Al motion rates in levitated, molten Al2O3 samples, measured by pulsed gradient spin echo 27Al NMR

Strong attenuation of 27Al NMR signals has been observed in a magnetic field gradient, in beads of alumina melted by a CO2 laser and levitated in flowing Ar. The standard NMR diffusivity measurement interpretation of the data leads to very large (∼1.5 × 10−7 m2/s) effective diffusivity values, approximately independent of temperature between melting and 2500 °C. These high values and their weak temperature dependence contrast sharply with estimates of ionic diffusivity arising from the known, activated temperature dependences of molten alumina's viscosity and 27Al NMR line width. Our anomalously large diffusivities are attributed to bulk displacive motions, which can be visually observed in the levitated molten beads and can lead to substantial Al displacements over experimental NMR time scales. Possible origins of these motions are briefly discussed.