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   10540篇
  免费   1191篇
  国内免费   1297篇
航空   5914篇
航天技术   3132篇
综合类   1192篇
航天   2790篇
  2023年   34篇
  2022年   137篇
  2021年   184篇
  2020年   195篇
  2019年   114篇
  2018年   181篇
  2017年   183篇
  2016年   165篇
  2015年   281篇
  2014年   427篇
  2013年   477篇
  2012年   565篇
  2011年   677篇
  2010年   676篇
  2009年   707篇
  2008年   726篇
  2007年   604篇
  2006年   508篇
  2005年   472篇
  2004年   347篇
  2003年   360篇
  2002年   353篇
  2001年   357篇
  2000年   351篇
  1999年   403篇
  1998年   389篇
  1997年   337篇
  1996年   268篇
  1995年   260篇
  1994年   300篇
  1993年   211篇
  1992年   224篇
  1991年   162篇
  1990年   136篇
  1989年   163篇
  1988年   100篇
  1987年   81篇
  1986年   56篇
  1985年   121篇
  1984年   87篇
  1983年   85篇
  1982年   67篇
  1981年   108篇
  1980年   27篇
  1979年   27篇
  1978年   33篇
  1977年   25篇
  1975年   39篇
  1972年   33篇
  1971年   25篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
A new method of nonlinear spectral analysis (called the method of global minimum: MGM), based on the best presentation (in sense of minimal squares) of a given time data set as a sum of sinusoids whose frequencies, amplitudes and phases are to be determined, has been used to find periodicities in annual Wolf sunspot numbers (W) during the period 1700–1995. The possible future behaviour of the 11-year solar cycle (based on an extrapolation of the calculated model) is also presented. The main characteristics of the 23rd solar cycle are as follows: the W maximum occurs about 2004, with a peak of nearly 220. An unusually large value of W will occur during the 23rd cycle, which should be characterised by the longest maximum, specifically, W will be greater than 100 during the 11-year period from 1997 to 2007. The first sharp rise will occur during the period 1996–1998, the second sharp during 2002–2004. The main features of the 24 year cycle are as follows: the next minimum in W, associated with the 24th solar cycle, should occur in the year 2008 and the maximum in 2014. W is expected to peak at about 180. The minimum value for the 25th year cycle is expected to occur in the year 2019. It is shown that the accuracy of these predictions depends, first of all, on the extrapolation of the hyperlong harmonic of the calculated polyharmonic model fit of observed annual sunspot numbers during the period 1700–1995. The error bars in the definition of the maximum and minimum epochs can be as large as two years.  相似文献   
53.
The Electron Radiation Belt   总被引:4,自引:0,他引:4  
Electron radiation belts can change dramatically in a few seconds or slowly over years. Important issues in understanding such changes are: (1) What is the source of electrons in the radiation belts? (2) How important is radial diffusion compared to other radial transport mechanisms? (3) What are the detailed changes in the magnetosphere that produce radial diffusion? (4) Why is the response of the electron radiation belt to changes in the solar wind different from that of substorms and of the ring current? (5) Are processes other than radial transport, such as wave-particle interactions, important in energizing electrons in the radiation belts?  相似文献   
54.
Eagle-Picher Energy Products (EPEP) has been manufacturing and testing large lithium ion cells (up to 100-Ah) for several years. Recently, work has focused on the testing of different chemistries at variable temperatures and designing and fabricating 100-Ah cylindrical cells. For the aircraft application the largest concern is irreversible capacity loss at elevated temperatures (70°C). In contrast, for the aerospace application shelf-life and cycle life are critical. EPEP has found that the major contributor to the loss in low temperature performance due to high temperature testing, was the positive electrode. EPEP discuss recent results of variable temperature cycling and 100-Ah cell performance  相似文献   
55.
An observer-type of Kalman innovation filtering algorithm to find a practically implementable "best" Kalman filter, and such an algorithm based on the evolutionary programming (EP) optima-search technique, are proposed, for linear discrete-time systems with time-invariant unknown-but-hounded plant and noise uncertainties. The worst-case parameter set from the stochastic uncertain system represented by the interval form with respect to the implemented "best" filter is also found in this work for demonstrating the effectiveness of the proposed filtering scheme. The new EP-based algorithm utilizes the global optima-searching capability of EP to find the optimal Kalman filter and state estimates at every iteration, which include both the best possible worst case Interval and the optimal nominal trajectory of the Kalman filtering estimates of the system state vectors. Simulation results are included to show that the new algorithm yields more accurate estimates and is less conservative as compared with other related robust filtering schemes  相似文献   
56.
With the increase of information capacity of an airborne system, high speed driving of laser diodes have been of interest. In order to match the low resistive LD, proposed in this paper is an exponential transmission line, which is used as preshaping to compensate the pattern effect in gain switching of laser diodes. It also will be used to overcome the effect of parasitic elements in high speed driving to improve the rise and trail edges of the output waveform. An accurate circuit model of the exponential line for the PSPICE program has been developed and simulations are conveniently carried out by employing PSPICE  相似文献   
57.
介绍一种高压快开阀装置。它可产生370MPa上升时间小于20μs的阶跃压力,用于对高压传感器进行动态校准。  相似文献   
58.
本文利用弹性力学和断裂力学方法对纤维增强复合材料的横向拉伸强度进行了分析,其中考虑了90度开裂层厚度和开裂邻层的约束效应的影响,推导出正交各向异性对称铺层中昨合就位横向拉伸强度的数学表达式,得出开裂层的厚度和约束邻层铺设角对就位横向拉伸强度都有影响的结论,理论计算与Flaggs实验结果吻合较好。  相似文献   
59.
We present new measurements concerning generation of light flash during hypervelocity impacts. We use iron particles (10−13 to 10−17 kg) with velocities over the range 1 to 42 km/s impacting semi-infinite targets (aluminium and molybdenum). The main results of previous work in the field are found to be reproduced with some slight deviations. For iron projectiles with given mass and velocity the energy of the flash (normalized to mass) is proportional to velocity to the power of 3.5 for aluminium targets and 3.9 for molybdenum targets. The duration of the flash is of order 1 microsecond. Simultaneous measurements of the generation of impact plasma do not change this. The onset of plasma generation of the bulk target material does not affect the total light flash energy. We discuss the duration of the flash compared to a simple calculation of temperature in the target and plasma vs time.  相似文献   
60.
This paper describes original research efforts in the design, simulation, and development of nanotechnology-based molecular test equipment (MTE). This is a research effort for testing printed circuit boards independent of traditional automatic test equipment (ATE) through the fabrication of MTE within integrated circuits (ICs). The MTE is embedded within the IC substrate and encapsulated within nanoprobes that connect between the surface and the substrate of the IC at various functional areas. A process is followed whereby IC device simulation is performed to assess the electrical, chemical, and structural properties of integrated and adjacent substrate devices. Through this approach the nominal and failed device performance parameters of interest to substrate-based MTE are found. Discussion of the development and application of MTE within IC architectures is provided, including such topics as the effect of substrate composition on the design and realization of MTE, interfaces between MTE and IC devices, and reporting of MTE results to the IC surface and technician. Potential application areas within different device functions will also be identified. A chemical structure diagram is also provided to illustrate the implementation of MTE using discrete device configurations with MTE-augmented logic  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