He在BCC铁中的扩散以及氦泡的成核生长试卷.pptxVIP

;主要内容;背景介绍;背景介绍;背景介绍;研究机理分析;研究机理分析;研究模型; In order to create cluster, n helium atoms are distributed over the lattice sites of a compact cluster of vacancies ( ) and then the unoccupied vacancies are allowed to perform diffusion jumps on bcc iron lattice with probabilities determined by Boltzmann factors appropriate for each particular jump.  The cluster diffusion coefficient is determined as the proportionality factor between the mean squared shift r2of the cluster‘center of mass’ during the time span and the time itself, After the end of each calculation run the code processed the cluster positions that had been periodically saved during the run and estimated the average shift for different time spans. ;A typical output is shown in Fig1.;  As can be seen, the shift follows the square root of time dependence only at time spans 10–20% of the average cluster lifetime, while at bigger the fluctuations strongly distort the shape of the curve, because the number of available points for estimating decreases too much.  If we present as where a it the iron lattice parameter, mV – the mono-vacancy jump frequency, and dc – the factor characterising cluster mobility, the lowest mobility limit detectable by the code is dc ~ 10-8. Cluster lifetime tc has been estimated as the average over all campaigns at each temperature. where is the atomic vibration frequency, the vacancy migration energy and has its usual meaning. For the estimates below we used and ; For all considered clusters it has the Arrhenian form, and The values of prefactors, and , the migration energy corrections, and the cluster dissociation energies , as obtained by fitting the calculated points, are summarized in Table 1. Note that according to Eqs. (2) and (3) the full activation energy for