Study on Pyrolytic Mechanisms of n-Perfluorosilanes Si_nF_2n+2 (2 ≤ n < 6) and Perfluorocyclosilanes Si_nF_2n (3 ≤ n ≤ 6)

Abstract

In this paper, the pyrolytic mechanisms of n-perfluorosilanes Si n F 2n+2 (2 ≤ n < 6) and perfluorocyclosilanes Si n F 2n (3 ≤ n ≤ 6) are studied in terms of kinetics and thermodynamics by theoretical calculation, and the pyrolytic reaction paths of Si n F 2n+2 (2 ≤ n < 6) and Si n F 2n (3 ≤ n ≤ 6) are obtained, which can be used to guide the experimental preparation research studies and separation operations of Si n F 2n+2 (2 ≤ n < 6), Si n F 2n (3 ≤ n ≤ 6), and their intermediate substances. The results of the kinetic analysis show that the pyrolytic mechanisms of Si n F 2n+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 SiF 2 at the same time, whereas the generation of double free radicals is the most difficult process. The pyrolytic mechanisms of Si n F 2n (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. SiF 2 is most easily formed by breaking during the chain transfer. In the pyrolytic processes of Si n F 2n+2 (2 ≤ n < 6) and Si n F 2n (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 Si n F 2n+2 (2 ≤ n < 6) and Si n F 2n (3 ≤ n ≤ 6) are endothermic. When Si n F 2n+2 (2 ≤ n < 6) undergoes a pyrolytic reaction and the temperature is higher, the main pyrolytic products are SiF 4 and SiF 2 . When 600 K < T < 1200 K, the main pyrolytic products of Si 4 F 10 are Si 3 F 8 and SiF 2 , and when 900 K < T < 1400 K, Si 5 F 12 can also convert to Si 3 F 8 and SiF 2 . The main pyrolytic products of Si n F 2n (3 ≤ n ≤ 6) are SiF 2 . When the temperature is higher, the pyrolytic order of Si n F 2n (3 ≤ n ≤ 6) is as follows: Si 3 F 6 (ring) < Si 4 F 8 (ring) < Si 5 F 10 (ring) < Si 6 F 12 (ring). However, if the temperature is in the range of 1000 K < T < 1200 K, the pyrolytic order is the opposite.