激光推力器喷管热防护实验研究

 基于激光推力器喷管热烧蚀因素分析，初步提出了反射式和吸收式两种喷管热防护方法，并相应设计了两种喷管。以10 kW级脉冲式CO₂激光器为光源，三次反射激光推力器为对象，进行了两种方式下两种喷管的热防护初步实验研究。结果表明：两种喷管均发生不同程度烧蚀，与相对聚焦位置和激光作用时间有关；反射式热防护通过喷管光学表面对透射激光能量进行二次聚焦，以及良好的循环冷却，可能成为将来激光推力器喷管热防护的备选方案之一。     

喷管长度对直筒型激光推力器推进性能的影响

"烧蚀模式"激光推进是利用强激光烧蚀推力器自身携带的工质产生的高温高压气团反喷进行驱动的,推力器的喷管构型对推进性能有重要影响。鉴于此,从实验和数值模拟两个方面研究了喷管长度对直筒型激光推力器推进性能的影响。实验发现,衡量推进性能的2个主要参数冲量耦合系数和比冲均随着喷管长度的增加而增加,不过前者的增长呈渐缓趋势。综合考虑推进参数和推力器自重的影响,导出了推力器获得最大单脉冲速度增量(ΔvT)的最佳喷管长度公式。数值模拟得到的不同喷管长度推力器推进参数的变化规律与实验结果基本吻合:若喷管长度过短,则高压气体未能充分作用于推力器就被排出筒外,造成了能量的浪费;若喷管长度过长,筒内压力的衰减则成为影响推进性能的主要因素,从而解释了直筒型推力器的推进性能的增长趋势随喷管长度增加而逐渐趋缓的原因。     

基于ZEMAX的二次反射式激光聚焦系统设计与模拟

反射式激光聚焦系统使激光推力器具备了推力矢量控制能力,并且聚焦系统不存在球差,具有广阔的应用前景。针对三种二次反射聚焦方式,抛物面-双曲面、平面-抛物面、抛物面-平面,在建立的聚焦系统坐标系下,通过几何光学方法分析了两个反射面母线参数之间的数学关系,以及聚焦特征参数与两个反射面母线参数的关系,并建立了程序设计流程。针对特定的聚焦特征参数设计要求,在光学软件ZEMAX中进行了反射面的建模和聚焦效果的验证,为下一步开展聚焦性能的研究打下了基础。     

掺杂红外染料聚叠氮缩水甘油醚工质激光烧蚀推进性能优化探索

选择含能聚合物聚叠氮缩水甘油醚(GAP)作为激光烧蚀微推力器的工质,分析了红外染料掺杂对激光烧蚀GAP工质推进性能的影响.通过对比掺杂红外染料GAP在不同激光功率密度、掺杂浓度、靶材厚度和激光烧蚀模式下的推进性能数据和烧蚀羽流,初步探索了掺杂红外染料GAP工质的推进性能优化方式.实验结果表明:透射式激光烧蚀模式下,激光能量的指数衰减特性和掺杂红外染料GAP的强黏性使得烧蚀羽流中易存在未充分烧蚀的工质; GAP的推进性能受红外染料掺杂浓度和靶材厚度的综合影响,当靶材厚度与激光吸收深度接近时,靶材充分吸收激光能量使中心烧蚀区达到化学能释放的温度阈值,同时沿激光传播方向未充分烧蚀的质量最少,此时推进性能达到最优值.反射式下掺杂红外染料的聚合物的激光烧蚀过程遵循"先吸收激光能量先喷射"的规律,工质分解充分,推进性能优于透射式.     

航空复合涂层材料的激光烧蚀效应

针对航空激光防护技术研究需求,开展了复合涂层材料的激光烧蚀效应综合研究。采用圆棒固体激光器作为测试光源,搭建了具有在线温度测量功能的烧蚀实验平台。在此基础上,对聚碳硅烷(PCS)涂层样品开展了激光烧蚀实验。通过对烧蚀区域形貌和温度数据的对比分析,证明了PCS复合材料具备显著的激光防护作用。同时,从理论方面对涂层的激光防护机理进行了研究,基于材料热传导方程,建立了激光烧蚀过程的热力学模型,对温度场变化进行了模拟。研究结果表明,在复合涂层的保护下,kW级激光仅产生百℃的金属基底升温。     

