Key Processes in the Formation of Hollow SiO2 Particle in Plasma-Electrolytic Spraying

Plasma-electrolyte spraying of an aqueous solution of liquid glass allows one to obtain hollow microspheres of silicon dioxide (SiO2) without organic templates and core removal steps. The aim of the work is to describe the key thermal and chemical processes in the zone of a pulsed microarc discharge and to determine the parameter window in which stable hollow shells are formed. It is shown that, at pulses of 300–700 V (τ = 200 μs, f = 100 Hz), local heating and the formation of a gas bubble lead to intense evaporation of water and an increase in the concentration of silicate complexes in the surface layer of the microdroplet. At the same time, active particles (OH•, H•, H2O2, etc.) form in the plasma–liquid region [14, 15, 32–35], which may be accompanied by a local decrease in alkalinity and acceleration of the transition of soluble silicates into solid SiO2. A shell is formed at the “steam–solution” boundary, after which water vapor and H2, which arises during plasmolysis of water, expand the internal cavity. In a series of experiments with varying voltage, silicate concentration, and processing time, the maximum proportion of intact hollow spheres was achieved at U = 500 V and C = 100 g/L: according to scanning electron microscopy estimates, 70–85% of the particles have a pronounced cavity, an outer diameter of 4–6 μm and a wall thickness of about 0.08–0.15 μm. A stage model is proposed that relates the transition between solid spheres, destroyed shells, and stable hollow particles with competition of SiO2 deposition rates and gas formation.