全文获取类型
收费全文 | 608397篇 |
免费 | 10190篇 |
国内免费 | 2843篇 |
专业分类
电工技术 | 11939篇 |
技术理论 | 5篇 |
综合类 | 1501篇 |
化学工业 | 94177篇 |
金属工艺 | 22788篇 |
机械仪表 | 17538篇 |
建筑科学 | 16666篇 |
矿业工程 | 2435篇 |
能源动力 | 15008篇 |
轻工业 | 60602篇 |
水利工程 | 5600篇 |
石油天然气 | 7527篇 |
武器工业 | 256篇 |
无线电 | 73873篇 |
一般工业技术 | 112890篇 |
冶金工业 | 113573篇 |
原子能技术 | 10024篇 |
自动化技术 | 55028篇 |
出版年
2021年 | 4635篇 |
2019年 | 4103篇 |
2018年 | 6490篇 |
2017年 | 6526篇 |
2016年 | 7098篇 |
2015年 | 5669篇 |
2014年 | 9164篇 |
2013年 | 27852篇 |
2012年 | 15911篇 |
2011年 | 22165篇 |
2010年 | 17403篇 |
2009年 | 19300篇 |
2008年 | 20801篇 |
2007年 | 21223篇 |
2006年 | 18938篇 |
2005年 | 17262篇 |
2004年 | 16152篇 |
2003年 | 15574篇 |
2002年 | 15258篇 |
2001年 | 15413篇 |
2000年 | 14411篇 |
1999年 | 14799篇 |
1998年 | 33600篇 |
1997年 | 24649篇 |
1996年 | 19323篇 |
1995年 | 14956篇 |
1994年 | 13427篇 |
1993年 | 13051篇 |
1992年 | 9991篇 |
1991年 | 9473篇 |
1990年 | 9213篇 |
1989年 | 8935篇 |
1988年 | 8540篇 |
1987年 | 7217篇 |
1986年 | 7202篇 |
1985年 | 8550篇 |
1984年 | 8164篇 |
1983年 | 7232篇 |
1982年 | 6650篇 |
1981年 | 6800篇 |
1980年 | 6341篇 |
1979年 | 6403篇 |
1978年 | 6034篇 |
1977年 | 6923篇 |
1976年 | 9165篇 |
1975年 | 5175篇 |
1974年 | 4918篇 |
1973年 | 4860篇 |
1972年 | 4008篇 |
1971年 | 3563篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
991.
Multiuser detection for DS-CDMA UWB in the home environment 总被引:20,自引:0,他引:20
We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that the adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interference for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate the adaptive MMSE is able to reject a narrowband IEEE 802.11a OFDM interferer, even for signal-to-interference ratio as severe as -30 dB. We show the adaptive MMSE exhibits only a 6 dB penalty relative to the single user case for the heavy multi-access interference (number of asynchronous users equal to spreading code length). The practical RAKE receivers were incapable of effectively rejecting either the strong narrowband interference or the heavily loaded wideband interference. Even more moderate levels of interference caused significant degradation in the performance of the practical RAKE receivers. 相似文献
992.
Krahenbuhl R. Howerton M.M. Dubinger J. Greenblatt A.S. 《Lightwave Technology, Journal of》2002,20(1):92-99
High-performance Y-branch digital optical switches realized in Ti:LiNbO3 are presented. Their switching response functions have been optimized in terms of switch voltage and crosstalk ratio. The optimization is based on analyzing different types of waveguide shaping and switching arrangements using coupled mode theory and computer simulations. Excellent switching characteristics are achieved with devices exploiting a specially shaped waveguide branch in a dilated switch arrangement. Demonstrated performances include switching voltage as low as 9 V with crosstalk suppression better than 45 dB and fiber-to-fiber losses as low as 4 dB. Polarization independence with crosstalk suppression better than 40 dB over a 1520- to 1570-nm wavelength range is achieved for any applied switch voltage greater than 18 V. These optimized digital optical switches have further demonstrated the capability to reshape electrical input signals at switching rates of several hundred megahertz 相似文献
993.
