全文获取类型
收费全文 | 174365篇 |
免费 | 8306篇 |
国内免费 | 3801篇 |
专业分类
电工技术 | 5736篇 |
技术理论 | 9篇 |
综合类 | 7151篇 |
化学工业 | 27588篇 |
金属工艺 | 9672篇 |
机械仪表 | 8598篇 |
建筑科学 | 9746篇 |
矿业工程 | 2711篇 |
能源动力 | 3748篇 |
轻工业 | 8800篇 |
水利工程 | 2825篇 |
石油天然气 | 5265篇 |
武器工业 | 558篇 |
无线电 | 20288篇 |
一般工业技术 | 27793篇 |
冶金工业 | 7522篇 |
原子能技术 | 1195篇 |
自动化技术 | 37267篇 |
出版年
2024年 | 393篇 |
2023年 | 1468篇 |
2022年 | 2646篇 |
2021年 | 3573篇 |
2020年 | 2636篇 |
2019年 | 2279篇 |
2018年 | 16529篇 |
2017年 | 15778篇 |
2016年 | 12294篇 |
2015年 | 3885篇 |
2014年 | 4651篇 |
2013年 | 5671篇 |
2012年 | 8781篇 |
2011年 | 15231篇 |
2010年 | 13410篇 |
2009年 | 10505篇 |
2008年 | 11424篇 |
2007年 | 12333篇 |
2006年 | 5122篇 |
2005年 | 5610篇 |
2004年 | 4076篇 |
2003年 | 3806篇 |
2002年 | 2871篇 |
2001年 | 2225篇 |
2000年 | 2573篇 |
1999年 | 2739篇 |
1998年 | 2533篇 |
1997年 | 1981篇 |
1996年 | 1854篇 |
1995年 | 1493篇 |
1994年 | 1252篇 |
1993年 | 913篇 |
1992年 | 680篇 |
1991年 | 576篇 |
1990年 | 418篇 |
1989年 | 377篇 |
1988年 | 329篇 |
1987年 | 182篇 |
1986年 | 171篇 |
1985年 | 116篇 |
1984年 | 99篇 |
1983年 | 63篇 |
1982年 | 60篇 |
1981年 | 55篇 |
1980年 | 41篇 |
1968年 | 45篇 |
1966年 | 43篇 |
1965年 | 46篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
The noise characteristics of semiconductor laser amplifiers (SLAs) in the Ga1-xInxAs/GaInAsP/InP strained quantum well (QW) system are theoretically calculated and analyzed using density-matrix theory and taking into account the effects of band mixing on both the valence subbands and the transition dipole moments. The numerical results show that a reduced noise figure can be obtained in both tensile and compressively strained QW structures due to the increase in differential gain and the decrease in transparent carrier density. From a comparison among compressively strained (x=0.70), unstrained (x=0.53), and tensile strained (x=0.40) QW SLAs at a fixed carrier density and optical confinement factor, it is found that the noise figure of the tensile strained QW reaches its lowest value of 3.4 dB at average input optical power of -20 dB 相似文献
172.
173.
A Linear Cross-Coupled Control System for High-Speed Machining 总被引:4,自引:0,他引:4
Q. Zhong Y. Shi J. Mo S. Huang 《The International Journal of Advanced Manufacturing Technology》2002,19(8):558-563
We present a linear cross-coupled controller to improve highspeed contouring accuracy independently of tracking accuracy in
a biaxial machine tool feed drive servomechanism. Unlike conventional cross-coupled controllers, the cross-coupled controller
presented here is a linear system, so it is very easy to perform the stability and steady-state error analysis, and to optimise
the controller parameters. The proposed controller is evaluated experimentally on a CNC LOM machine and compared to an uncoupled
controller and a conventional cross-coupled controller. Controller performance is evaluated for a circular contour at a feedrate
of 30 m min _1 . The experimental results show that the proposed controller can greatly reduce the contour error at large feedrates. The
linear cross-coupled controller is simple to implement and is practical. 相似文献
174.
