关于分子泻流条件的讨论

1 分子泻流存在的条件 分子泻流现象产生的分子束,在许多著名的物理实验里都有重要的应用[1].使用分子束技术,既使人们可以直接考察基本上孤立的分子、原子或者原子核的基本性质,又让人们得以探索容器中处于平衡态下的气体分子的某些特性.     

泻流速率分布函数及其应用举例

气体分子从容器壁上的小孔逃逸时,如果小孔直径远小于平均自由程,则容器内处于平衡的气体所受到的扰动可以忽略,这种分子从容器壁上小孔逸出的现象称为污流[1].泻流现象在分子速率分布的测定,同位素分离等方面有许多应用.泻流分子的速率分布与麦克斯韦速率分布有区别,初学者容易忽视以致产生混淆.本文简单介绍泻流速率分布函数,并以斯特恩-盖拉赫实验的分析为例说明其应用,供普通物理教学的参考. 如所周知,按麦克斯韦速率分布律,分子的速率在区间υ～υ dυ内的几率为式中dn为单位体积内该速率区间中的分子数,n为单位体积分子数.但泻流气体…     

利用分子动力学模拟研究气体双向泻流过程

本文利用分子动理论从理论上推导了理想气体的双向泻流过程。即在一个绝热容器中加入一绝热隔板,在隔板两边分别设置初始条件不同的理想气体。在两子系统达到平衡时开放隔板上一小孔,使两子系统进行双向泻流。除了理论推导,本文也引入分子动力学模拟计算了温度随时间变化曲线。这种双向泄流可能会产生一些违反常识的有趣的现象,我们发现在满足特定条件下,初始温度高的气体会先升温再降温,而初始温度低的气体会先降温再升温,直至温度相等为止。     

超导涡流制动装置液氦容器设计及分析

液氦容器是浸泡式超导涡流制动装置稳定运行不可或缺的部件, 其漏热损耗和结构强度关系着内部超导磁体能否安全稳定的工作. 针对超导涡流制动装置的液氦容器的各类漏热进行了分析计算, 详细讨论了超导线圈励磁过程中对不同壁厚的液氦容器产生的涡流损耗, 并采用 Ansys 有限元分析软件对不同壁厚液氦容器的强度进行了计算, 结合漏热及强度计算结果获得了最优的液氦容器壁厚参数. 结果表明4 mm 壁厚下的液氦容器总漏热在励磁速度为90 A/s 时可达1 .87 W, 液氦容器在各类综合力作用下的最大应力为393 MPa, 最大变形为2.2 mm,可满足所选材料的漏热及强度需求, 为超导涡流制动器装置总体设计提供了有力保证.     

泻流中气体性质变化的探讨

研究容器内气体性质在泻流中的变化，计算表明，在小孔足够小时，少量分子的逸出对容器里分子平衡分布的影响几乎可以忽略。     

GaAs-Ga_xAl_(1-x)As双异质结激光器的分子束外延生长

本文报道分子束外延生长GaAs-Ga_xAl_(1-x)As双异质结激光器的生长条件.其中包括低温生长的温度-时间循环,欧姆接触电极层的原位生长以及生长后的退火等程序.实验表明,生长过程中采用高纯度的源材料,氮化硼的泻流盒以及生长系统中的低温泵等对降低器件的阈电流密度,改善激光器的光电性能起到重要作用.     

二维GEL耦合方法的研究及对爆炸容器的数值模拟

 将以Euler方法为基础的MF PPM（Piecewise-Parabolic Method）程序和以Lagrange方法为基础的DEFEL（2-D Finite Elements Code,二维流体弹塑性动力有限元）程序,根据压力和法向速度连续准则进行耦合,发展了基于Level Set的GEL（Ghost-Fluid Euler-Lagrange）方法。该方法在处理大变形流场与小变形结构以及复杂流动与多物体相互作用等问题具有优越性。通过二维算例的计算结果与文献比较,检验了GEL方法和耦合程序的正确性,并对球形和椭球封头的爆炸容器进行了数值模拟,通过与实验结果的比较分析,表明本研究程序可以比较好地处理内爆引起的壳体流固耦合问题。     

1.5T MRI超导磁体低温容器的拉杆设计与校核

1.5T MRI磁体系统冷重约为4吨,为提高该系统稳定性,采用环氧跑道拉杆结构.分析了磁体正常运行时拉杆上承受的重力、冷缩力,运输过程中的动载力,以及拉杆安装过程中施加的预紧力.在分析的基础上,对拉杆的材料、安装角度进行了选取,并结合漏热对横截面积进行了优化.     

