Study of the dielectric breakdown properties of hot SF₆-CF ₄ mixtures at 0.01-1.6 MPa

The dielectric breakdown properties of SF₆-CF₄ mixtures were investigated at different ratios of SF₆, 0.01-1.6 MPa, and gas temperatures up to 3000 K. Initially, the equilibrium compositions of SF₆-CF₄ mixtures were calculated by minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium. Then the electron energy distribution function was obtained based on those data by solving the Boltzmann equation under the zero-dimensional two-term spherical harmonic approximation. Finally, the critical reduced electric field strength (E/N)_cr of SF₆-CF₄ mixtures, which is defined as the value for which total ionization reaction is equal to total attachment reaction, were determined and analyzed. The results confirm the superior breakdown properties of pure SF₆ at relatively low gas temperatures. However, for higher gas temperatures (i.e., T > 2200 K at 0.4 MPa), the (E/N)_cr in SF₆-CF₄ mixtures are obviously higher than that in pure SF₆ and the values of (E/N)_cr increase with the reduction of the ratio of SF₆.