Elucidating the microstructure evolution during hydrogen-based direct  reduction via a case study of single crystal hematite

This study examines the fundamental microstructural evolution during hydrogen-based direct reduction (HyDR) of dense single-crystal hematite (Fe₂O₃) at 700 °C, with the aim of disentangling the intrinsic reduction mechanisms from the complexities of industrial pellets. Unlike sintered polycrystalline pellets—which contain inherited porosity, grain boundaries, and gangue phases—single crystals (SCs) provide a clean model to probe reduction front morphology, phase transformations, and crystallographic effects.
Kinetics and macro-scale behaviour
Thermogravimetric analysis compared dense SCs with porous industrial pellets (~30 % porosity). Pellets reduced faster in early stages—reaching 55 % metallisation in 6 min versus 35 % for SCs—due to pre-existing pore networks enabling rapid gas transport. However, SCs approached similar final conversions (96 % vs. 91 %), with the last few percent hindered by oxide entrapment in dense Fe. For SCs, the reduction rate plateaued at ~0.1 s⁻¹ up to ~30 % metallisation, indicating overlapping phase transformations (Fe₂O₃→Fe₃O₄→Fe₁₋ₓO→α-Fe), contrasting with the sequential behaviour in pellets.
 

Crystallographic facet dependence
Partial reduction experiments (16 % and 80 %) on SCs with surfaces parallel to (0001)ₕₑₘ, (10-10)ₕₑₘ, and (12-10)ₕₑₘ planes revealed initial shrinking-core behaviour, with reduction fronts advancing ~10 % faster perpendicular to (0001) facets. At 16 % reduction, a core-shell structure was present; by 80 %, reduction was spatially uniform, with converging fronts and diagonal cracking. Microstructural analysis showed facet-dependent iron/oxide ratios and porosity: top basal planes had ~64 % Fe and 3 % porosity in reduced layers, whereas (10-10) planes had ~61 % Fe and 11 % porosity.
Hierarchical porosity and “cell” morphology
Near the hematite/magnetite interface, reduction produced a “cell-like” magnetite structure: finely nanoporous, strongly textured interiors (~1–2 µm across) enclosed by coarser, weakly textured “cell walls” with larger pores. These walls served as preferential nucleation sites for wüstite and α-Fe, which grew inward into the textured interiors. Nanopore channels (~28–35 nm diameter, ~70–80 nm spacing) extended 2–4 µm from the interface before coarsening; the bottom basal plane showed ~15–20 % larger pores, likely from altered gas flow.
Phase transformation sequence and retention
Despite thermodynamic expectations, Fe often formed adjacent to magnetite without an intervening wüstite layer, suggesting localised bypassing of the grain-model sequence. Magnetite and wüstite exhibited cube-on-cube orientation relationships, with no grain refinement during Fe₃O₄→Fe₁₋ₓO transformation. After 80 % overall reduction, ~30 % of oxide phases remained as elongated clusters trapped within Fe, impeding oxygen diffusion and slowing final reduction.
Porosity–phase correlations
Image analysis across reduced layers showed porosity peaking shortly after Fe nucleation, driven by ~41 % volume contraction in the wüstite→Fe step. Iron fraction maxima and distances from the hematite/magnetite interface varied: (10-10)/(12-10) facets initiated Fe formation closer to the interface (~10 µm) than basal facets (~20–30 µm).
Crystallographic orientation relationships
Electron backscatter diffraction revealed a dominant (112)ₘₐg//(0001)ₕₑₘ, [110]ₘₐg//[10-10]ₕₑₘ orientation (OR2), typical of porous cellular magnetite at intermediate temperatures, rather than the Shoji–Nishiyama OR common in lamellar morphologies. OR2 prevalence was consistent across facets, indicating a magnetite growth mode influenced by temperature and interface conditions.
Conclusions
HyDR of dense hematite single crystals proceeds via an initial shrinking-core regime followed by homogeneous reduction facilitated by newly formed pore networks. Reduction rates and microstructures are facet-dependent, with basal planes reducing faster and producing higher Fe fractions. The cell-like magnetite morphology, hierarchical porosity, and preferential Fe nucleation at coarse-pore walls are key microstructural motifs. Persistent oxide clusters, facet-specific pore metrics, and crystallographic OR2 dominance provide mechanistic insights relevant to dense regions in industrial pellets and natural ores, informing optimisation of pellet design and reactor conditions for efficient hydrogen-based steelmaking.