The thermodynamics and kinetics of the dehydration of CsH_(2)PO_(4) studied in the presence of SiO_2

Abstract

The thermodynamic and kinetics of the dehydration and hydration of CsH_(2)PO_(4) is investigated by thermogravimetric, differential scanning calorimetry and X-ray diffraction analysis in the temperature range of 200 to 400 °C, water partial pressure range of 0.06 to 0.90 atm. SiO_(2) powder was added to CsH_(2)PO_(4) to accelerate both dehydration and hydration. When dehydrated in the presence of SiO_(2), CsH_(2)PO_(4) has a fine microstructure (~200 nm feature size), and no reaction between CsH2PO4 and SiO_(2) is observed. By making use of the enhanced kinetics afforded by SiO_(2), the phase boundary between CsH_(2)PO_(4), CsPO_(3) and dehydrated liquid was precisely determined. The triple point connecting these phases is located at pH2O = 0.35 ± 0.2 atm and T = 267.5 ± 1.0 °C. The stability of CsH2PO4 and the liquid dehydrate, CsH_(2(1−x))PO_(4−x)(l), were confirmed by the complete reversal of dehydration to recover these phases in the appropriate temperature and water partial pressure ranges. Rehydration and conversion of CsPO3(s) to CsH2PO4(s) occur over a period of several hours, depending on temperature, water partial pressure, and morphology of the metaphosphate. High pH2O and small particles favor rapid dehydration, whereas the temperature dependence of the rehydration kinetics is non-monotonic, reaching its fastest rate in the vicinity of the superprotonic transition.

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