全文获取类型
收费全文 | 2257篇 |
免费 | 10篇 |
国内免费 | 10篇 |
专业分类
航空 | 1090篇 |
航天技术 | 817篇 |
综合类 | 10篇 |
航天 | 360篇 |
出版年
2021年 | 22篇 |
2018年 | 38篇 |
2017年 | 21篇 |
2016年 | 26篇 |
2014年 | 48篇 |
2013年 | 57篇 |
2012年 | 51篇 |
2011年 | 91篇 |
2010年 | 64篇 |
2009年 | 90篇 |
2008年 | 97篇 |
2007年 | 52篇 |
2006年 | 43篇 |
2005年 | 57篇 |
2004年 | 70篇 |
2003年 | 72篇 |
2002年 | 37篇 |
2001年 | 57篇 |
2000年 | 39篇 |
1999年 | 47篇 |
1998年 | 67篇 |
1997年 | 42篇 |
1996年 | 67篇 |
1995年 | 79篇 |
1994年 | 55篇 |
1993年 | 49篇 |
1992年 | 64篇 |
1991年 | 30篇 |
1990年 | 19篇 |
1989年 | 49篇 |
1988年 | 23篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 85篇 |
1984年 | 53篇 |
1983年 | 53篇 |
1982年 | 60篇 |
1981年 | 72篇 |
1980年 | 21篇 |
1979年 | 26篇 |
1978年 | 26篇 |
1977年 | 26篇 |
1976年 | 20篇 |
1975年 | 20篇 |
1974年 | 20篇 |
1973年 | 11篇 |
1972年 | 14篇 |
1970年 | 18篇 |
1969年 | 19篇 |
1967年 | 10篇 |
排序方式: 共有2277条查询结果,搜索用时 15 毫秒
831.
This paper reviews the work carried out at the Atomic Energy Research Establishment (AERE) since 1963 in connection with the development of isotope-powered thermoelectric generators, known as RIPPLEs. It outlines the criteria considered for the selection of strontium 90 as the radioisotope, the design of the generators, the choice of material for the biological shield, and the development of the thermal insulation. It briefly discusses the voltage conversion system employed and states safety design considerations. The paper concludes by discussing the effect of efficiency on the cost of generators and shows typical costs curves. 相似文献
832.
Gates David F. Haislmaier Robert J. Hill Lemmuel L. 《IEEE transactions on aerospace and electronic systems》1967,(2):303-308
A Doppler radar tracking system has been used successfully to measure impact drag coefficients for several water-entry configurations. Hemisphere-cylinder and cone-cylinder models were launched vertically into a tank of water at velocities between 100 and 200 feet per second. These launchings were evaluation tests for a system to be used in a new facility at the Naval Ordnance Laboratory?the Hydroballistics Tank. Planned launchings in that facility will be at velocities up to 3000 feet per second. Knowledge of the drag coefficient profile (CD versus depth of penetration) is important in the design of high-velocity water-entry weapons. 相似文献
833.
834.
four widely used self-oscillating dc-to-square-wave parallel inverters which employ an inductor-capacitor tuned network to determine the oscillation frequency are reduced to a common equivalent RLC network. The techniques of singular-point analysis and state-plane interpretations are employed to describe the steady-state and transient ient behavior of these circuits and to elucidate the three possible modes of operation: quasi-harmonic, relaxation, and discontinuous. Design guidelines are provided through a study of the influence of circuit parameter variations on the characteristics of oscillation and on frequency stability. Several examples are provided to illustrate the usefulness of this analysis when studying such problems as transistor emitter-to-base junction breakdown during oscillations and the design of starting circuits to insure self-excited oscillations in these inverters. 相似文献
835.
The frequency band from 5.925 to 6.425 GHz is served by fixed satellites and by terrestrial microwave links. There is a possibility of microwave links pointed at the horizon causing interference to the uplinks of domestic and international communications satellites sharing the same frequency band. A mathematical model has been derived for predicting the fields at geostationary orbit based on the known characteristics and known distribution of the terrestrial microwave relay system. The Applications Technology Satellite-6 (ATS-6) is sensitive to signals in the range of 10 dBW radiated in the direction of the satellite. Signals in the range of 10-30 dBW have been recorded over various parts of the United States. 相似文献
836.
