Study of the Equilibrium Composition of a Sulfur Hexafluoride–Air Plasma Mixture between 1,000 K and 15,000 K: An Application to High-Voltage Circuit Breakers

Abstract

Sulfur hexafluoride (SF₆) is widely used in high-voltage circuit breakers because of its insulating properties and interrupting capability, although concerns related to its environmental impact and decomposition products encourage efforts to limit its use. This study determines the equilibrium chemical composition of SF₆–air plasmas at atmospheric pressure over the temperature range 1,000–15,000 K under local thermodynamic equilibrium. Air is represented by 80% nitrogen and 20% oxygen, and different air proportions in the mixture are considered. The equilibrium number densities of the chemical species are calculated by Gibbs free energy minimisation using a computational code developed in MATLAB. The results indicate that molecular species predominate below 7,000 K. SF₆ begins to dissociate at approximately 1,700 K, while SF₄ and F₂ dissociate at around 1,900 K; SF₃ and SF₂ dissociate at approximately 2,000 K and 2,200 K, respectively. Between 7,000 K and 15,000 K, the dominant neutral species are N, O, S, and F, whereas electrons, N⁺, O⁺, S⁺, and F⁺ constitute the principal charged species. Increasing the air proportion increases the number densities of nitrogen- and oxygen-related species and their ions and slightly increases the electron number density, while several sulfur–fluorine species decrease. These equilibrium-composition trends clarify how air proportion modifies the species distribution of SF₆–air plasmas under the thermal conditions considered for high-voltage circuit-breaker applications.