Study on Pyrolytic Mechanisms of n-Perfluorosilanes Si n F2n+2 (2 ≤ n < 6) and Perfluorocyclosilanes Si n F2n (3 ≤ n ≤ 6).

Abstract

In this paper, the pyrolytic mechanisms of n-perfluorosilanes SinF2n+2 (2 ≤ n < 6) and perfluorocyclosilanes SinF2n (3 ≤ n ≤ 6) are studied in terms of kinetics and thermodynamics by theoretical calculation, and the pyrolytic reaction paths of SinF2n+2 (2 ≤ n < 6) and SinF2n (3 ≤ n ≤ 6) are obtained, which can be used to guide the experimental preparation research studies and separation operations of SinF2n+2 (2 ≤ n < 6), SinF2n (3 ≤ n ≤ 6), and their intermediate substances. The results of the kinetic analysis show that the pyrolytic mechanisms of SinF2n+2 (2 ≤ n < 6) are as follows: first, the silicon–silicon bond breaking induces the generation of free radicals; then, in the chain transfer, the related free radicals participate in F-abstraction transfer with the molecules; and finally, the free radicals form the molecules, and the chain terminates. The F-abstraction transfer is the easiest process to initiate in the low-order silicon–fluorine substance during the chain transfer while releasing SiF2 at the same time, whereas the generation of double free radicals is the most difficult process. The pyrolytic mechanisms of SinF2n (3 ≤ n ≤ 6) are as follows: first, the α–Si–Si bond breaking induces the generation of double free radicals; then, the α–Si–Si or β–Si–Si bond breaks continually in the chain transfer; and finally, the double free radicals form the molecules, and the chain terminates. SiF2 is most easily formed by breaking during the chain transfer. In the pyrolytic processes of SinF2n+2 (2 ≤ n < 6) and SinF2n (3 ≤ n ≤ 6), the chain initiation of silicon–silicon bond breaking requires the highest bond breaking energy, which is the control step of the pyrolytic reaction. The results of the thermodynamic analysis show that the pyrolytic reactions of SinF2n+2 (2 ≤ n < 6) and SinF2n (3 ≤ n ≤ 6) are endothermic. When SinF2n+2 (2 ≤ n < 6) undergoes a pyrolytic reaction and the temperature is higher, the main pyrolytic products are SiF4 and SiF2. When 600 K < T < 1200 K, the main pyrolytic products of Si4F10 are Si3F8 and SiF2, and when 900 K < T < 1400 K, Si5F12 can also convert to Si3F8 and SiF2. The main pyrolytic products of SinF2n (3 ≤ n ≤ 6) are SiF2. When the temperature is higher, the pyrolytic order of SinF2n (3 ≤ n ≤ 6) is as follows: Si3F6 (ring) < Si4F8 (ring) < Si5F10 (ring) < Si6F12 (ring). However, if the temperature is in the range of 1000 K < T < 1200 K, the pyrolytic order is the opposite.