The diffusion of Co in alumina was carried out at 1403 to 1873K in air, with and without the presence of a spinel phase. The penetration profiles were measured by using the secondary-ion mass spectrometry technique or (at the highest temperature) by performing tracer measurements. The results (table 42, figure 19) obtained using both methods could be described by:

D (m2/s) = 6.2 x 10-9 exp[-333(kJ/mol)/RT]

Formation of the spinel, Al2CoO4, was observed when the experiments were carried out below 1673K. This offered the useful possibility of calibrating the secondary ion mass spectrometry signal, and the solubility (metal atom fraction) of Co in alumina between 1403 and 1623K was found to range from 0.002 to 0.004.

Solubility and Diffusion of Cobalt in Alumina. E.G.Gontier-Moya, G.Erdélyi, F.Moya, K.Freitag: Philosophical Magazine A, 2001, 81[11], 2665-73