The oxidation resistance mechanism of SiC-B4C-xAl2O3 ceramics at 1400 ℃ in air atmosphere

The oxidation property of SiC-B₄C-xAl₂O₃ (x ranges from 0 wt% to 30 wt%) ceramics was studied in air at 1400 ℃. Results show that the porous oxide layer becomes dense and smooth with addition of Al₂O₃. When the content of Al₂O₃ is proper, the DOP (degree of polymerization) of borosilicate network can be improved with increase of Al₂O₃ content, inhibiting the migration of atoms and molecular groups. With that, the crystallization of SiO₂ and volatilization of B₂O₃ can be restrained. When the content of Al₂O₃ is excessive, the DOP of borosilicate network will be decreased, deteriorating the oxide layer morphology. It is believed that the damage of borosilicate network by excess Al₂O₃ should be responsible for this phenomenon. In this research, the SiC-B₄C ceramic with optimal oxidation resistant can be obtained when the content of Al₂O₃ is 15 wt%.     

Effect of low content sintering aids addition on β-SiC sintered by spark plasma sintering

The influence of low content of sintering aids addition on the properties of β-SiC sintered by spark plasma sintering was studied based on X-ray diffraction and SEM observations. Three dopants, respectively Al₂O₃, Al₂O₃-Y₂O₃ and Al₂O₃-AlN were chosen, and the investigation was undertaken in the [0.5–5 wt%] content range. The analyses revealed the possibility to increase the density of sintered pellets close to 100% of theoretical density (T.D.), even with the addition of very low sintering aids contents, as low as 0.5 wt%, for each dopant. The combination with high sintering pressure allowed keeping a fine microstructure and a pristine cubic crystalline structure while avoiding the formation of secondary phases.     

基于塑料闪烁体光纤阵列的中子探测系统设计

为了能探测高能中子，获得中子能谱信息，设计了一套基于塑料闪烁体光纤和多路复用技术的中子探测系统。利用梯度尺寸结构的阵列化探头感应中子入射的能量和方向。多路复用数据采集及读出单元接收探测器输出信号并产生触发信号，经处理后上传给上位机。数据采集及读出系统通过以太网实现实时通信。上位机程序基于LabWindows/CVI软件平台开发，实现数据读取与保存以及采集系统运行状态参数信息的实时显示。通过中子模拟源进行了现场测试，结果表明，设计的中子探测系统满足设计要求，能实现中子能量与入射方向的测量。     

Study on the influence of three-grid assembly thermal deformation on breakdown times and an ion extraction process

In order to study the influence of three-grid assembly thermal deformation caused by heat accumulation on breakdown times and an ion extraction process,a hot gap test and a breakdown time test are carried out to obtain thermal deformation of the grids when the thruster is in 5 k W operation mode.Meanwhile,the fluid simulation method and particle-in-cell-Monte Carlo collision(PICMCC) method are adopted to simulate the ion extraction process according to the previous test results.The numerical calculation results are verified by the ion thruster performance test.The results show that after about 1.2 h operation,the hot gap between the screen grid and the accelerator grid reduce to 0.25–0.3 mm,while the hot gap between the accelerator grid and the decelerator grid increase from 1 mm to about 1.4 mm when the grids reach thermal equilibrium,and the hot gap is almost unchanged.In addition,the breakdown times experiment shows that 0.26 mm is the minimal safe hot gap for the grid assembly as the breakdown times improves significantly when the gap is smaller than this value.Fluid simulation results show that the plasma density of the screen grid is in the range 6?×10~(17)–6?×?10~(18) m~(13) and displays a parabolic characteristic,while the electron temperature gradually increases along the axial direction.The PIC-MCC results show that the current falling of an ion beam through a single aperture is significant.Meanwhile,the intercepted current of the accelerator grid and the decelerator grid both increase with the change in the hot gap.The ion beam current has optimal perveance status without thermal deformation,and the intercepted current of the accelerator grid and the decelerator grid are 3.65 m A and 6.26 m A,respectively.Furthermore,under the effect of thermal deformation,the ion beam current has over-perveance status,and the intercepted current of the accelerator grid and the decelerator grid are 10.46 m A and 18.24 m A,respectively.Performance test results indicate that the breakdown times increase obviously.The intercepted current of the accelerator grid and the decelerator grid increases to 13 m A and 16.5 m A,respectively,due to the change in the hot gap after 1.5 h operation.The numerical calculation results are well consistent with performance test results,and the error comes mainly from the test uncertainty of the hot gap.     

