全文获取类型
收费全文 | 329篇 |
免费 | 33篇 |
国内免费 | 37篇 |
专业分类
航空 | 201篇 |
航天技术 | 79篇 |
综合类 | 27篇 |
航天 | 92篇 |
出版年
2022年 | 3篇 |
2021年 | 17篇 |
2020年 | 7篇 |
2019年 | 6篇 |
2018年 | 6篇 |
2017年 | 11篇 |
2016年 | 8篇 |
2015年 | 9篇 |
2014年 | 22篇 |
2013年 | 16篇 |
2012年 | 8篇 |
2011年 | 23篇 |
2010年 | 5篇 |
2009年 | 14篇 |
2008年 | 9篇 |
2007年 | 6篇 |
2006年 | 7篇 |
2005年 | 19篇 |
2004年 | 15篇 |
2003年 | 35篇 |
2002年 | 41篇 |
2001年 | 12篇 |
2000年 | 6篇 |
1999年 | 9篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1991年 | 2篇 |
1989年 | 6篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1975年 | 4篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 6篇 |
1971年 | 1篇 |
1970年 | 3篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1967年 | 4篇 |
1966年 | 2篇 |
1963年 | 2篇 |
1962年 | 1篇 |
排序方式: 共有399条查询结果,搜索用时 15 毫秒
361.
362.
硅氧氮陶瓷的先驱体法合成及性能的研究 总被引:3,自引:2,他引:3
用 Si Cl4为原料 ,通过水解和氨解的方法 ,制备了不同含氮量的硅氧氮先驱体。先驱体通过脱氨基原位聚合 ,再经过无机化转变成为成分均匀的硅氧氮粉体 ,用所得粉体热压烧结制备了硅氧氮材料。测试分析结果表明 ,氮的引入使氧化硅的析晶温度提高了 1 5 0℃ ;适量析晶显著提高材料的力学性能 ;烧结温度为 1 4 0 0℃时 ,氮的质量分数为 2 4 .3%材料的强度和韧性最大 ,分别达到 1 5 6 MPa和 1 .8MPa· m1 / 2 ,比 Si O2 基体的强度和韧性提高了 4 .5 8倍和 2 .2 5倍。 相似文献
363.
L. D. De Feiter 《Space Science Reviews》1974,16(1-2):3-43
In this paper a review is presented of the present status of our knowledge of solar flare phenomena with special emphasis on the production of suprathermal particles and their solar effects. Of these energetic particles electrons play an important role since they produce the X-ray and radiobursts observed during many flares. Also, during their slowing down to thermal energies they contribute to the heating of localized regions in the solar atmosphere, through energy exchange with the ambient electrons. Observable radiations of energetic protons, and other nuclei, are produced through nuclear interactions leading to the emissions of gamma-ray lines. Detectable fluxes of these gamma-ray lines are produced only in the most powerful flares. Also the nuclei that enter into deeper layers of the solar atmosphere transfer most of their kinetic energy to the ambient plasma. 相似文献
364.
面向空间在轨装配维修过程中的攀爬移动问题,基于空间细胞机器人(CSR)理念,提出连接细胞、转动细胞、末端执行细胞三种基本细胞单元。利用图论和拓扑学原理,在关联矩阵的基础上进行改进得到一种细胞迁移矩阵,直观准确地表达细胞机器人任意构型,以及不同节点之间的组织迁移过程。考虑桁架应用环境和细胞机器人自身的特点,划分出三种广义双杆攀爬工况,能够覆盖所有桁架攀爬任务需求。利用基于旋量理论的指数积公式进行不失一般性运动学分析。基于一致性、重复性以及无干涉原则,针对每种攀爬工况提出相应的攀爬步态,进行数值仿真分析,对比不同步态细胞机器人的关节力矩、能耗以及末端轨迹所占据工作空间等参数。仿真结果对空间机器人在轨攀爬路径规划以及控制方式的研究具有重要理论意义和工程价值。 相似文献
365.
