Composition-Dependent Structural Integrity of Hf6Ta2O17 Superstructure during Sintering in a Reducing Atmosphere

The applicability of a Hf6Ta2O17 superstructure to a sintering crucible in pyroprocessing was evaluated herein. Samples were prepared by simple oxidation of casted Hf-Ta alloys containing different Ta contents (20.2, 27.2, and 35.2 in at. %). The physical integrity of the oxidized samples was tested after passivation, which resulted in the formation of a Hf6Ta2O17 superstructure. As a result, only the sample with the lowest Ta content (20.2 at. %) was intact while others were destroyed by peeling off. Based on the XRD analysis, this originates from reduction of the Ta oxide phase from Ta2O5 to Ta2O2.2, which may lead to severe stresses due to changes in the crystal structure and lattice constant. It is therefore concluded that the Ta content should be lowered within a range permitting the formation of Hf6Ta2O17. This provides practical data for a beneficial composition design that can be extended to other applications prepared under reducing atmospheres.