175.
Certain new sulfide-smelting processes and coal-gasification processes generate high-strength sulfur dioxide streams, for
which a new process for converting sulfur dioxide to elemental sulfur needs to be developed because no process exists that
is generally and economically applicable to the treatment of such streams. A thermodynamic and experimental investigation
to develop a new process for converting sulfur dioxide to elemental sulfur by a cyclic process involving calcium sulfide and
calcium sulfate without generating secondary pollutants was carried out. In this process, the starting raw material, calcium
sulfate, is reduced by a suitable reducing agent, such as hydrogen, to produce calcium sulfide, which is used to reduce sulfur
dioxide to elemental sulfur vapor and calcium sulfate. The latter is, in turn, reduced to regenerate calcium sulfide. In this
Part I, detailed experimental results are presented on the kinetics of the reaction between sulfur dioxide and calcium-sulfide
pellets, which produces elemental sulfur and calcium sulfate. The experiments were carried out at temperatures between 1023
and 1088 K and sulfur-dioxide partial pressures between 9 and 60 kPa by the use of a thermogravimetric analysis (TGA) technique.
The rate of this reaction was demonstrated by the conversion of 40 pct calcium-sulfide pellets obtained from the hydrogen
reduction of fresh calcium sulfate in 10 minutes at 1073 K under a sulfur-dioxide partial pressure of 43 kPa. The reactivity
decreased somewhat during the first three cycles but remained largely unchanged thereafter up to the tenth cycle. This characteristic
of the pellets is important because the solids must be reusable for repeated cycles to avoid generating secondary pollutants.
A pore-blocking model was found to fit the reaction rate. The reaction is first order with respect to sulfur-dioxide partial
pressure and has an activation energy of 101 to 134 kJ/mol (24 to 32 kcal/mol) for calcium-sulfide pellets reacted and regenerated
several different times. Sulfur dioxide-containing streams from certain sources, such as the regenerator off-gas from an integrated-gasification,
combined-cycle, desulfurization unit and new sulfide-smelting plants, contain much higher partial pressures of SO2. In these cases, the rate of the first reaction is expected to be proportionally higher than in the test conditions reported
in this article. The reduction kinetics of calcium-sulfate pellets with hydrogen gas is reported in the accompanying Part
II. 相似文献
176.
T. T. Wang T. H. Huang 《Tunnelling and Underground Space Technology incorporating Trenchless Technology Research》2002,17(4):425-436
Many tunnels will be constructed in southwestern Taiwan in the upcoming decade to meet the huge demands of transportation, energy and water infrastructure projects. Mudstone strata cover more than a 1000 km2 area, consisting mainly of massive mudstone or alternation of mudstone and sandy layer, exhibiting unfavorable geological and hydrological characteristics, making it extremely difficult for tunnelling. This paper presents the lessons learned from three tunnelling projects in the 1990s in a mudstone area. In addition to discussing the rock behavior in the vicinity of tunnel and its failure patterns, the monitoring data during tunnelling are presented as well. Finally, the effective method of design and construction are recommended for tunnelling in mudstone area. 相似文献
177.
178.
油田污水中咪唑啉缓蚀剂浓度检测技术研究 总被引:5,自引:0,他引:5
研究评价了油田污水中咪唑啉缓蚀剂浓度检测的七种方法 ,其中紫外光分析法和显色反应法适合于低浓度咪唑啉缓蚀剂检测 ,且方法简便、速度快、成本低 ,检测浓度范围 2~ 5 0mg/L。 相似文献
179.
本文针对大型储罐罐顶整体桁架结构尺寸大、质量大、易变形的特点,详细介绍了一种切实可行的整体吊装及旋转就位方法。从选定吊装方案入手,阐叙了实现本方法的思路和技术细节,提出了实施过程中的注意事项。实践证明,该方法能保证安装质量,安全、可靠、可行。 相似文献
180.