全文获取类型
收费全文 | 84171篇 |
免费 | 15249篇 |
国内免费 | 11271篇 |
专业分类
航空 | 61049篇 |
航天技术 | 19253篇 |
综合类 | 7725篇 |
航天 | 22664篇 |
出版年
2024年 | 249篇 |
2023年 | 1115篇 |
2022年 | 3024篇 |
2021年 | 3540篇 |
2020年 | 3383篇 |
2019年 | 2699篇 |
2018年 | 3234篇 |
2017年 | 3656篇 |
2016年 | 2614篇 |
2015年 | 3341篇 |
2014年 | 4493篇 |
2013年 | 4703篇 |
2012年 | 5393篇 |
2011年 | 6095篇 |
2010年 | 6062篇 |
2009年 | 6399篇 |
2008年 | 6120篇 |
2007年 | 5444篇 |
2006年 | 5056篇 |
2005年 | 4059篇 |
2004年 | 2862篇 |
2003年 | 2363篇 |
2002年 | 2065篇 |
2001年 | 2346篇 |
2000年 | 2017篇 |
1999年 | 2085篇 |
1998年 | 1551篇 |
1997年 | 1140篇 |
1996年 | 1243篇 |
1995年 | 1334篇 |
1994年 | 1381篇 |
1993年 | 758篇 |
1992年 | 713篇 |
1991年 | 450篇 |
1990年 | 392篇 |
1989年 | 616篇 |
1988年 | 316篇 |
1987年 | 422篇 |
1986年 | 322篇 |
1985年 | 721篇 |
1984年 | 582篇 |
1983年 | 464篇 |
1982年 | 526篇 |
1981年 | 652篇 |
1980年 | 264篇 |
1979年 | 197篇 |
1978年 | 196篇 |
1975年 | 204篇 |
1974年 | 184篇 |
1972年 | 188篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
961.
962.
一类受周期扰动航天器的混沌姿态运动 总被引:1,自引:0,他引:1
研究了航天器从绕最小惯量主轴到最大惯量主轴旋转的姿态机动过程中的混沌现象。考虑到航天器内部或外部的振动部件的影响,假设两个主轴的转动惯量为时间的周期函数,同时还考虑了航天器内结构阻尼以及稀薄气体阻力的影响。应用高维的Melnikov方法,求解姿态机动过程中产生混沌的条件的解析表达式,且得到的阀值条件是扰动系统参数的函数。最后对该阀值条件进行了数值验证。 相似文献
963.
Aqueous corrosion of phosphide minerals from iron meteorites: a highly reactive source of prebiotic phosphorus on the surface of the early Earth 总被引:3,自引:0,他引:3
We present the results of an experimental study of aqueous corrosion of Fe-phosphide under conditions relevant to the early Earth. The results strongly suggest that iron meteorites were an important source of reactive phosphorus (P), a requirement for the formation of P-based life. We further demonstrate that iron meteorites were an abundant source of phosphide minerals early in Earth history. Phosphide corrosion was studied in five different solutions: deionized water, deionized water buffered with sodium bicarbonate, deionized water with dissolved magnesium and calcium chlorides, deionized water containing ethanol and acetic acid, and deionized water containing the chlorides, ethanol, and acetic acid. Experiments were performed in the presence of both air and pure Ar gas to evaluate the effect of atmospheric chemistry. Phosphide corrosion in deionized water results in a metastable mixture of mixed-valence, P-bearing ions including pyrophosphate and triphosphate, key components for metabolism in modern life. In a pH-buffered solution of NaHCO(3), the condensed and reduced species diphosphonate is an abundant corrosion product. Corrosion in ethanol- and acetic acid-containing solutions yields additional P-bearing organic molecules, including acetyl phosphonate and a cyclic triphosphorus molecule. Phosphonate is a major corrosion product of all experiments and is the only P-bearing molecule that persists in solutions with high concentrations of magnesium and calcium chlorides, which suggests that phosphonate may have been a primitive oceanic source of P. The stability and reactivity of phosphonate and hypophosphite in solution were investigated to elucidate reaction mechanisms and the role of mineral catalysts on P-solution chemistry. Phosphonate oxidation is rapid in the presence of Fe metal but negligible in the presence of magnetite and in the control sample. The rate of hypophosphite oxidation is independent of reaction substrate. 相似文献
964.
