TRACER DIFFUSION OF OXYGEN IN COO-SIO2 MELTS

Tracer diffusion of oxygen in CoO-SiO2 silicate melts has been studied in the composition range of 0.3 less than or equal to X(SiO2), less t han or equal to 0.45 (X = mole fraction) at 1450 to 1550 degrees C. Ex periments were carried out using a capillary-reservoir method, involvi ng a specially designed capillary to avoid convection effects. O-18 di ffusion profiles were obtained from secondary-ion mass spectrometric a nalysis. Effective diffusivities, D-O,eff(), were obtained by fitting these profiles to the appropriate solutions of Fick's second law. The effective diffusivity of oxygen was found to be smaller than that of cobalt, but larger than that of silicon. The composition dependence of the activation energies for oxygen diffusion, E(A), is significantly different from those of silicon or cobalt: the variation of E(A) With the composition was less in the case of oxygen. Experimentally determi ned diffusion profiles have been compared to those of computer simulat ions that are based on a kinetic model. The agreement between the shap es of the experimental and simulated concentration profiles, as well a s the similarity in the composition dependence of experimental and sim ulated D-O,D-eff, is evidence for the strong tendency towards polycon densation in cobalt silicate melts.