Nanoparticle Colloidal Suspension Optimization and Freeze-cast Forming

Nanoparticle suspension and forming are important areas. In this paper, the stability and rheology of A1203 nanoparticle suspensions at different dispersant concentration, suspension pH, and solids loading were studied. The most desirable suspension conditions were 7.5-9.5 for pH and 2.00-2.25 wt% of A1203 for poly(acry1ic acid) (PAA) dispersant. 45.0 vol% A1203 solids loading can be achieved while maintaining good suspension flow for freeze casting. The maximum solids loading of the A1203 nanoparticle suspension was predicted to be 50.7 ~01%. The preliminary results of the freeze-cast sample showed that suspension pre-rest before freezing was critical for achieving defect free microstructures. INTRODUCTION To overcome agglomeration and low packing density issues, colloidal processing is the preferred approach for nanoparticles. The unique ionic properties of water allow addition of ions to overcome the problematic particle-particle attraction; this ion-related stabilizing mechanism is called electrostatic stabilization. Another approach is to add a polymer dispersant to the suspension; polymer chains adsorb onto the particle surfaces and extend into water, and physically repel one another; this method of stabilization is called steric stabilization.' More effectively, an ionic polymer dispersant and the ionic properties of water can be used simultaneously to obtain a well-dispersed suspension; this method is called electrosteric stabilization.2 A delicate balance must be maintained in the pH and the dispersant concentration. Inappropriate pH will result in particle attraction, not rep~lsion.~ The adsorbed polymer must be thick enough to prevent close particle contact and counteract van der Waals forces. Too little polymer will cause bridging flocculation; too much polymer will cause depletion f l o c ~ u l a t i o n ~ ~ ~ ~ ~ ~ ~ Ideally, the adsorbed polymer layer should be just thick enough to prevent van der Waals bonding. Since nanoparticles have much larger specific surface area than micron size particles, more polymer molecules will be adsorbed to cover the nanoparticle surface of the same mass; this will correspondingly reduce the solids loading of the nanoparticles even with ideal polymer layer thickness. .Viscosity measures the ability of the solid particles to flow relative to one another. A decrease in the particle size leads to suspensions of high viscosity because the effective solids loading is increased in the nanoparticle suspensions. The effective solids loading increase occurs because the particle diameter decrease creates large electrical double layer or adsorbed polymer layer volume that accounts as part of the effective solids loading. For high solids loading, the polymer concentration must be controlled with a much higher precision for the nanoparticle suspension than for the conventional suspensions.* The suspension must flow well into the mold and fill in the complex details or cavities of the mold. Intuitively, the higher the solid loading, 1 Synthesis and Processing of Nanostructured Materials William M. Mullins Copyright