Abstract
Evaporation kinetics of fused silica were measured up to ≈3000K using CO(2) laser heating, while solid-gas phase chemistry of silica was assessed with hydrogen, air, and nitrogen. Enhanced evaporation in hydrogen was attributed to an additional reduction pathway, while oxidizing conditions pushed the reaction backwards. The observed mass transport limitations supported use of a near-equilibrium analysis for interpreting kinetic data. A semi-empirical model of the evaporation kinetics is derived that accounts for heating, gas chemistry and transport properties. The approach described should have application to materials laser processing, and in applications requiring knowledge of thermal decomposition chemistry under extreme temperatures.
Highlights
Silica plays a critical role in many industrial applications such as raw material in refractory linings, fiber optics, optical substrates and, in general, as a component in devices requiring inertness and toughness
The dependence of the evaporation kinetics on temperature and gas flow rate were derived from measurements of the surface temperature and shape profiles of the silica pits formed when exposed to continuous laser heating and controlled gas flow
The accuracy of this approach to derive R depends on the assumption that the depth at a particular location is the result of only the evaporation process, and not the result of flow of molten silica or material expulsion from explosive boiling [28]
Summary
Silica plays a critical role in many industrial applications such as raw material in refractory linings, fiber optics, optical substrates and, in general, as a component in devices requiring inertness and toughness. To determine the effect of a broad range of functional chemistries, air, hydrogennitrogen mixture, and pure nitrogen gases were selected to relate evaporation kinetics to an oxidizing, reducing, and inert atmosphere, respectively For these chemistries, the dependence of the evaporation kinetics on temperature and gas flow rate were derived from measurements of the surface temperature and shape profiles of the silica pits formed when exposed to continuous laser heating and controlled gas flow. Classical treatments of laser-based evaporation model evaporation kinetics based on the velocity distribution of escaped species within the Knudsen layer close to a hot surface [22] This type of analysis does not account for the specific chemical reactions that occur from a reacting gas, or any shift in the equilibrium of the evaporation reactions from the presence of a gas phase product. The analysis of the evaporation rate data described here provides a means to probe the role of chemical reaction thermodynamics and transport kinetics at the extreme temperatures reached during laser heating of materials in general
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