Max-Planck-Institut für Nachhaltige Materialien GmbH
The kinetics of the reduction of hematite to iron using hydrogen is limited by the outbound diffusion of oxygen/water during the reduction of wüstite. Controlled fracture by inter-pellet collision at the nano- and microscale could release trapped species and enhance the metallization kinetics. However, for this approach to be optimized, a thorough understanding of the mechanical properties and active deformation mechanisms of the primary iron oxide feedstock, i.e. Fe2O3, at small length-scales are necessary. The primary goal of this study is to gain insights into the nanomechanical characteristics of Fe2O3 using model single crystals. To achieve this, nanoindentation experiments were conducted at ambient temperature on the (0001) basal and (0-110) prismatic surface orientations of Fe2O3 utilizing Berkovich and spherical indentation tip geometries. The elastic modulus (E) and hardness (H) were measured for both orientations by continuous stiffness measurement method. Results with the Berkovich geometry revealed that while H was comparable (~15.8 GPa) for both orientations, E decreased from 273 ± 3 GPa in the (0001) plane to 245 ± 4 GPa in the (0-110) plane. Additionally, only the (0001) surface orientation displayed a discernible "pop-in" behavior in the load-displacement curve for both indenter geometries, attributed to incipient plasticity caused by dislocation nucleation or multiplication. Deformation in the plastic zone around the indent was characterized using electron backscattered diffraction (EBSD) and electron channeling contrast imaging (ECCI) to identify the dislocation orientation and active slip systems on both (0001) and (0-110) surface orientations. Rhombohedral twinning was also observed on the (0-110) plane, which was subsequently confirmed through EBSD. The coexistence of both slip and twinning as active deformation mechanism at room temperature in Fe2O3 indicates that the fracture of hematite, and at hematite-containing interfaces, may be complex at small length-scales, and is the target of current investigations.
Abstract
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