圆柱形容器内液体横向晃动的有限元法求解

提出了用变分有限元方法计算容器内液体晃动谐振频率的方法和步骤 ,这种方法可适用于各种形状复杂的容器内液体晃动问题 ,其正确性已经得到验证。按照这一方法计算了卧式圆柱形容器内液体的横向晃动问题 ,并对相关结果进行了总结 ,得出了该类型容器内液体晃动的一般规律。     

分子束外延动力学参数的测定

讨论了分子束外延的动力学生长模型。并以高能电子衍射的表面鉴定方法,为这一模型提供了证据.以GaAs为例研究了分子束外延的生长速率。并以四极质谱的测定,验证了该速率与Ga通量的关系。从理论与实验两个方面,研究了生长具有特定X值的Ga_(1-x)Al_xAs材料时,适宜的Ga和Al的分压比。并给出了掺杂元素Si,Sn,Be的掺杂浓度与相应元素泻流盒温度的关系.     

Hydrogen effusion: a probe for surface desorption and diffusion

Hydrogen effusion results are discussed for hydrogenated amorphous silicon (a-Si:H) and related alloys as well as for crystalline silicon (c-Si). It is demonstrated that depending on the microstructure of the material, hydrogen effusion gives information on hydrogen diffusion or surface desorption. The results suggest for compact a-Si:H and for ion implanted c-Si a similar hydrogen diffusion process, which is a trap limited motion of atomic hydrogen. Hydrogen effusion from defect-free c-Si and from void-rich amorphous semiconductors is limited by surface desorption. Both hydrogen diffusion and desorption depend on the Fermi energy if hydrogen bonds to the host material are broken.     

在线同位素分离装置的靶系统设计方法

产生放射性核束的在线同位素分离装置的靶系统设计是整个装置产生效率的关键环节.在ORNL的窄高斯分布的离子注入模型的基础上,研究了不同的靶的几何形状和尺寸、不同的温度条件、半衰期不同的放射性核素等对释放时间特性的影响,这对设计具有快速释放过程的靶系统,具有直接的实用价值.基于德国ZFK关于表面物理的实验测量数据,用MonteCarlo统计的方法来模拟放射性核素从靶材料表面到在线离子源的电离室的传输过程,对大量的放射性核素–传输管材料组合,和不同尺寸的传输管进行了统计计算,从这些统计计算中得到了可用于传输管工程设计的图表和经验公式.     

Procedural analysis of a new method for determining the Gibbs energy and experimental data on thermodynamic properties of liquid-metal coolants based on alkali metal alloys

A detailed procedural analysis is given and results of implementation of the new version of the effusion method for determining the Gibbs energy (thermodynamic activity) of binary and ternary systems of alkali metals Cs-Na, K-Na, Cs-K, and Cs-K-Na are presented. The activity is determined using partial pressures of the components measured according the effusion method by the intensity of their atomic beams. The pressure range used in the experiment is intermediate between the Knudsen and hydrodynamic effusion modes. A generalized version of the effusion method involves the pressure range beyond the limits of the applicability of the Hertz-Knudsen equation. Employment of this method provides the differential equation of chemical thermodynamics; solution of this equation makes it possible to construct the Gibbs energy in the range of temperatures 400 ≤ T ≤ 1200 K and concentrations 0 ≤ x _i ≤ 1.     

The influence of Fermi energy on structural and electrical properties of laser crystallized P-doped amorphous silicon

A series of phosphorous-doped hydrogenated amorphous silicon films (a-Si:H) were crystallized using step-by-step laser crystallization process. The structural changes during the sequential crystallization process were detected by Raman measurements. The dehydrogenation was monitored by measuring the Si-H local vibrational modes using Raman spectroscopy and hydrogen effusion measurements. Interestingly, hydrogen bonding is affected by doping of the amorphous material. The influence of doping concentrations, thus the Fermi energy on electronic properties has been investigated employing secondary ion mass spectroscopy (SIMS), dark-conductivity- and Hall-effect measurements. The results from hydrogen effusion are consistent with the results obtained from Raman spectroscopy, Hall-effect- and dark-conductivity measurements.     

Fabrication of axicons by cw laser effusion

The fabrication of axicons by cw laser effusion is introduced for the case of a chalcogenide alloy, and experimental results concerning the lateral resolution and the intensity distribution along the optical axis are reported.     

