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   410篇
  免费   75篇
  国内免费   64篇
航空   271篇
航天技术   140篇
综合类   32篇
航天   106篇
  2024年   4篇
  2023年   17篇
  2022年   12篇
  2021年   21篇
  2020年   21篇
  2019年   15篇
  2018年   20篇
  2017年   33篇
  2016年   17篇
  2015年   19篇
  2014年   26篇
  2013年   22篇
  2012年   37篇
  2011年   27篇
  2010年   19篇
  2009年   22篇
  2008年   27篇
  2007年   18篇
  2006年   26篇
  2005年   28篇
  2004年   8篇
  2003年   18篇
  2002年   8篇
  2001年   4篇
  2000年   11篇
  1999年   5篇
  1998年   7篇
  1997年   3篇
  1996年   10篇
  1995年   10篇
  1994年   12篇
  1993年   3篇
  1992年   7篇
  1991年   4篇
  1990年   2篇
  1989年   3篇
  1988年   1篇
  1984年   2篇
排序方式: 共有549条查询结果,搜索用时 15 毫秒
511.
A strong X-ray emission is one of the defining signatures of nuclear activity in galaxies. According to the Unified Model for Active Galactic Nuclei (AGN), both the X-ray radiation and the prominent broad emission lines, characterizing the optical and UV spectra of Type 1 AGNs, are originated in the innermost regions of the sources, close to the Super Massive Black Holes (SMBH), which power the central engine. Since the emission is concentrated in a very compact region (with typical size r?0.1r?0.1 pc) and it is not possible to obtain resolved images of the source, spectroscopic studies of this radiation represent the only valuable key to constrain the physical properties of matter and its structure in the center of active galaxies. Based on previous studies on the physics of the Broad Line Region (BLR) and on the X-ray spectra of broad (FWHMHβ ? 2000 km s−1) and narrow line (1000 km s−1 ?FWHMHβ ? 2000 km s−1) emitting objects, it has been observed that the kinematic and ionization properties of matter close to the SMBHs are related together, and, in particular, that ionization is higher in narrow line sources. Here we report on the study of the optical and X-ray spectra of a sample of Type 1 AGNs, selected from the Sloan Digital Sky Survey (SDSS) database, within an upper redshift limit of z=0.35z=0.35, and detected at X-ray energies. We present analysis of the broad emission line fluxes and profiles, as well as the properties of the X-ray continuum and Fe Kα emission and we use these parameters to assess the consistency of our current AGN understanding.  相似文献   
512.
Here we present a new method for subtracting the Balmer pseudocontinuum in the UV part of type 1 AGN spectra. We calculate the intensity of the Balmer pseudocontinuum using the prominent Balmer lines in AGN spectra. We apply the model on a sample of 293 type 1 AGNs from SDSS database, and found that our model of Balmer pseudocontinuum + power law continuum very well fits the majority of AGN spectra from the sample, while in ∼15% of AGNs, the model fits reasonable the UV continuum, but a discrepancy between the observed and fitted spectra is noted. Some of the possible reasons for the discrepancy may be a different value for the optical depth in these spectra than used in our model or the influence of the intrinsic reddening.  相似文献   
513.
介绍了用于天基空间碎片监测的主要载荷,包括可见光监测相机、微波雷达和激光雷达。根据评价指标体系,按照评价指标设计的原则,设计出评价指标的体系结构,建立基于AHP(采用层次分析法)的综合指标评估模型。利用评估模型实现对三种监测载荷进行评价分析,选择出效能最优的有效载荷。为天基监测载荷评估提供科学依据与评估方法。  相似文献   
514.
This article carries out synthetic measurements and analysis of the characteristics of the asymmetric surface dielectric barrier discharge plasma aerodynamic actuation.The rotational and vibrational temperatures of an N2 (C3Пu) molecule are measured in terms of the optical emission spectra from the N2 second positive system.A simplified collision-radiation model for N2(C) and N2+(B) is established on the basis of the ratio of emission intensity at 391.4 nm to that at 380.5 nm and the ratio of emission intensity at 371.1 nm to that at 380.5 nm for calculating temporal and spatial averaged electron temperatures and densities.Under one atmosphere pressure,the electron temperature and density are on the order of 1.6 eV and 1011cm-3 respectively.The body force induced by the plasma aerodynamic actuation is on the order of tens of mN while the induced flow velocity is around 1.3m/s.Starting vortex is firstly induced by the actuation;then it develops into a near-wall jet,about 70 mm downstream of the actuator.Unsteady plasma aerodynamic actuation might stimulate more vortexes in the flow field.The induced flow direction by nanosecond discharge plasma aerodynamic actuation is not parallel,but vertical to the dielectric layer surface.  相似文献   
515.
