Abstract

Introduction

Molten aluminum is very reactive and it reduces many oxide ceramics. In order to prevent and/or expect the reduction reaction of oxides by molten aluminum, an “Ellingham diagram” offers crucial guide and is usually used for a selection and modification of the refractory materials. According to the Ellingham diagram, TiO2 should be reduced and, MgO and Y2O3 should not be reduced by molten aluminum. In an actual reaction of the oxide ceramics with molten aluminum, a reaction interface between them is very important and should influence the reaction. Here we report our results of the reaction of TiO2, Y2Ti2O7, Y2O3, and MgO ceramics with molten aluminum.

Material and Methods

Raw materials used were TiO2, Y2O3 and MgO powders of high purity and they were CIPed into disks of 10mm diameter by 100MPa and then fired at 1500C for 2h to obtain dense ceramics. The relative densities of the TiO2, Y2O3 and MgO ceramics were 85, 94 and 93%, respectively. Y2Ti2O7 ceramic was fabricated by mixing TiO2 and Y2O3 in a ball mill, and CIPed, then fired at 1500C 2h. The resultant Y2Ti2O7 ceramic was 83% of its theoretical density. The fired ceramic was set in an alumina tube. Aluminum rod of 99% pure was set on the ceramic in the alumina tube. Then it was heated at 1000C for 25h in N2 gas flowing atmosphere. After the reaction experiment, it was cut and polished for microstructural observation using SEM and EDX. Another sample after the reaction was treated with aqueous NaOH solution to remove aluminum and the sediment was analyzed.

Results and Conclusions

Among the four oxide ceramics tested, TiO2 and Y2Ti2O7 ones were not reacted severely. It is attributed to a formation of corundum layer at the interface between the ceramics and molten aluminum. The thickness of the corundum layer formed on the TiO2 ceramic was only 1-2mm. Obviously the corundum layer was the result of the reduction of Ti4+ by aluminum and the layer prevented further reduction of TiO2 and Y2Ti2O7 ceramics.

Y2O3 ceramic was reacted with molten aluminum to form intermetallic compound of YAl3, which was detected in the precipitate after aqueous NaOH treatment. The remains of the ceramic consisted of YAlO3 and Y3Al5O12, and aluminum also existed inside the remains. It means that Y3+ ion was reduced by molten aluminum to form YAl3, which melted at the experiment temperature of 1000C. This is not expected from the Ellingham diagram, because the diagram does not include any information on intermetallics, such as YAl3.

MgO ceramic reacted with molten aluminum and spinel layer was formed at the interface. The thermodynamic calculation, 4MgO + 2Al → MgAl2O4 + 3Mg, is 66kJ at 1298K. In order to realize a reason of the formation of spinel, a formation of melt consisted of aluminum and magnesium should be a key.