全文获取类型
收费全文 | 426篇 |
免费 | 0篇 |
国内免费 | 1篇 |
专业分类
航空 | 242篇 |
航天技术 | 57篇 |
综合类 | 2篇 |
航天 | 126篇 |
出版年
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 2篇 |
2018年 | 67篇 |
2017年 | 37篇 |
2016年 | 2篇 |
2015年 | 3篇 |
2014年 | 10篇 |
2013年 | 11篇 |
2012年 | 13篇 |
2011年 | 32篇 |
2010年 | 16篇 |
2009年 | 20篇 |
2008年 | 15篇 |
2007年 | 26篇 |
2006年 | 17篇 |
2005年 | 8篇 |
2004年 | 13篇 |
2003年 | 11篇 |
2002年 | 6篇 |
2001年 | 13篇 |
2000年 | 5篇 |
1999年 | 9篇 |
1998年 | 8篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有427条查询结果,搜索用时 15 毫秒
161.
BiSbTe3-mixed crystals have been grown at normal and reduced gravity (during the MIR'97-mission) using a
-configuration of the TITUS facility. The distribution of the components in the melt, and so the homogeneity of the growing crystal, is strongly influenced by the flow in the melt even in the case of weak convection. The flow configuration in the melt especially in front of the solid-liquid phase boundary can be investigated by means of a segregation analysis of the system components and an additional Pb-dopant. The BiSbTe3-system is because of its hydro-dynamic properties a typical representative of semiconductor melts (low
number, high
number) but there are also some special properties relating to the segregationally caused enrichment of the lighter tellurium at the phase boundary and the resulting solutal destabilities. Experimental experiences from segregation analysis have shown that the mass transport in the melt at normal gravity is mainly influenced by convective mixing determined by thermally and solutally caused buoyancy forces. Numerical simulations have been performed for the real experimentally used configurations. These simulations have shown that a strong coupling of thermal and solutal effects exists and have given axial as well as radial segregation profiles being in excellent agreement with the experimental results for the vertical normal gravity grown crystals. For micro gravity conditions a reduction of the flow velocity of more than two orders of magnitude (depending on the micro gravity level and the direction of the residual acceleration) resulting in diffusion controlled component segregation has been predicted.The results of the two micro gravity grown crystals, especially the axial and radial segregation profiles as a sensitive indicator for the flow configuration in front of the phase boundary will be given and discussed in the paper. They will be compared with the results of numerical simulations of the melt flow for the real processing parameters measured during the TITUS growth processes and with experimental as well as numerical results for vertical normal gravity grown reference Samples. 相似文献
162.
H. Nieuwenhuijzen F. P. Israel C. Slottje L. B. F. M. Waters J. Kleczek K. Werner M. Barylak Patricia Whitelock Ľ Kresák G. Meynet K. A. van der Hucht D. Stickland 《Space Science Reviews》1992,61(3-4):393-417
The general significance of streamers of the solar corona is discussed in the frame of our knowledge of the solar wind phenomenon and the large-scale solar magnetic structure. Thermodynamical and geometric parameters of streamers observed and measured at total solar eclipses are reviewed. Both the low part (in the form of a helmet with a cusp) and the external part (in the form of a stalk extended at many solar radii) are considered. The modelling of streamers starts with the analysis of effects produced by the solar wind flow on a magnetic structure. Facts and arguments are presented in favor of a model with a current sheet and reconnection processes going on along the axis of the streamer, especially in the non-collisional part of the radially extended streamer. Further development of the Pneuman and Kopp (1971) model is discussed, including difficulties occurring in the interpretation of a stationary solution. An empirical model satisfying observations is presented. Future researchs on streamers were discussed with emphasis on observations to be done with the space-borne coronagraphs on the SOHO spacecraft. 相似文献
163.
Peter D. Zimmerman 《Space Policy》1990,6(4):297-310
The Bush administration is committed to achieving the rapid development and early deployment of some kind of space-based missile defence, with Brilliant Pebbles the favoured candidate. This article examines the conditions necessary to maintain stability during the extended period of transition from offence to defence dominance, and concludes that an agreement incorporating these conditions is most unlikely to be achieved. If the superpowers could agree to such a transition, with its prerequisite of virtually total strategic disarmament, they would agree to nuclear disarmament itself. 相似文献
164.
Three axis attitude stabilization of a satellite using a single spinning reaction wheel mounted on a two degree-of-freedom passively and actively torqued gimbal system is investigated. The passive control is assumed to be provided by a spring-loaded damper mounted on each of the gimbal axes, while active control results from both the wheel acceleration and the torque applied about the gimbal axes. The stability of the uncontrolled and passively controlled systems is investigated analytically. For constant wheel speed the pitch motion is decoupled from the roll-yaw and gimbal motions. Control laws for the roll-yaw motion are developed based on pole clustering and linear optimal control theory. For the pitch motion control laws are obtained based on classical second order system theory. Estimation techniques are applied to the roll-yaw system for the case when the complete state may not be directly observable (in the absence of a fine yaw position sensor). 相似文献
165.
