全文获取类型
收费全文 | 2545篇 |
免费 | 593篇 |
国内免费 | 460篇 |
专业分类
航空 | 1764篇 |
航天技术 | 571篇 |
综合类 | 376篇 |
航天 | 887篇 |
出版年
2024年 | 14篇 |
2023年 | 32篇 |
2022年 | 100篇 |
2021年 | 119篇 |
2020年 | 111篇 |
2019年 | 99篇 |
2018年 | 98篇 |
2017年 | 89篇 |
2016年 | 89篇 |
2015年 | 125篇 |
2014年 | 146篇 |
2013年 | 176篇 |
2012年 | 182篇 |
2011年 | 208篇 |
2010年 | 221篇 |
2009年 | 199篇 |
2008年 | 152篇 |
2007年 | 157篇 |
2006年 | 149篇 |
2005年 | 113篇 |
2004年 | 81篇 |
2003年 | 90篇 |
2002年 | 111篇 |
2001年 | 115篇 |
2000年 | 71篇 |
1999年 | 102篇 |
1998年 | 82篇 |
1997年 | 61篇 |
1996年 | 56篇 |
1995年 | 47篇 |
1994年 | 47篇 |
1993年 | 32篇 |
1992年 | 18篇 |
1991年 | 19篇 |
1990年 | 28篇 |
1989年 | 24篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1981年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有3598条查询结果,搜索用时 531 毫秒
721.
722.
飞机日历寿命试验的介质成分确定和加速方法 总被引:6,自引:1,他引:5
针对飞机使用环境,研究了飞机日历寿命试验中的腐蚀介质成分确定方法和加速方法。进行了5 ℃,25 ℃,50 ℃这3种温度和4种介质浓度下的30CrMnSiA结构钢和LY12CZ铝合金两种金属的纯腐蚀和腐蚀疲劳试验研究。得到的试验曲线表明,它们的腐蚀动力学规律都近似遵循线性关系,每条曲线的腐蚀速度也基本保持不变。从而认为,飞机日历寿命试验介质成分最好选用飞机所在机场使用环境的各腐蚀介质成分,加速腐蚀试验的最好方法,就是在此机场的各介质成分上加权,加权越高,加速越快。这个结论得到了试验验证。 相似文献
723.
724.
Two AhO3/Cu composites containing 0.24 wt.% A1203 and 0.60 wt.% A1203 separately are prepared by internal oxidation.Effectsof sliding speed and pressure on the fi-ictional characteristics of the composites and copper against brass are investigated and compared.The changes in morphology of the sliding surface and subsurface are examined with scanning electron microscope (SEM) and energy dispersive X-ray spectrum (EDS).The results show that the wear resistance of the AI203/Cu composites is superior to that of copperunder the same conditions.Under a given electrical current,the wear rate of AI203/Cu composites decreases as the AleO3-content increases.However,the wear rates of the Al203/Cu composites and copper increase as the sliding speed and pressure increase under drysliding condition.The main wear mechanisms for AleO3/Cu composites are of abrasion and adhesion;for copper,it is adhesion,although wear by oxidation and electrical erosion can also be observed as the speed and pressure rise. 相似文献
725.
726.
The growing amount of space debris poses a threat to operational spacecraft and the long-term sustainability of activities in outer space. According to the orbital mechanics, an uncontrolled space object will be tumbling, bringing great challenge to capture and remove it. In this paper, a dual-arm coordinated ‘‘Area-Oriented Capture"(AOC) method is proposed to capture a non-cooperative tumbling target. Firstly, the motion equation of the tumbling target is established, based on which, the dynamic properties are analyzed. Then, the ‘‘Area-Oriented Capture"concept is presented to deal with the problem of large pose(position and attitude) deviation and tumbling motion. An area rather than fixed points/devices is taken as the object to be tracked and captured. As long as the manipulators' end-effectors move to a specified range of the objective areas(not fixed points on the target, but areas), the target satellite will be hugged by the two arms.At last, the proposed method and the traditional method(i.e. fixed-point oriented capture method)are compared and analyzed through simulation. The results show that the proposed method has larger pose tolerance and takes shorter time for capturing a tumbling target. 相似文献
727.
Lean Blow-Off(LBO) prediction is important to propulsion system design. In this paper,a hybrid method combining numerical simulation and Da(Damk?hler) model is proposed based on bluffbody stabilized flames. In the simulated reacting flow, Practical Reaction Zone(PRZ) is built based on OH radical concentration, and it is considered to be the critical zone that controls LBO.Da number is obtained based on the volume-averaged parameters of PRZ. The flow time scale(s_f)indicates the residence time of the fresh mixture flowing through the PRZ. It is obtained based on the characteristic length and volume-averaged axial velocity of the PRZ. The chemical time scale(s_c) indicates the shortest time needed to trigger the reaction of the mixture. It is obtained by commercial software CHEMKIN through monitoring the transient variation of the reactor temperature. The result shows that the average Da number under different LBO conditions is 1.135(the Da number under each LBO condition ranges from 0.673 to 1.351). This indicates that the flow time scale and chemical time scale are comparable. The combustion is in a critical state where LBO is easy to occur. With the increase of the fuel mass flow rate(the global fuel/air ratio increases from 0.004761 to 0.01095), s_f increases from 0.001268 s to 0.007249 s, and s_c decreases from 0.00124 s to0.00089 s. Accordingly, Da number increases from 1.023 to 8.145, which shows that the combustion becomes more stable. The above results show that the method proposed in the present study can properly predict the LBO limits of combustors, which provides important technical supports for combustor design and optimization. 相似文献
728.
729.
Assessing ecohydrological factors variations and their relationships at different spatio-temporal scales in semiarid area,northwestern China 总被引:1,自引:0,他引:1
Yonghua Zhu Sheng Zhang Pingping Luo Feng Su Biao Sun Li'e Liang Juan Guo Rui Yang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2021,67(8):2368-2381
As a typical semiarid farming-pastoral ecotone sensitive to the environmennt, the Plain of West Liaohe Basin (WLBP) is currently experiencing drastic environmental changes. To identify how environmental change affect vegetation in the WLBP, we analysed spatiotemporal variation characteristics of Ecological environment factors based on monthly and annual air temperature (T), precipitation (P) and Normalized Difference Vegetation Index (NDVI) from 1982 to 2015. And the correlations between them were investigated by correlation analysis (Simple correlation, partial correlation and complex correlation) at temporal and spatial scale. The results showed that: (1) the vegetation growth of the WLBP showed ameliorated trend, with a change rate of 0.004/yr.; (2) P was more sensitive to NDVI than T; (3) and the influence of hydrothermal changes on vegetation growth was more significant than that of the change of single climate factors at time scales; (4) the effects of anthropogenic factors on vegetation change were 75.07% (1982–1993) and 98.08% (1994–2015), respectively. At the temp-special scales, P&T and land use type change (LUCC) were the main climatic and anthropogenic factors that affect vegetation changes, respectively. 相似文献
730.