全文获取类型
收费全文 | 482篇 |
免费 | 279篇 |
国内免费 | 50篇 |
专业分类
航空 | 472篇 |
航天技术 | 194篇 |
综合类 | 24篇 |
航天 | 121篇 |
出版年
2024年 | 2篇 |
2023年 | 22篇 |
2022年 | 29篇 |
2021年 | 34篇 |
2020年 | 50篇 |
2019年 | 39篇 |
2018年 | 38篇 |
2017年 | 45篇 |
2016年 | 49篇 |
2015年 | 28篇 |
2014年 | 36篇 |
2013年 | 21篇 |
2012年 | 30篇 |
2011年 | 40篇 |
2010年 | 38篇 |
2009年 | 35篇 |
2008年 | 21篇 |
2007年 | 63篇 |
2006年 | 20篇 |
2005年 | 14篇 |
2004年 | 13篇 |
2003年 | 12篇 |
2002年 | 13篇 |
2001年 | 14篇 |
2000年 | 7篇 |
1999年 | 14篇 |
1998年 | 21篇 |
1997年 | 15篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 1篇 |
排序方式: 共有811条查询结果,搜索用时 234 毫秒
21.
22.
半球深腔的加工质量是决定硅基MEMS半球陀螺精度的关键因素之一,及时检测半球深腔的形貌参数并将其反馈至加工过程,是确保高性能MEMS半球陀螺研制成功的重要措施。由于硅半球深腔的深度较大,台阶仪和光学显微成像系统无法对半球深腔的形貌特征进行有效测量。因此,需要将硅深腔结构剖开后采用扫描电镜(SEM)进行检测。这种检测方式时间周期长,且属于破坏性的样本检测,效率和测试精度都较低。提出以硅半球深腔为模具,利用PDMS铸模将硅半球深腔的结构尺寸和表面形貌转移到PDMS凸起的半球模型上,通过检测PDMS半球模型的尺寸结构和表面形貌,即可反推出硅半球深腔的尺寸特征。经实验验证,脱模后的PDMS模型可以准确地反映出半球深腔的尺寸信息,测量结果的不确定度小于5‰,有效解决了硅半球深腔无损检测的难题。 相似文献
23.
R.V. Semenyshyn A.N. VeklichI.L. Babich V.F. Boretskij 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Plasma of the free burning electric arc between Ag–SnO2–ZnO composite electrodes as well as brass electrodes were investigated. The plasma temperature distributions were obtained by Boltzmann plot method involving Cu I, Ag I or Zn I spectral line emissions. The electron density distributions were obtained from the width and from absolute intensity of spectral lines. The laser absorption spectroscopy was used for measurement of copper atom concentration in plasma. Plasma equilibrium composition was calculated using two independent groups of experimental values (temperature and copper atom concentration, temperature and electron density). It was found that plasma of the free burning electric arc between brass electrodes is in local thermodynamical equilibrium. The experimental verification of the spectroscopic data of Zn I spectral lines was carried out. 相似文献
24.
Marking arbitrary three-dimensional(3D) target curves on given objects with curved surface is required in many industrial fields, such as fabric prepreg placement in composite material part fabrication, product assembly, surface painting for decoration, etc. A shortcut to the solution of this intractable problem is proposed by utilizing a galvanometric laser scanner(GLS) with the aid of a camera. Without using the existing tedious GLS calibration procedures,the proposed method directly establishes a mapping between the 3D coordinates of the laser spots on the object surface and the control voltages of the scanner. A single-hidden layer feedforward neural network(SLFN) is employed to model the mapping. By projecting a dense grid of laser spots on the object to be marked and simultaneously taking only one image, the SLFN model is trained in minutes via a linear solving mechanism. Experiments demonstrate that the trained SLFN model has a good generalization performance for marking 3D target curves. The 3D laser marking errors on experimental objects are less than 0.5 mm. The proposed method is especially suitable for on-site use and can be conveniently extended to multiple GLSs for marking large complex objects. 相似文献
25.
介绍了激光器的主要性能指标,根据实际需要进行了激光性能测试设备的研究,提出了一种周全的激光性能测试设备研制方案,对该设备的结构设计、光路设计、以及测试方法作了详尽论述。设备中利用两对相互垂直的P分光镜和S分光镜以补偿偏振光在45°面上透反射率不同而造成的测量误差,进一步保证了激光能量测试的准确性。误差分析结果表明该设备满足实际应用中对激光器性能的测试要求。 相似文献
26.
