全文获取类型
收费全文 | 20572篇 |
免费 | 560篇 |
国内免费 | 832篇 |
专业分类
航空 | 11801篇 |
航天技术 | 5800篇 |
综合类 | 974篇 |
航天 | 3389篇 |
出版年
2021年 | 244篇 |
2019年 | 169篇 |
2018年 | 221篇 |
2016年 | 212篇 |
2015年 | 174篇 |
2014年 | 625篇 |
2013年 | 689篇 |
2012年 | 620篇 |
2011年 | 781篇 |
2010年 | 660篇 |
2009年 | 995篇 |
2008年 | 1012篇 |
2007年 | 537篇 |
2006年 | 584篇 |
2005年 | 524篇 |
2004年 | 486篇 |
2003年 | 553篇 |
2002年 | 518篇 |
2001年 | 625篇 |
2000年 | 479篇 |
1999年 | 597篇 |
1998年 | 565篇 |
1997年 | 452篇 |
1996年 | 463篇 |
1995年 | 526篇 |
1994年 | 472篇 |
1993年 | 458篇 |
1992年 | 365篇 |
1991年 | 337篇 |
1990年 | 300篇 |
1989年 | 431篇 |
1988年 | 245篇 |
1987年 | 267篇 |
1986年 | 247篇 |
1985年 | 643篇 |
1984年 | 510篇 |
1983年 | 396篇 |
1982年 | 489篇 |
1981年 | 607篇 |
1980年 | 243篇 |
1979年 | 182篇 |
1978年 | 189篇 |
1977年 | 144篇 |
1976年 | 155篇 |
1975年 | 181篇 |
1974年 | 181篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1969年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
The composition of Galactic Cosmic Ray Sources (GCRS) shows the following features: (i) an enhancement of the refractory elements
relative to the volatile ones, and (ii) an enhancement of the heavier volatile elements relative to the lighter ones; this
mass dependence should reflect a mass-to-charge (A/Q) dependence of the acceleration efficiency; among the refractory elements,
there is only a very weak enhancement of heavier species, or none at all. We consider it fortuitous that the GCRS composition
resembles that of the solar corona, which is biased according to first ionization potential. In a companion paper by Ellison
et al. (1998, this issue), this GCRS composition is interpreted in terms of a supernova shock wave acceleration of interstellar
and/or circumstellar (e.g., 22Ne-rich Wolf-Rayet wind) gas phase and especially dust grain material. These two papers summarize
and complement the content of two papers that recently appeared in Astrophys. J. (Meyer et al., 1997; Ellison et al., 1997).
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
942.
Mahaffy P.R. Donahue T.M. Atreya S.K. Owen T.C. Niemann H.B. 《Space Science Reviews》1998,84(1-2):251-263
The Galileo Probe Mass Spectrometer measurements in the atmosphere of Jupiter give D/H = (2.6 ± 0.7) × 10-5 3He/4He = (1.66 ± 0.05) × 10-4These ratios supercede earlier results by Niemann et al. (1996) and are based on a reevaluation of the instrument response at high count rates and a more detailed study of the contributions of different species to the mass peak at 3 amu. The D/H ratio is consistent with Voyager and ground based data and recent spectroscopic and solar wind (SW) values obtained from the Infrared Spectroscopic Observatory (ISO) and Ulysses. The 3He/4He ratio is higher than that found in meteoritic gases (1.5 ± 0.3) × 10-4. The Galileo result for D/H when compared with that for hydrogen in the local interstellar medium (1.6 ± 0.12) × 10-5 implies a small decrease in D/H in this part of the universe during the past 4.55 billion years. Thus, it tends to support small values of primordial D/H - in the range of several times 10-5 rather than several times 10-4. These results are also quite consistent with no change in (D+3He)/H during the past 4.55 billion years in this part of our galaxy. 相似文献
943.
通过使用Matlab编程处理日本东丽T300、国产GCF-1和GCF-2等3种碳纤维样品断面的扫描电镜照片,采用二值标签算法获得了碳纤维的直径及圆度等碳纤维物理特征定量数据。建立了采用直线方程切线法寻找并计算碳纤维表面沟槽的数量及尺度的方法,发现3种碳纤维样品的边缘沟槽的深度及宽度的分布均符合Weibull分布的特征。3种碳纤维的表面物理特征随温度变化的规律相同,说明其在结构上存在着一致性。经过不同温度热处理后3种碳纤维样品的圆度值变化不大,平均直径随着热处理温度的升高逐渐减小,并且表面沟槽的数量及分布的变化规律相同,说明其结构存在一致性。3种碳纤维的圆度等表面物理特征存在一定差异,T300的圆度小于国产GCF-1及GCF-2碳纤维。 相似文献
944.
