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

Abstract

This article describes investigations on the phase development (T ≤ 1500 °C) of phosphate-bonded Al 2 O 3 -MgAl 2 O 4 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.% P 2 O 5 . The binding mechanisms of these phosphate-bonded ceramics could be extensively deduced by analyses of phase developments during hardening, strength development, and hightemperature phase formation at T ≤ 1500 °C: The formation of a network of aluminium 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 °C, 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 polymerisation) 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 °C, these Ca, Mg and Zr phosphates begin to react with the ceramic material to form aluminium phosphates. The phosphate bond has a permanent effect until formation of the ceramic bond (T > 1000 °C). High-temperature phases are usually crystalline PO 4 sintered structures which are formed by reaction with MgAl 2 O 4 .

Effect of the P/Al Molar Ratio and Heating Rate on the Composi-Tion of Alumino-Phosphate Binders

“…Those binders are generally prepared by heat-treating alumino-phosphate solutions that are obtained by dissolving Al 2 O 3 or Al(OH) 3 in diluted phosphoric acid solutions. Such solutions, characterised by their P/Al molar ratio, their phosphorus concentration [P], and the presence of additives, have also been widely investigated [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. By heating, the alumino-phosphate species of the solutions react to form hydrated alumino-phosphate compounds (Al(H 2 PO 4 ) 3 , × H 2 O and AlH 2 P 3 O 10 , × H 2 O) around 150–350 °C.…”

“…The presence of Al(PO 3 ) 3 [A] is beneficial for the development of binders because this compound is stable and only melts at temperatures higher than 1200 °C. Several studies have been performed on solutions with a P/Al molar ratio of 3 because they present a similar P/Al ratio than the final Al(PO 3 ) 3 [A] phase [ 1 , 2 , 3 , 4 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Solutions with a P/Al ratio lower than 3 produce binders composed of Al(PO 3 ) 3 but also AlPO 4 compounds [ 3 , 4 ].…”

“…However, during heating, orthophosphoric acid (H 3 PO 4 ) will progressively condensate to H 4 P 2 O 7 , HPO 3 , and P 2 O 5 to finally evolve from the materials at higher temperatures. In all the previously mentioned studies, simple or complex thermal treatments were used [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ] but many papers only referred to the final curing temperature [ 1 , 4 , 11 , 12 , 14 ] with no real consideration for the effect of the heating rate used to reach this temperature. When the heating rate is mentioned [ 2 , 3 , 7 , 8 , 9 , 13 ], this parameter does not seem to be optimised, although it can be considered a key parameter.…”

“…The X-ray diffraction (XRD) technique has been the technique of choice for the investigation of these systems [ 1 , 2 , 3 , 9 , 12 , 13 ]. However, the efficiency of solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy has been clearly demonstrated in recent papers [ 10 , 11 , 14 ]. Indeed, 31 P and 27 Al NMR were able to confirm and complete the data obtained by XRD.…”

“…In addition, a qualitative analysis of the materials can be quickly performed using the chemical shift values observed for the main alumino-phosphate phases in previous studies ( Table 2 ) [ 10 , 15 , 16 ]. Quantification can also be quickly and efficiently achieved through the decomposition of the quantitative 31 P NMR spectra [ 10 , 11 ].…”