Rotation-coalescence of confined particles in Ni1–xO/NiAl2O4 composites

The Ni1–xO/NiAl2O4 powders of specified molar ratios (1:9, 32:1, 69:1, designated as N1S9, N32S1, N69S1) were sintered and annealed at 1873 K for 1–80 h and studied by transmission electron microscopy with regard to the orientation change of the intragranular particles. It was found that the N1S9 and N32S1 specimens contains only intergranular particles; whereas the N69S1 specimen contains both inter- and intragranular particles in the late stage of sintering. In N69S1 specimen, the intergranular spinel particles, either isolated or slightly coalesced as sausage-like chains, were able to detach from the Ni1–xO grain boundaries and became parallel-epitaxial with respect to the host Ni1–xO grains upon annealing. The orientation change can be ascribed to Brownian rotation and coalescence of the particles, and the rotation of the particles mostly proceeds over non-specific contact planes due to the dragging effect in the coalescence process. The epitaxial spinel particles could possibly be preserved in the Ni1–xO grains when the intergranular particles were coalesced to a larger size to suppress grain boundary separation.