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1.
王晓云  邓伟  张龙  苏鑫  赵世卓 《电信科学》2022,38(11):11-23
大气波导干扰是特定气象条件下发生的时分双工(time-division duplex,TDD)系统内干扰,是TDD移动通信系统大规模组网面临的顽疾。在总结分析大气波导干扰成因和分类等的基础上,对大气波导干扰进行建模和表征,验证了海量干扰源在时域和频域的功率集总特征,并结合大量4G/5G现网实测数据给出了典型条件下内陆波导和海面波导的量化干扰信号传播模型,对于干扰的预测和预防具有重要意义。基于干扰特征,给出了TDD系统预防大气波导干扰的帧结构与组网的4项设计原则,5G现网数据表明干扰控制方案有效,上行干扰下降10 dB以上,相关原则对于6G系统的设计也具有指导意义。  相似文献   
2.
《Ceramics International》2022,48(18):25984-25995
Design of architectured composites with layered-ordered structure can solve the strength-toughness mismatch problem of structural materials. In the present study, heterostructure Ti6Al4V/TiAl laminated composite sheets with different thicknesses of interface layer and TiAl composite layer were successfully produced by hot-pressing technology. The effects of interface regulation and laminated structure on their mechanical properties, crack propagation, and fracture behavior were studied. The results indicated that compressive strength of the sheets increased with the decrease in interface thickness. Compressive strength of TiAl composite sheet with thicker composite layer reached 1481.55 MPa at the arrester orientation with sintering holding time of 40 min, which was 25.96% higher than that of the sheet obtained at 120 min. Analysis indicated that the interface area transferred stress through slip bands and through-interface cracks. Compressive strength at the divider orientation reached 1443.06 MPa, which was 45.78% higher than that of the sheet obtained at 120 min. In this case, the interface area transferred stress through slip bands and along-interface cracks. For TiAl composite sheets with thinner composite layer, compressive strength was further improved to 1631.01 MPa and 1594.66 MPa at the arrester and divider orientations with sintering holding time of 40 min, respectively. The ductile metal layer exerted a significant toughening effect. Both interface regulation and laminated structure transformation could enhance the hetero-deformation induced (HDI) strengthening and improve the comprehensive mechanical properties of the composite sheets.  相似文献   
3.
This study presents an improved mathematical model to analyse the stress wave propagation in adhesively bonded functionally graded (FG) circular cylinders (butt joint) under an axial impulsive load. The volume fractions of the material constituents in the upper and lower cylinders were functionally tailored through the thickness of each cylinder using a power-law. The effective material properties of both cylinders, which are made of aluminum (Al) and silicon carbide (SiC), at any point were predicted by using the Mori–Tanaka homogenization scheme. In this improved model, the governing equations of the wave propagation include the spatial derivatives of local mechanical properties and were discretized by means of the finite difference method. The influence of these spatial derivatives and the compositional gradient exponent on the displacement and stress distributions of the joint was investigated. The material composition variations of both cylinders affected the displacement and stress fields whereas the compositional gradient exponent had a minor effect. The stress concentrations were alleviated in time, the displacement and stress distributions/variations around/along the upper and lower cylinder-adhesive interfaces were significantly affected by the adhesive layer. The spatial derivatives also affected the temporal histories of the displacement and stress components evaluated at the selected critical points of the upper cylinder, adhesive layer and lower cylinder. The consideration of the spatial local material derivatives provided a more accurate mathematical model of wave propagations through the graded layered structures.  相似文献   
4.
The phase shift characteristics reflect the state change of electromagnetic wave in plasma sheath and can be used to reveal deeply the action mechanism between electromagnetic wave and plasma sheath. In this paper, the phase shift characteristics of electromagnetic wave propagation in plasma were investigated. Firstly, the impact factors of phase shift including electron density,collision frequency and incident frequency were discussed. Then, the plasma with different electron density distribution profiles were employed to investigate the influence on the phase shift characteristics. In a real case, the plasma sheath around the hypersonic vehicle will affect and even break down the communication. Based on the hypersonic vehicle model, we studied the electromagnetic wave phase shift under different flight altitude, speed, and attack angle. The results indicate that the phase shift is inversely proportional to the flight altitude and positively proportional to the flight speed and attack angle. Our work provides a theoretical guidance for the further research of phase shift characteristics and parameters inversion in plasma.  相似文献   
5.
