全文获取类型
收费全文 | 707525篇 |
免费 | 9992篇 |
国内免费 | 2100篇 |
专业分类
电工技术 | 13166篇 |
综合类 | 830篇 |
化学工业 | 105197篇 |
金属工艺 | 27019篇 |
机械仪表 | 20930篇 |
建筑科学 | 17180篇 |
矿业工程 | 3094篇 |
能源动力 | 19064篇 |
轻工业 | 61593篇 |
水利工程 | 7025篇 |
石油天然气 | 12007篇 |
武器工业 | 100篇 |
无线电 | 83541篇 |
一般工业技术 | 135732篇 |
冶金工业 | 138155篇 |
原子能技术 | 14055篇 |
自动化技术 | 60929篇 |
出版年
2021年 | 6193篇 |
2020年 | 4542篇 |
2019年 | 5902篇 |
2018年 | 10014篇 |
2017年 | 9882篇 |
2016年 | 10360篇 |
2015年 | 7106篇 |
2014年 | 11833篇 |
2013年 | 32240篇 |
2012年 | 18814篇 |
2011年 | 25957篇 |
2010年 | 20442篇 |
2009年 | 22961篇 |
2008年 | 23851篇 |
2007年 | 23469篇 |
2006年 | 20691篇 |
2005年 | 18965篇 |
2004年 | 18255篇 |
2003年 | 17971篇 |
2002年 | 17071篇 |
2001年 | 17061篇 |
2000年 | 16072篇 |
1999年 | 16884篇 |
1998年 | 42838篇 |
1997年 | 30134篇 |
1996年 | 23204篇 |
1995年 | 17547篇 |
1994年 | 15352篇 |
1993年 | 15053篇 |
1992年 | 11141篇 |
1991年 | 10392篇 |
1990年 | 10130篇 |
1989年 | 9731篇 |
1988年 | 9119篇 |
1987年 | 8115篇 |
1986年 | 7981篇 |
1985年 | 8988篇 |
1984年 | 8433篇 |
1983年 | 7508篇 |
1982年 | 7087篇 |
1981年 | 7234篇 |
1980年 | 6827篇 |
1979年 | 6662篇 |
1978年 | 6332篇 |
1977年 | 7616篇 |
1976年 | 9777篇 |
1975年 | 5606篇 |
1974年 | 5350篇 |
1973年 | 5485篇 |
1972年 | 4430篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
1-GHz-spaced 16-channel arrayed-waveguide grating for a wavelength reference standard in DWDM network systems 总被引:3,自引:0,他引:3
We fabricated a 1-GHz-spaced 16-channel arrayed-waveguide grating (AWG) by using a new AWG configuration where the path of each arrayed waveguide winds backward and forward across a 4-in diameter wafer without crossing any other waveguides. The ultra-narrow (< 1 GHz) and stable transmission bands of this AWG can be used to construct a wavelength reference standard covering the S, C, and L bands in the dense wavelength-division-multiplexing network systems whose frequency deviation is /spl plusmn/160 MHz. 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.
Beyond stabilizer codes II: Clifford codes 总被引:1,自引:0,他引:1
Klappenecker A. Rotteler M. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(8):2396-2399
For pt. I see ibid., vol.48, no.8, p.2392-95 (2002). Knill (1996) introduced a generalization of stabilizer codes, called Clifford codes. It remained unclear whether or not Clifford codes can be superior to stabilizer codes. We show that Clifford codes are stabilizer codes provided that the abstract error group has an Abelian index group. In particular, if the errors are modeled by tensor products of Pauli matrices, then the associated Clifford codes are necessarily stabilizer codes. 相似文献
994.
Portable system for data acquisition and transmission based on handheld PC technology 总被引:1,自引:0,他引:1
A portable system is developed to monitor physical activity by acquiring and storing data from body-mounted sensors; a handheld PC is used for managing data acquisition and storage, including radio-transmission to a remote host. An application related to the use of a gyro-accelerometric sensor for determining walking distance is discussed. 相似文献
995.
Tonchev V.D. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(4):975-977
It is proved that a class of q-ary (2n,n) formally self-dual codes obtained from symmetric matrices over GF (q), contains codes that meet the Varshamov-Gilbert bound. The codes are self-dual with respect to the symplectic inner product and yield quantum codes encoding one state with n q-ary qubits and having minimum distance proportional to n 相似文献
996.
Oliveira M. Dart-Mao Cheung C. Al-Zayed A. Chio I.-F.F. Swisher R.R. Lecuyer F. DeLisio M.P. 《Antennas and Propagation, IEEE Transactions on》2002,50(5):698-702
Grid-mixer arrays can achieve high linearity and dynamic range through quasi-optical power combining. We present a 100-element single-ended diode grid mixer operating at 2.45 GHz. Each element incorporates two diodes in series. We measure an input third-order intercept of 11 W (40.5 dBm), and output third-order intercept of 3.4 W (35.4 dBm), and an associated conversion loss of 5.1 dB. The power-handling capability of the array is 100 times larger than that of a microstrip mixer using a single element. The local oscillator (LO) drive requirement for the entire array is 1.4 W (31.6 dBm). The angular dependence of the array's IF power is also presented and is in agreement with theory 相似文献
997.
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
998.
Timo M. Kauppinen 《Journal of Housing and the Built Environment》2002,17(2):173-197
Finland has experienced a rapid increase inimmigration since the beginning of the 1990s.Almost half of the foreign population isconcentrated in the metropolitan area of thecapital, Helsinki. The development andexplanations of their settlement patterns havenot yet been thoroughly studied. It ishypothesised in this study that social housingcan explain the settlement patterns. Indices ofdissimilarity were calculated to study thelevel and development of spatial segregation ofimmigrant groups in the Helsinki metropolitanarea in 1995–99, and the association betweenthe settlement patterns of immigrants andsocial housing was studied by regressionanalysis. Neighbourhood-level data were used.The results indicate that the Helsinkimetropolitan area has avoided extreme ethnicsegregation so far and that segregation isdecreasing. Especially immigrants from poorcountries are clearly associated with socialhousing in their settlement patterns, evenafter adjustment for socio-economic anddemographic differentiation. Also newconstruction of social housing seems to haveaffected the spatial distribution of thisgroup. Despite this dependence, the group isnot extremely segregated, which is probablybecause the social housing sector is nottotally marginalised. 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