全文获取类型
收费全文 | 18554篇 |
免费 | 37篇 |
国内免费 | 122篇 |
专业分类
航空 | 9927篇 |
航天技术 | 5605篇 |
综合类 | 252篇 |
航天 | 2929篇 |
出版年
2021年 | 156篇 |
2018年 | 222篇 |
2016年 | 154篇 |
2014年 | 440篇 |
2013年 | 524篇 |
2012年 | 423篇 |
2011年 | 587篇 |
2010年 | 406篇 |
2009年 | 761篇 |
2008年 | 807篇 |
2007年 | 384篇 |
2006年 | 434篇 |
2005年 | 402篇 |
2004年 | 451篇 |
2003年 | 543篇 |
2002年 | 488篇 |
2001年 | 586篇 |
2000年 | 375篇 |
1999年 | 466篇 |
1998年 | 450篇 |
1997年 | 336篇 |
1996年 | 410篇 |
1995年 | 491篇 |
1994年 | 471篇 |
1993年 | 360篇 |
1992年 | 353篇 |
1991年 | 251篇 |
1990年 | 243篇 |
1989年 | 424篇 |
1988年 | 209篇 |
1987年 | 241篇 |
1986年 | 241篇 |
1985年 | 652篇 |
1984年 | 531篇 |
1983年 | 422篇 |
1982年 | 492篇 |
1981年 | 616篇 |
1980年 | 248篇 |
1979年 | 194篇 |
1978年 | 189篇 |
1977年 | 147篇 |
1976年 | 155篇 |
1975年 | 196篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 38 毫秒
951.
B. Aschenbach 《Space Science Reviews》1985,40(3-4):447-465
A progress report is given on our current interpretation of the X-ray emission from supernova remnants. Previous results from earlier experiments, the Einstein Observatory in particular, are reviewed and supplemented by the most recent data from the Exosat mission for a selection of remnants (Puppis-A, Cas-A, SN 1006, RCW103, W49B). Major improvements come from using the high energy spectra obtained with Exosat which indicate the presence of a very hot electron component in both young and old thermal remnants. Despite the fact that non-equilibrium ionization has been found in some cases, the spectra of most remnants investigated so far are not well represented by single non-equilibrium models, but require at least two components. An impression of the variety of plasma states which can be found in remnants is obtained from Exosat low energy filter spectroscopy of Puppis-A, which shows temperature variations on scales as small as 1. 相似文献
952.
We report the results of a 1.4 104s observation of the region of 4U 1323-62 with the EXOSAT ME. The source has a flux of 7–8 10-11 erg/cm2s (2–10 keV) and a power-law spectrum with 1.1 < < 1.8. During our observation, the source showed a symmetric 60% dip in its X-ray flux of R~1 hr. The spectrum hardens during the dip. Inside the dip we observed an X-ray burst with a 2–10 keV peak flux of 7 10-10 erg/cm2s. The burst spectrum is black-body, and shows evidence of cooling during the burst decay. The discovery of a burst from 4U 1323-62 settles the classification of the source; the observation of a dip suggests that we may be able to measure its orbital period in the near future. 相似文献
953.
EXOSAT observed LMC X-4 on November 17/19, 1983 for one 1.4 day binary period during the high state of the 30.5 day cycle. An eclipse with sharp ingress and slow egress was detected with an eclipse angle of 27.1±1.0 dgr. In the medium energy experiment the source showed a hard power law spectrum. Outside eclipse the source was remarkably constant and only one flare was detected on November 17 at 19 UT lasting for about 1 h. The energy spectrum of the source softens considerably during that time and shows an emission line of cold iron. 13.5 sec pulsations are strongly present during the flare and have also been detected during the quiescent period and during several 1 min flares in another EXOSAT LMC X-4 observation on November 22, 1983. A pulse delay time analysis results in the determination of the pulse period (13.5019±0.0002) s and of the semimajor axis of the orbit of the X-ray star (26.0±0.6) It-sec. These results, together with other available information on LMC X-4, allowed to improve the binary parameters. The mass of the neutron star is found to be 1.34
±0.44
0.48
Mo (95% confidence errors). 相似文献
954.
The results of a previous discrete-time model of the series resonant dc-dc converter are reviewed and from these a small signal dynamic model is derived. This model is valid for low frequencies and is based on the modulation of the diode conduction angle for control. The basic converter is modeled separately from its output filter to facilitate the use of these results for design purposes. Experimental results are presented. 相似文献
955.
956.