This paper presents results of experimental investigations on spherical and cylindrical flame propagation in pre-mixed H2/air-mixtures in unconfined and semi-confined geometries. The experiments were performed in a facility consisting of two transparent solid walls with 1 m2 area and four weak side walls made from thin plastic film. The gap size between the solid walls was varied stepwise from thin layer geometry (6 mm) to cube geometry (1 m). A wide range of H2/air-mixtures with volumetric hydrogen concentrations from 10% to 45% H2 was ignited between the transparent solid walls. The propagating flame front and its structure was observed with a large scale high speed shadow system. Results of spherical and cylindrical flame propagation up to a radius of 0.5 m were analyzed. The presented spherical burning velocity model is used to discuss the self-acceleration phenomena in unconfined and unobstructed pre-mixed H2/air flames.  相似文献   
6.
王腾  张哲 《岩土工程学报》2019,41(10):1921-1927
竖向循环荷载作用下桩土界面的作用机理是研究桩土摩擦疲劳的关键。针对循环荷载作用下桩-粉土界面的剪切性能,使用改进的剪切试验装置在恒刚度条件下进行循环剪切试验,研究循环次数、累积位移和法向刚度对其摩擦疲劳性能、循环后单调剪切性能的影响。试验结果表明,粉土在循环剪切过程中,法向应力和剪应力在初始10个循环内随循环数增加快速衰减,随着循环进行,逐渐趋于稳定;单次循环内在剪切位移方向变化时,土体呈现表现出剪缩-剪胀-剪缩交替现象,总体变形呈现剪缩的趋势;循环荷载作用下,粉土界面的法向应力和剪应力随法向刚度增大衰减速率增大,达到稳定的累积循环位移越小;粉土循环后的单调剪切、法向应力恢复的单调剪切的剪应力比小于首次单调剪切试验值,且法向应力恢复的循环后剪切试验的剪胀程度较小,表明循环剪切过程中界面处粉土颗粒棱角破碎,颗粒变得光滑。在对试验数据分析的基础上,提出了与累积位移、法向刚度和初始应力相关的无量纲累积位移,建立了法向应力和界面摩擦角随累积位移的衰减方程。  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.
Anisoplanatic electromagnetic (EM) propagation across a turbulent atmosphere has been recently examined for an unmodulated carrier propagating over an image-bearing transparency through optical lensing, and for the embedded information inside a carrier recovered using heterodyning and digital demodulation. Carrier modulation yielded better recovery than simple lens-based imaging. A possible mitigation strategy is proposed whereby the image information is encrypted on an RF chaotic carrier, thereafter secondarily embedded onto an optical carrier. Results based on the modified von Karman (MVKS) and the Hufnagel-Valley (H-V) models showed that the signal/image recovery under turbulence is improved compared with non-chaotic propagation. The case of time-varying/dynamic images is also taken up; it is demonstrated via cross-correlation products that turbulence is mitigated by the use of chaotic carrier encryption. Overall, transmission via chaos offers mitigation against distortions due to turbulence along with the security feature inherent via the chaos keys which prevent signal recovery without key-matching.  相似文献   
9.
熵模糊物元分析法在水库正常高水位决策中的应用   总被引:4,自引:0,他引:4       下载免费PDF全文
基于信息熵理论和模糊物元分析理论,探讨了水库正常高水位的熵模糊物元决策模型.以待选方案与最优方案间的海明贴近度来决策水库正常高水位.在确定评价综合指标信息熵的熵权和专家知识权重的的过程中,充分挖掘待选方案的固有信息和避免个人主观因素的弊端.实例应用表明,该方法有效、可行.  相似文献   
10.
多裂纹混凝土等效断裂模型研究   总被引:2,自引:0,他引:2  
根据混凝土材料断裂过程区存在尺寸效应这一特点,结合混凝土断裂韧度的尺寸效应规律,对混凝土有效裂缝扩展量进行了研究,得到反映尺寸效应规律的有效裂缝扩展量表达式,进而得到与试件尺寸相关的混凝土多裂纹等效断裂模型,使得多裂纹等效断裂模型更符合断裂过程区的尺寸效应规律。同时,通过与在实际工程中已用的其他模型的计算结果相比较,得出此模型比较可靠的结论。  相似文献   
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