Effect of Pore Formation on Redox-Driven Phase Transformation

When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the reduction of iron oxide by hydrogen as a model system. The redox product (water) accumulates inside the pores and shifts the local equilibrium at the already reduced material back toward reoxidation into cubic ${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ (where $x$ refers to Fe deficiency, space group $Fm\overline{3}m$). This effect helps us to understand the sluggish reduction of cubic ${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ by hydrogen, a key process for future sustainable steelmaking.

Figure 1

Microstructure of a reduced single-crystalline cubic ${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ imaged by using SEM after partial reduction at $700°\mathrm{C}$ for 2 h under a ${\mathrm{H}}_2$ flux of $30\mathrm{L}/\mathrm{h}$; (a) top view and (b) cross-section view. The surface exposed to the reducing ${\mathrm{H}}_2$ environment is perpendicular to the $c$ direction.

Figure 2

(a) A representative HAADF image of the hydrogen-reduced cubic-${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ sample. Areas marked with red arrows appear to assume a faceted Wulff shape. Regions of interest for 4D STEM scans (Fig. 3) are outlined in color. The rectangular areas in orange, red, and blue are regions close to the surface, in the middle of the lamella, and away from the surface, respectively. Some channels between adjacent pores are marked by blue arrows. (b) 3D reconstruction of the ET results. The regions in royal blue color coding are the bcc-Fe matrix, while the salmon-colored ones are the pores. The reconstruction is based on 29 HAADF images (tilting from $-70°$ to 70° for every 5°). (c),(d) Magnified images of the rectangular area highlighted in blue in (a). (e) EDS mapping of the purple rectangular region in (c).

Figure 3

(a) 4D STEM reconstruction to show (i)–(iii) the virtual bright-field images (iv)–(vi) the phase maps, and (vii)–(ix) the orientation maps (i.e., inverse pole figure maps) of three regions outlined with colored frames in Fig. 2. In the phase and orientation maps, the regions with a confidence index below 0.1 are shown in black. (b) Selected diffraction patterns from points 1–6 are highlighted in magenta in the virtual bright-field images (i)–(iii). The red and green spot matrices correspond to bcc Fe and cubic ${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$, respectively.

Figure 4

(a)(i)–(a)(iv) top-to-bottom rows show the phase field simulation results for the molar fraction of oxygen and water, and the bcc-Fe phase and cubic-${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ phase fractions, respectively, at a different reduction time at $700°\mathrm{C}$. (b)(i)–(b)(iii) Contour plots of the oxygen and water content, and cubic-${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ phase evolution at different oxidation times during the cooling process from 700 to $25°\mathrm{C}$. Since the cooling time is comparatively small, the oxidation reaction rate is treated as constant. (c) Pressure change of different pores under the reduction reaction at $700°\mathrm{C}$. (d) Fractions of water vapor and the cubic-${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ phase fraction of different pores during the simulated cooling, where the initial fraction value of the cubic ${\mathrm{Fe}}_{1\text{−}x}\mathrm{O}$ is based on the final status of the reduction process, and the initial water fraction is around 15% which is the thermodynamic equilibrium value during reduction.