Effect of Surface Passivation in Spinel Slurry Toward Hydrolysis: Neutron Scattering and Rheological Studies.

Aqueous colloidal forming of magnesium aluminate (MgAl2O4) spinel offers much potential for various applications; however, these advantages are generally offset by the basic nature of the powder and its affinity for hydrolysis. Hydrolysis in the presence of water generally imparts surface chemical changes resulting in the degradation of colloidal stability. In the present study, spinel powders were subjected to thermally assisted surface passivation and evaluated for the effectiveness of preventing hydrolysis through quasielastic neutron scattering (QENS) technique and correlated with rheological measurements. In order to evaluate the extent of hydrolysis, spinel slurries prepared with (SP) and without surface passivation (WSP) were studied by rheological and QENS measurements at regular intervals of time. While WSP slurry exhibited a steep enhancement in viscosity from 1.02 to 19.4 Pa · s and fraction of the elastic intensity from 0.20 to 0.38 for 96 and 200 hours, respectively, a negligible change in viscosity for SP slurries from 0.313 to 0.345 Pa · s and fraction of the elastic intensity from 0.16 to 0.17 for the similar period confirmed the inhibition of hydrolysis, revealing change in surface chemistry due to hydrolysis. Microscopic details as obtained from neutron scattering data revealed that dynamical behavior of water molecules in both the slurries could be described very well by the Singwi–Sjolander model of jump diffusion. Further analysis showed lower diffusivity ∼1.82 × 10−5 cm2/sec and higher residence time ∼6.39 ps for WSP slurry in comparison with 2.16 × 10−5 cm2/sec and 5.80 ps, complimenting the inhibition of hydrolysis in case of SP slurry. [ABSTRACT FROM AUTHOR]