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

Abstract:

Aluminum powders were mixed with sodium chloride having particle sizes between 150-212 μm and weight fractions of aluminun (fAl) between 0.30 and 0.55. The mixtures were compacted at a pressure (Pc) 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 fAl and Pc are determining factors in the conservation of the final form of aluminum sponges. A relatively low Pc (5 MPa) does not allow sufficiently strong binding of the particles during the sintering process. On the other hand, samples prepared at higher fAl 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 fAl 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.