首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3553篇
  免费   288篇
  国内免费   98篇
电工技术   99篇
综合类   82篇
化学工业   151篇
金属工艺   315篇
机械仪表   143篇
建筑科学   266篇
矿业工程   24篇
能源动力   254篇
轻工业   12篇
水利工程   18篇
石油天然气   12篇
武器工业   69篇
无线电   1474篇
一般工业技术   111篇
冶金工业   46篇
原子能技术   110篇
自动化技术   753篇
  2024年   1篇
  2023年   28篇
  2022年   44篇
  2021年   43篇
  2020年   57篇
  2019年   41篇
  2018年   45篇
  2017年   48篇
  2016年   73篇
  2015年   83篇
  2014年   209篇
  2013年   170篇
  2012年   156篇
  2011年   321篇
  2010年   290篇
  2009年   256篇
  2008年   298篇
  2007年   306篇
  2006年   214篇
  2005年   193篇
  2004年   174篇
  2003年   134篇
  2002年   123篇
  2001年   76篇
  2000年   85篇
  1999年   63篇
  1998年   75篇
  1997年   48篇
  1996年   48篇
  1995年   42篇
  1994年   46篇
  1993年   31篇
  1992年   31篇
  1991年   18篇
  1990年   20篇
  1989年   14篇
  1988年   11篇
  1987年   6篇
  1986年   10篇
  1985年   3篇
  1984年   2篇
  1983年   1篇
  1981年   1篇
  1974年   1篇
排序方式: 共有3939条查询结果,搜索用时 15 毫秒
1.
The use of a Pt-based catalyst was evaluated for autocatalytic hydrogen recombination. The Pt was supported on a mixture of Ce-, Zr- and Y-oxides (CZY) to yield nanosized Pt particles. The Pt/CZY/AAO catalyst was then prepared by the spray-deposition of the Pt/CZY intermediate onto an anodized aluminium oxide (AAO) layer on a metallic aluminum core. The Pt/CZY/AAO catalyst (3 × 1 cm) was evaluated for hydrogen combustion (1–8 vol% hydrogen in the air) in a recombiner section testing station. The thermal distribution throughout the catalyst surface was investigated using an infrared camera. The maximum temperature gradient (ΔT) for the examined hydrogen concentrations did not exceed 36 °C. The Pt/CZY/AAO catalyst was also evaluated for prolonged hydrogen combustion duration to assess its durability. An average combustion temperature of 239.0 ± 10.0 °C was maintained for 53 days of catalytic hydrogen combustion, suggesting that there was limited, or no, catalyst deactivation. Finally, a Pt/CZY/AAO catalytic plate (14.0 × 4.5 cm) was prepared to investigate the thermal distribution. An average surface temperature of 212.5 °C and a maximum ΔT of 5.4 °C was obtained throughout the catalyst surface at a 3 vol% hydrogen concentration.  相似文献   
2.
A novel spiral micromixer with sinusoidal channel walls was designed to enhance the mixing index in the low to intermediate Reynolds number range (1 < Re < 100). To analyze the fluid flow, a set of numerical simulations were performed using the finite-difference method. The microchip was fabricated from polydimethylsiloxane, employing the soft-lithography technique. The degree of mixing was increased by 99.11 % when using the proposed micromixer, compared to 59.44 % for a simple spiral micromixer. The introduced microchannel drastically reduced the mixing length, increasing the mixing index of a 0.5-loop spiral-sinusoidal microchannel compared to that of the simple spiral microchannel with 1.5 loops. The mixing index of the 3-loop mixer was higher than that of the microchannel with 1.5 loops, and its pressure drop was increased.  相似文献   
3.
【】:针对NB基站和新建基站采用C-RAN接入,导致城区光缆网纤芯资源紧张,提出利用无源波分复用设备方式进行扩容纤芯,但无源波分复用设备插入损耗,造成了全程链路衰耗增大,理论分析最大传输距离,给出了无源波分复用设备和彩光模块选型,最后结合光缆网现状,提出无源波分复用设备的应用方案。  相似文献   
4.
There are numerous researcher-developed mathematical models for single slope passive solar stills which considered the effect of heat capacities of basin and glass cover as negligible. The negligence parameter also plays a vital role in the prediction of the thermal performance of the system. In this paper, an effort has been made to evaluate the cumulative yield, energy efficiency and exergy efficiency of a single slope passive solar still with and without considering the heat capacities of basin and glass cover by using various thermal models such as Dunkle’s model, Zheng Hongfei et al.’s model and Tsilingiris model. These models were studied and compared with and without considering the heat capacity of solar components such as basin and glass cover. The thermodynamic model of this system is developed, and executed in a MATLAB code. By comparing the theoretical values of the cumulative yield, energy efficiency and exergy efficiency, it was found that consideration of the heat capacities of glass cover and basin gives a higher output in all the thermal models.  相似文献   
5.