Extraterrestrial organic matter may have been chemically altered into forms more ameanable for prebiotic chemistry in the wake of a meteor after ablation. We measured the rate of cooling of the plasma in the meteor wake from the intensity decay just behind a meteoroid by freezing its motion in high frame-rate 1000 frames/s video images, with an intensified camera that has a short phosphor decay time. Though the resulting cooling rate was found to be lower than theoretically predicted, our calculations indicated that there would have been insufficient collisions to break apart large organic compounds before most reactive radicals and electrons were lost from the air plasma. Organic molecules delivered from space to the early Earth via meteors might therefore have survived in a chemically altered form. In addition, we discovered that relatively small meteoroids generated far-ultraviolet emission that is absorbed in the immediate environment of the meteoroid, which may chemically alter the atmosphere over a much larger region than previously recognized. 相似文献
166.
167.
Peter W. A. Roming Thomas E. Kennedy Keith O. Mason John A. Nousek Lindy Ahr Richard E. Bingham Patrick S. Broos Mary J. Carter Barry K. Hancock Howard E. Huckle S D. Hunsberger Hajime Kawakami Ronnie Killough T Scott Koch Michael K. Mclelland Kelly Smith Philip J. Smith Juan Carlos Soto Patricia T. Boyd Alice A. Breeveld Stephen T. Holland Mariya Ivanushkina Michael S. Pryzby Martin D. Still Joseph Stock 《Space Science Reviews》2005,120(3-4):95-142
The Ultra-Violet/Optical Telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (∼1 min) UV and optical photons from the afterglow of gamma-ray
bursts in the 170–600 nm band as well as long term observations of these afterglows. This is accomplished through the use
of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey–Chrétien design with micro-channel plate intensified
charged-coupled device detectors that record the arrival time of individual photons and provide sub-arcsecond positioning
of sources. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument. 相似文献
168.
C. M. S. Cohen R. A. Mewaldt R. A. Leske A. C. Cummings E. C. Stone M. E. Wiedenbeck T. T. von Rosenvinge G. M. Mason 《Space Science Reviews》2007,130(1-4):183-194
Solar abundances can be derived from the composition of the solar wind and solar energetic particles (SEPs) as well as obtained
through spectroscopic means. Past comparisons have suggested that all three samples agree well, when rigidity-related fractionation
effects on the SEPs were accounted for. It has been known that such effects vary from one event to the next and should be
addressed on an event-by-event basis. This paper examines event variability more closely, particularly in terms of energy-dependent
SEP abundances. This is now possible using detailed SEP measurements spanning several decades in energy from the Ultra Low
Energy Isotope Spectrometer (ULEIS) and the Solar Isotope Spectrometer (SIS) on the ACE spacecraft. We present examples of
the variability of the elemental composition with energy and suggest they can be understood in terms of diffusion from the
acceleration region near the interplanetary shock. By means of a spectral scaling procedure, we obtain energy-independent
abundance ratios for 14 large SEP events and compare them to reported solar wind and coronal abundances as well as to previous
surveys of SEP events. 相似文献
169.
Despite provisions declarations that outer space shall be free for exploration and use by all states, it is a finite resource. This article argues for the need to ensure rational use of this resource via a major restructuring of space law. Focusing on telecommunications it proposes a change in the assignment of radio-spectrum and suggests that space must be treated as a means of advancing genuine globalisation. 相似文献
170.
Jill Tarter Peter Backus Gary Heiligman John Dreher Sam LaRoque Project Phoenix Team 《Acta Astronautica》2000,46(10-12)
During 16 weeks of continuous SETI observing at the Parkes Observatory in New South Wales, Australia, a set of time-averaged data with 643 Hz resolution were recorded and returned to the SETI Institute for post-processing. These data are the 14 second (10 frame) average powers in each of 15,552 “subband” channels covering 10 MHz of the spectrum in both right and left circular polarizations that were used by the signal detection hardware to baseline and threshold the 1 Hz high resolution SETI spectra. The observations covered frequencies from 1.2 to 3 GHz, tracking 209 stellar targets across the sky. The data at each frequency were averaged over all directions and then interrogated to attempt to determine the prevalence of radio frequency interference (RFI). Estimates were made for the probability of encountering RFI at a particular frequency. Particular attention has been paid to those portions of the spectrum that are allocated as primary use status, or footnote protection for radioastronomy. This sixteen-week snapshot of the RFI situation at Parkes is by now out of date. Unfortunately, a year later, the situation has undoubtedly worsened. 相似文献