The microstructure studies also revealed that the fracture surfaces
showed a layered texture, but not the expected clear grain boundary for
ceramic materials. This texture was supposed due to the fracture across
the grain rather than the grain boundary. The formation of layered
texture was attributable to the ZnO grain growth preferring along
specific direction b002N [2] and the formed liquid ZnAl2O4 preventing
the ZnO grain growth through diffusion across this liquid barrier. The
anisotropic growth of the ZnO grains was also observed in the ZnO–
V2O5 ceramics due to the early formation of liquid phase V2O5 [20] and
in the growth of ZnO film with addition of Al and Ce elements [21].
Fig. 3 shows the resistivity changes with Al2O3 content at room
temperature. A minimum electrical resistivity was observed at an Al2O3
addition level of 0.25% for samples sintered at both 1400 and 1300 °C.
A similar observation has also been reported for ZnO with addition of
NiO [18]. The resistivities were weakly affected by the Al2O3 content
variation until 3% for samples sintered at both temperatures. However,
sintering at higher temperature resulted in a lower resistivity at the same
Al2O3 content.