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   85497篇
  免费   4612篇
  国内免费   2918篇
电工技术   3364篇
技术理论   5篇
综合类   3581篇
化学工业   13572篇
金属工艺   5301篇
机械仪表   4026篇
建筑科学   4402篇
矿业工程   1199篇
能源动力   2577篇
轻工业   7062篇
水利工程   1271篇
石油天然气   3404篇
武器工业   401篇
无线电   8749篇
一般工业技术   15167篇
冶金工业   8879篇
原子能技术   1335篇
自动化技术   8732篇
  2024年   322篇
  2023年   782篇
  2022年   1497篇
  2021年   2071篇
  2020年   1596篇
  2019年   1437篇
  2018年   1907篇
  2017年   2020篇
  2016年   2015篇
  2015年   2314篇
  2014年   3011篇
  2013年   5059篇
  2012年   4461篇
  2011年   5214篇
  2010年   4434篇
  2009年   4644篇
  2008年   4532篇
  2007年   4401篇
  2006年   3920篇
  2005年   3382篇
  2004年   2655篇
  2003年   2224篇
  2002年   2176篇
  2001年   2042篇
  2000年   1827篇
  1999年   1894篇
  1998年   2832篇
  1997年   2249篇
  1996年   1955篇
  1995年   1434篇
  1994年   1089篇
  1993年   1020篇
  1992年   813篇
  1991年   728篇
  1990年   637篇
  1989年   587篇
  1988年   455篇
  1987年   446篇
  1986年   418篇
  1985年   405篇
  1984年   328篇
  1983年   312篇
  1982年   313篇
  1981年   311篇
  1980年   367篇
  1979年   349篇
  1977年   416篇
  1976年   636篇
  1975年   312篇
  1973年   313篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In this paper, we propose a search technique that takes advantage of multipath, which has long been considered deleterious for efficient communication, to aid the sequence acquisition in dense multipath channels. We consider a class of serial-search strategies and use optimization and convexity theories to determine fundamental limits of achievable mean acquisition times (MATs). In particular, we derive closed-form expressions for both the minimum and maximum MATs and the conditions for achieving these limits. We prove that a fixed-step serial search, a form of nonconsecutive serial search, achieves a near-optimal MAT. We also prove that the conventional serial search, in which consecutive cells are tested serially, should be avoided as it results in the maximum MAT. Our results are valid for all signal-to-noise ratio (SNR) values, regardless of the specifics of the detection layer and the fading distributions  相似文献   
992.
A versatile exposure system which can be used to investigate the effects of a 60-Hz alternating magnetic field or traveling magnetic field on small laboratory animals is described. Special attention was given to the design of the electric circuit to simplify the supply requirements and to keep the overall current to a minimum. A magnetic field having a flux density up to 30 mT (300 G) can be generated, and a traveling magnetic field is obtained by reconnecting the coils to a three-phase supply. The total magnetic field due to the currents in the three coils used in the exposure system is analyzed. A computer program was written to evaluate the magnetic field in the region where the animals are located. The difference observed between measured and computed values is attributed to the representation of the multiturn, multilayer coils by equivalent single-turn coils  相似文献   
993.
Cathode interlayer is essential to inverted bulk heterojunction polymer solar cells (PSCs). A series of zwitterionic ammonium and neutral amino organic molecules are introduced into inverted PSCs as cathode interlayer and power conversion efficiency (PCE) as high as 8.07% is demonstrated. Compared to the devices without interlayer, all the devices exhibit significant improvements of the device parameters by reducing the work function of indium tin oxide (ITO) cathode. It is striking that the devices with neutral amino molecules as interlayer exhibit remarkably higher PCEs than the devices with zwitterionic ammonium molecules as interlayer. We attribute the improved performance to the better photoactive morphology induced by the hydrophobic properties of the neutral amino derivatives through research of ultraviolet photoelectron spectroscopy, atomic force microscopy, and contact angle measurements. Interestingly, the PCEs of the inverted PSCs with cathode interlayer are positively correlated with the hydrophobic properties of the interlayer materials, since devices with neutral amino molecules or molecules with a more hydrophobic alkyl pendant (piperidine) as interlayer exhibit higher PCEs. These results pave the way to the design of effective cathode interlayer materials.  相似文献   
994.
Motion tracking is of great importance in a wide range of fields such as automation, robotics, security, sports and entertainment. Here, a self‐powered, single‐electrode‐based triboelectric sensor (TES) is reported to accurately detect the movement of a moving object/body in two dimensions. Based on the coupling of triboelectric effect and electrostatic induction, the movement of an object on the top surface of a polytetrafluoroethylene (PTFE) layer induces changes in the electrical potential of the patterned aluminum electrodes underneath. From the measurements of the output performance (open‐circuit voltage and short‐circuit current), the motion information about the object, such as trajectory, velocity, and acceleration is derived in conformity with the preset values. Moreover, the TES can detect motions of more than one objects moving at the same time. In addition, applications of the TES are demonstrated by using LED illuminations as real‐time indicators to visualize the movement of a sliding object and the walking steps of a person.  相似文献   
995.
