BP神经网络在元器件贮存可靠性参数预测中的应用

元器件贮存可靠性受多个非线性因素的影响。时间序列预测实质是实现一个非线性映射。应用BP神经网络模型对某元器件贮存可靠性性能参数时间序列进行了预测,仿真表明BP神经网络预测模型有较高的精度。     

Negative VTEC with spherical harmonic expansion model under different solar activity conditions

The Earth’s ionosphere can be described by a spherical harmonic (SH) expansion up to a specific degree. However, there exist negative vertical total electron content (VTEC) values in the global ionosphere map (GIM) with the SH expansion model. In this contribution, we specifically investigated the negative VTEC values that are induced by the SH expansion model and validated the performance of the inequality-constrained least squares (ICLS) method in eliminating the negative VTEC values. The GPS data from 2004 to 2017 was selected to cover one solar cycle and the experiments under different solar activity conditions were analyzed. The results in our work show that the occurrence of the negative VTEC values is attributed to the deficiency of the SH expansion model when the VTEC itself is small instead of the unevenly distribution of the GNSS stations. The negative VTEC values appear periodically in the temporal domain, showing apparently one year and half year periods. During one year, two peaks in June and December can be observed in the time series of the negative VTEC values. The number of negative VTEC values in June is obvious larger than that in December. During one solar cycle, the number of negative VTEC values under quiet solar activity condition is obvious larger than that under strong solar activity condition. In the spatial domain, the appearance of the negative VTEC values is strongly related with the movement of the subsolar point. In the latitude of the subsolar point has the largest magnitude, the negative values will appear on the opposite hemisphere and the further from the subsolar point the more negative values. The maximum number of the negative VTEC values in the southern hemisphere appears in June, while the peak value in the northern hemisphere appears in December. The maximum number of negative VTEC values in the southern hemisphere is generally larger than that in the northern hemisphere. In addition, the negative VTEC values are distributed both at middle latitude and high latitude in the southern hemisphere, while they are mainly distributed at high latitude in the northern hemisphere. When the ICLS method is used, the negative VTEC values can be eliminated efficiently and it has nearly no influence on the positive VTEC values. The ICLS method can also improve the receiver’s differential code bias (DCB) and significantly decrease the unreasonable negative slant TEC (STEC) values along the lines of sight. Using the final GIM product of the Jet Propulsion Laboratory (JPLG) as a reference, the root mean square (RMS) of the ICLS solution shows maximum 25%, 20% and 45% improvement relative to the least squares (LS) solution at northern high latitude, southern middle latitude and southern high latitude, respectively.     

Development and coupling test of active spacecraft potential control – Next generation (ASPOC-NG)

The active spacecraft potential control (ASPOC) system developed in the 1990s emits positive ions to neutralise the spacecraft potential, such as used in several missions like Geotail, Equator-S, Cluster, Doublestar and MMS. With the experience gained, the next generation of the active spacecraft potential control (ASPOC-NG) instrument has been developed over the last three years. Thereby, three emission technologies were tested including Liquid Metal Ion Source (LMIS), Liquid Metal Electron Source (LMES) and Solid Metal Electron Source (SMES). The development of the emitter module by FOTEC and the corresponding electronics control unit by IWF is presented. Optimisations were carried out with the focus on the reduction of mass and power consumption to comply with the requirements of future scientific missions. Coupling tests of the modules and the electronics control unit were performed including range, accuracy and lifetime tests. Both ASPOC-NG instruments for positive and negative charge compensation and their performance values show excellent results.     

Comparison of electron density measurements from CSES and Swarm satellites with GNSS ionospheric tomography data

Electron density measurements obtained from China Seismo‐Electromagnetic Satellite (CSES) and Swarm-B can play an increasingly important role in the study of ionosphere above F2 peak height. This study presented a comprehensive comparison of electron density products obtained from Langmuir probe mounted on CSES and Swarm-B with ionospheric tomography for a whole year period of 2019. CSES was fully compared with Swarm-B on a global scale, including both absolute and relative differences, and a new index called NFI was developed to better quantify the similarity between two latitudinal profiles of electron density. CSES and Swarm-B were then compared with tomography respectively in four regions, roughly located in America, Europe, Australia and China. Results indicated that CSES data are consistent with Swarm-B, as NFI values exceed 0.6 for most of the analyses. Tomography and Swarm-B were found to have a good agreement as their biases are less than 0.2 × 10⁵ el/cm³ in general. For the comparison between CSES and tomography, the bias increased to around 0.6 × 10⁵ el/cm³ but the standard deviation changed slightly, validating the underestimation of electron density by CSES. The spatiotemporal comparisons of CSES and Swarm-B with tomography showed that: 1) the differences in electron density were relatively low in middle latitudes and increased rapidly in the regions of equatorial ionization anomaly; 2) Swarm-B has a better consistent with tomography than CSES, but both are capable of detecting ionosphere anomalies such as midlatitude arcs; and 3) CSES and Swarm-B both can capture the seasonal changes of electron density, while their values are basically smaller than those from tomography in Spring and Summer months.     

热障涂层在航空发动机涡轮叶片上的应用研究

从热障涂层在涡轮叶片的应用需求出发,研究了陶瓷热障涂层的材料与制备方法,介绍了热障涂层的基本原理和主要工艺特点,解决了热障涂层在喷涂过程中所产生的堵塞气膜孔、减小排气面积、降低疲劳性能、遮挡等方面的难点问题,给出了热障涂层叶片在试验和试车中的考核结果。     

负电子亲和半导体的二次电子发射

根据0.8keV≤Epomax≤5keV的负电子亲和(negative electron affinity,NEA)半导体二次电子发射(secondary electron emission,SEE)的特性，初级电子产额R，现有的次级电子产额δ的通用公式和实验数据，分别推导并实验证明了NEA金刚石的δ在0.5Epomax≤Epo≤10Epomax, GaN在2keV≤Epomax≤5keV, NEA金刚石的δ在08keV≤Epo≤3keV, GaN在08keV≤Epomax≤2keV的特殊公式;其中Epomax为δ达到最大值时的Epo, Epo为初级电子入射能。推导出了08keV≤Epomax≤5keV时NEA半导体的内部二次电子到达发射极表面后逃逸到真空中的概率B。还提出了计算08keV≤Epomax≤5keV NEA半导体1/α的方法;其中1/α为二次电子的平均逃逸深度。分析结果表明，B和1/α的理论研究有助于研究不同样品制备方法对NEA半导体中SEE的定量影响，从而生产出理想的NEA发射体，如NEA金刚石。