Layers of dormant endospores of Bacillus subtilis HA101 were applied to eight different spacecraft materials and exposed to martian conditions of low pressure (8.5 mbar), low temperature (-10 degrees C), and high CO(2) gas composition and irradiated with a Mars-normal ultraviolet (UV-visible- near-infrared spectrum. Bacterial layers were exposed to either 1 min or 1 h of Mars-normal UV irradiation, which simulated clear-sky conditions on equatorial Mars (0.1 tau). When exposed to 1 min of Mars UV irradiation, the numbers of viable endospores of B. subtilis were reduced three to four orders of magnitude for two brands of aluminum (Al), stainless steel, chemfilm-treated Al, clear-anodized Al, and black-anodized Al coupons. In contrast, bacterial survival was reduced only one to two orders of magnitude for endospores on the non-metal materials astroquartz and graphite composite when bacterial endospores were exposed to 1 min of Mars UV irradiation. When bacterial monolayers were exposed to 1 h of Mars UV irradiation, no viable bacteria were recovered from the six metal coupons listed above. In contrast, bacterial survival was reduced only two to three orders of magnitude for spore layers on astroquartz and graphite composite exposed to 1 h of Mars UV irradiation. Scanning electron microscopy images of the bacterial monolayers on all eight spacecraft materials revealed that endospores of B. subtilis formed large aggregates of multilayered spores on astroquartz and graphite composite, but not on the other six spacecraft materials. It is likely that the formation of multilayered aggregates of endospores on astroquartz and graphite composite is responsible for the enhanced survival of bacterial cells on these materials. 相似文献
965.
966.
967.
968.
969.
The short- and long-term effects of spacecraft explosions, as a function of the end-of-life re-orbit altitude above the geostationary orbit (GEO), were analyzed in terms of their additional contribution to the debris flux in the GEO ring. The simulated debris clouds were propagated for 72yrs, taking into account all the relevant orbital perturbations.The results obtained show that 6–7 additional explosions in GEO would be sufficient, in the long term, to double the current collision risk with sizable objects in GEO. Unfortunately, even if spacecraft were to re-orbit between 300 and 500km above GEO, this would not significantly improve the situation. In fact, an altitude increase of at least 2000km would have to be adopted to reduce by one order of magnitude the long-term risk of collision among geostationary satellites and explosion fragments. The optimal debris mitigation strategy should be a compromise between the reliability and effectiveness of spacecraft end-of-life passivation, the re-orbit altitude and the acceptable debris background in the GEO ring. However, for as long as the re-orbit altitudes currently used are less than 500km above GEO, new spacecraft explosions must be avoided in order to preserve the geostationary environment over the long term. 相似文献
970.
Microgravity effects on water supply and substrate properties in porous matrix root support systems.
G E Bingham S B Jones D Or I G Podolski M A Levinskikh V N Sytchov T Ivanova P Kostov S Sapunova I Dandolov D B Bubenheim G Jahns 《Acta Astronautica》2000,47(11):839-848
The control of water content and water movement in granular substrate-based plant root systems in microgravity is a complex problem. Improper water and oxygen delivery to plant roots has delayed studies of the effects of microgravity on plant development and the use of plants in physical and mental life support systems. Our international effort (USA, Russia and Bulgaria) has upgraded the plant growth facilities on the Mir Orbital Station (OS) and used them to study the full life cycle of plants. The Bulgarian-Russian-developed Svet Space Greenhouse (SG) system was upgraded on the Mir OS in 1996. The US developed Gas Exchange Measurement System (GEMS) greatly extends the range of environmental parameters monitored. The Svet-GEMS complex was used to grow a fully developed wheat crop during 1996. The growth rate and development of these plants compared well with earth grown plants indicating that the root zone water and oxygen stresses that have limited plant development in previous long-duration experiments have been overcome. However, management of the root environment during this experiment involved several significant changes in control settings as the relationship between the water delivery system, water status sensors, and the substrate changed during the growth cycles. 相似文献