全文获取类型
收费全文 | 1119篇 |
免费 | 162篇 |
国内免费 | 184篇 |
专业分类
航空 | 644篇 |
航天技术 | 304篇 |
综合类 | 157篇 |
航天 | 360篇 |
出版年
2023年 | 8篇 |
2022年 | 12篇 |
2021年 | 17篇 |
2020年 | 30篇 |
2019年 | 22篇 |
2018年 | 19篇 |
2017年 | 12篇 |
2016年 | 33篇 |
2015年 | 27篇 |
2014年 | 64篇 |
2013年 | 62篇 |
2012年 | 58篇 |
2011年 | 73篇 |
2010年 | 76篇 |
2009年 | 69篇 |
2008年 | 70篇 |
2007年 | 53篇 |
2006年 | 50篇 |
2005年 | 55篇 |
2004年 | 37篇 |
2003年 | 43篇 |
2002年 | 67篇 |
2001年 | 39篇 |
2000年 | 35篇 |
1999年 | 33篇 |
1998年 | 48篇 |
1997年 | 33篇 |
1996年 | 42篇 |
1995年 | 46篇 |
1994年 | 28篇 |
1993年 | 18篇 |
1992年 | 27篇 |
1991年 | 22篇 |
1990年 | 12篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 4篇 |
1972年 | 2篇 |
1971年 | 4篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1967年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有1465条查询结果,搜索用时 125 毫秒
151.
针对目前膜生物反应器运行过程中膜易污染的问题,研制了一种新型的双功能陶瓷膜生物反应器.该生物反应器的主要特点是采用一种具有双重功能的陶瓷膜作为生物反应器的主要元件,陶瓷膜可以交替地进行曝气和过滤,将膜的在线反冲清洗和生物反应器的供氧曝气结合在一起.考察了不同运行条件下该反应器对COD、氨氮的去除效果.对陶瓷膜的充氧能力进行了实验,测得小孔陶瓷膜、大孔陶瓷膜、砂型曝气头的氧传递系数分别为:0.2315min-1,0.1587min-1,0.0991min-1. 相似文献
152.
Z. Peeters D. Vos I.L. ten Kate F. Selch C.A. van Sluis D.Yu. Sorokin G. Muijzer H. Stan-Lotter M.C.M. van Loosdrecht P. Ehrenfreund 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
Halophilic archaea are of interest to astrobiology due to their survival capabilities in desiccated and high salt environments. The detection of remnants of salty pools on Mars stimulated investigations into the response of haloarchaea to martian conditions. Natronorubrum sp. strain HG-1 is an extremely halophilic archaeon with unusual metabolic pathways, growing on acetate and stimulated by tetrathionate. We exposed Natronorubrum strain HG-1 to ultraviolet (UV) radiation, similar to levels currently prevalent on Mars. In addition, the effects of low temperature (4, −20, and −80 °C), desiccation, and exposure to a Mars soil analogue from the Atacama desert on the viability of Natronorubrum strain HG-1 cultures were investigated. The results show that Natronorubrum strain HG-1 cannot survive for more than several hours when exposed to UV radiation equivalent to that at the martian equator. Even when protected from UV radiation, viability is impaired by a combination of desiccation and low temperature. Desiccating Natronorubrum strain HG-1 cells when mixed with a Mars soil analogue impaired growth of the culture to below the detection limit. Overall, we conclude that Natronorubrum strain HG-1 cannot survive the environment currently present on Mars. Since other halophilic microorganisms were reported to survive simulated martian conditions, our results imply that survival capabilities are not necessarily shared between phylogenetically related species. 相似文献
153.
Qile Zhao Jing Guo Zhigang Hu Chuang Shi Jingnan Liu Hua Cai Xianglin Liu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The GRACE (Gravity Recovery And Climate Experiment) monthly gravity models have been independently produced and published by several research institutions, such as Center for Space Research (CSR), GeoForschungsZentrum (GFZ), Jet Propulsion Laboratory (JPL), Centre National d’Etudes Spatiales (CNES) and Delft Institute of Earth Observation and Space Systems (DEOS). According to their processing standards, above institutions use the traditional variational approach except that the DEOS exploits the acceleration approach. The background force models employed are rather similar. The produced gravity field models generally agree with one another in the spatial pattern. However, there are some discrepancies in the gravity signal amplitude between solutions produced by different institutions. In particular, 10%–30% signal amplitude differences in some river basins can be observed. In this paper, we implemented a variant of the traditional variational approach and computed two sets of monthly gravity field solutions using the data from January 2005 to December 2006. The input data are K-band range-rates (KBRR) and kinematic orbits of GRACE satellites. The main difference in the production of our two types of models is how to deal with nuisance parameters. This type of parameters is necessary to absorb low-frequency errors in the data, which are mainly the aliasing and instrument errors. One way is to remove the nuisance parameters before estimating the geopotential coefficients, called NPARB approach in the paper. The other way is to estimate the nuisance parameters and geopotential coefficients simultaneously, called NPESS approach. These two types of solutions mainly differ in geopotential coefficients from degree 2 to 5. This can be explained by the fact that the nuisance parameters and the gravity field coefficients are highly correlated, particularly at low degrees. We compare these solutions with the official and published ones by means of spectral analysis. It is found that our solutions are, in general, consistent with others in the spatial pattern. The water storage variations of the Amazon, Chari and Ganges river basins have also been computed. The variations computed with the NPARB approach are closer to those produced by JPL and DEOS solutions, while the variations produced with the NPESS approach are in good agreement with those produced by the CSR and GFZ solutions. A simulation study is implemented with considering realistic noise and low-frequency error. The two approaches are used to recover the true model. The NPESS solution appears closer to the true one. Therefore we are inclined to estimate the nuisance parameters simultaneously with the geopential coefficients. 相似文献
154.
155.
156.
Eiges P. E. Zastenker G. N. Safrankova J. Nemecek Z. Eismont N. A. 《Cosmic Research》2001,39(5):432-438
Based on simultaneous measurements of ion fluxes made onboard the closely separated satellites Interball-1and Magion-4, the propagation velocity of middle-scale plasma structures in the Earth's foreshock relative to the solar wind flow is estimated. The derived value of this velocity allows these structures to be identified as a fast magnetosonic wave propagating upstream of the solar wind inflowing the Earth's bow shock. An evaluation is also made of the correlation length of these disturbances in the plane perpendicular to the Sun–Earth line. This length is approximately equal to 2R
E. 相似文献
157.
158.
充液挠性多体航天器的变结构控制 总被引:4,自引:1,他引:4
本文首先给出了充液中心刚体上铰接有多个挠性附件的开链多体系统的简化动力学方程,并以跟踪与数据中继卫星为例,用变结构控制方法设计了系统的控制器。控制任务是驱动单址天线使之精确地跟踪期望的运动规律,同时保持星体稳定,并且有效地抑制弹性附件的振动。数值算例证明了所设计控制规律的有效性。 相似文献
159.
160.