全文获取类型
收费全文 | 12705篇 |
免费 | 2576篇 |
国内免费 | 2032篇 |
专业分类
航空 | 10536篇 |
航天技术 | 2261篇 |
综合类 | 1332篇 |
航天 | 3184篇 |
出版年
2024年 | 43篇 |
2023年 | 173篇 |
2022年 | 325篇 |
2021年 | 434篇 |
2020年 | 452篇 |
2019年 | 369篇 |
2018年 | 443篇 |
2017年 | 534篇 |
2016年 | 645篇 |
2015年 | 600篇 |
2014年 | 758篇 |
2013年 | 706篇 |
2012年 | 961篇 |
2011年 | 994篇 |
2010年 | 774篇 |
2009年 | 853篇 |
2008年 | 947篇 |
2007年 | 1065篇 |
2006年 | 928篇 |
2005年 | 798篇 |
2004年 | 665篇 |
2003年 | 605篇 |
2002年 | 470篇 |
2001年 | 365篇 |
2000年 | 345篇 |
1999年 | 352篇 |
1998年 | 287篇 |
1997年 | 247篇 |
1996年 | 171篇 |
1995年 | 196篇 |
1994年 | 145篇 |
1993年 | 152篇 |
1992年 | 130篇 |
1991年 | 123篇 |
1990年 | 95篇 |
1989年 | 66篇 |
1988年 | 62篇 |
1987年 | 26篇 |
1986年 | 8篇 |
1984年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
53.
复杂系统的可靠性分析方法研究 总被引:1,自引:1,他引:0
本文对未来的运载器的复杂控制系统的可靠性建模、分配和预计的一般方法和程序及综合分析方法进行了讨论和研究。并简述了复杂系统的计算机辅助可靠性分析。 相似文献
54.
55.
56.
57.
Mercury’s unusually high mean density has always been attributed to special circumstances that occurred during the formation of the planet or shortly thereafter, and due to the planet’s close proximity to the Sun. The nature of these special circumstances is still being debated and several scenarios, all proposed more than 20 years ago, have been suggested. In all scenarios, the high mean density is the result of severe fractionation occurring between silicates and iron. It is the origin of this fractionation that is at the centre of the debate: is it due to differences in condensation temperature and/or in material characteristics (e.g. density, strength)? Is it because of mantle evaporation due to the close proximity to the Sun? Or is it due to the blasting off of the mantle during a giant impact? In this paper we investigate, in some detail, the fractionation induced by a giant impact on a proto-Mercury having roughly chondritic elemental abundances. We have extended the previous work on this hypothesis in two significant directions. First, we have considerably increased the resolution of the simulation of the collision itself. Second, we have addressed the fate of the ejecta following the impact by computing the expected reaccretion timescale and comparing it to the removal timescale from gravitational interactions with other planets (essentially Venus) and the Poynting–Robertson effect. To compute the latter, we have determined the expected size distribution of the condensates formed during the cooling of the expanding vapor cloud generated by the impact. We find that, even though some ejected material will be reaccreted, the removal of the mantle of proto-Mercury following a giant impact can indeed lead to the required long-term fractionation between silicates and iron and therefore account for the anomalously high mean density of the planet. Detailed coupled dynamical–chemical modeling of this formation mechanism should be carried out in such a way as to allow explicit testing of the giant impact hypothesis by forthcoming space missions (e.g. MESSENGER and BepiColombo). 相似文献
58.
针对飞机全机主操纵系统疲劳试验载荷谱中小操纵位移频次影响问题进行了分析和讨论,确定了某机全机主操纵系统小操纵位移频次的级别。同时,在原某机全机主操纵系统疲劳试验载荷谱的基础上再次对其小操纵位移频次进行了计算,去掉了这一级别以下对全机主操纵系统疲劳试验无影响的小操纵位移频次,从而完善了主操纵系统疲劳试验载荷谱。另外,对小操纵位移频次问题研究结果在其全机主操纵系统疲劳试验中的实施效果进行了阐述。 相似文献
59.
The architecture of digital sheet metal manufacturing system is proposed based on the classification of sheet metal manufacturing information.The essence of digital manufacturing is the definition,management and transfer of information,and the key technologies are brought forward and described.It is pointed out that knowledge-based manufacturing elements design is necessary to make digital technology efficient.The management of all kinds of sheet metal manufacturing element information is to build single source of manufacturing data.Multi-state model-based digital transfer and coordination method is designed to provide a foundation for digital manufacturing of aircraft sheet metal part.The application of digital sheet metal manufacturing is exemplified with an aircraft sheet metal part.The application result is compared to that of the traditional analog transfer technology.It is shown that the developed technology can improve part quality,shorten manufacturing time and lower manufacturing cost. 相似文献
60.
近期远程监控系统在工业中逐步推广,针对工业系统的特点和工业网络的实际情况,在原有工业系统的基础上,增加现场设备区域的图像监控和数据传输功能,提出了一种工业远程监控系统实施方案。本文论述了该系统的结构、原理及实现方法,经具体实施证明,本文提出的方案已基本能够满足一个通用的基于公用网络的远程视频监控系统的要求,体现了较重要的理论价值和很强的工程实用价值。 相似文献