全文获取类型
收费全文 | 176637篇 |
免费 | 2383篇 |
国内免费 | 704篇 |
专业分类
电工技术 | 3290篇 |
综合类 | 101篇 |
化学工业 | 27876篇 |
金属工艺 | 7775篇 |
机械仪表 | 5120篇 |
建筑科学 | 4410篇 |
矿业工程 | 875篇 |
能源动力 | 4677篇 |
轻工业 | 16372篇 |
水利工程 | 1705篇 |
石油天然气 | 3126篇 |
武器工业 | 2篇 |
无线电 | 19845篇 |
一般工业技术 | 33313篇 |
冶金工业 | 33113篇 |
原子能技术 | 4291篇 |
自动化技术 | 13833篇 |
出版年
2021年 | 1312篇 |
2019年 | 1246篇 |
2018年 | 2095篇 |
2017年 | 2077篇 |
2016年 | 2186篇 |
2015年 | 1570篇 |
2014年 | 2716篇 |
2013年 | 7779篇 |
2012年 | 4548篇 |
2011年 | 6360篇 |
2010年 | 5033篇 |
2009年 | 5881篇 |
2008年 | 5861篇 |
2007年 | 5836篇 |
2006年 | 5013篇 |
2005年 | 4681篇 |
2004年 | 4473篇 |
2003年 | 4145篇 |
2002年 | 4102篇 |
2001年 | 4125篇 |
2000年 | 3922篇 |
1999年 | 4054篇 |
1998年 | 10381篇 |
1997年 | 7389篇 |
1996年 | 5654篇 |
1995年 | 4281篇 |
1994年 | 3617篇 |
1993年 | 3565篇 |
1992年 | 2604篇 |
1991年 | 2537篇 |
1990年 | 2416篇 |
1989年 | 2435篇 |
1988年 | 2376篇 |
1987年 | 2126篇 |
1986年 | 2066篇 |
1985年 | 2369篇 |
1984年 | 2185篇 |
1983年 | 2012篇 |
1982年 | 1880篇 |
1981年 | 1944篇 |
1980年 | 1796篇 |
1979年 | 1816篇 |
1978年 | 1776篇 |
1977年 | 2109篇 |
1976年 | 2689篇 |
1975年 | 1557篇 |
1974年 | 1549篇 |
1973年 | 1607篇 |
1972年 | 1351篇 |
1971年 | 1262篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
The scale along the coordinate axes in the equations of motion hitherto used for an H-maser (in relative units) is shown to depend on Q as the quality factor for an H-maser. This hinders both analysis of the operation of the H-maser and optimization of its parameters. The modified equations of motion proposed here do not have that shortcoming. We determine the atomic flux into the bulb that gives the H-maser the best metrological characteristics. 相似文献
132.
Spectrum asked three IEEE members from the telecommunications sector to comment on the industry's current woes and prospects. Roch Guerin discusses bandwidth as a commodity and how to deal with it. The impact of technological change on telecommunications is enormous and calls for a shift in business models. Technology is making bandwidth a commodity and it should not matter who supplies it. Competition is vital and commodity providers must be decoupled from value providers. Frank Ferrante points out that the Year 2000 crisis resulted in manufacturers being deluged with orders for new upgraded systems. Vast sums were spent to meet the demand in time. However, when Y2K was over, production stayed high and inventories continued to build, while demand effectively stopped overnight. Dazzling new innovations are needed to compel the masses to spend and manufacturers must apply their resources to deliver quickly and efficiently. Jules A. Bellisio discusses the role of engineers in bringing new ideas to the market place. Many engineers have put together ventures for the sole purpose of attracting capital. The underlying technology and business propositions were there solely to lure the unsuspecting. By allowing engineers to use technology simply to cover for a scam, then the profession is also to blame for the current problems. 相似文献
133.
134.
A novel driver-assist stability system for all-wheel-drive electric vehicles is introduced. The system helps drivers maintain control in the event of a driving emergency, including heavy braking or obstacle avoidance. The system comprises a fuzzy logic system that independently controls wheel torque to prevent vehicle spin. Another fuzzy wheel slip controller is used to enhance vehicle stability and safety. A neural network is trained to generate the required reference for yaw rate. Vehicle true speed is estimated by a sensor data fusion method. The intrinsic robustness of fuzzy controllers allows the system to operate in different road conditions successfully. Moreover, the ease of implementing fuzzy controllers gives a potential for vehicle stability enhancement. 相似文献
135.
