The extremely high temperature at which alumina -- a white crystalline powder used in the production of glasses, ceramics, abrasives and aluminum -- melts (2054°C) has meant that our understanding of the properties of alumina in its liquid form has been limited by our inability to study it at such temperatures.

Until now, the use of a container within a conventional furnace was precluded by the high reactivity of the alumina with the vessel containing it.

This discovery by Professor Neville Greaves and materials scientists in the Department of Physics at UWA was achieved using an aerodynamic levitator furnace to heat the alumina and to levitate it in a neutron beam.

Use of a special computer simulation procedure that directly predicts the experimental results then enabled Professor Greaves and his colleagues to undertake the first neutron diffraction study of molten alumina in contactless conditions and to obtain the first reliable structural model of liquid alumina.

The studies have shown that, in liquid alumina, each aluminum atom has just 4 neighboring oxygens, and each oxygen just 3 aluminums. This is totally different from the structure of crystalline alumina that furnaces are made from.

Moreover, the precise distribution of the different atomic configurations in the liquid state may explain the enormous (30%) drop in density (from 3.8 g/cm 3 to 2.8 g/cm 3) when alumina melts. The change in structure and the resultant change in density between the liquid and crystalline solid helps to explain the shrinkage seen when alumina ceramics are fabricated.

The study was made possible by a collaboration with colleagues at CRMHT, Orleans, France, who developed the levitator furnace and by using the SANDALS neutron diffractometer at the Rutherford Appleton Laboratory, Oxfordshire.

This discovery of the structure of alumina in its molten state will enable materials scientists to begin to understand the properties of this and other high temperature materials and to incorporate that knowledge in the development of ceramics and glasses for the future.

Alumina usually constitutes only 1-3% of most commercial glasses used for glasses, bottles or windows, but may form up to 20% of ceramics and glasses for specialist purposes.

Alumina has a very high melting point (2054°C), hence materials containing high levels of alumina have the capability to withstand high temperatures and to resist thermal shock. As a result, alumina is used to line furnaces and tunnel kilns and to provide the backing lining within ladles used by ceramics makers and also for the production of advanced ceramics.

(Reference: Physical Review Letters, Volume 86, Number 21, May 2001.)


[Contact: Professor Neville Greaves, Arthur Dafis]

05-Jun-2001