采用固相填充+搅拌摩擦焊复合工艺进行了2219铝合金搅拌摩擦焊缝匙孔形缺陷的修补,焊后对接头的微观组织和力学性能进行了分析。结果显示:匙孔修补位置焊接接头焊核区呈现细小的等轴晶,热力影响区组织发生了较大程度的弯曲变形,接头抗拉强度≥335 MPa,延伸率≥8.0%,断裂机理为韧性断裂。 相似文献
945.
Almost all theoretical and numerical models for the modulation of cosmic ray in the heliosphere are based on Parker's transport equation which contains all the important basic physical processes. The relative importance of the various mechanisms is however not established and may vary significantly over 22 years. The simultaneous measurements of solar wind parameters, heliospheric magnetic field properties and cosmic rays over a wide range of energies and positions in the heliosphere have brought the realization that modulation is much more complicated than what the original drift models predicted. In the process the sophistication of models based on solving Parker's equation has increased by orders of magnitude. A short review of the global modulation of cosmic rays is given from a theoretical and modelling point of view. 相似文献
946.
947.
Solar Wind Electron Proton Alpha Monitor (SWEPAM) for the Advanced Composition Explorer 总被引:1,自引:0,他引:1
McComas D.J. Bame S.J. Barker P. Feldman W.C. Phillips J.L. Riley P. Griffee J.W. 《Space Science Reviews》1998,86(1-4):563-612
The Solar Wind Electron Proton Alpha Monitor (SWEPAM) experiment provides the bulk solar wind observations for the Advanced
Composition Explorer (ACE). These observations provide the context for elemental and isotopic composition measurements made
on ACE as well as allowing the direct examination of numerous solar wind phenomena such as coronal mass ejections, interplanetary
shocks, and solar wind fine structure, with advanced, 3-D plasma instrumentation. They also provide an ideal data set for
both heliospheric and magnetospheric multi-spacecraft studies where they can be used in conjunction with other, simultaneous
observations from spacecraft such as Ulysses. The SWEPAM observations are made simultaneously with independent electron and
ion instruments. In order to save costs for the ACE project, we recycled the flight spares from the joint NASA/ESA Ulysses
mission. Both instruments have undergone selective refurbishment as well as modernization and modifications required to meet
the ACE mission and spacecraft accommodation requirements. Both incorporate electrostatic analyzers whose fan-shaped fields
of view sweep out all pertinent look directions as the spacecraft spins. Enhancements in the SWEPAM instruments from their
original forms as Ulysses spare instruments include (1) a factor of 16 increase in the accumulation interval (and hence sensitivity)
for high energy, halo electrons; (2) halving of the effective ion-detecting CEM spacing from ∼5° on Ulysses to ∼2.5° for ACE;
and (3) the inclusion of a 20° conical swath of enhanced sensitivity coverage in order to measure suprathermal ions outside
of the solar wind beam. New control electronics and programming provide for 64-s resolution of the full electron and ion distribution
functions and cull out a subset of these observations for continuous real-time telemetry for space weather purposes.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
948.
D. Ruciński A. C. Cummings G. Gloeckler A. J. Lazarus E. Möbius M. Witte 《Space Science Reviews》1996,78(1-2):73-84
The rates of the most important ionization processes acting in interplanetary space on interstellar H, He, C, O, Ne and Ar atoms are critically reviewed in the paper. Their long-term modulations in the period 1974 – 1994 are reexamined using updated information on relevant cross-sections as well as direct or indirect data on variations of the solar wind/solar EUV fluxes based on IMP 8 measurements and monitoring of the solar 10.7 cm radio emission. It is shown that solar cycle related variations are pronounced (factor of 3 between maximum and minimum) especially for species such as He, Ne, C for which photoionization is the dominant loss process. Species sensitive primarily to the charge-exchange (as H) show only moderate fluctuations 20% around average. It is also demonstrated that new techniques that make use of simultaneous observations of neutral He atoms on direct and indirect orbits, or simultaneous measurements of He+ and He++ pickup ions and solar wind particles can be useful tools for narrowing the uncertainties of the He photoionization rate caused by insufficient knowledge of the solar EUV flux and its variations. 相似文献
949.
950.