Phase transformations during plasma synthesis of sodium polyaluminates

The synthesis of sodium beta-alumina with the use of a low-temperature nitrogen plasma is described. The major plasma synthesis product is the sodium polyaluminate Na 2 O· n Al 2 O 3 with an unidentified structure and n =3.5–9.5. The phase transformations of the plasma synthesis products are studied during heating in the range 600–1400°C. The phase with the rhombohedral beta-alumina structure (β″-Al 2 O 3 ) is shown to form in several steps, through metastable sodium polyaluminates. The results are analyzed in relation to the phase equilibria in the Na 2 O-Al 2 O 3 system.