飞秒激光烧蚀氟化钙晶体表面特性

采用飞秒激光对氟化钙晶体表面进行加工。通过调控激光参数,采用静止聚焦和动态扫描两种方式在晶体表面加工出一系列微结构(烧蚀孔和烧蚀线)。分别对两种加工方式烧蚀后的氟化钙晶体表面微结构进行系统研究,包括参数依赖关系、材料表面烧蚀阈值等。计算结果表明:在静止聚焦情况下,累积因子为0.0033;在动态扫描情况下,当扫描方向与激光偏振方向垂直或平行时,累积因子分别为0.0043和0.0052。飞秒激光加工过程中的脉冲累积效应能够对晶体的烧蚀产生重要影响。     

线性喷管构形对激光推力器冲量耦合系数的影响

 喷管是激光推力器的重要组成部分。在设计喷管构形时,可以使其与聚光系统一体化设计,也可以把聚光系统和喷管分离设计。针对聚光系统与喷管分离设计的工作模式,建立了一种辅助聚焦系统的点火模型。通过改变喷管的构形,分析了圆锥形、圆台形喷管的冲量耦合系数与喷管顶部直径与出口直径之比以及喷管长度与出口直径之比之间的关系。通过对推力曲线的分析,阐述了喷管结构参数对其性能影响的原因。研究结果显示,圆台形喷管的推进性能优于圆锥形和圆筒形。     

多脉冲飞秒激光对锗材料的烧蚀效应

 开展了脉宽为40 fs的不同数量激光脉冲对锗材料的烧蚀效应实验,采用扫描电镜、激光共聚焦显微镜等方法对不同数量的飞秒激光脉冲作用下锗材料表面烧蚀区进行了检测,并对作用后材料烧蚀形貌演化规律进行了分析,初步分析了锗材料烧蚀区周围形成的不同环区的形貌特征及成因,对各环区烧蚀形貌特征随激光作用脉冲数的增加而产生的形貌演化过程进行了观测。并给出单脉冲飞秒激光对锗材料的烧蚀阈值为1.2 J·cm^-2,采用激光共聚焦显微镜测得该阈值条件下单个飞秒激光脉冲对锗材料的烧蚀深度约为150 nm。     

用全息光学元件实现对眼睛的激光防护

反射式全息滤光片具有很好的滤光特性,可作为一种激光防护元件.本文对这种元件的波长选择性,角度选择性,所能承受的功率密度进行分析,并讨论了实际应用的可能性.     

长脉冲激光辐照下环氧树脂的热烧蚀规律

 研究了树脂基复合材料中常用的E-51环氧树脂在ms级脉冲激光辐照下热烧蚀率及质量烧蚀率的变化规律,分析了脉冲激光的辐照时间、峰值功率密度、重复频率以及脉宽对烧蚀率的影响。研究结果表明：随辐照时间的增加热烧蚀率逐渐增大,但辐照一定时间后,热烧蚀率趋于稳定;峰值功率密度的增加能明显提高热烧蚀率,但随峰值功率密度的增加,热烧蚀率的增幅减小直至趋于一定值;热烧蚀率不随脉冲激光重复频率和脉宽的变化而改变,当峰值功率密度一定时,热烧蚀率一定,质量烧蚀率与频率和脉宽成正比。     

高温SHPB冲击实验技术及其应用

 为研究高温下材料的动态力学性能,研制了一套适用于分离式霍普金森压杆（Split Hopkinson Pressure Bar,SHPB）高温冲击实验的温控系统。利用该温控系统和Φ100 mm常规SHPB装置,对混凝土在高温下的动态力学性能进行了实验研究,实验温度分别为20、200、400、600、800和1 000 ℃。结果表明：由管式实时加热装置和箱式预加热炉组成的温控系统操作方便,实验效率高,试件组装方法简便可行;热传导导致的试件温度分布不均匀和压杆局部温升对实验结果产生的影响可以忽略,实验技术可靠;高温下混凝土动态力学性能的温度效应十分明显,相同冲击速率下,随温度升高,平均应变率逐渐增大,动态应力-应变曲线逐渐表现出塑性特性,动态抗压强度随温度升高先增大后减小,动态峰值应变随温度升高不断增大。     

