Crack formation and self-healing behavior during the drying of alumina gels: Experimental studies

The high shrinkage of alumina gels during the drying process leads to cracks. In this work, the behavior of four alumina gel formulations was experimentally studied following two drying procedures: (i) at ambient temperature and ambient humidity, outside a convective dryer and (ii) inside the dryer, allowing the application of different conditions of temperature and humidity. During the experiments, the observations in real time showed shrinkage of gels and the formation of cracks on them due to drying. Two of the gels displayed a capacity for self-healing, requiring numerical treatment of pictures to distinguish the closure of cracks due to shrinkage from those due to self-healing. The results show the precise determination of shrinkage steps, the estimation of the “shrinkage diffusivity” and proved that the manifestation of the chemical reaction is responsible for self-healing, activated by both applied temperature conditions and residual water of gels. Additional in situ investigations at microscopic scale using an environmental scanning electronic microscope corroborate the self-healing phenomenon noticed at macroscopic scale.