Ultra-low-temperature sintering of TiO2 via grain boundary diffusion enabled by nanosecond laser irradiation

Sintering of metal oxide ceramics typically requires high temperatures to achieve densification; however, excessive heat often leads to grain coarsening and phase instability. In this study, nanosecond (ns) laser irradiation is employed for the first time as a pre-treatment step of TiO2 nanoparticle to lower the sintering temperature by tailoring the microstructure at the nanoscale. During ns laser exposure, the localized high-energy input lowers the activation energy for dislocation nucleation, thereby increasing dislocation density. Subsequently, with optimized exposure duration, heat accumulation induces localized annealing, which facilitates dislocation annihilation and initiates in situ recrystallization during irradiation. This process leads to the formation of new nanoscale grains within individual nanoparticles prior to sintering. During subsequent furnace sintering at low temperature (750 °C), these laser-induced nanograins remain stable and serve as diffusion-active pathways, promoting a transition from surface diffusion to grain boundary diffusion, as confirmed by diffusion coefficient analysis. This mechanism enhances densification, reduces porosity, and improves relative density. At elevated temperatures (∼1050 °C), extreme annealing destabilizes the laser-induced nanoscale grains, effectively suppressing grain boundary-mediated diffusion. Overall, the findings demonstrate that grain boundary diffusion can drive densification at low temperatures, bypassing the conventional grain growth typically associated with ceramic sintering.