强流脉冲电子束轰击对单晶Si表面形貌的影响

用强流脉冲电子束技术对两种取向的单晶Si片进行了表面轰击,对电子束诱发的表面形貌进行了分析.实验结果表明,当能量密度～3 J/cm2时,轰击表面开始形成大量的熔坑.能量密度～4 J/cm2时,表面开始出现微裂纹,微裂纹的形态与单晶Si的晶体取向密切相关;强流脉冲电子束轰击能够诱发表层强烈的塑性变形,[111]取向单晶Si表面出现剪切带结构,而[001]取向单晶Si表面变形结构则以微条带为主;此外,变形区域内还出现大量<100 nm的微孔洞形貌,这些微孔洞的形成为制备表面多孔材料提供了可能.     

强脉冲电子束在聚酯薄膜表面产生的化学气相沉积

一、引言 高功率密度脉冲电子束(>10~9W/cm~2)轰击金属,将引起金属剧烈蒸发,产生高温等离子体。等离子体中的金属离子获得一定能量后将与本底气体分子相互作用而形成化合物。本文介绍在低压氮气环境中,利用强脉冲电子束轰击铜靶,产生了氮铜化合物沉积在聚酯薄膜表面的实验结果。并用ESCA分析束处理过的聚酯薄膜表面的化学组成的变化。     

强流脉冲电子束纯铝表面改性过程的热力学模拟

针对强流脉冲电子束表面改性的特点,建立了相应的数学物理模型,以纯铝材表面改性实验为基础,计算了由强流脉冲电子束辐照材料表面所产生快速升降温与熔凝过程,对其中的熔化、蒸发、热应力波等现象分别进行了数值模拟。计算结果表明,熔化深度约在1-10μm,与实验结果接近;蒸发作用影响较小,汽化层厚度仅为纳米量级;热应力波的幅值约在0．1MPa量级,且与脉冲的能量密度大致成正比,但是对材料结构和性能有重要影响。     

计算脉冲电子束辐照下能量沉积剖面的新方案

脉冲电子束辐照材料试验研究中,束流电子具有不同的速度和角度分布。但数值模拟计算一般都考虑电子束垂直入射靶材料,这可能导致数值计算结果与试验结果不符。针对该问题,提出了一种计算电子束辐照下能量沉积剖面的新方案,利用MCNP(Monte Carlo N Particle Transport Code)软件对铝、铜、钽金属材料在电子束辐照下的能量沉积进行模拟,分析了电子束垂直入射与带有角度分布入射时能量沉积的差异,为解释电子束辐照试验测量数据与理论计算结果之间的差异提供了依据。     

多束脉冲作用下韧致辐射转换靶照射量模拟

20MeV的强流脉冲电子束经加速聚焦后轰击韧致辐射转换靶,在产生脉冲X光的同时将大量能量沉积在靶内,导致靶材迅速膨胀飞散。在多个束脉冲作用下,后续脉冲由于靶材密度的降低可能无法产生足够的X光照射量。本文采用Monte-Carlo软件MCNP以及流体动力学软件ANSYSAUTODYN对靶材的动力学响应以及各脉冲所能产生的照射量进行了数值模拟,结果表明,在3个间隔500ns的20MeV、2kA、70ns、束斑1.5mm（FWHM）的电子束脉冲作用下,X光照射量无明显降低。     

脉冲激光沉积薄膜生长机制的计算机模拟研究

建立了一个分子动力学模型以研究脉冲激光沉积金属薄膜的成膜过程，探讨激光沉积中极主的瞬时沉积率和载能粒子轰击对成模机制的影响，以能量为１０ｅＶ的Ｃｕ原子入射到Ｃｕ（１００）表面为例，进行了分子动力学模拟的计算，结果表明，激光成膜过程中载能粒子的瞬时高能量沉积极大地增加了外延表面的原子扩散活性，促使薄膜能在低温下以原子层尺度逐层生长。     

严重事故下安全壳内氢气浓度场分布

利用计算流体力学程序FLuENT和GASFLOW,采用不同的湍流模型,研究了核电站严重事故下氢气在安全壳内的传输与混合过程.计算结果表明,FLUENT中的RNG k-ε模型能够较好的模拟氢气的质量扩散,动量扩散和湍流脉动特征;FLUENT中的标准k-ε模型和GASFLOW中的k-ε模型能得到工程上可以接受的计算结果;而GASFLOW中代数模型未能较好地模拟氢气的质量扩散和动量扩散,氢气的浓度场分布与其他模型的计算结果存在较大的差别.同时,本文对混合气体中的水蒸汽浓度和气体的质量流速对安全壳内氢气浓度分布的影响进行了初步研究.研究表明,破口气体的密度和流速是影响氢气浓度场的重要因素;混合气体密度越小、流速越大,则有更大的浮力和初始动量作用于气体.湍流模型的选择和对浮力驱动的湍流射流的模拟是影响严重事故下氢气在安全壳内的分布模拟结果的重要因素.     

