A synthetic hydrated form, AlPO4·1.5H2O is also known.

To elucidate the segregation process of aluminum phosphate and the microstructural changes caused by alumina crystallization, the surface and cross-sectional structures of heat-treated films before and after acid treatment were evaluated using SEM. 30 The microstructure after heating reflects the segregated morphology of aluminum phosphate, while the microstructure after acid treatment reflects the framework of alumina and the segregated morphology characteristic of aluminum phosphate. Figure 5.10A–E show the surface and cross-sectional SEM images of the heat-treated films. Figure 5.10F–J show the surface and cross-sectional SEM images of the heat- and acid-treated films. The particles observed after heat treatment were completely removed by acid treatment, and these particles were identified as aluminum phosphate crystals. Based on the membrane surface, aluminum phosphate detaches from cell boundary ridges formed after the first anodization (black arrows in Fig. 5.10A), and morphological changes are observed on the cell wall (depressed slopes, white arrows in Fig. 5.10A) Due to the segregation of aluminum phosphate the total heat increases as the heating rate decreases (top panels of Fig. 5.10A-D). Meanwhile, for the film heat-treated at 1400 °C for 4 h, significant segregation of aluminum phosphate was observed, and the pores were completely covered by aluminum phosphate crystals.