全文获取类型
收费全文 | 51502篇 |
免费 | 1704篇 |
国内免费 | 251篇 |
专业分类
电工技术 | 976篇 |
综合类 | 299篇 |
化学工业 | 7966篇 |
金属工艺 | 1703篇 |
机械仪表 | 1284篇 |
建筑科学 | 1313篇 |
矿业工程 | 213篇 |
能源动力 | 1080篇 |
轻工业 | 5740篇 |
水利工程 | 419篇 |
石油天然气 | 313篇 |
武器工业 | 11篇 |
无线电 | 4141篇 |
一般工业技术 | 9318篇 |
冶金工业 | 12942篇 |
原子能技术 | 570篇 |
自动化技术 | 5169篇 |
出版年
2023年 | 343篇 |
2022年 | 342篇 |
2021年 | 763篇 |
2020年 | 632篇 |
2019年 | 582篇 |
2018年 | 1135篇 |
2017年 | 1035篇 |
2016年 | 1159篇 |
2015年 | 972篇 |
2014年 | 1229篇 |
2013年 | 2569篇 |
2012年 | 1837篇 |
2011年 | 2094篇 |
2010年 | 1676篇 |
2009年 | 1689篇 |
2008年 | 1938篇 |
2007年 | 1893篇 |
2006年 | 1444篇 |
2005年 | 1370篇 |
2004年 | 1079篇 |
2003年 | 1096篇 |
2002年 | 1072篇 |
2001年 | 1013篇 |
2000年 | 904篇 |
1999年 | 1047篇 |
1998年 | 4250篇 |
1997年 | 2692篇 |
1996年 | 1940篇 |
1995年 | 1263篇 |
1994年 | 1046篇 |
1993年 | 1046篇 |
1992年 | 518篇 |
1991年 | 514篇 |
1990年 | 484篇 |
1989年 | 495篇 |
1988年 | 428篇 |
1987年 | 420篇 |
1986年 | 370篇 |
1985年 | 491篇 |
1984年 | 380篇 |
1983年 | 356篇 |
1982年 | 307篇 |
1981年 | 377篇 |
1980年 | 368篇 |
1979年 | 277篇 |
1978年 | 250篇 |
1977年 | 482篇 |
1976年 | 842篇 |
1975年 | 206篇 |
1973年 | 226篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
This paper presents a 20-Gb/s 1:4-demultiplexer for future fiber-optic transmission systems. It uses an 0.4-μm emitter double polysilicon 21-GHz fT Si bipolar foundry process. This is the highest data rate of a 1:4-DEMUX reported so far in any technology. The 1:4-DEMUX features a tree-type architecture with one frequency divider and a channel switch circuit. The circuit design was carefully optimized to achieve high speed and moderate power dissipation. It consumes 1.4 W with a single -4.5-V supply 相似文献
83.
High-power passively mode-locked semiconductor lasers 总被引:8,自引:0,他引:8
Haring R. Paschotta R. Aschwanden A. Gini E. Morier-Genoud F. Keller U. 《Quantum Electronics, IEEE Journal of》2002,38(9):1268-1275
We have developed optically pumped passively mode-locked vertical-external-cavity surface-emitting lasers. We achieved as much as 950 mW of mode-locked average power in chirped 15-ps pulses, or 530 mW in 3.9-ps pulses with moderate chirp. Both lasers operate at a repetition rate of 6 GHz and have a diffraction-limited output beam near 950 nm. In continuous-wave operation, we demonstrate an average output power as high as 2.2 W. Device designs with a low thermal impedance and a smooth gain spectrum are the key to such performance. We discuss design and fabrication of the gain structures and, particularly, their thermal properties 相似文献
84.
A Dick G Adam E Spüntrup A Prescher A Mühler RW Günther 《Canadian Metallurgical Quarterly》1996,165(4):392-397
PURPOSE: In an experimental pyogenic liver abscess model, the signal intensities were compared intraindividually and interindividually after the application of a new blood pool contrast agent, 24-gadolinium-DTPA (diethylenetriamine-pentaacetic acid) cascade polymer, and after the application of gadopentetate dimeglumine. METHODS: In 20 rabbits with experimentally induced liver abscesses, the relative signal intensities of the liver, abscess centre, abscess wall and portal vein were assessed before and between 30 seconds and 60 minutes after injection of a 25 mumol/kg dose of gadolinium polymer and of 100 mumol/kg of gadolinium-DTPA, respectively. Measurements were performed at 1.5 Tesla, using a head coil and a Flash-2-D sequence. RESULTS: The interindividual comparison (unpaired T-test, p < 0.05) yielded significant differences of the relative signal intensities of the abscess centre (at any time point after contrast-media application), abscess wall (between 15 and 60 minutes after contrast media application), and portal vein (between 30 seconds and 7.5 minutes after contrast media application). The interindividual comparison showed a significantly higher abscess centre-liver contrast (between 30 seconds and 12.5 minutes after contrast media application) and a significantly higher abscess wall-centre contrast (between two and 7.5 minutes after contrast media application) after the application of gadolinium polymer compared with gadopentetate dimeglumine. CONCLUSION: In this animal model, the higher abscess centre-liver contrast after the application of gadolinium polymer was the basis for a better and prolonged visibility of the abscesses, as compared with images acquired after injection of gadopentetate dimeglumine. 相似文献
85.
This paper presents an analysis of the sensitivity of coupled resonator filters in which some direct couplings are missing. The effect of changes in the coupling coefficients and resonant frequencies of the resonators is investigated by directly computing the gradient of the scattering parameters. It is shown that structures that are modular in the input-to-output direction are much less sensitive than those with modularity in the orthogonal direction for the same frequency response. 相似文献
86.
87.
88.
Survival of pure and mixed cultures of Yersinia enterocolitica and Escherichia coli in sterile spring water stored at 4 degrees C was evaluated. Y. enterocolitica increased two or three logs during the first 3 weeks of incubation, and at the end of 64 weeks viable cells in numbers corresponding to the initial inoculum level were still detectable. However, after 1 week of incubation, the number of E. coli in water started to decrease and in 13 weeks time no E. coli was detected. More rapid decrease of E. coli was observed in mixed cultures. 相似文献
89.
90.
Madhow U. Pursley M.B. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》1991,37(2):291-306
The problem of communication over a channel with unknown characteristics is addressed. The true channel is from a known set of channels, but the transmitter and receiver do not know which of these channels is actually in effect. The goal of a universal receiver is to provide nearly optimal demodulation regardless of the channel that is actually in effect. A parallel receiver implementation is proposed for a universal scheme to cope with such uncertainty. The parallel system consists of a finite number of receivers with the property that, for each channel in the set, the performance of at least one of the receivers will be within a specified performance range. Data verification is accomplished by an appropriate coding system. Sufficient conditions for the existence of such a universal receiver for a prescribed set of channels are established, procedures are outlined for the receiver design, and an example is given to illustrate the applicability of the theory. For M -ary signaling it is shown that, from an information-theoretic viewpoint, the data verification can be achieved at no extra cost by use of the intrinsic side information that is provided by an appropriate coding scheme that also provides error correction 相似文献