Fig. 2(a)-(c) show the fracture SEM images of the pure and doped ZnO samples sintered at 1150 °C. The result shows that a porous sample with significant grain growth can be achieved by conventional sintering HPT powders. For the doped samples, some fine particles with sizes varying from 100 nm to 300 nm were observed to precipitate mostly along the gain boundaries. It reveals that Al reacted with ZnO to form the ZnAl2O4 phase at grain boundaries due to the limited solubility [18], and the amount of ZnAl2O4increases with increasing content of Al dopant which is in accordance with the results of XRD, as shown in Fig. 2(b) and (c).