A kinetic study of the I to II phase transition in ammonium chloride, ammonium bromide and ammonium iodide

• Published in: Journal of non-crystalline solids Vol. 451; pp. 161 - 167
• Main Author: Clark, S.M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-11-2016
• Subjects: Activation energy; Ammonium bromides; Ammonium chlorides; Ammounium halides; Differential scanning calorimetry; Iodides; Kinetics; Phase transformations; Powder diffraction; Shape factor; Synchrotron radiation; Transformations; Differential scanning calorimetry; Powder diffraction; Kinetics; Ammounium halides; Activation energy; Synchrotron radiation
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/j.jnoncrysol.2016.06.008

A kinetic study of phase transition between phases I and II in ammonium chloride and ammonium bromide has been carried out using time-resolved powder diffraction (TRPD) and differential scanning calorimetry (DSC). The same transformation in ammonium iodide has been studied using DSC only. Reaction onset temperatures for NH4Cl, NH4Br and NH4I of about: 458K, 412K, and 253K and enthalpies of transformation of: 4.4(2) kJmol−1, 3.3(1) kJmol−1 and 2.6(5) kJmol−1 were determined. The Avrami equation adequately described the shape of the degree of reaction curve for all three materials. For NH4Cl and NH4Br, the Avrami shape factor was equal to two. Taken in conjunction with previous microscopic studies, this implies very rapid nucleation followed by two-dimensional growth of the new phase. The value of the shape factor for NH4I switched from 2 to 4 about 55s after the commencement of the transformation. Values of 50(20) kJmol−1 and 20(10) kJmol−1 were obtained for the activation energy before and after the switch. Activation energies for NH4Cl and NH4Br were found to be 400(80) kJmol−1 and 170(80) kJmol−1 from the DSC data. We conclude that for all three ammonium halides this transformation probably has the same mechanism.
•A kinetic study of the phase I to phase II transformation in ammonium chloride, bromide and iodide has been undertaken.•Non-isothermal data were collected using time resolved powder diffraction and differential scanning calorimetry.•The data were found to be well described by the Avrami equation.•Values for the Activation Energy, Change in Configurational Entropy and Enathalpy were determined for each compound.•It was concluded that the mechanism of transformation is the same for each compound in the series.