27.
28.
随着核磁共振陀螺技术的发展,高精度核磁共振陀螺对原子气室性能提出了更高要求.原子气室内Xe核自旋的横向弛豫时间(T2)是衡量原子气室性能的重要参数之一,T2的常用测量方法为自由感应衰减法(Free Induction Decay,FID).当T2较短时,由于自旋进动信号易受外界干扰,FID方法难以对T2进行精确测量.根据磁共振线宽理论以及自旋进动信号检测技术,针对T2较短的原子气室,提出了基于磁共振线宽的Xe核自旋横向弛豫时间测量方法,构建了测试装置,对Xe核自旋进行了测试.测试结果表明,该测量方法能够有效获得Xe核自旋的横向弛豫时间,克服了FID方法对T2较短的原子气室难以测量的局限性,为检验核磁共振陀螺中原子气室的性能提供了有效测试手段. 相似文献
29.
Matthew Wilkinson Graham Appleby 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The navigation and geodetic satellites that orbit the Earth at altitudes of approximately 20,000 km are tracked routinely by many of the Satellite Laser Ranging (SLR) stations of the International Laser Ranging Service (ILRS). In order to meet increasing demands on SLR stations for daytime and nighttime observations, any new mission needs to ensure a strong return signal so that the target is easily acquirable. The ILRS has therefore set a minimum effective cross-section of 100 million square metres for the on-board laser retro-reflector arrays (LRAs) and further recommends the use of ‘uncoated’ cubes in the arrays. Given the large number of GNSS satellites that are currently supported by SLR, it is informative to make an assessment of the relative efficiencies of the various LRAs employed. This paper uses the laser ranging observations themselves to deduce and then compare the efficiencies of the LRAs on the COMPASS-M1 navigation satellite, two satellites from the GPS and three from the GLONASS constellations, the two GIOVE test satellites from the upcoming Galileo constellation, the two Etalon geodetic spheres and the geosynchronous communications test satellite, ETS-8. All the LRAs on this set of satellites employ back-coated retro-reflector cubes, except those on the COMPASS-M1 and ETS-8 vehicles which are uncoated. A measure of return signal strength, and thus of LRA-efficiency, is calculated using the laser-range full-rate data archive from 2007 to 2010, scaled to remove the effects of variations in satellite range, atmospheric attenuation and retro-reflector target total surface area. Observations from five SLR stations are used in this study; they are Herstmonceux (UK), Yarragadee (Australia), Monument Peak and McDonald (USA) and Wettzell (Germany). Careful consideration is given to the treatment of the observations from each station in order to take account of local working practices and system upgrades. The results show that the uncoated retro-reflector cubes offer significant improvements in efficiency. 相似文献
30.
P. Lejba S. Schillak 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The positions and velocities of the four Satellite Laser Ranging (SLR) stations: Yarragadee (7090), Greenbelt (7105), Graz (7839) and Herstmonceux (7840) from 5-year (2001–2005) SLR data of low orbiting satellites (LEO): Ajisai, Starlette and Stella were determined. The orbits of these satellites were computed from the data provided by 20 SLR stations. All orbital computations were performed by means of NASA Goddard’s GEODYN-II program. The geocentric coordinates were transformed to the topocentric North–South, East–West and Vertical components in reference to ITRF2005. The influence of the number of normal points per orbital arc and the empirical acceleration coefficients on the quality of station coordinates was studied. To get standard deviation of the coordinates determination lower than 1 cm, the number of the normal points per site had to be greater than 50. The computed positions and velocities were compared to those derived from LAGEOS-1/LAGEOS-2 data. Three parameters were used for this comparison: station coordinates stability, differences from ITRF2005 positions and velocities. The stability of coordinates of LEO satellites is significantly worse (17.8 mm) than those of LAGEOS (7.6 mm), the better results are for Ajisai (15.4 mm) than for Starlette/Stella (20.4 mm). The difference in positions between the computed values and ITRF2005 were little bit worse for Starlette/Stella (6.6 mm) than for LAGEOS (4.6 mm), the results for Ajisai were five times worse (29.7 mm) probably due to center of mass correction of this satellite. The station velocities with some exceptions were on the same level (≈1 mm/year) for all satellites. The results presented in this work show that results from Starlette/Stella are better than those from Ajisai for station coordinates determination. We can applied the data from LEO satellites, especially Starlette and Stella for determination of the SLR station coordinates but with two times lower accuracy than when using LAGEOS data. 相似文献