Lowering hexacelsian sintering temperature with additives: solid or liquid phase mechanisms

The sintering temperature of an oxide/oxide composite, comprising a barium aluminosilicate (BaAl2Si2O8 or BAS) matrix reinforced with alumina fibers, is constrained to avoid a decrease in the mechanical properties of the fibers. This paper highlights the potential of sintering additives in lowering the sintering temperature of the hexagonal BAS (hexacelsian), thereby safeguarding the integrity of the fibers. Consequently, the role of sintering additives, MgO, TiO2, and B2O3, is investigated to determine their impact on the BAS matrix sintering. The addition of 1 wt% of these additives successfully achieved this objective, as confirmed through thermal and microstructural analyses. MgO and TiO2 facilitated sintering kinetics through a solid-phase mechanism. Concerning the addition of B2O3, its effect on BAS sintering accelerated densification through a transient liquid-phase mechanism. Another consequence of incorporating these sintering additives is the destabilization of the hexagonal BAS. To limit this phenomenon, the addition of 1 wt% Rb2CO3 served as a stabilizer through steric hindrance, partially substituting the Ba element in the BAS matrix with Rb.