Reduction of trace oxygen by hydrogen leaking during selective vaporization to produce ultra-pure cadmium for electronic applications

摘要

A viable method utilizing leaking of high pure hydrogen gas into the distillation retort ambience maintained at 8.0 x 10~(-3) Torr was devised to minimize the incorporation of trace oxygen in cadmium and the possibility of forming cadmium oxides during purification using selective vaporization. Hydrogen leaking at the rate of 5 standard cubic centi-metre per minute (sccm) during distillation at soaking temperature of 450 ℃ reduced trace oxygen from 230-350 ppm for raw cadmium of 3N8 (99.98%) purity and from 5-10 ppm for distilled cadmium of 6N purity. This can be compared to the presence of 30-38 ppm oxygen in cadmium distilled under simple-vacuum of 2.1 x 10~(-3) Torr achievable in the system. A detailed comparison between distillation under simple-vacuum and hydrogen leaking is presented with reference to purity, distillation rate, soaking temperature, and mean free path of distillate vapors.