全文获取类型
收费全文 | 7289篇 |
免费 | 11篇 |
国内免费 | 33篇 |
专业分类
航空 | 3427篇 |
航天技术 | 2745篇 |
综合类 | 24篇 |
航天 | 1137篇 |
出版年
2021年 | 60篇 |
2019年 | 47篇 |
2018年 | 183篇 |
2017年 | 117篇 |
2016年 | 82篇 |
2015年 | 34篇 |
2014年 | 148篇 |
2013年 | 200篇 |
2012年 | 182篇 |
2011年 | 278篇 |
2010年 | 188篇 |
2009年 | 327篇 |
2008年 | 370篇 |
2007年 | 198篇 |
2006年 | 167篇 |
2005年 | 206篇 |
2004年 | 212篇 |
2003年 | 259篇 |
2002年 | 154篇 |
2001年 | 246篇 |
2000年 | 155篇 |
1999年 | 191篇 |
1998年 | 209篇 |
1997年 | 164篇 |
1996年 | 192篇 |
1995年 | 247篇 |
1994年 | 243篇 |
1993年 | 119篇 |
1992年 | 178篇 |
1991年 | 65篇 |
1990年 | 76篇 |
1989年 | 167篇 |
1988年 | 57篇 |
1987年 | 69篇 |
1986年 | 72篇 |
1985年 | 217篇 |
1984年 | 155篇 |
1983年 | 138篇 |
1982年 | 167篇 |
1981年 | 207篇 |
1980年 | 67篇 |
1979年 | 50篇 |
1978年 | 53篇 |
1977年 | 46篇 |
1976年 | 41篇 |
1975年 | 41篇 |
1974年 | 44篇 |
1972年 | 36篇 |
1970年 | 33篇 |
1969年 | 36篇 |
排序方式: 共有7333条查询结果,搜索用时 15 毫秒
61.
R Hemmersbach B Bromeis I Block R Braucker M Krause N Freiberger C Stieber M Wilczek 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,27(5):893-898
Experiments under varied gravitational accelerations as well as in density-adjusted media showed that sensation of gravity in protists may be linked to the known principles of mechanosensation. Paramecium, a ciliate with clear graviresponses (gravitaxis and gravikinesis) is an ideal model system to prove this hypothesis since the ciliary activity and thus the swimming behaviour is controlled by the membrane potential. It has also been assumed that the cytoplasmic mass causes a distinct stimulation of the bipolarly distributed mechano-sensitive K+ and Ca2+ ion channels in the plasma membrane in dependence of the spatial orientation of the cell. In order to prove this hypothesis, different channel blockers are currently under investigation. Gadolinium did not inhibit gravitaxis in Paramecium, showing that it does not specifically block gravireceptors. Further studies concentrated on the question of whether second messengers are involved in the gravity signal transduction chain. Exposure to 5 g for up to 10 min led to a significant increase in cAMP. 相似文献
62.
A representative switched-capacitor DC-DC converter topology is presented, circuit operation is explained, and control strategies are identified. State-space averaging is used to analyze steady-state performance and to develop control criteria and design equations. The analytical results are verified by SPICE simulation 相似文献
63.
The Freja ultraviolet imager 总被引:2,自引:0,他引:2
J. S. Murphree R A. King T. Payne K. Smith D. Reid J. Adema B. Gordon R. Wlochowicz 《Space Science Reviews》1994,70(3-4):421-446
Auroral images acquired by satellite instrumentation have proven to be a crucial component of the scientific equiry into the physical processes of the Earth's magnetosphere. TheFreja mission provided an opportunity to extend these measurements into the temporal and spatial regime commonly associated with ground-based optical imagers. Employing the basic procedure used successfully in theViking program allowed image repetition rates of 6 s to be achieved with simultaneous exposure of all pixels within the field-of-view. Typical exposure times of 0.3 s required development of an improved image intensifier system and operational requirements resulted in image formats covering approximately one-third of a spin. Simultaneously exposing two broad-band UV cameras, the instrument generates in its normal operational mode 264.6 kbytes per image pair. Results from initial operations confirm the design approach and suggest such imagers can be routinely included on challenging low cost missions such asFreja. 相似文献
64.
Changes in the procurement environment and developments in technology that will require the adoption of new development certification procedures within the next few years are examined. Characteristics of safety-critical computer systems and the safety problems posed by digital computers are described. The way in which the changes have influenced the preparation of the new draft defence standards offered for public debate in the UK prior to their formal adoption by the Ministry of Defence is discussed. Principle features of the future safety-critical systems policy are outlined. The use of Ada for safety-critical software is considered 相似文献
65.
