Carbon nanotubes prevent the coagulation at high shear rates of aqueous suspensions of equiaxed ceramic nanoparticles

Equiaxed ceramic nanoparticles and their mixtures are expected to exhibit shear-thinning behaviour when dispersed colloidally in aqueous media, whereas shear-thickening is the expectation for large aspect ratio phases such as, for example, carbon nanotubes (CNTs). Here, contrary experimental evidence is presented demonstrating the occurrence of severe coagulation at high shear rates in colloidally stable, semi-concentrated, aqueous suspensions of equiaxed SiC nanoparticles (major phase) mixed with equiaxed Y3Al5O12 nanoparticles (liquid-phase sintering additive), and how CNT addition prevents this coagulation if sufficient sonication is applied. It is also shown that although shear-thinning is the natural behaviour of the ceramic suspension up to moderate shear rates, coagulation is eventually a phenomenon inherent to the aqueous colloidal processing of these suspensions, with the critical shear rate for coagulation increasing and the rheopexy decreasing the better is the initial dispersion state achieved with the sonication. It is also shown that the critical shear rate for coagulation depends on the exact condition of shear rate increase, and that the re-sheared suspensions coagulate more significantly and at lower shear rates than the fresh suspensions. The mechanisms by which this coagulation occurs and is impeded by the CNTs are discussed, together with broader implications of these phenomena for the environmentally friendly processing of nanostructured ceramics and ceramic composites.