Fahr H. J. Neutsch W. Grzedzielski S. Macek W. Ratkiewicz-Landowska R. 《Space Science Reviews》1986,43(3-4):329-381
Existing heliopause models are critically rediscussed under the new aspect of possible plasma mixing between the solar wind and the ambient ionized component of the local interstellar medium (LISM). Based on current kinetic plasma theories, effective diffusion rates across the heliopause are evaluated for several models with turbulence caused by electrostatic or electromagnetic interactions that could be envisaged in this context. Some specific cases that may lead to high diffusion rates are investigated, especially in regard to their LISM magnetic field dependence.For weak fields (less than 10–7 G), macroscopic hydrodynamic instabilities, such as of Rayleigh-Taylor or Kelvin-Helmholtz-types, can be excited. The resulting plasma mixing rates at the heliopause may amount to 20–30% of the impinging mass flow.Recently, an unconventional new approach to the problem for the case of tangential magnetic fields at the heliopause was published in which a continuous change of the plasma properties within an extended boundary layer is described by a complete set of two-fluid plasma equations including a hybrid MHD-formulation of wave-particle interaction effects. If a neutral sheet is assumed to exist within the boundary layer, the magnetic field direction is proven to be constant for a plane-parallel geometry. Considering the electric fields and currents in the layer, an interesting relationship between the field-reconnection probability and the electric conductivity can be derived, permitting a quantitative determination of either of these quantities.An actual value for the electrical conductivity is derived here on the basis of electron distribution functions given by a superposition of Maxwellians with different temperatures. Using two-stream instability theory and retaining only the most unstable modes, an exact solution for the density, velocity, and magnetic and electric fields can be obtained. The electrical conductivity is then shown to be six orders of magnitude lower than calculated by conventional formulas. Interestingly, this leads to an acceptable value of 0.1 for the reconnection coefficient.By analogy with the case of planetary magnetopauses, it is shown here for LISM magnetic fields of the order of 10–6 G or larger that field reconnection processes may also play an important role for the plasma mixing at the heliopause. The resulting plasma mixing rate is estimated to amount to an average value of 10% of the incident mass flow. It is suggested here that the dependence of the cosmic-ray penetration into the heliosphere on the distribution of reconnecting areas at the heliopause may provide a means of deriving the strength and orientation of the LISM field.A series of observational implications for the expected plasma mixing at the heliopause is discussed in the last part of the paper. In particular, consequences are discussed for the generation of radio noise at the heliopause, for the penetration of LISM neutrals into the heliosphere, for the propagation of cosmic rays towards the inner part of the solar system and for convective electric field mergings into the heliosphere during the course of the solar cycle, depending on the solar cycle variations. With concern to a recent detection of electrostatic plasma waves by plasma receivers on Voyagers 1 and 2, we come to an interesting alternate explanation: the heliopause, rather than the heliospheric shock front, could be responsible for the generation of these waves. 相似文献
957.
Time of arrival (TOA) estimation of narrowband signals is a problem of considerable practical interest in radar and sonar applications. A new technique is presented to analyze the mean square error (MSE) performance of TOA estimation schemes, based on recently developed lower bound. We obtain a complete characterization of the MSE as a function of the signal and noise parameters. The results are given in a simple closed-form analytical expression. 相似文献
958.
Mr.WANG ZHENHUA 《航空动力学报》1987,2(4):384-384
The Journal of Aerospace Power is a leading publication of its kind in China devoted to the advancement of the science and technology of aerospace power.It provides a common medium through which academic and industrial researchers and designers can communicate results, review progress and identify requirements for further work in this increasingly significant field. 相似文献
959.
湿热对单向复合材料层合板Ⅱ型分层特性的影响 总被引:1,自引:0,他引:1
利用横向裂纹拉伸试验模型(Transverse Crack Tension-tension Test),在3种湿热环境(室温、85℃和85℃/95%RH)条件下,对2种单向碳纤维增强树脂基复合材料(T300/QY8911和HTA/6376)层合板的Ⅱ型分层特性进行了试验研究。结果表明湿热环境导致复合材料层间断裂韧性降低,引起Ⅱ型应变能释放率门槛值显着下降,裂纹扩展速率大大提高,温度的影响较湿度更为明显。利用电镜(SEM)对试样分层表面进行了检测,湿度对断面特征无明显的影响。 相似文献
960.
Lazarus A. J. Belcher J. W. Paularena K. I. Richardson J. D. 《Space Science Reviews》1998,83(1-2):87-92
We discuss the solar wind parameters measured in the distant heliosphere from the Voyager 2 spacecraft. Periodic variations in the speed of the wind observed at roughly the solar rotation period may correspond to interaction regions between slower and faster streams of wind. Since the interplanetary magnetic field is enhanced in such regions, they are important for the study of modulation of cosmic rays. Unfortunately, direct observation of the enhanced magnetic field from Voyager 2 has been made difficult by spacecraft-associated noise since 1989. 相似文献