全文获取类型
收费全文 | 49354篇 |
免费 | 15861篇 |
国内免费 | 546篇 |
专业分类
电工技术 | 1510篇 |
综合类 | 690篇 |
化学工业 | 18921篇 |
金属工艺 | 918篇 |
机械仪表 | 1459篇 |
建筑科学 | 2507篇 |
矿业工程 | 279篇 |
能源动力 | 1171篇 |
轻工业 | 7959篇 |
水利工程 | 569篇 |
石油天然气 | 344篇 |
武器工业 | 116篇 |
无线电 | 8422篇 |
一般工业技术 | 12832篇 |
冶金工业 | 1514篇 |
原子能技术 | 119篇 |
自动化技术 | 6431篇 |
出版年
2024年 | 52篇 |
2023年 | 223篇 |
2022年 | 448篇 |
2021年 | 814篇 |
2020年 | 1841篇 |
2019年 | 3492篇 |
2018年 | 3487篇 |
2017年 | 3802篇 |
2016年 | 4221篇 |
2015年 | 4403篇 |
2014年 | 4483篇 |
2013年 | 5797篇 |
2012年 | 3565篇 |
2011年 | 3207篇 |
2010年 | 3368篇 |
2009年 | 3224篇 |
2008年 | 2775篇 |
2007年 | 2581篇 |
2006年 | 2243篇 |
2005年 | 1868篇 |
2004年 | 1741篇 |
2003年 | 1659篇 |
2002年 | 1634篇 |
2001年 | 1407篇 |
2000年 | 1296篇 |
1999年 | 643篇 |
1998年 | 283篇 |
1997年 | 247篇 |
1996年 | 158篇 |
1995年 | 100篇 |
1994年 | 114篇 |
1993年 | 96篇 |
1992年 | 58篇 |
1991年 | 43篇 |
1990年 | 41篇 |
1989年 | 51篇 |
1988年 | 43篇 |
1987年 | 38篇 |
1986年 | 41篇 |
1985年 | 35篇 |
1984年 | 24篇 |
1983年 | 17篇 |
1982年 | 10篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 15篇 |
1974年 | 4篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
101.
102.
A route to synthesize ZSM‐5 crystals with a bimodal micro/mesoscopic pore system has been developed in this study; the successful incorporation of the mesopores within the ZSM‐5 structure was performed using tetrapropylammonium hydroxide (TPAOH)‐impregnated mesoporous materials containing carbon nanotubes in the pores, which were encapsulated in the ZSM‐5 crystals during a solid rearrangement process within the framework. Such mesoporous ZSM‐5 zeolites can be readily obtained as powders, thin films, or monoliths. 相似文献
103.
Generalized multiuser orthogonal space-division multiplexing 总被引:8,自引:0,他引:8
Zhengang Pan Kai-Kit Wong Tung-Sang Ng 《Wireless Communications, IEEE Transactions on》2004,3(6):1969-1973
This paper addresses the problem of performing orthogonal space-division multiplexing (OSDM) for downlink, point-to-multipoint communications when multiple antennas are utilized at the base station (BS) and (optionally) all mobile stations (MS). Based on a closed-form antenna weight solution for single-user multiple-input multiple-output communications in the presence of other receiver points, we devise an iterative algorithm that finds the multiuser antenna weights for OSDM in downlink or broadcast channels. Upon convergence, each mobile user will receive only the desired activated spatial modes with no cochannel interference. Necessary and sufficient conditions for the existence of OSDM among the number of mobile users, the number of transmit antennas at the BS, and the number of receive antennas at the MS, are also derived. The assumption for the proposed method is that the BS knows the channels for all MS's and that the channel dynamics are quasi-stationary. 相似文献
104.
Hambleton P.J. Ng B.K. Plimmer S.A. David J.P.R. Rees G.J. 《Electron Devices, IEEE Transactions on》2003,50(2):347-351
The nonlocal enhancement in the velocities of charge carriers to ionization is shown to outweigh the opposing effects of dead space, increasing the avalanche speed of short avalanche photodiodes (APDs) over the predictions of a conventional local model which ignores both of these effects. The trends in the measured gain-bandwidth product of two short InAlAs APDs reported in the literature support this result. Relatively large speed benefits are predicted to result from further small reductions in the lengths of short multiplication regions. 相似文献
105.
T. Choi J.‐H. Jang C.K. Ullal M.C. LeMieux V.V. Tsukruk E.L. Thomas 《Advanced functional materials》2006,16(10)
The probing of the micromechanical properties within a two‐dimensional polymer structure with sixfold symmetry fabricated via interference lithography reveals a nonuniform spatial distribution in the elastic modulus “imprinted” with an interference pattern in work reported by Tsukruk, Thomas, and co‐workers on p. 1324. The image prepared by M. Lemieux and T. Gorishnyy shows how the interference pattern is formed by three laser beams and is transferred to the solid polymer structure. The elastic and plastic properties within a two‐dimensional polymer (SU8) structure with sixfold symmetry fabricated via interference lithography are presented. There is a nonuniform spatial distribution in the elastic modulus, with a higher elastic modulus obtained for nodes (brightest regions in the laser interference pattern) and a lower elastic modulus for beams (darkest regions in the laser interference pattern) of the photopatterned films. We suggest that such a nonuniformity and unusual plastic behavior are related to the variable material properties “imprinted” by the interference pattern. 相似文献
106.
Woo‐Seok Cheong 《ETRI Journal》2003,25(6):503-509
Selective epitaxial growth (SEG) of silicon has attracted considerable attention for its good electrical properties and advantages in building microstructures in high‐density devices. However, SEG problems, such as an unclear process window, selectivity loss, and nonuniformity have often made application difficult. In our study, we derived processing diagrams for SEG from thermodynamics on gas‐phase reactions so that we could predict the SEG process zone for low pressure chemical vapor deposition. In addition, with the help of both the concept of the effective supersaturation ratio and three kinds of E‐beam patterns, we evaluated and controlled selectivity loss and nonuniformity in SEG, which is affected by the loading effect. To optimize the SEG process, we propose two practical methods: One deals with cleaning the wafer, and the other involves inserting dummy active patterns into the wide insulator to prevent the silicon from nucleating. 相似文献
107.
108.
109.
Kun-Wah Yip Tung-Sang Ng 《Communications Letters, IEEE》2003,7(2):58-60
This letter analyzes Bluetooth's power-control algorithm with a goal to study the mean transmit power required in the presence of lognormal shadowing. The following results are found. 1) A smaller power-control step size yields a lower mean transmit power. 2) When the standard deviations of lognormal shadowing are 3 and 6 dB, respectively, a Bluetooth device needs to consume 11.1 dB and 15.0 dB more in the transmit energy than the minimum one required in the absence of shadowing. 3) The transmit energy consumption varies by around 6 dB among Bluetooth devices as a result of the /spl plusmn/6 dB tolerance in the golden receive power range, which has a nominal size of 20 dB. 相似文献
110.
Polyaryloxydiphenylsilanes were prepared from phosphorus‐containing diols and diphenydichlorolsilane through solution polymerization. With a stoichiometric imbalance in feed monomers, the resulting polymers exhibited moderate melting points and good processing properties. The polymers prepared showed initial decomposition temperatures above 340 °C, excellent thermal stability, high char yields at 850 °C and very high limited oxygen index values of 56–59. The polymers' char yields and their (P + Si) contents showed linear relationships. © 2003 Society of Chemical Industry 相似文献