高温处理后活性粉末混凝土动力学行为及本构模型研究

 利用分离式霍普金森压杆系统,采用铅片作为整形器,分别对常温下及400、600、800 ℃高温处理后的活性粉末混凝土（Reactive Powder Concrete,RPC）试样进行单轴冲击压缩实验,研究高温后RPC材料的动态力学性能,建立高温处理后材料的率型本构模型。结果表明：经不同高温处理后的RPC材料的动态抗压强度和韧性指标均有较明显的应变率敏感性,而峰值应变、初始弹性模量受应变率影响不大;不同应变率下,400 ℃以上高温处理后RPC材料的单轴动态压缩力学性能有所降低。扫描电镜分析表明,高温处理后RPC材料微观结构的劣化是宏观力学性能降低的根本原因。对ZWT粘弹性本构模型进行了修正,修正后的模型适用于混凝土材料经高温处理后的率型本构关系的分析。     

High temperature—high pressure neutron scattering experiments on expanded liquid alkali metals

Abstract

We present neutron diffraction investigation of the static structure factor S(Q) of liquid alkali metals expanded by heating towards conditions close to their liquid-vapour critical point. The experiments were designed to obtain the characteristic changes of the macroscopic structure as a function of temperature and density. The data give information on the density dependent changes in the interatomic forces. as the metal-nonmental transition is approached in the expanded liquid metal.     

Radiation scattering data on liquid metals

The scattering of radiation by liquid metals provides data on both the microscopic and the macroscopic properties of the metal. The macroscopic properties are obtained as a limit in which the momentum transferred by the radiation to the liquid is vanishingly small, while the microscopic structure and dynamical behaviour are obtained from data at higher momenta.

An outline of the theory of radiation scattering will be given which emphasizes the relation between neutron and x-ray results. The possibility of studying electron shell movements by measuring the ratio of x-ray to neutron intensities is discussed.

In general there are two types of experiment: those in which the intensity is measured as a function of the momentum transfer only (S(Q)), and those in which the intensity is measured as a function of both energy and momentum transfer (S(Q, θ)). The former experiments are used to obtain information on atomic positions while the latter experiments (coupled with the former) are used to derive dynamical information. Both kinds of experiment will be discussed and some methods of interpreting (S(Q)) and S(Q, θ)) will be described.

The cooperative modes of motion in the system, particularly for wavelengths of the order of the spacing between atoms, may be observed as peaks in S(Q, θ). Several recent experiments of this kind on liquid lead will be discussed, particularly those in which the temperature variation was studied. In addition, information on the velocity correlation function for the metal atoms may be obtained. The spectral density of the velocity correlation will be considered and compared to predictions based on the cooperative mode picture.

It is concluded that while much information has been obtained from these experiments there is a more detailed and extensive work to be done with both neutrons and x-rays.     

Effect of grain boundary on the mechanical behaviors of irradiated metals: a review

The design of high irradiation-resistant materials is very important for the development of next-generation nuclear reactors. Grain boundaries acting as effective defect sinks are thought to be able to moderate the deterioration of mechanical behaviors of irradiated materials, and have drawn increasing attention in recent years. The study of the effect of grain boundaries on the mechanical behaviors of irradiated materials is a multi-scale problem. At the atomic level, grain boundaries can effectively affect the production and formation of irradiation-induced point defects in grain interiors, which leads to the change of density, size distribution and evolution of defect clusters at grain level. The change of microstructure would influence the macroscopic mechanical properties of the irradiated polycrystal. Here we give a brief review about the effect of grain boundaries on the mechanical behaviors of irradiated metals from three scales: microscopic scale, mesoscopic scale and macroscopic scale.     

