The Influence of Crucible Material on Aluminum Composition

The freezing point of aluminum is one of the ITS-90 defining fixed points widely used for thermometer calibration. However, long-term investigations have revealed slow temperature depressions of the aluminum freezing temperature and alterations of the metal structure that are probably due to metallic contamination caused by the partial dissolution of crucible material. The objective of this work was to study the iInteraction of liquid aluminum with graphite and boron nitride in order to select a nonreactive material. Two crucibles made of high purity graphite and boron nitride were filled with Al of 6N5 purity; then, 40 melting-freezing cycles were carried out. The total time of contact between aluminum in the liquid phase and the crucible was 240 h. After that, the composition of Al, graphite, and boron nitride was studied and the results were compared with the analysis of the initial samples. The Al ingot in contact with boron nitride was found to be contaminated by boron.