Phase transition of RbH2PO4 and its composite with SiO2 studied by thermal analysis

The phase transition at T _p (~109 °C) of RbH₂PO₄ and its composite with SiO₂ has been investigated by thermal analysis here. In the case of neat RbH₂PO₄, there is a linear relationship between endothermic peak temperature (T _m) and square root of heating rate (Φ ^1/2), from which the onset temperature of phase transition can be determined. Besides, Kissinger method and another calculation method were employed to obtain the activation energy of phase transition. The detailed deduction process was presented in this paper, and the estimated activation energies are E ₁ ≈ 126.3 kJ/mol and E ₂ ≈ 129.2 kJ/mol, respectively. On the other hand, the heterogeneous doping of RbH₂PO₄ with SiO₂ as dopant facilitates its proton conduction and leads to the disappearance of jump in conductivity at T _p. The heats of transition in the composites decrease gradually with increasing the molar fraction of SiO₂ additives. In the cooling process, a new and broad exothermic peak appeared between ~95 and ~110 °C, and its intensity also changes with the SiO₂ amount. These phenomena might be related to the formation of amorphous phase of RbH₂PO₄ on the surface of SiO₂ particles due to the strong interface interaction.