地基雷达部署对探测临近空间高超声速目标影响研究   总被引:1,自引:0,他引:1  
针对不同地基雷达(GBR)部署方式对探测临近空间高超声速目标的性能影响问题,该文建立临近空间高超声速目标模型和GBR探测模型,依据目标雷达截面积(RCS)、探测距离和观测角随时间的变化情况,提出检测概率、跟踪系数和资源冗余率3种雷达探测性能评估指标,仿真分析GBR前沿部署、接力部署和要地部署方式对临近空间高超声速目标探测性能的影响。结果表明,前沿部署和接力部署相结合的探测效果好,前沿部署首次发现目标距离远,能提供的预警时间长,要地部署跟踪时间短,资源冗余率高。研究结果具有一定现实意义和工程实践性,能为临近空间预警系统中GBR部署提供理论依据和技术支撑。  相似文献   
6.
当干扰与目标方位夹角较小时,基于单圆阵的方位全向米波雷达在进行干扰对消时,目标回波的对消损失较大,从而降低了干扰对消后的目标检测概率和目标方位估计精度。针对此问题,该文提出一种基于双圆阵的干扰对消和目标方位角估计算法。该算法先利用小圆阵激励出的-1,0和1阶相位模式初步估计干扰的方位角,后利用大圆阵激励出对称高阶相位模式获得精确的干扰方位角并利用其激励出的非对称高阶相位模式进行干扰对消和目标方位角的估计。该算法相对于单圆阵的干扰对消和测角方法可以减小目标回波的对消损失,提高干扰对消后的信噪比,从而增大目标检测概率和提高目标方位角的估计精度,仿真结果证明了该算法的有效性。  相似文献   
7.
A number of worms, named P2P (peer-to-peer) passive worms, have recently surfaced, which propagate in P2P file-sharing networks and have posed heavy threats to these networks. In contrast to the majority of Internet worms, it is by exploiting users’ legitimate activities instead of vulnerabilities of networks in which P2P passive worms propagate. This feature evidently slows down their propagation, which results in them not attracting an adequate amount of attention in literature. Meanwhile, this feature visibly increases the difficulty of detecting them, which makes it very possible for them to become epidemic. In this paper, we propose an analytical model for P2P passive worm propagation by adopting epidemiological approaches so as to identify their behaviors and predict the tendency of their propagation accurately. Compared with a few existing models, dynamic characteristics of P2P networks are taken into account. Based on this proposed model, the sufficient condition for the global stability of the worm free equilibrium is derived by applying epidemiological theories. Large scale simulation experiments have validated both the proposed model and the condition.  相似文献   
8.
姚琳元  宋飞  张宏科 《电子学报》2015,43(3):557-567
无源光网络(Passive Optical Network,PON)作为当今接入网的主要技术解决方案,具有带宽使用效率高、传输距离远、抗干扰能力强等特点.通过研究PON技术的发展动态,本文首先归纳了各种PON技术的产生背景和应用特点,整理出各技术间的连接关系及主要标准;其次介绍了PON技术的帧结构,并对带宽、波长、传输模式等PON技术的主要参数进行了汇总;然后将国内外研究热点进行划分,围绕媒体访问控制协议、帧结构、动态带宽分配算法、节能机制等关键技术,阐述了其研究现状及在PON中的重要作用;最后对PON技术的发展趋势进行了展望.  相似文献   
9.
针对一般结构的分布式全相参雷达,研究了基于相关法的时延差和相位差估计性能以及输出信噪比增益(oSNRg)。首先分别建立多输入多输出(MIMO)模式和全相参(FC)模式下的信号模型。然后重点分析估计相位差时的相位模糊问题,并提出一种有效的解模糊方法。数值实验结果表明:当输入信噪比足够高时,相关法能够获得稳健的相干参数估计,且oSNRg接近理想值。  相似文献   
10.
Hydrogen, producing electricity in fuel cells, is a versatile energy source, but with risks associated with flammability. Fuel cells use enclosures for protection which need ventilating to remove hydrogen emitted during normal operation or from supply system leaks. Passive ventilation, using buoyancy driven flow is preferred to mechanical systems. Performance depends upon vent design, size, shape, position and number. Vents are usually plain rectangular openings, but environmentally situated enclosures use louvres for protection. The effect of louvres on passive ventilation is not clear and has therefore been examined in this paper. Comparison ‘same opening area’ louvre and plain vent tests were undertaken using a 0.144 m3 enclosure with opposing upper and lower vents and helium leaking from a 4 mm nozzle on the base at rates from 1 to 10 lpm, simulating a hydrogen leak. Louvres increased stratified level helium concentrations by typically in excess of 15%. The empirical data obtained was also used in a validation exercise with a SolidWorks: Flow Simulation CFD model, which provided a good qualitative representation of flow behaviour and close empirical data correlations.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号