首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2521篇
  免费   29篇
  国内免费   7篇
航空   1349篇
航天技术   916篇
综合类   22篇
航天   270篇
  2018年   30篇
  2017年   25篇
  2014年   37篇
  2013年   56篇
  2012年   41篇
  2011年   79篇
  2010年   56篇
  2009年   71篇
  2008年   140篇
  2007年   42篇
  2006年   37篇
  2005年   51篇
  2004年   74篇
  2003年   83篇
  2002年   55篇
  2001年   72篇
  2000年   55篇
  1999年   38篇
  1998年   78篇
  1997年   56篇
  1996年   64篇
  1995年   74篇
  1994年   90篇
  1993年   52篇
  1992年   72篇
  1991年   29篇
  1990年   34篇
  1989年   71篇
  1988年   27篇
  1987年   32篇
  1986年   51篇
  1985年   90篇
  1984年   59篇
  1983年   63篇
  1982年   61篇
  1981年   73篇
  1980年   35篇
  1979年   29篇
  1978年   27篇
  1977年   26篇
  1975年   25篇
  1974年   24篇
  1973年   25篇
  1972年   22篇
  1971年   32篇
  1970年   19篇
  1969年   29篇
  1968年   19篇
  1967年   23篇
  1966年   19篇
排序方式: 共有2557条查询结果,搜索用时 15 毫秒
201.
In reviewing discussions of future directions for space activity, it becomes obvious that there are a large number of groups formulating a wide diversity of plans for the future use of space. These plan alternatives are being made to account for user needs, technology development constraints, economic constraints, and launch support, and each of the plans will have direct or indirect effects on the orbital debris environment in terms of mass to orbit, deposition of operational debris, and control of accidental breakups. Thus it is important to develop the ability to project future debris states for a range of possible space traffic scenarios. The impact that these possible traffic environments would have on space operations forms the basis for studies of alternative options for the usage of space. In this paper, the effects on the orbital debris environment of a base-line mission model and two alternatives are investigated, using a numerical debris environment simulation code under development at JSC.  相似文献   
202.
Development of components for bioregenerative life-support systems is a vital step toward long-term space exploration. The culturing of plants in a microgravity environment may be optimized by the use of appropriate sensors and controllers. This paper describes a sensor developed for determining the amount of fluid (nutrient solution) available on the surface of a porous ceramic nutrient delivery substrate to the roots of conventional crop plants. The sensor is based on the change in thermal capacitance and thermal conductance near the surface as the moisture content changes. The sensor could be employed as a data acquisition and control sensor to support the automated monitoring of plants grown in a microgravity environment.  相似文献   
203.
During the initial period of high voltage biasing of the SPEAR-3 sounding rocket payload, it was observed that electrical breakdown occurred in the gas surrounding the rocket. The breakdown occurred almost all the way to apogee of 289 km on the upleg, but did not recur on the downleg until the payload reached an altitude of 100 km. It is suggested that this behavior can be attributed to payload outgassing on the upleg leading to abnormally high gas pressure near the payload skin. Consideration of a modified Paschen discharge process with varying pressure along the discharge path was found to be consistent with the results.  相似文献   
204.
An absorption feature at 3.4 micrometers has been observed in various lines-of-sight through the diffuse interstellar medium. Its position and width lead to an identification with the C-H stretching mode of solid organic material. A possible mechanism for the production of organic solids in the interstellar medium is UV photoprocessing of icy mantles which accrete on dust grains in dense clouds. Furthermore, thermally induced reactions involving formaldehyde molecules in the mantles could be an important source of organics. Laboratory simulation of these processes shows that a large variety of oxygen- and nitrogen-rich species may be produced. It is shown that the occurrence of periodic transient heating events plays an important role in the production of organic material in the ice mantles. Finally, it is pointed out how future missions like the Infrared Space Observatory (ISO) as well as analysis of comet material by Rosetta may be able to clarify the nature and evolution of interstellar organics.  相似文献   
205.
206.
The multiparametric dosimetry system that we are developing for medical radiological defense applications could be adapted for spaceflight environments. The system complements the internationally accepted personnel dosimeters and cytogenetic analysis of chromosome aberrations, considered the best means of documenting radiation doses for health records. Our system consists of a portable hematology analyzer, molecular biodosimetry using nucleic acid and antigen-based diagnostic equipment, and a dose assessment management software application. A dry-capillary tube reagent-based centrifuge blood cell counter (QBC Autoread Plus, Becton [correction of Beckon] Dickinson Bioscience) measures peripheral blood lymphocytes and monocytes, which could determine radiation dose based on the kinetics of blood cell depletion. Molecular biomarkers for ionizing radiation exposure (gene expression changes, blood proteins) can be measured in real time using such diagnostic detection technologies as miniaturized nucleic acid sequences and antigen-based biosensors, but they require validation of dose-dependent targets and development of optimized protocols and analysis systems. The Biodosimetry Assessment Tool, a software application, calculates radiation dose based on a patient's physical signs and symptoms and blood cell count analysis. It also annotates location of personnel dosimeters, displays a summary of a patient's dosimetric information to healthcare professionals, and archives the data for further use. These radiation assessment diagnostic technologies can have dual-use applications supporting general medical-related care.  相似文献   
207.
An ultracapacitor system for an electric vehicle has been implemented. The device allows higher accelerations and decelerations of the vehicle with minimal loss of energy and minimal degradation of the main battery pack. The system uses a DC-DC power converter, which is connected between the ultracapacitor and the main battery pack. The design has been optimized in weight and size, by using water-cooled heat sinks for the power converter, and an aluminum coil with air core for the smoothing inductance. The ratings of the ultracapacitor are: nominal voltage: 300 Vdc; nominal current: 200 Adc; capacitance: 20 Farads. The amount of energy stored allows us to have 40 kW of power during 20 seconds, which is enough to accelerate the vehicle without the help of the traction batteries. The vehicle uses a brushless DC motor with a nominal power of 32 kW and a peak power of 53 kW. A control system based on a Digital Signal Processor (DSP) manipulates all the aforementioned variables and controls the Pulse Width Modulation (PWM) switching pattern of the converter transistors. The car used for the implementation of this system is a Chevrolet LUV truck.  相似文献   
208.
Twenty-first century littoral and open-sea missions present US Navy (USN) shipboard-radar systems with the challenge of detecting small targets in severe clutter and against multiple sources of interference. In Fiscal Year 2000 (FY00), the Office of Naval Research (ONR) sponsored a program to develop an active array radar that includes a digital beamforming (DBF) architecture. The DBF radar system has the potential for improved time-energy management, improved signal-to-clutter (S/C) ratios, improved reliability and reduced life-cycle costs. This paper summarizes the latest developments of the program during FY00  相似文献   
209.
210.
A ceramic material having a large dielectric constant at 77 K, ε=8000-12000, has been developed for capacitive energy storage at this temperature. A large matrix of multilayer ceramic capacitors were fabricated using conventional tape-casting methods to optimize the dielectric breakdown strength at 77 K, and measured energy storage values on these capacitors range up to 6 J/cm3 at 77 K. An unfused bank of these capacitors was voltage-cycled 105 times at 77 K without failure, and the heating effects during cycling were immeasurably small (i.e., nitrogen boiloff was monitored). An electrocaloric effect on discharge (ΔT~1 K) contributes to the thermal stability. Measurements of the frequency dependence of the dielectric properties of the ceramic at 77 K indicate a fundamental limit of about 8 μs for the switching repetition rate. Improved capacitor-manufacturing methods are discussed which can increase the energy density to the 20-30 J/cm3 range  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号