介绍了天线系数的有关特性。阐述了天线系数变化对电场辐射发射测量的影响。最后分析了天线系数变化的原因。  相似文献   
516.
现有关于空间太阳能电站(SpaceSolarPowerStation,SSPS)轨道动力学的研究中,均将其放置于地球静止轨道(GeostationaryOrbit,GEO〖BF〗),然而这并非最优的工作轨道。文章提出了一种优于GEO的地球同步拉普拉斯(GeosynchronousLaplacePlane,GLP)轨道。首先,建立了轨道运动模型及影响轨道运动的摄动模型,包括地球非球形引力摄动、日月引力摄动、太阳光压力摄动及微波反冲力摄动;然后,提出了评估空间太阳能电站轨道的3个指标:接收功率、轨道适用性和安全性,并据此分析了GLP轨道相对于GEO的优势。最后,给出了数值仿真算例。结果表明:在发电功率大致相同且满足供电需求的情况下,工作在GLP上的SSPS每年大约能节省用于轨道保持的燃料364534kg。  相似文献   
517.
低重力状态下末级火箭剩余推进剂空间排放干扰分析   总被引:2,自引:0,他引:2  
廖少英 《上海航天》1995,12(4):13-18,54
根据流体力学原理、流型分析,对低重力状态下剩余推进剂空间排放干扰进行了分析、计算。分析认为,采取对称的排放平衡器能满足空间排放姿态稳定控制系统的要求。  相似文献   
518.
一种半导体致冷/热模糊逻辑温度控制系统   总被引:1,自引:0,他引:1  
介绍一种用MCS-51系列单片机汇编语言软件编程方法,实现用温度模糊逻辑控制算法构成的微波移相器和微波衰减器恒温控制器。在环境温度为-10~40℃范围内,温度设定值在15℃时,该控制器控温精度达到±0.5℃。  相似文献   
519.
F-12芳纶纤维缠绕壳体水压实验AE检测及剩余强度预报   总被引:1,自引:0,他引:1  
采用声发射仪,对F-12芳纶纤维缠绕复合材料壳体水压实验过程进行了低压AE检测,选用设计爆破压强的43%以下的AE数据,建立了一个低压下的AE参量与爆玻压力之间的多元线性回归方程式。该方程式对壳体爆破压力的预测值与实测值符合较好,从而为降低复合材料壳体水压安全评定实验压强提供了新途径。它既保证了经过水压实验后的复合材料壳体有足够的承载能力,又能可靠地预报其爆破压强。  相似文献   
520.
Solar and stellar flares are highly structured in space and in time, as is indicated for example by their radio signatures: the narrowband spikes, type III, type II and IV, and pulsation events. Structured in time are also the not flare related type I events (noise storms). The nature of this observationally manifest fragmentation is still not clear. Either, it can be due to stochastic boundary or initial conditions of the respective processes, such as inhomogeneities in the coronal plasma. Or else, a deterministic non-linear process is able to cause complicated patterns of these kinds.We investigate the nature of the fragmentation in time. The properties of processes we enquire are stationarity, periodicity, intermittency, and, with dimension estimating methods, we try to discriminate between stochastic and low-dimensional deterministic processes. Since the measured time series are rather short, the dimension estimate methods have to be used with care: we have developed an extended dimension estimate procedure consisting of five steps. Among others, it comprises again the questions of stationarity and intermittency, but also the more technical problems of temporal correlations, judging scaling and convergence, and limited number of data points (statistical limits).We investigate 3 events of narrowband spikes, 13 type III groups, 10 type I storms, 3 type II bursts and 1 type IV event of solar origin, and 3 pulsation-like events of stellar origin. They have in common that all of them have stationary phases, periodicities are rather seldom, and intermittency is quite abundant. However, the burst types turn out to have different characteristics. None of the investigated time series reveals a low-dimensional behaviour. This implies that they originate from complex processes having dimensions (degrees of freedom) larger than about 4 to 6, which includes infinity,i. e. stochasticity. The lower limit of the degrees of freedom is inferred from numerical experiments with known chaotic systems, using time series of similar lengths, and it depends slightly on the burst types.  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