Electrical Resistivity of Aluminum Carbide at 990-1240 K

In the conventional aluminum electrolytic reduction process, a molten metal pool is contained in a carbon cell bottom. Since the standard free energy of the reaction 4A1 + 3C--> AhC:~ is about-145 kJ/mol at 1273 K (1), it is not surprising that aluminum carbide (A14C~) can form at the interface be-tween the metal and the carbon. An A14C: ~ reaction layer is thought to be responsible for a significant voltage drop, thereby reducing the energy efficiency of the reduction process (2). To gain a better understanding of the voltage drop that could be expected in this region, the electrical resistivity of A14C ~ was measured at temperatures of inter-est (990-1240 K). Several methods of preparing ALC~, including reactions of carbon with AI~O:~, AlPO4, and aluminum metal at tem-peratures up to 1770 K, were tried before one was found that produced material of sufficient purity and yield. The process finally used consisted of heating a mixture of 0.5 mm diam aluminum shot, high purity graphite, and natu-ral cryolite in a graphite boat to about 1570 K in purified argon. The temperature was then raised to about 1770 K, allowing the cryolite to volatilize. The amount of carbon used was only 97.8 % of theoretical stoichiometric quan-tity, as it was found that a slight excess of aluminum was necessary, especially when conducting the reaction in a graphite boat. When the boats were removed from the fur-nace, a dark crust covered the A14C3, which was bright orange in color. The A14C3 reaction product was analyzed for aluminum and carbon and found to contain 75.0 weight percent (w/o) A1 and 24.2 w/o C, indicating that the purity was at least 99%. X-ray diffraction analysis of the material suggested a trace of carbon impurity