66.
67.
M M Saulmon K F Reardon W Z Sadeh 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,18(1-2):289-292
As space missions become longer in duration, the need to recycle waste into useful compounds rises dramatically. This problem can be addressed by the development of Controlled Ecological Life Support Systems (CELSS) (i.e., Engineered Closed/Controlled Eco-Systems (ECCES)), consisting of human and plant modules. One of the waste streams leaving the human module is urine. In addition to the reclamation of water from urine, recovery of the nitrogen is important because it is an essential nutrient for the plant module. A 3-step biological process for the recycling of nitrogenous waste (urea) is proposed. A packed-bed bioreactor system for this purpose was modeled, and the issues of reaction step segregation, reactor type and volume, support particle size, and pressure drop were addressed. Based on minimization of volume, a bioreactor system consisting of a plug flow immobilized urease reactor, a completely mixed flow immobilized cell reactor to convert ammonia to nitrite, and a plug flow immobilized cell reactor to produce nitrate from nitrite is recommended. It is apparent that this 3-step bioprocess meets the requirements for space applications. 相似文献
68.
In extension to common applications such as groundtrack displays and antenna steering, the SGP4 orbit model is proposed for operational orbit determination in small satellite missions. SGP4 is an analytical orbit model for Low-Earth orbiting satellites that is widely used for the propagation of NORAD twoline elements. Twoline elements may hence be generated completely independent of NORAD. Their use as exclusive source of orbital information simplifies the operations concept and reduces mission costs through the extensive use of existing low-cost mission support software. Due to small computer resource requirements of 8–10kByte, the SGP4 model may also be applied for onboard orbit computations making use of e.g. a 80186 processor, thus ensuring full compatibility of ground-based and onboard operations. The proposed approach is particularly suited in combination with a space-borne GPS receiver, were the C/A-code navigation solutions are treated as measurements that are adjusted in a least-squares sense using the SGP4 model. As consequence, inherent drawbacks of the pure navigation solutions such as data gaps and scatter as well as limited velocity accuracy are avoided, while the operational navigation activities are kept at a minimum. The feasibility of the concept is illustrated based on real GPS navigation data from the TOPEX/Poseidon and the MIR space station with an inherent data quality of 50–100 m. It is shown that 3 hours of data within a 4 day period are sufficient to keep the position error within 4 km, that is considered sufficient for most applications. 相似文献
69.
E M Nedukha 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,17(6-7):37-45
Space and clinostatic experiments revealed that changes of plant cell wall structure and its function depend on type of tissue and duration of influence. It was shown that clinostat conditions reproduce the part of weightlessness biological effects. It is established that various responses of wall structural-metabolic organization occur at microgravity: changes of cell walls ultrastructure and organelles structure; decrease of synthesis of primary plant cell wall; rearrangements of polysaccharides content. It is shown that mechanisms of plant cell wall changes at microgravity are connected with decrease of cellulose crystallization, activation of pectolytic enzymes and rearrangement of calcium balance of apoplast and cytoplasm. 相似文献
70.
T W Tibbitts 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1996,18(4-5):197-201
The use of controlled environments is subject to problems from contaminants emitted from materials of the system and from plants. Many contaminants are difficult to identify because injurious dosages are very low, there is a lack of information on what compounds injure plants, because species and cultivars differ greatly in their sensitivity to injury and injury symptoms often are not distinctive. Plastics have been shown to emit many different volatile compounds. The compound, di-butyl phthalate, contained in certain flexible plastics, has been shown to be very toxic to plants. Other injuries have been produced by caulking compounds and bonded screening. Paints have been shown to release xylene that is toxic to plants. Steam for humidification can cause problems because of hydroxylamines and other compounds added to steam used for heating to control fungal growth in return lines. Mercury, from broken thermometers is a particular problem in growth chambers because small quantities can collect in cracks and slowly volatilize to slow growth of plants. Plants themselves release large quantities of volatile hydrocarbons, with ethylene being the commonly recognized chemical that can be damaging when allowed to accumulate. People release large quantities of carbon dioxide which can cause variations in the rate of growth of plants. Contaminant problems can be controlled through filtering of the air or ventilation with make-up air, however the potential for problems is always present and careful testing should be undertaken with the particular species and cultivars being grown insure that there are no toxic agents altering growth in each particular controlled environment being utilized. 相似文献