Formation of superficial alumina in aluminum sponges manufactured by sintering and dissolution process: Effect of the sinterization time

Aluminum powders were mixed with sodium chloride having particle sizes between 150-212 μm and weight fractions of aluminun (f_Al) between 0.30 and 0.55. The mixtures were compacted at a pressure (P_c) between 5 and 25 MPa and sintered at 640°C for 2, 6 or 8 h in a stream of N₂. The resulting preforms were cooled down to room temperature and sodium chloride particles were dissolved in hot water. Visual inspection of the materials allowed us to demonstrate that the f_Al and P_c are determining factors in the conservation of the final form of aluminum sponges. A relatively low P_c (5 MPa) does not allow sufficiently strong binding of the particles during the sintering process. On the other hand, samples prepared at higher f_Al above 0,50 showed a uniform metallic look while maintaining the cylindrical shape without malformations or shedding their profiles. However, it was observed a breakdown of the preform during dissolution process when lower f_Al were used. SEM micrographs of samples prepared showed, firstly, the formation of interconnected pores which indicates the formation of open-cell cellular materials. In addition, it was observed the formation of a thin layer of alumina at the surface; the quality of the alumina shape (whiskers) is improved when the sintering time is raised from 2 to 6 and 8 h. This surface could be used for the controlled growth of alumina and posterior anchorage of a catalyst.