Tailoring Radial Dopant Homogeneity in Silica‐Based Laser Active Media Using Variable SiO 2 Precursors in Atmospheric Pressure Plasma‐Based Nanoparticle Synthesis

ABSTRACT In this study, we created silica nanoparticle aggregates doped with Yb 2 O 3 and Al 2 O 3 in an atmospheric microwave plasma (12 kW) using two different SiO 2 ‐precursor systems: (i) tetraethoxysilane (TEOS) ethanolic solutions and (ii) octamethyl cyclotetrasiloxane (OMCTS) surfactant‐stabilized aqueous emulsions. The effect of precursor choice on the resulting dopant distribution in vitrified preforms for fiber optical applications was investigated. In such plasma‐assisted droplet conversion of silica precursors into silica, the molecular‐scale liquid‐phase homogeneity governs uniform dopant incorporation. EPMA measurements on vitrified material revealed a 20–40x higher dopant homogeneity when using TEOS‐based sol–gel precursors as compared to OMCTS‐based emulsions. Despite comparable residence times of vaporized droplets of ∼2.6 ms in the plasma plume, the two precursor systems exhibited significant differences in evaporation behaviors and thus in resulting silica condensates, as confirmed by BET, SEM analysis, and Raman spectroscopy. The fictive temperatures of the as‐obtained powder were 1300°C and 1190°C when using OMCTS and TEOS, respectively, for an estimated droplet size in terms of Sauter mean diameter of 90 and 108 µm. Thermodynamic considerations revealed that the water content in the OMCTS‐based emulsion led to a lower evaporation and produced a much less uniform dopant distribution in the vitrified silica product.