机械泵驱动两相回路的储液器控温策略研究

本文阐述了一种机械泵驱动两相回路的储液器的控温原理,并对待选的两种控温策略进行了解释:一种是基于电压的PI控制,另一种是基于功率的PI控制。文中将它们作了近似等效转换,并用SINDA/FLUINT软件进行模拟。结果显示:当控温点上升时,两种不同控制策略的控温效果大体一致;但当控温点下降时,由于基于功率的PI控制策略包含有Peltier的制冷功能,可以比基于电压的PI控制策略更快到达控温点。另外,基于功率的PI控制策略在死区内的跳跃更少,其应当为优先选择的控制策略。     

Effect of structural vacancies on lattice vibration,mechanical, electronic,and thermodynamic properties of Cr5BSi3

In recent years,transition metal silicides have become the potential high temperature materials.The ternary silicide has attracted the attention of scientists and researchers.But their inherent brittle behaviors hinder their wide applications.In this work,we use the first-principles method to design four vacancy defects and discuss the effects of vacancy defects on the structural stability,mechanical properties,electronic and thermodynamic properties of hexagonal Cr;BSi;silicide.The data of lattice vibration and thermodynamic parameters indicate that the Cr;BSi;with different atomic vacancies can possess the structural stabilities.The different atomic vacancies change the mechanical properties and induce the Cr;BSi;to implement the brittle-to-ductile transition.The shear deformation resistance and volume deformation resistance of Cr;BSi;are weakened by different vacancy defects.But the brittleness behavior is remarkably improved.The structural stability and brittle-to-ductile transition of Cr;BSi;with different vacancies are explored by the electronic structures.Moreover,the thermal parameters indicate that the Cr;BSi;with vacancies exhibit different thermodynamic properties with temperature rising.     

High temperature embrittlement of metals by helium

The present knowledge of the influence of helium on the high temperature mechanical properties of metals to be used as structural materials in fast fission and in future fusion reactors is reviewed. A wealth of experimental data has been obtained by many different experimental techniques, on many different alloys, and on different properties. This review is mostly concentrated on the behaviour of austenitic alloys—especially austenitic stainless steels, for which the data base is by far the largest—and gives only a few examples of special bcc alloys. The effect of the helium embrittlement on the different properties—tensile, fatigue and, with special emphasis, creep—is demonstrated by representative results. A comparison between data obtained from in-pile (-beam) experiments and from post-irradiation (-implantation) experiments, respectively, is presented. Theoretical models to describe the observed phenomena are briefly outlined and some suggestions are made for future work to resolve uncertainties and differences between our experimental knowledge and theoretical understanding of high temperature helium embrittlement.     

以修正WLF方程研究压力作用下丁腈橡胶的阻尼性能

时温等效原理表明固定频率下温度越高,模量越低,而相同温度下频率越低,模量越低,即升高温度与降低频率具有同等效应。根据这一规律,可将聚合物的力学性能随温度的变化转化为这些性能随频率的变化,从而可通过不同温度下的力学性能测试数据,换算成宽频率范围内的材料力学性能表现。为了研究压力作用下橡胶阻尼性能的基本变化规律,通过自由体积理论推导出加压后的修正WLF方程,采用动态热机械分析实验,测试得到丁腈橡胶在不同温度下的损耗因子tanδ对频率ω的曲线,根据计算得到不同压力下的测试温度至室温的平移因子,便可做出加压后的丁腈橡胶的损耗因子-频率谱的主拟合曲线,其曲线的频率跨度达10个数量级以上。结果表明,丁腈橡胶的tanδ测试段在高于参考温度以后出现,而随着压力的增加,玻璃化温度相应升高,峰值往高频移动达1.5个数量级。此结果为研究压力作用下橡胶材料阻尼性能的定量变化提供了理论依据。     

Strain rate sensitivity of ultra-fine and nanocrystaline metals and alloys

The mechanical behavior of metals and alloys is strongly related to grain size. In particular, the grain refining leads to the increase in yield strength in the ultra-fine grain (<1 μm) and nanocrystalline (<100 nm) regimes.Instrumented nanoindentation measurements allow a rapid evaluation of mechanical properties of materials, and the possibility to perform tests in a very wide range of loads. The strain rate sensitivity of ultra-fine and nanocrystalline metals can be derived by changing loading rates. The present paper presents the results on the strain rate sensitivity of ultra-fine grain metals produced by equa-channel angular pressing and nanocrystalline materials produced via electrodeposition. The results were obtained by systematic experiments performed at different loading rates (3, 30 and 300 mN/s) showing broad ranges of variations for the investigated metals. Also, the strain rate sensitivity of the studied materials was derived from the load vs. depth curves.