Tailored rheological behavior of mullite and BSAS suspensions using a cationic polyelectrolyte

The effects of a cationic polyelectrolyte, polyethylenimine, on the rheological behavior of aqueous mullite and celsian phase, barium strontium aluminosilicate (BSAS) suspensions have been studied to optimize a dip-coating process. The surface of the ceramic particles was characterized in aqueous media using zeta potential measurements. The ionization behavior and hydrodynamic radius of polyethylenimine in aqueous media was characterized by potentiometric titration and dynamic light scattering measurements, respectively. The rheological behavior of concentrated ceramic suspensions containing polyethylenimine was characterized by stress viscometry and elastic modulus measurements. Polyethylenimine imparts repulsive, electrosteric interactions between mullite and BSAS particles in aqueous suspension, resulting in shear thinning flow behavior, linear elastic moduli, and yield stress values that are minimized for suspensions with a critical PEI concentration of 0.2 mg PEI per m² of mullite and 0.4 mg PEI per m² of BSAS, respectively. The elastic moduli and yield stress of mullite and BSAS suspensions were nearly identical at equivalent fractions of their critical PEI concentration. Uniform coatings were obtained upon dipping SiC, Si₃N₄, and Si-metal substrates into shear-thinning suspensions containing PEI at 81-88% of the critical concentration. Dense mullite coatings were demonstrated on SiC substrates after sintering at 1400°C.