(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.