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对蒸汽的超音速膨胀形成金属团簇进行了研究。在实验中，金属（银或锌）在坩埚中汽化，蒸汽通过喷嘴（直径通常为2mm）流入高真空。为了检测由绝热膨胀引起的过饱和蒸汽中的团簇形成，采用了缓速场法。研究发现，电离和电加速可以破坏星团。对圆柱型喷嘴、宽发散型喷嘴和窄发散型喷嘴进行了试验，发现宽发散型喷嘴对簇的生成效果最好。进一步研究了喷嘴和坩埚之间温差的影响。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，说明喷嘴壁面向蒸汽流动的传热影响了团簇的形成。在实验的基础上，通过分子动力学模拟研究了过饱和蒸汽中团簇的性质，揭示了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表面张力的降低，基于喷嘴流动的数值模拟得到的温度和压力场，分析了簇的形成。表面张力的降低大大提高了成核速率，但预测的团簇尺寸过大。因此，在分析中考虑了凝结潜热产生的热量以及团簇与蒸汽之间的换热。计算结果较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.