Structural and microstructural transitions during phase separation and crystallization of Al2O3-SiO2-P2O5-Y2O3 glass

A highly refractory glass in the system Al₂O₃-SiO₂-P₂O₅-Y₂O₃ has been designed and produced such that, upon heating, an essentially fully crystalline glass—ceramic evolves containing mullite (nominally 3Al₂O₃·2SiO₂) and xenotime (YPO₄) as the final principal phases. Phase separation in this glass occurred during cooling from the melt and continued during annealing. XPS of the Al 2p, P 2p and the Y 3d electrons revealed that the average chemical environment of each of these elements is measurably different in the annealed glass and in the completely crystallized material. This indicates that the compositions of the separated glass phases are very different from those of the crystal phases which form from them. Additional rearrangement of the glass structure was observed at 1173 K. Extensive formation of mullite was initially detected at 1223 K and was followed by the crystallization of xenotime and the transient compounds of Y₄Al₂O₉, Y₂Si₂O₇, AlPO₄ and YP₅O₁₄. The optimum crystal nucleation and crystallization temperatures of 1173 and 1473 K, respectively, were determined from DTA, XRD, SEM and TEM studies.