Densification behavior of freeze-casted alumina with grain boundary segregation of impurities

• Coble’s densification model has been adopted to investigate the densification behavior of a freeze-casted Al 2 O 3 sample. • A shift of the densification mechanism from grain boundary to lattice diffusion has been elucidated at 1500 °C. • Glassy phases at grain boundary would facilitate densification when grain boundary diffusion dominates below 1500 °C. • Densification also would be facilitated by oxygen vacancies formed at grain boundary due to segregation of impurities. • Proper selection of dopants is important for densification to obtain beneficial grain boundary characteristics. Coble’s densification theory has been adopted to investigate the densification behavior of a freeze-casted Al 2 O 3 sample. The successful adoption of bulk theory determined the dominant densification mechanisms of the porous structure. We noted a shift of the densification mechanism from grain boundary to lattice diffusion at 1500 °C. Interestingly, a very low activation energy value of 72.17 kJ/mol was measured for grain boundary diffusion while a general value of 455.83 kJ/mol for lattice diffusion. This outcome was accompanied by the presence of glassy phases at the grain boundary that would facilitate densification when grain boundary diffusion dominates the overall densification kinetics. Otherwise, the concentration of oxygen vacancies was high at the grain boundaries without the glassy phases. Both resulted from the segregation of intrinsic impurities, emphasizing the need for the proper selection of dopants to determine the grain boundary characteristics and hence potential densification strategies based on sintering science.