半导体中正电子向表面的扩散

考虑到半导体中内电场的存在对半无限介质和有限厚度薄膜中的正电子扩散过程，在各种边界条件下，利用δ函数法计算了正电子向表面扩用分数，这些结果为半导体多层复合结构中的慢正电子束实验结果的分析提供了理论基础。     

脉冲离子束轰击靶的热效应数值模拟

分析了脉冲离子轰击靶膜和衬底的热效应,在能量较低的情形下,离子轰击处理为靶膜表面热流输入。采用有限元程序,对能量为600keV、束流为12mA的不同束斑半径的脉冲离子流轰击Cu基Ti靶的热传导进行了数值计算,得到了热传导清晰物理图象。     

低温条件下金属薄膜外延生长过程中瞬时扩散机制的研究

采用分子动力学方法和混合的紧束缚势模拟了Ｃｕ在Ｃｕ（１００）表面外延生长过程的初始阶段段。重点研究低温上外延表面原子的瞬时扩散机制，对瞬时扩散运动进行分类检验，并定量报道了各类瞬时扩散过程在低温条件下的相对发生比例，由此得知，轰击引起的级联扩散机制对外延表面原子的扩散活动起重要作用，同时在模拟在还观察到了温度低至１００Ｋ时的衍射强度振荡，显示了该成膜初始阶段是一个以原子层为尺度的准逐层生长过程。     

Improved wear resistance of Al-15Si alloy with a high current pulsed electron beam treatment

A hypereutectic Al-15Si alloy (Si 15 wt.%, Al balance) was irradiated by high current pulsed electron beam (HCPEB). The HCPEB treatment causes ultra-rapid heating, melting and cooling at the top surface layer. As a result, the special “halo” microstructure centering on the primary Si phase is formed on the surface due to interdiffusion of Al and Si elements. The composition of the “halo” microstructure is distributed continuously from the center to the edge of the “halo”. Compared to an untreated matrix, the remelted layer underneath the surface presents single contrast because of the compositional homogeneity after HCPEB treatment. The thickness of the remelted layer increases slightly from 4.4 μm (5 pulses) to 5.6 μm (25 pulses). HCPEB treatment broadens and shifts the diffraction peaks of Al and Si. The lattice parameters of Al decreases due to the formation of a supersaturated solid solution of Al in the melted layer. Through analysis of Raman spectra and transmission electron microscopy (TEM), the amorphous Si (a-Si) and nanocrystalline Si are formed in the near-surface region under multiple bombardments of HCPEB. The relative wear resistance of a 15-pulse sample is effectively improved by a factor of 9, which can be attributed to the formation of metastable structures.     

D2钢电子束表面改性抗微动磨损性能研究

利用强流脉冲电子束（HCPEB）技术对D2钢进行了表面改性处理。结果表明，电子束聂击试样近表层组织结构发生了很大变化，且亚表层显微硬度明显增加。微动磨损结果显示，电子束处理可以改善D2钢抗微动磨损性能，预涂覆TiN和Cr层效果更明显。磨痕SEM分析表明，涂覆Cr层的样品微动磨损主要机制是粘着和氧化，而TiN涂层及电子束直接处理试样主要磨损机制则是表面疲劳微裂纹的扩展和断裂产生的剥落颗粒，在摩擦面造成磨粒磨损，同时也伴随轻微的粘着和氧化，其磨损机制相对比较复杂。     

SKD11模具钢的强流脉冲电子束辐照改性

本文介绍强流脉冲电子束（HCPEB）材料表面改性原理，采用模具钢SKD11进行辐照处理实验。对处理样品表面进行形貌观察，发现熔坑面密度及粗糙度随脉冲处理次数的增加而减小。X射线衍射分析证实，多脉冲处理条件下表面改性层中碳化物发生溶解，形成高奥氏体含量的重熔组织，而过多的能量注入会使奥氏体发生再度分解。磨损性能测试表明，在低电压（19．8kV）处理情况下，耐磨损性能得到改善，脉冲次数为8次时处理样品的耐磨性达到最佳，这与表面改性层中碳化物及奥氏体的相对含量变化密切相关。     

On the release and ionization efficiency of catcher-ion-source systems in isotope separation on-line

It is shown that the critical steps in isotope separation on-line - diffusion, effusion, and ionization - can be studied in a very efficient way by stimulating the separation process using beam particles of the UNILAC accelerator as tracer instead of reaction products. The analysis of the measured release profiles readily yields the ionization efficiency and the half-life dependence of the separation efficiency on an absolute scale. For the cases in which surface desorption is a fast process compared to solid state diffusion, additionally the decisive delay parameters μ₀ for diffusion and ν for effusion can be extracted. These are closely related to material constants such as diffusion coefficients and enthalpies for surface adsorption and in favourable cases permit extraction of the Arrhenius coefficients. Thus the assumed release model can be tested by comparison with literature values or, in turn, enables the conversion of known material constants into estimates for the half-life dependence of the separation efficiency.

