna Luca Salemi Osman Balci Alisson Ronieri Cadore Sachin Maruti Shinde Lianhe Li Cinzia di Franco Alexander Giles Davies Edmund Harold Linfield Andrea Carlo Ferrari Gaetano Scamarcio Miriam Serena Vitiello 《Advanced functional materials》2021,31(10):2008039
An electrically switchable graphene terahertz (THz) modulator with a tunable-by-design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation-optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies. 相似文献
77.
Wavelength conversion due to gain saturation in a travelling wave semiconductor laser amplifier is analyzed using a small-signal model. An analytic expression is developed showing that the small-signal bandwidth of wavelength conversion is not limited simply to the carrier modulation bandwidth, but also depends on the single-pass gain of the device 相似文献
78.
79.
Tsz Ho Tse Z. Elhawary H. Zivanovic A. Rea M. Paley M. Bydder G. Davies B.L. Young I. Lamperth M.U. 《Mechatronics, IEEE/ASME Transactions on》2008,13(3):316-324
The "magic angle" effect consists of the increase in signal intensity observed at a tendon or cartilage in a magnetic resonance image, when the tissue is oriented at an angle of approximately 55deg with respect to the main magnetic field B0. The exploitation of this phenomenon is often used to assist diagnosis of tendinous and other diseases, although practical difficulties derived from positioning target tissue at the desired orientation inside closed-bore scanners has made this exploitation hard to implement. A 3-DOF MR-compatible mechatronic system has been developed to position a variety of limbs at the magic angle inside a closed- bore scanner, actuated by a custom-developed pneumatic air motor. The system is capable of locating the desired anatomy with high accuracy, and is designed to position the target tissue at a minimal distance from the isocenter. The compatibility of the system is demonstrated, producing negligible artifacts and an insignificant reduction in signal to noise of the image. Preliminary clinical trials scanning the Achilles tendon of healthy volunteers prove the functionality of the device. An increase in signal intensity of up to 21-fold has been recorded in the tendon at the magic angle. 相似文献
80.
Xiang Zhu D.T. Cassidy M.J. Hamp D.A. Thompson B.J. Robinson Q.C. Zhao M. Davies 《Photonics Technology Letters, IEEE》1997,9(9):1202-1204
InGaAsP-InP strained multiple-quantum-well (MQW) lasers for extended wavelength tunability in external cavity operation were designed, fabricated, and tested. The active layer was a strain compensated structure consisting of three 3.2/spl plusmn/0.3 nm and three 6.4/spl plusmn/0.3 nm 1.0% compressive strained wells and five 10.3/spl plusmn/0.3 nm 0.45% tensile strained barrier layers. A 2-/spl mu/m-wide ridge waveguide laser of length 250 /spl mu/m, when used in a grating external cavity and with no coatings to alter the reflectivity of the facets, was observed to operate over a range >110 nm. The lasers were designed for applications in trace gas and liquid detection with the goal to maximize the tunable range when operated in external cavities and with no facet coatings. 相似文献