Mosesso P Schuber M Seibt D Schatz A Fosci A Fonti E Palitti F 《Acta Astronautica》1998,42(1-8):473-488
In the present investigation we report the effects of simulated microgravity conditions (clinostat) on the induction of chromosomal aberrations in human lymphocytes in vitro by (R) Bleomycin. Chromosomal aberrations have been analysed by means of fluorescent in situ hybridisation (FISH) and chromosome-specific composite DNA probes (chromosome painting). The results obtained show that, under simulated microgravity conditions, the levels of both symmetrical and asymmetrical (dicentrics, rings), the number of cells bearing "complex" aberrations and hence the total numbers of aberrations were significantly elevated at any of the dose-levels assayed, compared to the parallel treatments performed as 1g control ("ground"). Furthermore, the ratio symmetrical:asymmetrical translocations was markedly elevated under simulated microgravity conditions, compared to the findings usually observed under "normal" 1g conditions. On these bases, we are much inclined to believe that simulated microgravity, rather than limiting the resealing of DNA double strand breaks (DSB's) induced by genotoxic agents is influencing in terms of enhancement the misrejoining of DSB's which is actually responsible for the fixation of the original lesions to DNA into chromosomal aberrations. In addition, the possible different misrepair processes leading to the formation of symmetrical and asymmetrical translocations might be differentially influenced by microgravity being the symmetrical translocations significantly more represented. 相似文献
837.
J. F. Crifo 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(12):203-206
The “Vega” Soviet flyby probes to comet Halley will carry a French infrared sounder, called “I.K.S.”. In order to assess its observing capabilities, a theoretical model of the comet infrared emission was constructed. We show how the experiment results will be used to derive the nucleus size and radiative properties, and to study the distribution of gas and dust in the inner coma and circumnuclear area. A preliminary discussion is made of the relevance of the data in instances where the cometary phenomena would be more complex than assumed in the model. 相似文献
838.
Results of almost two years (January 1999–October 2000) of continuous observations of auroral kilometric radio emission with the instrument AKR-X onboard the high-apogee satellite of the Earth Interball-1 are presented. The observations were conducted at the growth stage (in 1999) and in the maximum (2000) of solar activity within the 100–1500 kHz frequency band. The results of AKR detection in the vicinity of the maximum of its spectrum at a frequency of 252 kHz are presented. Both similarity (for example, the character of global directivity) and important differences from the AKR emission observed during the solar activity minimum [5] are found. Together with very high sporadicity, strong seasonal changes in the intensity are typical for the emission. It is completely absent in the spring-summer period in the Northern Hemisphere and is strongly suppressed in this period in the Southern Hemisphere. Probable nature of these features of AKR is discussed. 相似文献
839.
Atmospheric gaseous constituents play an important role in determining the surface temperatures and habitability of a planet. Using a global climate model and a parameterization of the carbonate-silicate cycle, we explored the effect of the location of the substellar point on the atmospheric CO(2) concentration and temperatures of a tidally locked terrestrial planet, using the present Earth continental distribution as an example. We found that the substellar point's location relative to the continents is an important factor in determining weathering and the equilibrium atmospheric CO(2) level. Placing the substellar point over the Atlantic Ocean results in an atmospheric CO(2) concentration of 7 ppmv and a global mean surface air temperature of 247 K, making ~30% of the planet's surface habitable, whereas placing it over the Pacific Ocean results in a CO(2) concentration of 60,311 ppmv and a global temperature of 282 K, making ~55% of the surface habitable. 相似文献
840.
Abstract The survival strategies of one cyanobacteria colony and three terricolous lichen species from the hot subdesert of Tabernas, Spain, were studied along with topographical attributes of the area to investigate whether the protective strategies adopted by these pioneer soil colonizers are related to the environmental stressors under which they survive. A handheld Raman spectrometer was used for biomolecular characterization, while the microclimatic and topographic parameters were estimated with a Geographic Information System (GIS). We found that the survival strategies adopted by those organisms are based on different combinations of protective biomolecules, each with diverse ecophysiological functions, such as UV-radiation screening, free-energy quenching, antioxidants, and the production of different types and amounts of calcium oxalates. Our results show that the cyanobacteria community and each lichen species preferentially colonized a particular microhabitat with specific moisture and incident solar radiation levels and exhibited different adaptive mechanisms. In recent years, a number of studies have provided consistent results that suggest a link between the strategies adopted by those extremophile organisms and the microclimatic environmental parameters. To date, however, far too little attention has been paid to results from Raman analyses on dry specimens. Therefore, the results of the present study, produced with the use of our miniaturized instrument, will be of interest to future studies in astrobiology, especially due to the likely use of Raman spectroscopy at the surface of Mars. Key Words: Hot desert-Raman spectroscopy-Topography-Terricolous lichens-Cyanobacteria-Planetary exploration. Astrobiology 12, 743-753. 相似文献