Electron attachment to perfluorocarbon compounds. II. <i>c</i>-C5F8, <i>c</i>-C6F10, <i>c</i>-C6F12, C7F8, and C8F16: Relevance to gaseous dielectrics

Abstract

Electron attachment rates αw as a function of the pressure-reduced electric field have been measured for c-C5F8 (perfluorocyclopentene), c-C6F10 (perfluorocyclohexene), c-C6F12 (perfluoro-1,2,-dimethylcyclobutane), C7F8 (perfluorotoluene), and C8F16 (perfluoro-1,3-dimethylcyclohexane). The thermal values (αw)ther of the attachment rate for these compounds are 12.8, 12.9, 5.0, 9.2, and 2.4×109 sec−1 Torr−1, respectively. The electron attachment cross sections σa(ε) as functions of the electron energy ε have been determined using the swarm-unfolding technique; they show distinct structure at ε&amp;lt;1.5 eV. The electron attachment properties for the perfluorocarbons (PFCs) studied in this and the preceding paper depend strongly on the molecular structure. Thus (i) open-chain saturated PFCs do not attach electrons as efficiently as the unsaturated PFCs; (ii) for saturated PFCs an increase in molecular size (chain length) increases αw; (iii) the presence of multiple bonds in the molecule dramatically increases the magnitude of αw for the saturated open-chain PFCs, but it has only a small effect for cyclic PFCs; (iv) the cyclic nature of the PFCs seems to increase greatly σa(ε); and (v) for single-bonded cyclic PFCs, substitution of CF3 groups for F atoms increases (αw)ther more than does an increase in molecular size; in double-bonded cyclic compounds increase in ring size increases σa(ε) at thermal energies. The PFCs studied in this and the preceding paper have higher dielectric strengths than SF6. This improvement in dielectric strength can be attributed to their large σa(ε) over a wider energy range (to ∼1.5 eV). Their use as unitary gases and/or as additives to multicomponent gas mixtures is discussed.