A new dual-quantisation sigma-delta modulator is proposed, which introduces an additional feedback path in the input of the second integrator. In this way, unlike other dual-quantisation architectures, larger signal-to-noise ratios can be obtained by means of aggressive noise-shaping, like in a conventional multibit modulator. The proposed modulator is also shown to be more robust against non-idealities than other dual-quantisation architectures. 相似文献
136.
Demiguel S. Giraudet L. Joulaud L. Decobert J. Blache F. Coupe V. Jorge F. Pagnod-Rossiaux P. Boucherez E. Achouche M. Devaux F. 《Lightwave Technology, Journal of》2002,20(12):2004-2014
The design, fabrication, and performance of double-stage taper photodiodes (DSTPs) are reported. The objective of this work is to develop devices compatible with 40-Gb/s applications. Such devices require high efficiency, ultrawide band, high optical power handling capability, and compatibility with low-cost module fabrication. The integration of mode size converters improves both the coupling efficiency and the responsivity with a large fiber mode diameter. Responsivity of 0.6 A/W and 0.45 A/W are achieved with a 6-/spl mu/m fiber mode diameter and cleaved fiber, respectively, providing relaxed alignment tolerances (/spl plusmn/1.6 /spl mu/m and /spl plusmn/2 /spl mu/m, respectively), compatible with cost-effective packaging techniques. DSTPs also offer a wide bandwidth greater than 40 GHz and transverse-electric/transverse-magnetic polarization dependence lower than 0.2 dB. Furthermore, a DSTP saturation current as high as 11 mA results in optical power handling greater than +10 dBm and a high output voltage of 0.8 V. These capabilities allow the photodiode to drive the decision circuit without the need of a broad-band electrical amplifier. The DSTP devices presented here demonstrate higher responsivities with large fiber mode diameter and better optical power handling capabilities and are compared with classical side-illuminated photodiodes. 相似文献
137.
Martinez-Munoz D. Rosa-Zurera M. Cruz-Roldan F. Lopez-Ferreras F. Ruiz-Reyes N. 《Electronics letters》2002,38(16):932-933
The problem of computing, in a subband audio coder, the maximum quantisation noise power that can be injected in each band to ensure transparent coding when low selectivity filter banks are used, is addressed. A low complexity strategy, taking into account the frequency responses of the synthesis filter bank, is proposed for achieving an overall distortion due to quantisation noise always below the masking threshold (provided by a psycho-acoustic model) for any length prototype filters. 相似文献
138.
K. P. Lee S. J. Pearton M. E. Overberg C. R. Abernathy R. G. Wilson S. N. G. Chu N. Theodoropolou A. F. Hebard J. M. Zavada 《Journal of Electronic Materials》2002,31(5):411-415
In p-GaN implanted with Mn (3×1016 cm−2 at 250 keV), the material after annealing shows ferromagnetic properties below 250 K. Cross-sectional transmission electron
microscopy (TEM) revealed the presence of platelet structures with hexagonal symmetry. These regions are most likely GaxMn1−xN, which produce the ferromagnetic contribution to the magnetization. In p-GaN implanted with Fe, the material after annealing
showed ferromagnetic properties at temperatures that were dependent on the Fe dose, but were below 200 K in all cases. In
these samples, TEM and diffraction analysis did not reveal any secondary phase formation. The results for the Fe implantation
are similar to those reported for Fe doping during epitaxial growth of GaN. 相似文献
139.
Mine K. Kubota N. Morimoto F. Sanada M. Zhang Qi 《Electromagnetic Compatibility, IEEE Transactions on》1994,36(3):253-255
This short paper discusses the method of effectively canceling equal status normal mode noise not only on a sensor line but also on a transmission line of an optical instrument using a sensor with a sensordummy resistance 相似文献
140.
1/f noise sources 总被引:2,自引:0,他引:2
This survey deals with 1/f noise in homogeneous semiconductor samples. A distinction is made between mobility noise and number noise. It is shown that there always is mobility noise with an α value with a magnitude in the order of 10-4. Damaging the crystal has a strong influence on α, α may increase by orders of magnitude. Some theoretical models are briefly discussed none of them can explain all experimental results. The α values of several semiconductors are given. These values can be used in calculations of 1/f noise in devices 相似文献