Low-loss (<6 dB), flat-top, 16-wavelength, 100-GHz channel spacing DWDM (de)multiplexers are reported. The bulk-optics architecture used allows excellent thermal stability for operating temperature [-10, 60°C] 相似文献
994.
The interfacial reactions between liquid In and Cu substrates at temperatures ranging from 175°C to 400°C are investigated
for the applications in bonding recycled sputtering targets to their backing plates. Experimental results show that a scallop-shaped
Cu16In9 intermetallic compound is found at the Cu/In interface after solder reactions at temperatures above 300°C. A double-layer
structure of intermetallic compounds containing scallop-shaped Cu11In9 and continuous CuIn is observed after the Cu/In interfacial reaction at temperatures below 300°C. The growth of all these
intermetallic compounds follows the parabolic law, which implies that the growth is diffusion-controlled. The activation energies
for the growth of Cu16In9, Cu11In9, and CuIn intermetallic compounds calculated from the Arrhenius plot of growth reaction constants are 59.5, 16.9, and 23.5
kJ/mole, respectively. 相似文献
995.
Sung D. Kwon Sung J. Song Dong H. Bae Young Z. Lee 《Journal of Mechanical Science and Technology》2002,16(9):1084-1092
The frequency dependency of Rayleigh surface wave is investigated indirectly by measuring the angular dependency of the backward
radiation of the incident ultrasonic wave in two kinds of degraded specimens by scuffing or corrosion. Then, the frequency
dependency is compared with the residual stress distribution or the corrosion-fatigue characteristics for the scuffed or corroded
specimens, respectively. The width of the backward radiation profile increases with the increase of the variation in residual
stress distribution for the scuffed specimens. In the corroded specimens, the profile width decreases with the increase of
the effective aging layer thickness and is inversely proportional to the exponent, m, in the Paris’ law that can predict the
crack size increase due to fatigue. The result observed in this study demonstrates high potential of backward radiated ultrasound
as a tool for nondestructive evaluation of subsurface gradient of material degradation generated by scuffing or corrosion. 相似文献
996.
997.
The stability of the reactive interface during the solid-state displacement reaction, Cu2O+Co1−X
Fe
X
=2Cu+(Co1−X
Fe
X
)O, is studied as a function of Co-Fe alloy composition at 1223 K. For X≤0.03, the reaction zone has a layered structure, and the cation diffusion in (Co, Fe)O is the rate-limiting step. The interface
is unstable in the early stages of the reaction; the instability decreases with time as the oxide thickness increases, and
the interface becomes planar at long times. The time required for the attainment of interface planarity increases with the
value of X. The reaction kinetics are consistent with the available cation-diffusion data in (Co, Fe)O. For X≥0.045, the product zone is a composite of Cu+(Co, Fe)O, and the rate is limited by the oxygen transport in copper. The transition
to interface instability occurs when the oxide can support a cation flux that exceeds the maximum possible oxygen flux in
copper. During the reaction, composition gradients develop in (Co, Fe)O because of higher diffusion rates for iron than for
cobalt. 相似文献
998.
本文设计了用于千兆以太网基带铜缆接收器均衡的甚高频自适应连续时间Gm-C二阶带通滤波器。基于最陡梯度下降算法,带通滤波器的零点在57-340MHz的频率范围内可以自适应地调节,中心频率为1.278GHz。通过外接电阻伺服环路,滤波器中跨导转换器的跨导值不受工艺偏差和温度变化的影响,采用CSMC-HJ0.6μm CMOS工艺器件模型,用Cadence Spectres仿真器仿真了设计的自适应滤波器电路,仿真结果验证了设计原理和设计的电路。系统的最长学习时间为880个参考时钟周期。 相似文献
999.
1000.
This report deals with direct observations of microcrack linkage at the crack tip prior to macroscopic crack initiation. It is shown that this process is directly linked to the inclusion distribution in the material and can be considered in terms of an R curve to describe microcrack development. 相似文献