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   1160篇
  免费   274篇
  国内免费   194篇
航空   838篇
航天技术   286篇
综合类   169篇
航天   335篇
  2024年   6篇
  2023年   14篇
  2022年   44篇
  2021年   55篇
  2020年   52篇
  2019年   25篇
  2018年   31篇
  2017年   38篇
  2016年   44篇
  2015年   66篇
  2014年   59篇
  2013年   77篇
  2012年   78篇
  2011年   71篇
  2010年   82篇
  2009年   67篇
  2008年   65篇
  2007年   67篇
  2006年   49篇
  2005年   55篇
  2004年   41篇
  2003年   32篇
  2002年   47篇
  2001年   46篇
  2000年   42篇
  1999年   49篇
  1998年   40篇
  1997年   35篇
  1996年   32篇
  1995年   24篇
  1994年   33篇
  1993年   28篇
  1992年   27篇
  1991年   36篇
  1990年   17篇
  1989年   20篇
  1988年   9篇
  1987年   14篇
  1986年   4篇
  1983年   1篇
  1981年   1篇
  1977年   1篇
  1972年   2篇
  1970年   1篇
  1965年   1篇
排序方式: 共有1628条查询结果,搜索用时 15 毫秒
51.
A straw-soil co-composting and evaluation for plant substrate in BLSS   总被引:1,自引:0,他引:1  
Material closure is important for the establishment of Bioregenerative Life Support System, and many studies have focused on transforming candidate plant residues into plant culture medium. For the limitations of using wheat straw compost as substrate for plant cultivation, a straw-soil co-composting technique was studied. The changes of pH, C/N value, germination index, cellulose, lignin and so on were monitored during the co-composting process. The maturity was evaluated by the C/N value and the germination index. The result showed that after 45 days’ fermentation, the straw-soil final co-compost with inoculation (T1) became mature, while the co-compost without inoculation (T0) was not mature. In the plant culture test, the T1 substrate could satisfy the needs for lettuce’s growth, and the edible biomass yield of lettuce averaged 74.42 g pot−1 at harvest. But the lettuces in T0 substrate showed stress symptoms and have not completed the growth cycle. Moreover, the results of nitrogen (N) transformation experiment showed that about 10.0% and 3.1% N were lost during the T1 co-composting and plant cultivation, respectively, 23.5% N was absorbed by lettuce, and 63.4% N remained in the T1 substrate after cultivation.  相似文献   
52.
储氢合金的性质及发展趋势   总被引:5,自引:0,他引:5  
本文介绍了储氢合金的性质,Ni-MH电池原理,以及储氢合金对Ni-MH电池所产生的影响,根据目前科学技术的发展以及市场的需求,提出了未来储氢合金及Ni-MH电池的发展趋势。  相似文献   
53.
结合洪都航空工业集团科研和生产的实际情况,从型号研制与生产的角度人笔,阐述了电子档案管理存在的问题,并提出了解决办法。  相似文献   
54.
针对航空关键动部件生产制造及验收试验中出现的提前失效问题,采用多种疲劳失效分析方法和工具,进行了疲劳试 验出现失效的原因分析,从宏观和微观分析、金相分析、能谱分析及疲劳试验过程中的应力分析等方面对产生的缺陷进行了系统 研究。了解了其疲劳开裂的原因,并对缺陷性质及零件制造工艺过程进行分析。结果表明:应力集中区是最容易出现开裂的位 置,也是检测的关键部位,要进一步加强在制件及在役件的表面缺陷的检测,细化其检测实施方式及关键控制点,是保证动部件试 验质量的关键。通过加强受力部位的无损检测,为及早发现和及时处理潜在的缺陷提供重要依据,从而降低关键动部件的试验失 败的风险,提高产品试验质量及使用寿命,有助于其它机型更好地改进检测的设计要求及应用,为其提供了重要的依据和研究方向。  相似文献   
55.
本文以WM-80/φ273毫米火箭炮的扭力平衡机为背景,讨论了扭力平衡机的基本原理及其设计的一般情况。通过对Mp,Mq0和Mq等参数的确定,分析,为同类型产品的设计计算提供了理论指导。  相似文献   
56.
Grasping Strategy in Space Robot Capturing Floating Target   总被引:3,自引:3,他引:0  
When the space robot captures a floating target, contact impact occurs inevitably and frequently between the manipulator hand and the target, which seriously impacts the position and attitude of the robot and grasping security. “Dynamic grasping area” is introduced to describe the collision process of manipulator grasping target, and grasping area control equation is established. By analyzing the impact of grasping control parameters, base and target mass on the grasping process and combining the life experience, it is found that if the product of speed control parameter and dB adjustment parameter is close to but smaller than the minimum grasping speed, collision impact in the grasping process could be reduced greatly, and then an ideal grasping strategy is proposed. Simulation results indicate that during the same period, the strategy grasping is superior to the accelerating grasping, in that the amplitude of impact force is reduced to 20%, and the attitude control torque is reduced to 15%, and the impact on the robot is eliminated significantly. The results would have important academic value and engineering significance.  相似文献   
57.
考虑轨道摄动的外热流计算分析   总被引:1,自引:0,他引:1  
王宇宁  魏承  赵阳 《上海航天》2012,29(5):48-53,66
为改进传统外热流算法未考虑外热流经多次反射后的吸收和国外商业软件未考虑轨道摄动及太阳矢量变化对外热流的影响等不足,重新定义了外热流角系数和外热流辐射传递因子,综合SGP4轨道模型和Gebhart方法,给出了考虑摄动等因素的辐射传递因子和轨道外热流计算模型,以及相应的计算流程。通过仿真分析论证了考虑多次反射及摄动等因素的重要性。  相似文献   
58.
通过对影响飞机纵向平衡速度的结构特性和气动原理的分析,指出了飞机纵向平衡速度调整中存在的问题,给出了不同飞机有效调整纵向平衡速度的方法。  相似文献   
59.
Remote sensing images and technologies have been widely applied to environmental monitoring, in particular landuse/landcover classification and change detection. However, the uncertainties involved in such applications have not been fully addressed. In this paper two hypothesis-test-based change detection methods, namely the bivariate joint distribution method and the conditional distribution method, are proposed to tackle the uncertainties in change detection by making decisions based on the desired level of significance. Both methods require a data set of class-dependent no-change pixels to form the basis for class-dependent hypothesis test. Using an exemplar study area in central Taiwan, performance of the proposed methods are shown to be significantly superior to two other commonly applied methods (the post-classification comparison and the image differencing methods) in terms of the overall change detection accuracies. The conditional distribution method takes into consideration the correlation between digital numbers of the pre- and post-images and the effect of the known pre-image digital number on the range of the post-image digital number, and therefore yields the highest change detection accuracy. It is also demonstrated that the class-dependent change detection is crucial for accurate landuse/landcover change detection.  相似文献   
60.
简述了美国雷达武器化的研究现状;介绍了超宽带雷达(UWBR)和高功率微波武器(HPMW)的组成、基本原理;结合国内外现有技术成果,分析了UWBR和HPMW一体化设计的可行性;参照数字阵列雷达,提出了一种UWBR和HPMW一体化概念设计结构图,并重点讨论了一体化设计的难点及关键技术。  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