Abstract
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 coordination numbers and the structure has network-like characteristics. Under contactless conditions, substantial under cooling can be achieved. Melting involves a significant decrease in density which is approximately recovered on recrystallization, 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 〈μ2〉 can be followed up to the melting point and upon freezing using in situ X-ray diffraction. As melting is approached 〈μ2〉 for α-Al2O3 exceeds the harmonic approximation of the Lindemann–Galvarry law and for liquid Al2O3 above the melting point 〈μ2〉 appears to be greater still. On freezing from the undercooled state α-Al2O3 is initially decompressed with the T - P point falling on the melting curve extrapolated to negative pressures.
Published Version
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