Intelligent UUVs: Some issues on ROV dynamic positioning 总被引:1,自引:0,他引:1
Intelligent unmanned underwater vehicles (UUVs) fall under two main group categories: the remotely operated vehicles (ROVs), which are characterized by remote operation and presence of a tether cable; and the autonomous underwater vehicles (AUVs), which are characterized by their autonomous behavior and absence of a tether cable. One fundamental issue of the UUV design is the dynamic position control system. This system plays a crucial role together with the sensor architecture in the degree of system autonomy that can be achieved. This paper is concerned with a few issues when dynamically positioning remotely operated underwater vehicles (ROVs). By restricting the operating regime of ROVs to slow velocity requirements the paper investigates the implementation of a few decentralized control strategies and compare their performance measures, which are assessed by simulating a nonlinear ROV system model for each control strategy. Issues concerning input tracking, disturbance rejection, and plant variations are discussed. The evaluations consider the use of linear PID feedback and feedforward variants, and a robust nonlinear control strategies applied to a full order, fully coupled, and nonlinear vehicle model. These evaluations consider a vehicle undertaking standard mission activities where the tether cable dynamics, with load estimates obtained from a lumped mass cable model, and the vehicle actuator system are present. The paper shows that much of the performance deterioration may be attributed mainly due to cable inertia. The authors also verify that the nonlinear robust control strategy does not necessarily allow for better performance over the linear feedback control strategies implemented when vehicle motions are confined to slow velocity profiles. These and other partial results will aid the design of the control system for an underwater vehicle currently under construction 相似文献
366.
In part I (De Greve and Packet) we have investigated the occurrence of reversed phases of mass-transfer during Case A evolution in close binaries. If the initial period of a system is shorter than 1–2 days (Early Case A) the reversed phase starts before core hydrogen exhaustion of the gainer (part I). This type of evolution is characterized by at least two phases of slow mass-transfer. We have computed the evolution of four Early Case A systems with initial masses of the loser equal to 3 Mo and 5 Mo. These four systems start mass-exchange when Xc of the primary has decreased to 0.525 (75% of its initial value). They all experience two phases of slow mass-transfer. We find that both phases have about the same duration for all systems. The mass ratios are clearly distinct, being closer to unity during the first phase. In the Hertzsprung-Russell, mass-radius and mass-luminosity diagrams both components remain close to the main-sequence band during slow mass-transfer. Evolution as an Algol is ended when both components overflow their outer critical surface after a second reversal of the mass-transfer. Observed Algol systems evolving in Early Case A are scarce. A search thruogh the catalogue by Giuricin et al. gives us the following candidates: X Tri, SX Aur and V Pup. Based on their mass ratios, SX Aur can tentatively be assigned to the first phase of slow mass transfer and X Tri to the second phase. For V Pup (which is more massive) this choice can not be made with certainty. 相似文献
367.
在气动弹性领域内对结构 /控制一体化设计作了初步的探索与研究。针对采用加速度传感器的气动伺服弹性模型 ,建立了在拉普拉斯域中颤振速度对结构设计变量及控制设计变量的解析导数表达式。同时也建立了奇异值对结构设计变量的解析导数表达式。对一矩形机翼以输出反馈的形式进行了结构 /控制一体化设计。目标函数为结构重量最轻 ,满足闭环系统颤振速度和以奇异值为基础的鲁棒稳定性指标约束 ,取得了较好的计算结果 相似文献
368.
D. A. Paige M. C. Foote B. T. Greenhagen J. T. Schofield S. Calcutt A. R. Vasavada D. J. Preston F. W. Taylor C. C. Allen K. J. Snook B. M. Jakosky B. C. Murray L. A. Soderblom B. Jau S. Loring J. Bulharowski N. E. Bowles I. R. Thomas M. T. Sullivan C. Avis E. M. De Jong W. Hartford D. J. McCleese 《Space Science Reviews》2010,150(1-4):125-160
The Diviner Lunar Radiometer Experiment on NASA’s Lunar Reconnaissance Orbiter will be the first instrument to systematically map the global thermal state of the Moon and its diurnal and seasonal variability. Diviner will measure reflected solar and emitted infrared radiation in nine spectral channels with wavelengths ranging from 0.3 to 400 microns. The resulting measurements will enable characterization of the lunar thermal environment, mapping surface properties such as thermal inertia, rock abundance and silicate mineralogy, and determination of the locations and temperatures of volatile cold traps in the lunar polar regions. 相似文献
369.
370.