Transition metal atom doped C2N as catalyst for the oxygen reduction reaction: A density functional theory study

The catalytic mechanism and activity of transition metal atom doped C₂N (M-C₂N, M = Fe, Co, Ni, and Cu) for the oxygen reduction reaction (ORR) are investigated in detail by density functional theory method. All the screened M-C₂N are thermodynamically stable based on the binding energy calculations. The adsorption energy results indicate that the adsorption strength of O₂ and ORR intermediates are decreased in the order of Fe-C₂N ˃ Co-C₂N ˃ Ni-C₂N ˃ Cu-C₂N, in which the adsorption energy values on Cu-C₂N are most close to those on the Pt(111). Based on the relative energy diagram of ORR, the energetically favorable pathway on Fe-C₂N and Co-C₂N is direct 4e⁻ mechanism, in which the O–O bond is directly dissociated after the second electron transfer. While for Ni-C₂N and Cu-C₂N, the most favorable pathway is indirect 4e⁻ mechanism, in which the H₂O₂ is formed as the intermediate product. For all studied M-C₂N, the Ni-C₂N and Cu-C₂N hold better catalytic activity, which could attribute to the contribution of metal atom and part of its activated nitrogen atoms.     

Effects of isothermal treatment parameters on the microstructure of semisolid alloys

Cold-rolled 6061 aluminium alloys are used to prepare semisolid billets by recrystallisation and partial melting. The effects of isothermal treatment parameters on the microstructures were investigated. The results indicated that the high isothermal holding temperature increases the experimental liquid fraction, average grain size and shape factor. A long isothermal holding time also increases the experimental liquid fraction and improves the spheroidisation degree of solid grains, but the average grain size also increases. Moreover, when the isothermal holding time at high temperature is increased, the size of intragranular liquid droplets increases, but their quantity decreases. The optimal isothermal holding temperature and time during semisolid isothermal treatment were 635°C and 5?–?10?min, respectively.     

控制棒水压驱动步升过程流动阻力影响分析

控制棒水压驱动系统（CRHDS）是清华大学为NHR200发明的新型内置式控制棒驱动系统。组合阀是驱动系统关键的流量控制部件,其流动阻力对控制棒水压驱动机构的步进性能有重要影响。本文完成了驱动机构不同流动阻力工况步升过程性能试验,建立了驱动机构水压缸步升过程理论模型,得到了水压缸步升过程关键性能参数的动态变化曲线,理论结果与试验结果符合很好。在此基础上,利用模型分析了热态工况驱动机构水压缸变流阻步升动态过程。结果表明：随着流动阻力的增大,步升平均速度逐渐减小,步升时间、步升后压力迟滞时间和步升过程总时间均增大。研究成果为CRHDS组合阀流动阻力的设计和优化奠定基础。     

Electrochemical Study on the Reduction Mechanism of Uranium Oxide in a LiCl-Li2O Molten Salt

By means of a linear sweep voltammetry, a cyclic voltammetry and a chronopotentiometry, the electrolytic reduction of uranium oxide has been studied to establish the reduction mechanisms, which are based on a simultaneous uranium oxide reduction and a Li₂O electrowinning, and the formation and electrolysis of lithium uranate. From the voltammograms, the reduction potentials of the uranium oxide and Li₂O were obtained. From the chronopotentiometries based on the results of the voltammograms, the uranium oxide was reduced to uranium metal through the reduction mechanisms showing a more than 99% conversion. For a verification of the reduction mechanisms feasibility, basic data on the electrolytic reduction of the uranium oxide was obtained from the experiments and the characteristics of the closed recycle of Li₂O were discussed.     

Ionization probability of secondary ions sputtered from Si(1 1 1) and Ge(1 1 1) surfaces

We have measured the energy distributions of the secondary ions sputtered from the Si(1 1 1) and Ge(1 1 1) surfaces and investigated the ionization probabilities of sputtered Si⁺ and Ge⁺ ions for clarifying their ionization mechanisms. The observed ionization probabilities depend on the velocity of Si⁺ and Ge⁺ ions. This velocity dependence can be successfully analyzed by a theoretical expression, which was proposed originally for the metal surfaces. This implies that the ionization mechanism of Si⁺ and Ge⁺ ions is the same as ions sputtered from the metal surface, i.e., the resonant electron transfer in the high velocity regime and the thermal excitation process in the low velocity regime. The difference in the ionization probability between Si⁺ and Ge⁺ ions is well explained by the difference in the band gap energy.     

控制棒驱动机构响应时间测试与数据分析

针对控制棒驱动机构（CRDM）机电响应时间快且测试环境存在噪声干扰等问题,搭建了一套集PXI数据采集硬件平台和MATLAB数据分析的快速机电响应时间测试系统,能够实现对高频控制棒棒位实时信号的快速采集。利用滑动平均滤波和分区间拟合方法对棒位位移信号进行滤波和拟合,并利用快速傅里叶变换（FFT）频谱分析法验证去噪效果。结果表明,本套测试系统能被成功应用于钍基熔盐液态堆（TMSR-LF1）第一停堆系统CRDM的机电响应时间测试中,在可行性、可靠性和可用性等方面均满足CRDM机电响应时间测试要求。      

碳化硅结势垒肖特基二极管的质子辐射效应

碳化硅结势垒肖特基二极管(SiC JBS)是新一代航天器电推进系统的关键部件,但高能粒子辐射严重威胁其可靠性与稳定性。为揭示其辐射损伤机理,为其抗辐射加固设计与考核评估储备数据,本研究基于加速器开展了先进商用SiC JBS 10~20 MeV中能质子地面辐照实验,并提取器件辐照前后的正向伏安特性、反向伏安特性、电容电压等电学参数及缺陷特性。系统分析器件关键特性随辐照条件的改变规律。结果显示,质子辐照引起了器件肖特基势垒升高、载流子浓度降低,且10 MeV较低能质子导致的位移损伤退化更严重。分析认为,PN结界面缺陷导致高性能商用SiC JBS反向电学性能对中能质子的辐照更加敏感,正向特性相对稳定,辐照生碳缺陷造成载流子去除效应是引起SiC JBS性能退化的主要机制。     

低能重离子束注入小麦种子诱导根尖细胞有丝分裂畸变的研究

比较研究了低能重离子束注入小麦干种子和萌动的种子对种子发芽及根尖细胞有丝分裂畸变的影响。结果表明,离子注入对萌动的种子的发芽有明显的影响,而对干种子的发芽影响不明显。在一定剂量范围内(0-6×1016/cm2),N+注入诱导的萌动种子的有丝分裂畸变率略高于干种子,并且随辐射剂量的升高而升高;但超过一定的剂量(>6×1016/cm2),无论是干种子还是萌动种子的有丝分裂畸变率达到一定程度的饱和。就实验结果对N+离子束注入诱变的机理进行了一些探讨。