Study of the thermal plasma etching at atmospheric pressure on silica rods

Etching of SiO2 rods has been obtained with a dc torch with argon as the process gas in an air environment at atmospheric pressure; the high temperature of the plasma jet causes vaporization of the exposed area. The apparatus and torch operative parameters have been set up to obtain a depth etch rate of up to 0.6 mm min−1 corresponding to 0.826 g min−1.An enthalpy probe has been employed to monitor the plasma conditions before the thermal plasma etching process and from the experimental etch rate a surface rod temperature of Tsur = 2057 K has been derived. Etching has been obtained with uniformity over the entire exposed area with peak to peak differences below 1%.The plasma to rod heat transfer has been simulated using a commercial CFD code Fluent©. The model consists of a non-steady two-dimensional simulation for a compressible turbulent fluid, with an adapted grid calculation. Boundary conditions have been set out using the enthalpy probe plasma jet map. In particular for the flow, the continuity equation for chemical species, the species transport equations (Fick's Law), the momentum conservation equations, the energy equation and the turbulent kinetic energy have been solved. In the solid regions the convective energy transfer due to the rotational motion of the rod, the conduction and radiation energies have been considered. The heat transfer between the fluid and the silica walls (continuity temperature between the fluid and the solid) accounts for the energy loss due to surface radiation (qr) and vaporization (qv) obtained from experimental data.A rod surface temperature of 2057 K obtained from experimental etch rate data agrees well with the calculated temperature obtained using Fluent© (Tsim = 2010 K).