全文获取类型
收费全文 | 8029篇 |
免费 | 17篇 |
国内免费 | 14篇 |
专业分类
航空 | 3946篇 |
航天技术 | 2675篇 |
综合类 | 194篇 |
航天 | 1245篇 |
出版年
2021年 | 67篇 |
2018年 | 147篇 |
2017年 | 90篇 |
2016年 | 98篇 |
2014年 | 170篇 |
2013年 | 192篇 |
2012年 | 211篇 |
2011年 | 299篇 |
2010年 | 194篇 |
2009年 | 338篇 |
2008年 | 392篇 |
2007年 | 201篇 |
2006年 | 167篇 |
2005年 | 196篇 |
2004年 | 195篇 |
2003年 | 249篇 |
2002年 | 244篇 |
2001年 | 314篇 |
2000年 | 142篇 |
1999年 | 189篇 |
1998年 | 233篇 |
1997年 | 165篇 |
1996年 | 188篇 |
1995年 | 248篇 |
1994年 | 236篇 |
1993年 | 141篇 |
1992年 | 183篇 |
1991年 | 91篇 |
1990年 | 84篇 |
1989年 | 191篇 |
1988年 | 82篇 |
1987年 | 93篇 |
1986年 | 81篇 |
1985年 | 218篇 |
1984年 | 184篇 |
1983年 | 159篇 |
1982年 | 180篇 |
1981年 | 239篇 |
1980年 | 89篇 |
1979年 | 72篇 |
1978年 | 79篇 |
1977年 | 52篇 |
1976年 | 60篇 |
1975年 | 65篇 |
1974年 | 61篇 |
1973年 | 46篇 |
1972年 | 66篇 |
1971年 | 62篇 |
1970年 | 44篇 |
1969年 | 47篇 |
排序方式: 共有8060条查询结果,搜索用时 15 毫秒
21.
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
22.
H J Durand-Manterola B Mendoza R Diaz-Sandoval 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,28(4):679-684
An increasing number of studies have appeared in the last ten years indicating that some pathologies may correlate with geomagnetic activity and cosmic rays. However a physical mechanism binding both phenomena has not been proposed. In the present work we obtain the amplitude of the magnetic fields at different frequencies, that may induce inside biological cells currents of the same magnitude of the currents generated by the cells themselves. We compare these values with the wave amplitudes produced in geomagnetic and atmospheric phenomena, and found that micropulsations, whistlers and lightning are capable of produce the same or larger values. 相似文献
23.
This discusses the implementation issues of installing a commercially available hand geometry system in the current infrastructure of Purdue University's Recreational Sports Center. In addition to a performance analysis of the system, a pre- and post-data collection survey was distributed to the 129 test subjects gathering information on perceptions of biometrics, in particular hand geometry, as well as participants' thoughts and feelings during their interaction with the hand geometry device. The results of the survey suggest that participants were accepting of hand geometry. Additionally, analyses of the participants' survey responses revealed that 93% liked using hand geometry, 98% thought it was easy to use, and 87% preferred it to the existing card-based system, while nobody thought the device invaded their personal privacy. System performance achieved a 3-try match rate of 99.02% (FRR 0.98%) when "gaming"/potential impostor attempts were removed from analysis. The failure to enroll rate was zero. Statistical analyses exposed a significant difference in the scores of attempts made by users with prior hand geometry usage, and subjects that could not straighten out their hand on the device. However, there were no statistical difference in the effects of rings/no rings, improper hand placements/proper hand placements, or gender on hand geometry score. 相似文献
24.
25.