Instantaneous Radiation Energy Flux and Radiation Power Near the Event Horizon of Slowly Changing Vaidya Black Hole

By the thin film model of the black hole and the assumption of the local thermal equilibrium, the instantaneous radiation energy flux and radiation power of the slowly changing Vaidya black hole have been studied. The result has been obtained that the thermal radiation of the Vaidya black hole satisfies the generalized Stefan-Boltzmann law. When the cut-off distance and the thin film thickness are both fixed, the instantaneous radiation energy flux of the scalar field near the event horizon of the Vaidya black hole is not only related to the black hole mass, but also to the rate of the change of its event horizon and the average effusion velocity of the radiation particles in the thin film. While its instantaneous radiation power is related to the rate of the change of the event horizon and the average radial effusion velocity of the radiation particles in the thin film. These results indicate that the gravitational field around the black hole and the change of its event horizon will both affect the thermal radiation of the black hole.     

STUDIES OF THE TRANSPIRATION COOLING THROUGH A SINTERED STAINLESS STEEL PLATE

This study concerns the liquid transpiration cooling effect on thermal protection of a porous plate wall. The results indicate that the effectiveness reaches more than 95% for a very weak effusion rate, about 0.1%, that is to say 50 times weaker than that of gas effusion. The concentration profile in the boundary layer is calculated experimentally and the rate of liquid evaporated is then calculated numerically, using a model based on utilization of momentum equations in laminar flow for the boundary layer. The results of this numerical study confirm evaporation rates calculated by semi-empirical relations.     

Experimental thermionic energy converter without enclosure: A molecular beam supplied converter

Experimental thermionic energy converters of an open type, i.e. with no closely located surrounding walls, as proposed here, have many advantages compared to conventional enclosed converters. Interactions with and problems of the enclosure do not interfere with the fundamental plasma and surface processes, which can be studied efficiently in an open converter, for example by mass spectrometry, optical spectroscopy, electron energy analysis and surface analytical methods. An open converter has been built and tested, where the necessary alkali vapour is supplied by a molecular beam type source with 400 laser bored effusion holes. The source acts as collector in front of a directly heated emitter. An ignited mode behaviour can be reached easily. The examples of results presented for Mo and Pt emitters at 1600–1700 K, a Cs plasma and nickel collector give a power density of 1–2 W cm^–2 and a minimum barrier index of 1.95–2.1 eV and indicate a behaviour similar to enclosed converters. Thus, results for new surfaces, new additives etc. found in open converters should be of direct relevance for the development of normal enclosed converters.     

Corrosion behavior of high-level waste container materials Ti and Ti–Pd alloy under long-term gamma irradiation in Beishan groundwater

Titanium and titanium–palladium alloys are important potential materials for nuclear waste container, which will endure both intense γ-irradiation and groundwater erosion. Therefore, it is very important to investigate the corrosion behavior of the container materials. In this research, the cumulative dose effect of TA8-1 type titanium–palladium alloy(TA8-1) and TA2-type pure titanium(TA2) under γ-irradiation was studied based on the geological disposal of nuclear wastes. The irradiation experiments were performed at room temperature using60 Co gamma sources with a 5.0-k Gy·h-1 intensity for 40, 80 or 160 days, respectively. The p H value and conductivity of Beishan groundwater were investigated.The results showed that the p H value changed from alkaline(8.22) to acidic(2.46 for TA8-1 and 2.44 for TA2), while the un-irradiated solution remained alkaline(8.17 for TA8-1 and 8.20 for TA2) after 160 days. With the increase of irradiation dose, the conductivity increases rapidly and then tends to become stable, which indicates that the titanium dioxide corrosion layer formed on the surface of the sample surface effectively prevents further corrosion. Meanwhile, XRD and SEM–EDS analysis results show that the main components of corrosion products are Ti O_2 and Ti O. The titanium on the surface of the sample is oxidized, resulting in slight uneven local corrosion. The results show that TA8-1 and TA2 are suitable to be used as candidate materials for high-level waste(HLW) disposal containers due to their excellent performance under long-term and high-dose irradiation corrosion.     

Open‐cage fullerenes as tailor‐made container for a single water molecule

Water is the most important liquid on earth. Clusters of water have been investigated extensively in an effort to understand the bulk property of water. But the behavior of single water molecule without H‐bond has been rarely studied. Open‐cage [60]fullerenes have been shown to trap a single water molecule selectively over molecules with comparable size and act as the smallest “water bottle”. Copyright © 2013 John Wiley & Sons, Ltd.