In situ structural studies of alumina during melting and freezing

Modern levitation furnaces are enabling melting and freezing of refractory materials like Al2O3 to be studied in depth with synchrotron radiation techniques. Whilst α-Al2O3 is a close packed Debye-like solid, liquid Al2O3 has smaller co-ordination numbers and the structure has networklike characteristics. Under contactless conditions, substantial under cooling can be achieved. Melting involves a significant decrease in density which is approximately recovered on recrystallisation, both of which can be followed with high speed video imaging. Freezing occurs with a burst of energy – recalescence – which substantially raises the temperature momentarily. Using Rietveld refinement the density of α-Al2O3 and the mean square displacement of the average atom can be followed up to the melting point and upon freezing using in situ X-ray diffraction. As melting is approached for α-Al2O3 exceeds the harmonic approximation of the Lindemann-Galvarry law and for liquid Al2O3 above the melting point appears to be greater still. On freezing from the undercooled state α-Al2O3 is initially decompressed with the TP point falling on the melting curve extrapolated to negative pressures.