Evaluating the IRI topside model for the South African region: An overview of the modelling techniques

The representation of the topside ionosphere (the region above the F2 peak) is critical because of the limited experimental data available. Over the years, a wide range of models have been developed in an effort to represent the behaviour and the shape of the electron density (Ne) profile of the topside ionosphere. Various studies have been centred around calculating the vertical scale height (VSH) and have included (a) obtaining VSH from Global Positioning System (GPS) derived total electron content (TEC), (b) calculating the VSH from ground-based ionosonde measurements, (c) using topside sounder vertical Ne profiles to obtain the VSH. One or a combination of the topside profilers (Chapman function, exponential function, sech-squared (Epstein) function, and/or parabolic function) is then used to reconstruct the topside Ne profile. The different approaches and the modelling techniques are discussed with a view to identifying the most adequate approach to apply to the South African region’s topside modelling efforts. The IRI-2001 topside model is evaluated based on how well it reproduces measured topside profiles over the South African region. This study is a first step in the process of developing a South African topside ionosphere model.     

冷却通道预氧化处理抑制碳氢燃料热裂解结焦的研究

主动冷却超燃冲压发动机用吸热碳氢燃料应用过程中不可避免产生裂解结焦,这是制约燃料热沉能力和过程可靠性的重要因素。冷却通道表面进行预处理,是减少裂解结焦的重要途径之一。系统研究了预氧化321不锈钢管(Φ3 mm×0.5 mm)的元素迁移和微观结构。利用超临界航空煤油RP-3的热裂解反应为探针,研究预氧化处理对结焦的影响。实验发现,高温氧化后(900℃),管表面形成一层致密的Cr2O3膜,能减少碳沉积,几乎不生成纤维碳;较低温(600~800℃)氧化时,虽然Cr元素不断向金属表面迁移,Fe,Ni元素向金属内部迁移,但同时产生了大量空缺位提供渗碳通道,最终导致结焦量高于空管。     

EFFECT OF DEPOSITIONS CONDITION ON THE MICROSTRUCTURE OF C／C MINICOMPOSITE

C/C minicomposites were manufactured by CVI with propylene as resource gas and Ar as diluent gas. Effects of deposition temperature, the flow rate of propylene and total system pressure on theof C/C minicomposites were investigated. Deposition conditions are as follows: deposition temperature ranging from 870℃ to 1000℃. the flow rate of propylene from 10 to 60 ml/min, and total system pressure from 3 to 16kPa. The results revealed : The deposit distributes non-uniformly as increasing the deposition temperature or total system pressure. The effect of the flow rate of propylene on the distribution of deposit observes different principles at different deposition temperatures. If the deposition temperature rises to 900℃, the deposit distributes more non-uniformly as increasing the flow rate of propylene. The morphology of deposit varies with deposition condition, such as smooth, droplet-like or spike-like. Deposition mechanism varies with deposition condition. It can be surface reaction nucleation and growth, heterogeneous nucleation of liquid droplet on carbon fiber or gas-borne nuclei-growth. With the principle of nuclei-growth of crystal and the method of phenomenology, the changes of deposition mechanism and the morphology of pyrolytic carbon with deposition condition were explained successfully. The effect of deposition condition on deposit homogeneity stems from the effect of deposition condition on the CVI kinetics and the deposition mechanism of pyrolytic carbon.     

AF1410钢电子束焊接接头组织及性能研究

采用电子束扫描焊和修饰焊工艺对5mm AF1410钢板进行电子束焊接。针对热处理前、后的接头,采用光学显微镜、扫描电镜、硬度仪等对其组织、硬度及拉伸断口进行分析。结果表明:接头热影响区分为浅腐蚀区和深腐蚀区,分别为单一马氏体(M)、M+少量逆转奥氏体(Ar)组织;焊缝区柱状晶分为重熔区、正火区和回火区,晶内为马氏体,晶界附近为残留奥氏体。热处理后,接头组织、显微硬度与母材趋于一致,抗拉强度也达到了母材98%以上,失效裂纹始于热影响区,呈约60°方向扩展并贯穿整个接头。     

Inconel617合金表面电子束熔覆WC-CoCr显微组织和耐磨性研究

Inconel617合金材料在实际应用中以耐腐蚀性、高温抗氧化性能和耐磨性为主。但是,其本身耐磨性比较低。为了提高Inconel617合金表面耐磨性,采用高速火焰喷涂与电子束表面改性技术在Inconel617合金表面制备了WC–Co Cr陶瓷涂层。分析了合金层的微观组织结构和元素分布情况,测试了合金层的硬度与耐磨性。结果表明,在电子束熔覆处理过程中,涂层重熔与基体形成冶金结合,使其耐磨性能大大提高。熔覆层硬度相比Inconel617合金硬度高出620HV0.3。     

