Effect of aluminum on Na5P3O10 (Form-II→Form-I) thermal transformation

The presence of Al3+, as an impurity, on the temperature of the polymorphic transformation of pentasodium tripolyphosphate Na5P3O10-II (low-temperature Form-II) into Na5P3O10-I (high-temperature Form-I) has been investigated using X-ray powder diffraction, IR spectroscopy, and scanning electron microscopy methods. The effect of Al-3(+) on the phase transformation was recorded between ignition temperatures of 350 and 450 degreesC. Al-free powders, ignited up to 450 degreesC, did not contain Form-1, while powders containing 0.5 or 1.0 wt % of Al were mixtures of Form-I/Form-II indicating that the presence of Al acted in favor of Form-I formation. Phase composition at temperatures of ignition higher than 500 degreesC was not dependent on Al presence. Na5P3O10 in all of the products ignited at 500-550 degreesC, whether Al-free or containing Al, was in Form-1. The presence of Al caused lower crystallinity and inhibited the grain growth of pentasodium tripolyphosphate while powder was ignited. Both phenomena are believed to act in favor of phase transformation at lower ignition temperatures.