Silicon Isotope Separation by Distillation of Silicon Tetrafluoride

Abstract

Abstract Silicon isotopes were separated by distilling silicon tetrafluoride in a 150-cm packed column at temperatures of 189–206 K. The data show a reverse isotope effect with heavy isotopes more volatile in this temperature range. At total reflux, the maximum column separations were 0.926 for 28/29Si and 0.872 for 28Si/30Si. Carbon monoxide was also distilled in the column to obtain an estimated number of separating stages. From an estimated HETP of 2 cm for the packing, the volatility of 28Si is 0.9990 relative to 29Si and 0.9982 relative to 30Si. The SiF4 isotope effect is the largest yet found in distilling silicon compounds, and the effect is opposite that seen for SiH4. The magnitude of the isotopic relative volatilities is large enough to consider SiF4 distillation as a practical method for silicon isotopic enrichment. This effect of higher mass isotopes being more volatile has analogues in isotope effects in SF4 and BF3.