HFCVD系统热丝参数遗传优化

在热丝化学气相沉积金刚石系统中，衬底温度是影响金刚石成膜质量的关键因素之一。为了沉积大面积优质金刚石膜，必须对衬底表面温度的大小及均匀性实施有效控制。中通过应用遗传算法对热丝参数进行优化设计，保证了衬底在半径为Rs＝10cm的范围内具有较均匀的温度场，从而为大面积生长优质金刚石膜提供了可能。     

精密模具大面积微结构电铸制备工艺研究

对精密模具大面积微结构的电铸制备工艺进行了研究。研究了掩膜厚度、化学微蚀刻、二次辅助阴极对精密模具大面积微结构电铸成型的影响。结果表明,化学微蚀刻能进一步去除显影残胶,提高镀层微结构与模具基板的结合力。在曝光时间为100s,曝光能量为750~810mJ/cm2的曝光工艺条件下,掩膜厚度在130~160μm时,可以得到线宽为100μm侧壁陡直度较好的精密模具微结构。采用外加电势的二次辅助阴极三电极电铸体系可以提高铸层的均匀性。     

黄麻纤维表面原位沉积纳米SiO2工艺以及机理探究

采用超声-溶胶凝胶法在黄麻纤维表面原位沉积纳米SiO_2,通过红外光谱分析,微观形貌分析以及沉积量测试,讨论了不同工艺参数对纳米SiO_2沉积效果的影响。结果表明:随着正硅酸乙酯(TEOS)浓度或氨水浓度的增加,纳米SiO_2的沉积量逐渐增多,粒径逐渐增大;随着沉积温度的升高,纳米SiO_2的沉积量逐渐减少,粒径逐渐减小;与沉积温度为20℃相比,当沉积温度为60℃时,纳米SiO_2的沉积量减少了36.4%、粒径减小了37.8%;沉积时间主要影响纳米SiO_2的沉积量,对其粒径的影响不明显。通过实验探究了纳米SiO_2成核与生长的机理:黄麻纤维表面的孔隙结构为纳米SiO_2提供了成核位点;TEOS经过水解缩合反应形成短链交联结构,通过氢键或化学键沉积于黄麻纤维表面的孔隙中;短链交联结构经过成核与生长过程,逐渐形成纳米SiO_2颗粒。因此,通过对工艺参数合理地选择,可以调控纳米SiO_2在黄麻纤维表面成核与生长阶段的形貌与沉积量。     

离子推力器栅极系统电子反流阈值的数值分析

阻止束流等离子体中电子反流到加速栅上游区域是离子推力器加速栅负电压的主要作用之一,能够阻止电子反流的加速栅电压最小值称为电子反流阈值。加速栅电压的选择直接影响到离子推力器的工作性能和运行寿命,电子反流阈值电压是确定加速栅电压的重要参考参数。基于PIC方法计算了20cm氙离子推力器加速栅电子反流阈值,并分析了加速栅孔径、栅间距、单孔引出束流电流大小对加速栅电子反流阈值电压的影响,计算结果与试验测量值符合较好。该数值模型为加速栅参数的选择和降低电子反流失效风险方法提供了参考,为下一步电子反流现象对加速栅寿命的预测分析奠定了基础。     

A different method to update monthly median hmF2 values

The height, h_mF2, and the electron density, N_mF2, of the F2 peak are key model parameters to characterize the actual state of the ionosphere. These parameters, or alternatively the propagation factor, M3000F2, and the critical frequency, f_oF2, of the F2 peak, which are related to h_mF2 and N_mF2, are used to anchor the electron density vertical profile computed with different models such as the International Reference Ionosphere ( Bilitza, 2002), as well as for radio propagation forecast purposes. Long time series of these parameters only exist in an inhomogeneous distribution of points over the surface of Earth, where dedicated instruments (typically ionosondes) have been working for many years. A commonly used procedure for representing median values of the aforementioned parameters all over the globe is the one recommended by the ITU-R ( ITU-R, 1997). This procedure, known as the Jones and Gallet mapping technique, was based on ionosondes measurements gathered from 1954 to 1958 by a global network of around 150 ionospheric stations (  and ). Even though several decades have passed since the development of that innovative work, only few efforts have been dedicated to establish a new mapping technique for computing h_mF2 and N_mF2 median values at global scale or to improve the old method using the increased observational database. Therefore, in this work three different procedures to describe the daily and global behavior of the height of the F2 peak are presented. All of them represent a different and simplified method to estimate h_mF2 and are based on different mathematical expressions. The advantages and disadvantages of these three techniques are analyzed, leading to the conclusion that the recommended procedure to represent h_mF2 is best characterized by a Spherical Harmonics expansion of degree and order equal to 15, since the differences between the h_mF2 values obtained with the Jones and Gallet technique and those obtained using the abovementioned procedure are of only 1%.