Rheology of alumina suspensions subjected to alternating current electric fields for freeze‐casting

Abstract Alternating current (AC) electric field can extrinsically tune freeze‐cast microstructure, originating from field‐induced increase in viscosity of ceramic suspensions. However, the changes that occur in a ceramic suspension and rheological behavior, ultimately affecting freeze‐cast microstructure, are not well understood. Moreover, the effects of AC electrokinetic forces and temperature on viscosity need to be decoupled. The viscosity and temperature of ceramic suspensions subjected to AC field and direct heating were measured, revealing that the increase in viscosity is due to AC dielectrophoretic forces rather than field‐induced heating of suspension. The shear thinning behavior of suspensions characterized using a power‐law model reveals that the greater the viscosity, the greater the shear thinning. Although visual inspections and very high viscosity of suspensions indicated strong flocculation, in situ x‐ray imaging pinpointed the field time window during which strong flocculation occurred. With increasing field duration, freeze‐cast microstructure became increasingly dendritic; however, it was absent at higher field durations. For a short field duration, weak flocculation did not affect viscosity but enhanced the propensity for particle entrapment within ice crystals, resulting in a dendritic freeze‐cast morphology. Longer field durations resulted in a strongly flocculated state with a strong network of particles, preventing freeze‐cast microstructure formation.