中文摘要：

在中真空条件（18~50 Pa）下,从原子碰撞与碰撞凝聚的角度分析系统温度、铝分压及冷凝区温度梯度对铝蒸气相变形核及冷凝的影响。采用真空蒸发冷凝法（不充入惰性气体）制备出铝珠及铝粉,并利用扫描电镜与能谱进行表征。结果表明：在中真空条件（18~50 Pa）下,蒸气过饱和度增大,利于液相形核,这与温度对单位时间铝原子碰撞次数的影响一致,但当系统温度降到液气转变最低温度后,铝蒸气将冷凝成固态。中真空下,其影响蒸气冷凝方式的重要因素之一是系统中冷凝区的温度梯度。通过真空蒸发冷凝法,在冷凝区温度梯度约1.4 K/mm时制得金属铝珠,温度梯度约19.5 K/mm时制得金属铝粉。

英文摘要：

The nucleation of aluminum vapor in phase transition and its condensation in medium vacuum,ranging from 18 Pa to 50 Pa,were studied.The impacts of the condensing conditions,including the Al partial pressure,temperature,and temperature gradient in the condensation zone,on Al nucleation and condensation were discussed from the point of view of atom collision.The Al powders and beads,grown by vacuum evaporation-condensation without inert gas protection,were characterized with scanning electron microscopy and energy dispersive spectroscopy.The results show that the condensing conditions strongly affect the characteristics of the Al condensation.For instance,in medium vacuum,high degree of super-saturation promotes Al liquid-phase nucleation;however,when temperature drops to the critical temperature of liquid-gas transition,Al vapor will rapidly condenses into solid phase.Moreover,the temperature gradient in the condensation zone significantly affects the condensation;at a gradient of 1.4 K/mm,Al beads form,and at 19.5 K/mm,formation of Al powder dominates.