Freeze Casting of Aqueous PAA-Stabilized Carbon Nanotube-Al2O3 Suspensions

Freeze casting is a colloidal processing technique that shows great promise for development of nanostructured materials. A ceramic nanopowder is dispersed with a polymer in water, under carefully controlled pH. The suspension is cast into a suitable mold and frozen, then de-molded and exposed to a vacuum to sublimate and remove the water. Polymer adsorption and rheology were studied to optimize and characterize a colloidal suspension of a 38 nm Al2O3 powder. The dispersant, dispersant amount, pH and solids loading were examined to determine the best conditions for freeze casting. Based on adsorption and viscosity data, optimal conditions for freeze casting were found with Poly(acrylic acid) (PAA) dispersant, at 2.00 wt% (of Al2O3), pH of 9.5, and a solids loading of 40 vol%. Carbon nanotubes (CNTs) were added to that suspension in increments of 0.14, 0.28, 0.53, 1.30 and 2.60 vol%. The viscosity increased dramatically upon addition of 1.30 vol% CNTs. The colloidal CNT-Al2O3 suspension was successfully freeze cast and the microstructure showed a very smooth fracture surface. It was determined that upon resting, the suspension undergoes a physical change which must be completed to obtain advantageous microstructure. Freeze cast Al2O3 discs with and without CNTs were measured using a concentric ring test, with strengths on the order of one MPa. The freeze cast sample was successfully debinded, but the heating profile attempted was not effective in obtaining full density.