Flash cosintering of a lanthanum strontium cobalt ferrite nanofibre/Gd-doped ceria bilayer structure

For solid oxide fuel cells, an important structural requirement is that the electrolyte layer needs to be dense and the electrode layer porous, which is difficult to obtain by conventional cosintering. In this work, flash cosintering of a double layer structure consisting of a Gd-doped ceria substrate with a lanthanum strontium cobalt ferrite nanofibre coating is investigated. Experimental and finite element modelling results reveal that when the LSCF layer is connected to the electrode, the heat is concentrated in the LSCF layer, which leads to a huge temperature gradient and introduces severe cracking. When the LSCF layer is electrically isolated from the electrode, the heat is concentrated in the GDC layer, and the temperature gradient is dramatically reduced. In this situation, the density of GDC can reach 92.86% while a high porosity of 52.26% is maintained in the LSCF layer, which is higher than that of the conventional cosintered sample.     

Numerical research of a 2D axial symmetry hybrid model for the radio-frequency ion thruster

Since the high efficiency discharge is critical to the radio-frequency ion thruster (RIT),a 2D axial symmetry hybrid model has been developed to study the plasma evolution of RIT.The fluid method and the drift energy correction of the electron energy distribution function (EEDF) are applied to the analysis of the RIT discharge.In the meantime,the PIC-MCC method is used to investigate the ion beam current extraction character for the plasma plume region.The beam current simulation results,with the hybrid model,agree well with the experimental results,and the error is lower than 11％,which shows the validity of the model.The further study shows there is an optimal ratio for the radio-frequency (RF) power and the beam current extraction power under the fixed RIT configuration.And the beam extraction efficiency will decrease when the discharge efficiency beyond a certain threshold (about 87 W).As the input parameters of the hybrid model are all the design values,it can be directly used to the optimum design for other kinds of RITs and radio-frequency ion sources.     

气动分置式斯特林制冷机被动减振技术研究

论述了斯特林制冷机的振动来源和振动特性,利用梁模型计算了冷指的自然频率,理论上给出了动力吸振器的设计方法,得出在气动分置式斯特林制冷机中被动减振方法具有优势.根据分析设计了动力吸振器,通过实验验证了动力吸振器可以有效的抑制制冷机地振动输出.     

添加稀土Nd改善金刚石/铜复合材料界面

目的添加稀土Nd改善金刚石/铜复合材料界面间的缺陷,抑制金刚石与铜之间的弱润湿性,增强复合材料的界面结合。方法采用放电等离子烧结(SPS)技术制备含有不同质量分数Nd的镀钛金刚石/铜复合材料,采用扫描电子显微镜观察界面处的微观形貌,采用X射线衍射仪和X射线能谱仪分析界面处组织,采用排水法测试复合材料的密度和致密度。结果添加稀土Nd后,金刚石-铜两相界面间促生了Cu_5Nd、NdCu_2、Cu_3Ti等相。界面间的Cu_5Nd、NdCu_2、Cu_3Ti、TiC填补了镀钛金刚石/铜复合材料界面处原有的空隙、孔洞等缺陷。未添加稀土Nd的镀钛金刚石/铜复合材料的密度为4.589 g/cm~3,致密度为81%;添加3wt%的Nd元素后,镀钛金刚石/铜复合材料的密度和致密度分别达到了5.569 g/cm~3和98%,密度较未添加Nd的复合材料提升了21%。随着Nd含量的增加,金刚石-铜界面间的缺陷逐渐减少,界面结合效果逐渐转好。结论稀土Nd极大地改善了镀钛金刚石/铜复合材料两相界面处的缺陷,很好地修饰了原本润湿性较差的金刚石-铜两相界面。添加Nd元素后,复合材料两相界面结合紧密。     

超高真空下小型四极质谱计的性能研究

在超高真空下对QMS200小型四极质谱计进行了实验研究。实验通过调节离子源的发射电流、聚焦电压和场轴电压,测量了质谱计灵敏度、线性以及图样系数的相应变化。实验结果表明,不同的参数设置对质谱计性能有不同的影响。文章结论对四极质谱计应用具有一定的参考价值。     

开式雾化喷射冷却技术的应用研究

雾化喷射冷却技术是一种高效快速的冷却手段,具有冷却效率高、冷却温度均匀、没有接触热阻等优点。从开式雾化喷射冷却系统的方案入手,从雾化特性、制冷工质、环境外热流3个方面分析并研究了开式雾化喷射冷却系统冷却效果的影响因素,开式雾化喷射冷却系统非常适合于高速飞行器等短期内工作的高热流密度设备的散热及热防护。