A Study on the Formation of Metal Clusters in a Supersonic Expansion of Vapor

本文从实验和理论两方面研究了蒸汽超音速膨胀形成金属团簇的过程。在实验中，金属(银或锌)在坩埚中蒸发，蒸气通过喷嘴(通常直径为2毫米)流入高真空。为了探测绝热膨胀引起的过饱和蒸汽中团簇的形成，采用了一种延迟场方法。结果表明，电离和电加速可以使团簇被破坏。试验了三种喷嘴，即圆柱形喷嘴、宽扩散型喷嘴和窄扩散型喷嘴，发现宽扩散型喷嘴最有利于团簇的形成。此外，还研究了喷嘴与坩埚之间的温差对熔化效果的影响。结果表明，当喷嘴温度比坩埚温度低约100K时，团簇形成最为明显，这表明喷嘴壁面向蒸汽流动的换热影响着团簇的形成。在实验的基础上，对团簇在过饱和蒸气中的性质进行了分子动力学模拟，发现表面张力系数随团簇尺寸的减小而减小。在估算成核速率时考虑了表面张力降低的影响，根据喷管流动数值模拟得到的温度和压力场，分析了成核过程中团簇的形成。表面张力的降低大大提高了成核率，但预测的团簇尺寸太大。因此，在分析中考虑了凝结潜热的产生和团簇与水蒸气之间的换热。计算结果很好地解释了低熔点金属锌的实验结果。

The formation of metal clusters by a supersonic expansion of vapor has been investigated both experimentally and theoretically. In the experiment, metal (silver or zinc) was vaporized in a crucible, and the vapor effused into a high vacuum through a nozzle (typically 2mm in diameter). In order to detect the cluster formation in the supersaturated vapor caused by the adiabatic expansion, a retarding field method was used. It was found that the cluster could be destroyed by the ionization and electrical acceleration. Three kinds of nozzles were tested, namely, a cylindrical nozzle, and wide and narrow divergent nozzles, and it was found that the wide divergent nozzle is most effective for the production of clusters. Further, the effect of temperature difference between nozzle and crucible was investigated. It was found that the cluster formation was most pronounced when the nozzle temperature is about 100 K lower than that of crucible, which indicates that the heat transfer from the nozzle wall to the vapor flow affects the cluster formation. In addition to the experiments, a molecular dynamic simulation was performed to investigate the properties of cluster in a supersaturated vapor, and the reduction of surface tension coefficient with the decrease of cluster size was revealed. By considering the surface tension reduction in the estimation of nucleation rate, the cluster formation was analyzed based on the temperature and pressure field obtained by a numerical simulation of the nozzle flow. The surface tension reduction drastically increases the nucleation rate, however, the predicted cluster size was too large. Therefore, the heat generation due to the latent heat of condensation and heat transfer between cluster and vapor were taken into account in the analysis. The calculated result well explains experimental results for the low melting point metal, Zn.