The Effect of Cryolite on the Formation of Aluminum Carbide at the Carbon Aluminum Interface

Abstract

The effect of cryolite on the formation of aluminum carbide was studied by an aluminum-carbon diffusion couple experiment. The interface between the aluminum and carbon was coated by a thin layer of cryolite, and the diffusion couple experiments were performed at 1030 °C in stagnant argon atmosphere. The micro structure of the relatively thick and porous aluminum carbide formed at the solid-liquid interface was investigated by optical and scanning electron microscopy. It was shown that the presence of cryolite has a pronounced effect on the formation of carbide, although fluorides could not be found at the interface after the experiment. The porous and needle like morphology of the carbide layer suggests the mass transport during growth is through the gas or liquid phase. The stability of cryolite and possible gas species involved in the growth process have been analyzed thermodynamically to identify possible gaseous species present at both global equilibrium and metastable conditions.