全文获取类型
收费全文 | 9129篇 |
免费 | 3172篇 |
国内免费 | 1274篇 |
专业分类
航空 | 8318篇 |
航天技术 | 1664篇 |
综合类 | 688篇 |
航天 | 2905篇 |
出版年
2024年 | 69篇 |
2023年 | 280篇 |
2022年 | 668篇 |
2021年 | 721篇 |
2020年 | 656篇 |
2019年 | 595篇 |
2018年 | 621篇 |
2017年 | 748篇 |
2016年 | 511篇 |
2015年 | 687篇 |
2014年 | 586篇 |
2013年 | 669篇 |
2012年 | 754篇 |
2011年 | 798篇 |
2010年 | 723篇 |
2009年 | 681篇 |
2008年 | 696篇 |
2007年 | 647篇 |
2006年 | 578篇 |
2005年 | 484篇 |
2004年 | 436篇 |
2003年 | 266篇 |
2002年 | 292篇 |
2001年 | 176篇 |
2000年 | 154篇 |
1999年 | 66篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1990年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
991.
992.
994.
利用磷酸的全面腐蚀特性和氯离子的活化作用,设计了磷酸-氯化镍浸镍反应体系,通过表面电位监测及微观形貌表征对不同磷酸浓度与反应温度下的浸镍过程进行分析。结果表明:磷酸浓度是影响浸镍层表面电位及微观形貌的关键因素,当磷酸浓度为25%,反应温度为30℃时,可制得化学性质稳定、包覆性良好且晶粒尺寸均匀的浸镍层。在此反应体系下,浸镍层在形核后通过球状方式叠层生长,在反应进行600 s后得到厚度约1 μm的浸镍层,其表面电位可达到-0.51 V左右。 相似文献
995.
996.
It is well known that the Two-step Weighted Least-Squares(TWLS) is a widely used method for source localization and sensor position refinement. For this reason, we propose a unified framework of the TWLS method for joint estimation of multiple disjoint sources and sensor locations in this paper. Unlike some existing works, the presented method is based on more general measurement model, and therefore it can be applied to many different localization scenarios.Besides, it does not have the initialization and local convergence problem. The closed-form expression for the covariance matrix of the proposed TWLS estimator is also derived by exploiting the first-order perturbation analysis. Moreover, the estimation accuracy of the TWLS method is shown analytically to achieve the Cramér-Rao Bound(CRB) before the threshold effect takes place. The theoretical analysis is also performed in a common mathematical framework, rather than aiming at some specific signal metrics. Finally, two numerical experiments are performed to support the theoretical development in this paper. 相似文献
997.
998.
999.
Influence of pre-tension on ballistic impact performance of multi-layer Kevlar 49 woven fabrics for gas turbine engine containment systems 总被引:1,自引:0,他引:1
In this paper, ballistic impact tests on wrapped multi-layer Kevlar 49 woven fabric systems were carried out with a flat blade projectile to investigate the impact response during a fan blade out event. The influences of the number of Kevlar layers and pre-tension were discussed particularly. Test results were used to analyze failure modes and energy absorption characteristics of multi-ply Kevlar fabrics. Results show that there are two kinds of impact damage for fabrics: global deformation mainly involving stretching of yarns in the impact region and fabric wrinkle from both sides to the impact zone, and local damage characterized by yarn fracture, yarn pull-out, and yarn unraveling. The energy absorption capability of Kevlar 49 woven fabrics improves with the number of fabric layers. The energy absorbed by multi-layer fabrics increases slightly at the beginning and then decreases substantially with pre-tension. The work in this paper can provide guidance for designing light-weight multi-layer fabrics containment systems. 相似文献
1000.