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   6974篇
  免费   15篇
  国内免费   19篇
航空   3179篇
航天技术   2407篇
综合类   21篇
航天   1401篇
  2021年   73篇
  2019年   45篇
  2018年   211篇
  2017年   133篇
  2016年   114篇
  2015年   51篇
  2014年   182篇
  2013年   225篇
  2012年   215篇
  2011年   331篇
  2010年   233篇
  2009年   333篇
  2008年   370篇
  2007年   223篇
  2006年   156篇
  2005年   185篇
  2004年   183篇
  2003年   205篇
  2002年   145篇
  2001年   222篇
  2000年   127篇
  1999年   162篇
  1998年   190篇
  1997年   109篇
  1996年   171篇
  1995年   203篇
  1994年   189篇
  1993年   123篇
  1992年   150篇
  1991年   47篇
  1990年   48篇
  1989年   139篇
  1988年   59篇
  1987年   56篇
  1986年   62篇
  1985年   183篇
  1984年   145篇
  1983年   107篇
  1982年   115篇
  1981年   214篇
  1980年   49篇
  1979年   45篇
  1978年   50篇
  1977年   42篇
  1975年   50篇
  1974年   38篇
  1973年   32篇
  1972年   35篇
  1971年   35篇
  1970年   37篇
排序方式: 共有7008条查询结果,搜索用时 15 毫秒
991.
MultiWaveLink is an interactive, computerized data base that was developed to facilitate a multiwavelength approach to studying astrophysical sources. It can be used to access information about multiwavelength resources (observers, telescopes, data bases and analysis facilities) or to organize observing campaigns that require either many telescopes operating in different spectral regimes or a network of similar telescopes circumspanning the Earth.  相似文献   
992.
Mass motions below the photosphere drive the solar cycle which is associated with variations in the magnetic field structure and accompanying phenomena. In addition to semi-empirical models, dynamo theories have been used to explain the solar cycle. The emergence of magnetic field generated by these mechanisms and its expansion into the corona involves many plasma physical processes. Magnetic buoyancy aids the expulsion of magnetic flux. The corona may respond dynamically or by continually adjusting to a quasi-static force-free or pressure-balanced equilibrium. The formation and disruption of current sheets is significant for the overall structure of the coronal magnetic field and the physics of quiescent prominences. The corona has a fine structure consisting of magnetic loops. The structure and stability of these are important as they are one of the underlying elements which make up the corona.  相似文献   
993.
The high magnetic energy stored in rare earth-cobalt magnets allows the design of lightweight motors and magnetic bearings for high-speed rotors. Magnetic bearings are not subject to wear and with the ability to operate under high vacuum conditions, they appear ideal for applications requiring high rotational speeds such as 100 000 r/min. Important applications are for turbomolecular pumps, laser scanners, centrifuges, momentum rings for satellite stabilizations, and other uses in space technology. This paper presents a two-dimemsional nonlinear numerical analysis of the magnetic fields in a magnetic bearing, based on magnetostatic assumptions and finite-difference iterative techniques.  相似文献   
994.
Structural features of germanium-silicon solid solution crystals have been investigated and silicon distribution in the crystals has been studied. All the crystals obtained under microgravity had, in spite of good external shape and facetting, a poorer internal structure than those obtained on Earth. The distribution of silicon have been shown to be non-uniform. High dislocation densities were observed.  相似文献   
995.
The application of Fourier Transform Spectroscopy (FTS) to planetary research is reviewed. The survey includes FTS observations of the Sun, all the planets except Uranus and Pluto, the Galilean satellites and Saturn's rings. Instrumentation and scientific results are considered. The prospects and limitations of FTS for planetary research in the forthcoming years are discussed.  相似文献   
996.
A low frequency, small signal model, valid for all types of converters, both in the heavy and the light mode of conduction, is developed in block diagram form. The model provides a clear insight into the behavior of such converters and enhances the respective contributions of input and output filters.  相似文献   
997.
The backscattering of an elliptically polarized plane wave from two kinds of complex radar objects is considered from the standpoint of coherency theory, and the antenna polarizations that give rise to maximum correlated and uncorrelated orthogonally polarized components in the receiving channels are discussed. The resulting observations may find application in radar systems which use the correlation between the received orthogonally polarized components for detecting targets in natural and man-made interference.  相似文献   
998.
Aminoacylation of the “internal” 2′-hydroxyl groups of poly(A) with the imidazolide of racemic DL-leucine resulted in the formation of an enantiomeric excess of the D-ester, whereas aminoacylation with the imidazolide of racemic N-3,5-dinitrobenzoyl-DL-leucine gave an enantiomeric excess of the L-ester. Comparison of these results with those obtained earlier for alanine shows that the larger side chain tends to favor the formation of the D-ester. High resolution proton magnetic resonance spectroscopy was performed on the D- and L-alanyl “internal” 2′-esters of 3′,5′-ApA. The resonance of the side chain methyl group of the D-ester is upfield from that of the L-ester, and the separation of the peaks increases with a decrease in temperature.  相似文献   
999.
1000.
The matched filter ambiguity function is presented for a burst waveform composed of repeated subbursts, each one of which consists of N pulses in which the phase is varied quadratically from pulse to pulse. The resulting ambiguity function exhibits small residual ambiguities along the delay axis separated by the reciprocal of the pulse repetition frequency (PRF). A cross-ambiguity function is derived which reduces these ambiguities to zero amplitude. A third cross-ambiguity function is presented for a receiver matched to a generalized Hamming weighted repeated quadratic burst. The location in the delay/Doppler plane of the waveform ambiguities for these waveforms is compared with that of an uncoded pulse burst.  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