Wright R.G. Zgol M. Keeton S. Kirkland L.V. 《Aerospace and Electronic Systems Magazine, IEEE》2001,16(6):15-19
This paper describes original research efforts in the design, simulation, and development of nanotechnology-based molecular test equipment (MTE). This is a research effort for testing printed circuit boards independent of traditional automatic test equipment (ATE) through the fabrication of MTE within integrated circuits (ICs). The MTE is embedded within the IC substrate and encapsulated within nanoprobes that connect between the surface and the substrate of the IC at various functional areas. A process is followed whereby IC device simulation is performed to assess the electrical, chemical, and structural properties of integrated and adjacent substrate devices. Through this approach the nominal and failed device performance parameters of interest to substrate-based MTE are found. Discussion of the development and application of MTE within IC architectures is provided, including such topics as the effect of substrate composition on the design and realization of MTE, interfaces between MTE and IC devices, and reporting of MTE results to the IC surface and technician. Potential application areas within different device functions will also be identified. A chemical structure diagram is also provided to illustrate the implementation of MTE using discrete device configurations with MTE-augmented logic 相似文献
26.
27.
A flat panel multifunction display (MFD) that offers the size and reliability benefits of liquid crystal display technology while achieving near-CRT display quality is presented. Display generation algorithms that provide exceptional display quality are being implemented in custom VLSI components to minimize MFD size. A high-performance processor converts user-specified display lists to graphics commands used by these components, resulting in high-speed updates of two-dimensional and three-dimensional images. The MFD uses the MIL-STD-1553B data bus for compatibility with virtually all avionics systems. The MFD can generate displays directly from display lists received from the MIL-STD-1553B bus. Complex formats can be stored in the MFD and displayed using parameters from the data bus. The MFD also accepts direct video input and performs special processing on this input to enhance image quality 相似文献
28.
R D Govorun I V Koshlan N A Koshlan E A Krasavin N L Shmakova 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2002,30(4):885-890
The induction of HPRT-mutations and survival of Chinese hamster cells (line B11ii-FAF28, clone 431) were studied after irradiation by 4He and 12C-ions of various LET (20-360 keV/micrometers), produced by the U-200 heavy ion accelerator. The RBE increases with LET up to the maximum at 100-200 keV/micrometers and then decreases. Cytogenetic analysis was performed on the HPRT-mutant subclones selected from unirradiated Chinese hamster V-79 cells and from HPRT-mutant subclones that arose after exposure to gamma-rays, 1 GeV protons and 14N-ions (LET-77 keV/micrometers), produced by the synchrophasotron and the U-400M heavy ion accelerator. Slow growing mutant subclones were observed. The cytogenetic properties of individual clones were highly heterogeneous and chromosome instability was observed in both spontaneous and radiation-induced mutants. Chromosome instability was highest among spontaneous mutants and decreased with increasing LET. 相似文献
29.
An electric vehicle (EV) energy management system (EMS) developed to accurately predict and extend the usable driving range and the life of the battery pack is discussed. The EMS monitors and records battery state of charge, vehicle performance, power consumption of key components, and driver's actions. Convenient drive inputs in conjunction with memorized energy consumption profiles allow accurate prediction of driving range and selection of appropriate recharging profiles. EV range extension is accomplished by identifying inefficient use of energy, resulting in EMS modification of energy usage by offending subsystems or EMS suggestion to the driver of improved driving habits or vehicle use. Factors affecting the accuracy of range prediction and the amount of range extension are described 相似文献
30.
G Horneck R Facius G Reitz P Rettberg C Baumstark-Khan R Gerzer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,31(1):87-95
Human missions to Mars are planned to happen within this century. Activities associated therewith will interact with the environment of Mars in two reciprocal ways: (i) the mission needs to be protected from the natural environmental elements that can be harmful to human health, the equipment or to their operations; (ii) the specific natural environment of Mars should be protected so that it retains its value for scientific and other purposes. The following environmental elements need to be considered in order to protect humans and the equipment on the planetary surface: (i) cosmic ionizing radiation, (ii) solar particle events; (iii) solar ultraviolet radiation; (iv) reduced gravity; (v) thin atmosphere; (vi) extremes in temperatures and their fluctuations; and (vii) surface dust. In order to protect the planetary environment, the requirements for planetary protection as adopted by COSPAR for lander missions need to be revised in view of human presence on the planet. Landers carrying equipment for exobiological investigations require special consideration to reduce contamination by terrestrial microorganisms and organic matter to the greatest feasible extent. Records of human activities on the planet's surface should be maintained in sufficient detail that future scientific experimenters can determine whether environmental modifications have resulted from explorations. 相似文献