(526c) Formation and Mechanism Study of Micro-Sized Core-Shell Particles Via Cyclic Gas-Solid Reactions
This work reports and discusses the formation and mechanism of core-shell structured micro-particles via cyclic gas-solid reactions. Unlike most conventional core-shell formation strategies involving coating and coating-like processes, this formation strategy uses the irreversible solid-phase ionic diffusion that occurs during a gas-solid reaction cycle (e.g. reduction and oxidation of Fe) to extract the shell species from a core and shell powders mixture to the surface of micro-particle. Without the need for solvent as opposed to many conventional processes, this novel process only involves gas-solid reactions, thereby posing minimum environmental impact. To prove this proposed strategy, a micro-particle (micron-level) made of Fe2O3 and Al2O3 powder mixture is first reduced by H2 and oxidized by O2 for 50 cycles at 900 oC, where the reactions proceed mainly through the diffusion of Fe cations. SEM and EDX analyses show clearly the formation of a desired Al2O3 core - Fe2O3 shell structure. To support the proposed formation mechanism, a micro-particle made of Fe2O3 and TiO2 powders mixture go through the identical 50 cycles, where the reactions proceeds mainly through diffusion of O anions. The second sample shows no formation of core-shell structure, which highlights the dominating role of solid-phase ionic diffusion. In addition, a 2-D continuum diffusion model is applied to simulate the inter-Fe-particle bridging and directional product layer growth during an oxidation reaction, which is vital to this core-shell formation strategy.