车辆电气系统智能故障诊断仪由电参量、继电器状态、发动机起动和蓄电池测试电路,信号调理板、数据采集卡、工控机组件及专家系统组成.设置扫描方式、采样频率和点数并进行A/D转换后,通过程序可绘制起动电流、电压曲线,计算起动时间,单位时间内电池端电压变化率及蓄电池剩余电量.基于诊断基元和产生式规则的专家系统,根据初始故障现象找出最底层故障基元,通过多层正反向混合推理实现故障最终定位.  相似文献   
996.
重复使用运载器 (RLV)将大幅度降低现役航天运载器的发射准备时间、成本 ,并提高其可靠性、安全性 ,是航天运输系统的发展方向。制导、导航与控制 (GN&C)系统是RLV的核心技术之一 ,先进的RLV的GN&C技术不仅能缩短设计周期、降低成本 ,而且对复杂多变的飞行环境具有较强的鲁棒性 ,从而提高了飞行器的安全性。指出了RLV的GN&C系统的要求、特点 ,总结了近年来该领域的研究成果 ,提出了下一步需研究的问题。  相似文献   
997.
本文研究了受激喇曼散射(SRS)对有级联掺铒光纤放大器(EDFA)的波分复用系统若干性能的影响.根据通信系统几率的特性,建立了SRS对通信系统误码率影响的理论模型,分析了它对输入光功率、信道数、信道间隔以及放大器个数和放大器间距的影响及相应限制.研究表明,合理采用较少的放大器数和较小的输入光功率,有利于保证系统的性能指标,降低系统造价,提高系统性价比;由最小误码率来综合确定合理的信道间隔和信道容量;通过合理的设置放大器及系统参数,可以使受激喇曼散射的影响减至最小,从而实现性能良好的长距离、大容量通信.  相似文献   
998.
We report a new method to fabricate self‐organized nanoporous titania films (pore diameter ≈ 30 nm; ≈ 1100 nm thick) and ordered titania nanorod arrays (rod diameter ≈ 30–60 nm; 70–260 nm high) by combined anodizing of superimposed Al/Ti layers sputter‐deposited on glass substrates. The titania nanostructures mimic the ordered nanoporous anodic alumina films via a through‐mask anodization. We propose a new anodizing electrolyte, i.e., a diluted nitric acid solution, for fabricating uniform, self‐organized, ordered nanoporous titania films with parallel cylindrical pores and without any thickness limit. More significantly, the nanoporous titania films contain a small amount of titanium nitride and dissociated nitrogen, and exhibit a moderate transparency and an enhanced absorption throughout the UV and visible light regions of the electromagnetic spectrum. After heating at 600 °C for 2 h, the nanoporous titania films develop a small absorption red‐shift and exhibit high photocatalytic activity under UV illumination.  相似文献   
999.
Z-source inverter for motor drives   总被引:2,自引:0,他引:2  
This paper presents a Z-source inverter system and control for general-purpose motor drives. The Z-source inverter system employs a unique LC network in the dc link and a small capacitor on the ac side of the diode front end. By controlling the shoot-through duty cycle, the Z-source can produce any desired output ac voltage, even greater than the line voltage. As a result, the new Z-source inverter system provides ride-through capability during voltage sags, reduces line harmonics, improves power factor and reliability, and extends output voltage range. Analysis, simulation, and experimental results will be presented to demonstrate these new features.  相似文献   
1000.
HfO2 films were deposited at low temperature (400 °C) by UV assisted injection metal-organic chemical vapor deposition (UVI-MOCVD). A three-step process was used for this study, consisting of (A) Pre-deposition anneal for nitridation; (B) Deposition step; (C) Post-deposition annealing in oxygen. Special attention was paid to the effect of UV exposure during these steps. Films were characterized by physical, optical and electrical techniques. Thickness was determined by different methods (X-ray Reflectrometry (XRR), spectroscopic ellipsometry and transmission electron microscopy) and a good agreement was found for all samples. The HfO2 permittivity, equivalent oxide thickness (EOT), flat-band voltage (Vfb) and total charge (Qt) were extracted from the CV response at high frequency taking into account the HfO2 and SiO2 thicknesses obtained by XRR. The calculated permittivity values were in the range 7–13, i.e. lower than theoretical values for the monoclinic phase. Explanations are suggested in the context of the other characterizations. JEeff characteristics were constructed taking into account the EOT values (Eeff = V/EOT). Effective breakdown fields range between 8.7 and 16.9 MV/cm. No dependence of Eeff with UV exposure was found.  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