Reducing the Sintering Temperature for MgO–Al ₂ O ₃ Mixtures by Addition of Cryolite (Na ₃ AlF ₆ )

The preparation of near stoichiometric spinel and alumina‐rich spinel composites from Al 2 O 3 and MgO powders with the addition of Na 3 AlF 6 up to 4 wt% in the temperature range 700°–1600°C was studied; 98 wt% spinel containing 72 wt% Al 2 O 3 can be produced from the mixture of 72 wt% (50 at.%) Al 2 O 3 + 28 wt% (50 at.%) MgO powders with the addition of 1 wt% Na 3 AlF 6 fired at 1300°C for 1 h. Spinels containing 81–85 wt% Al 2 O 3 can be produced from either the mixture of 90 wt% (78 at.%) Al 2 O 3 + 10 wt% (22 at.%) MgO or the mixture of 95 wt% (88 at.%) Al 2 O 3 + 5 wt% (12 at.%) MgO powders with the addition of 4 wt% Na 3 AlF 6 in the temperature range 1300°–1600°C by using a torch‐flame firing for 3 min, followed by quenching in water, while the same system under slow cooling in a furnace results in spinel containing 74–76 wt% Al 2 O 3 . Microscopic studies indicate that the alumina‐rich spinel composites consist of a continuous majority spinel phase and an isolated minority corundum phase, regardless of slow cooling in a furnace or quenching in water.