Phase development of phosphate‐bonded Al2O3‐MgAl2O4 high‐temperature ceramics: XRD and solid‐state NMR investigations

Abstract

AbstractThis article describes investigations on the phase development (T ≤ 1500℃) of phosphate‐bonded Al2O3‐MgAl2O4 high‐temperature ceramics by a combination of solid‐state MAS NMR and X‐ray diffraction analyses. The ceramic body was bonded with inorganic hydrogen orthophosphates (Al, Mg, Ca, Zr) with a total binder content of 3 wt.% P2O5. The binding mechanisms of these phosphate‐bonded ceramics could be extensively deduced by analyses of phase developments during hardening, strength development, and high‐temperature phase formation at T ≤ 1500℃: The formation of a network of aluminum phosphate compounds by acid and condensation reactions of the phosphate with the ceramic body is proven to be the active binding process. The binding process is initiated by the formation of active phosphoric acid phosphate phases, which in principle prerequisites water solubility of the binder. Thermal treatment, especially at T ≤ 600℃, promotes the degree of P‐cross‐linking of the phosphate structures. In addition to these phosphate‐ceramic reactions, it was also possible to identify parallel pure phosphate–phosphate conversions of the initial phosphates (via condensation and polymerization) without reactions with ceramic components. The contribution of these structures to the binding effect can be estimated to be minor. Only at temperatures of T ≥ 600℃, these Ca, Mg and Zr phosphates begin to react with the ceramic material to form aluminum phosphates. The phosphate bond has a permanent effect until the formation of the ceramic bond (T > 1000℃). High‐temperature phases are usually crystalline PO4‐sintered structures which are formed by reaction with MgAl2O4.