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，蒸汽通过喷嘴（通常直径为2 mm）流出到高真空中。为了探测过饱和蒸汽绝热膨胀引起的团簇形成，采用了阻滞场方法。发现团簇可以被电离和电加速破坏。对圆柱形喷嘴、宽发散喷嘴和窄发散喷嘴三种喷嘴进行了试验，发现宽发散喷嘴对产生团簇最有效。此外，还研究了喷嘴和坩埚之间的温差对反应的影响。结果表明，当喷嘴温度比坩埚温度低100 K左右时，团簇的形成最为明显，这表明喷嘴壁面向蒸汽流的传热影响了团簇的形成。在实验的基础上，采用分子动力学模拟方法研究了过饱和蒸汽中团簇的性质，揭示了团簇的表面张力系数随团簇尺寸的减小而减小的规律。通过对喷嘴流场进行数值模拟，得到了喷嘴内温度场和压力场，并在此基础上考虑了表面张力降低对成核速率的影响，分析了喷嘴内团簇的形成过程。表面张力的降低大大增加了成核速率，然而，预测的簇尺寸太大。因此，在分析中考虑了凝结潜热产生的热量以及团簇与水蒸气之间的传热。计算结果很好地解释了低熔点金属Zn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.