Since ion sources have reached a high degree of sophistication, the ionization efficiency is hardly ever the limiting factor of on-line mass separation. This is in general also true for the diffusion step, if the distribution of the reaction products in the catcher is either homogeneous or very close to the surface. As a consequence it turns out that effusion decisively causes the limitations in mass separation on-line, i.e. surface adsorption for refractory elements and the intrinsic delay for very short half-lives.     

Diffusion coefficients for LMFBR cells calculated with MOC and Monte Carlo methods

The present work discusses the calculation of the diffusion coefficient of a lattice of hexagonal cells, with both “sodium present” and “sodium absent” conditions. Calculations are performed in the framework of lattice theory (also known as fundamental mode approximation). Unlike the classical approaches, our heterogeneous leakage model allows the calculation of diffusion coefficients under all conditions, even if planar voids are present in the lattice. Equations resulting from this model are solved using the method of characteristics (MOC). Independent confirmation of the MOC result comes from Monte Carlo calculations, in which the diffusion coefficient is obtained without any of the assumptions of lattice theory. It is shown by comparison to the Monte Carlo results that the MOC solution yields correct values of the diffusion coefficient under all conditions, even in cases where the classic calculation of the diffusion coefficient fails. This work is a first step in the development of a robust method to calculate the diffusion coefficient of lattice cells. Adoption into production codes will require more development and validation of the method.     

针对不锈钢表面60Co污染的电化学去污研究

核设施退役与三废治理过程中会产生大量表面被沾污的不锈钢金属部件,作业环境和人员将面临潜在的放射性污染和受照剂量问题。针对不锈钢金属表面的60Co污染问题,设计了手持式电化学试验装置对不锈钢表面污染进行合理有效的原位去污;采用正交实验法优化电解液浓度、电流密度和电极间距等电化学工艺参数,并对不锈钢表面60Co污染进行去污验证。研究结果表明,本文建立的电化学原位去污方法具有去污时间短、效率高等优点;在电解时间为30 s、电解液为10 mol/L硝酸、电流密度为0.3 A/cm2、电极间距为0.4 cm的条件下,此方法对不锈钢表面60Co污染的去污效率可达99.9%以上,腐蚀深度大于10 μm,可使污染降低至环境本底辐射水平。      

Comments on “Stability of the Gas Bubbles in Solids,by G.P. Tiwari”

A new difference equation for the numerical solution of the one-dimensional diffusion equation was obtained by using a semi-analytic method, in which the only approximation employed is that the source distribution within a mesh-region is represented by a linear function.

A test program, EXX-1, was prepared and run for BWR calculations, and comparisons were made with the conventional method. The results show that by using the new difference equation, a very coarse mesh model (1 mesh point per material region) can be applied without seriously impairing the computational accuracy.

It is also shown that the conventional difference equation becomes identical to the new expression if group-constants are multiplied by correction factors and the treatment of the source term appropriately modified.

New difference equations for spherical and cylindrical geometries are also given, in an appendix.     

Cessation of environmentally-assisted cracking in a low-alloy steel: experimental results

The presence of dissolved metallurgical sulfides in pressure vessel and piping steels has been linked to environmentally-assisted cracking (EAC), a phenomenon observed in laboratory tests that results in fatigue crack growth rates as high as 100 times that in air. Previous experimental and analytical work based on diffusion as the mass transport process has shown that surface cracks that are initially clean of sulfides will not initiate EAC in most applications. This is because the average crack tip velocity would not be sufficiently high to expose enough metallurgical sulfides per unit time and produce the sulfide concentration required for EAC. However, there is a potential concern for the case of a relatively large embedded crack breaking through to the wetted surface. Such a crack would not be initially clean of sulfides, and EAC could initiate. Previous experiments have suggested that under some conditions, EAC could be persistent. This paper presents the results of a series of experiments conducted on two heats of an EAC susceptible, high-sulfur, low-alloy steel in 243°C low-oxygen water to further study the phenomenon of EAC persistence at low crack tip velocities. A load cycle profile that incorporated a significant load dwell period at minimum load was used. Experiments using compact tension specimens with various initial precrack depths were employed to simulate the breakthrough of embedded cracks. The results showed that EAC ceased after several hundred hours of cycling. This indicates that significant dwell periods can allow sufficient time for sulfur diffusion to turn off EAC provided that the initial crack tip velocities are not unusually high.     

拉格朗日烟团模型的大气扩散系数自适应修正

在核事故后果实时评价系统中,拉格朗日烟团模型作为大气扩散模型得到了广泛应用。大气扩散系数是影响烟团模型的重要参数之一,本文提出一种动态修正拉格朗日烟团模型的大气扩散系数的自适应方法,以提高放射性核素浓度分布计算的准确性。该方法利用观测的核素浓度数据、气象数据和源项释放数据,以最小二乘法实时地对大气扩散系数进行了估计。使用大气扩散模型验证工具MVK中的Kincaid实验数据,将动态大气扩散系数自适应修正方法与传统的以Pasquill-Gifford（P-G）曲线为基础的方法相比较,结果表明,大气扩散系数自适应修正方法能提高拉格朗日烟团模型计算结果的准确性。