Seeding with ferric-oxide for enhanced transformation and microstructure development in boehmite-derived alumina

• Main Author: McArdle, James Landis
• Format: Dissertation
• Language: English
• Subjects: engineering, materials science

Enhanced transformation and microstructure development in boehmite-derived alumina gels resulted from the addition of $\alpha$-Fe$\sb2$O$\sb3$ seed particles. Boehmite ($\gamma$-ALOOH) sols were produced by dispersion of commercial powder and by hydrolysis of aluminum sec-butoxide. Single-crystal seed particles of $\alpha$-Fe$\sb2$O$\sb3$ were produced by hydrothermal aging of nitrate-derived precipitates, and were added to boehmite sols at concentrations up to 10$\sp{14}$ particles/cm$\sp3$ of dried gel ($\sim$4 wt%).
Isothermal kinetics measurements showed a 21% decrease in apparent activation energy for transformation of $\gamma$- to $\alpha$-Al$\sb2$O$\sb3$, from $\sim$600 kJ/mole (unseeded) to $\sim$474 kJ/mole (10$\sp{14}$/cm$\sp3$ seeded). Incubation periods for seeded samples were reduced by a decade compared with unseeded samples at the same temperature. Order of magnitude increase in isothermal transformation rate constants for seeded samples over unseeded materials was attributed to increased nucleation frequency in the seeded systems.
TEM of seeded gel films demonstrated that growth of $\alpha$-Al$\sb2$O$\sb3$ on $\alpha$-Fe$\sb2$O$\sb3$ occurs epitactically, with the orientation relationship $\{11\bar 20\}\sb{\rm A}{//}\{11\bar 20\}\sb{\rm F}$, (0001) $\sb{\rm A}$//(0001) $\sb{\rm F}$; Moire pattern analysis showed a rotation of less than 0.5$\sp\circ$ from exact orientation. Weak-beam imaging of dislocation arrays at the $\alpha$-Al$\sb2$O$\sb{\rm 3}$:$\alpha$-Fe$\sb2$O$\sb3$ interphase interface confirmed the semi-coherent nature of the nucleation interface.
Microstructure measurements made as a function of sintering temperature showed that seedin had no effect on coarsening and rearrangement behavior in transition alumina microstructures. Fully transformed seeded materials retained significantly greater surface area (22 m$\sp2$/g) compared with unseeded samples ($<$10 m$\sp2$/g) for similar transformation rates. Seeded gel samples (10$\sp{14}$/cm$\sp3$) sintered to full density after 120 min at $<$1250$\sp\circ$C, and unseeded materials achieved no better than 80% density up to 1300$\sp\circ$C.
Microscopy of bulk sintered materials revealed development of submicron, uniform microstructures in seeded gels, and the characteristic vermicular grain morphology in unseeded materials. TEM showed that vermicular grain colonies in unseeded gel films were single crystals, with few high-angle grain boundaries evident in fully transformed regions. TEM also showed regions of enhanced densification where transformation growth fronts with different crystallographic orientations impinge; the effect was attributed to formation of high-angle grain boundaries along the line of impingement.