Discussion on magnetic-induced polarization Ampere's force by in situ observing the special particle growth of alumina during microwave sintering

Internal microstructure evolution during alumina microwave sintering was in situ investigated with synchrotron radiation computed tomography (SR-CT). Two special microstructure evolution phenomena were continuously observed from the experimental images of the sample at different sintering times throughout the entire process of the sintering, which we called “suppressed particle growth” and “particle homogenization”. These two special phenomena were further confirmed by the two curves of “average particle radius” and “particle radius standard deviation” versus sintering time which were directly extracted from the full-field SR-CT results. A polarization Ampere's force model was proposed to provide a possible explanation for these special phenomena, which introduced the effect of magnetic field on insulating ceramic materials, a topic rarely discussed in previous studies. The polarization Ampere's force model may explain these two special sintering phenomena observed in the in situ experiment. On one hand, ceramic particles may sustain “Ampere's force” that pointed toward the particle center according to this model, thereby possibly leading to the special suppressed-particle-growth phenomenon; on the other hand, large particles may sustain a strong force in our model, which may explain the other special phenomenon of particle homogenization. In return, these two special phenomena can also serve as probable experimental evidence of our polarization Ampere's force model. This study may offer some help for revealing the complex mechanisms during microwave sintering and for preparing materials with expected microstructure and excellent properties.