Microstructure and phase evolution of Fe-20Ni-20W foams during high-temperature redox cycling

Freeze-cast Fe-20Ni-20 W (at%) foams suitable for high-temperature redox cycling show excellent microstructural stability and regenerative porosity formation, but they exhibit slower reduction kinetics as compared to previously studied Fe-25 W foams. The Fe-20Ni-20 W lamellar foams, after initial hydrogen reduction, consist of a two-phase mixture of µ-Fe7W6 and γ-Fe(Ni,W), with significant microporosity due to the sintering inhibition of W. During oxidation by steam at 800 °C, these phases are oxidized to a three-phase mixture of (Fe,Ni)WO4, Fe3O4, and γ-Ni(Fe); upon subsequent reduction by H2, the foams return to their initial composition. The chemical vapor transport reduction of FeWO4 results in the formation of submicron pores during each reduction half-cycle which accelerate subsequent reaction and limit sintering in the reduced state. The reduction of mixed oxide is relatively sluggish, which is likely due to the increased stability of the (Fe,Ni)WO4 phase brought on by the substitution of Ni in Fe sites in FeWO4.