Preparation and Thermal Performances of 1-Alkylammonium Tetrachlorocobaltate Eutectoid Mixtures as Phase Change Materials

The thermotropic phase-transition compounds 1-alkylammonium tetrachlorocobaltate (1-CnH2n+1NH3)2CoCl4 (n=10, 18) and a series of their binary mixtures were prepared by solution reflux at 353 K in ethanol solutions. Binary-mixture (1-C10H21NH3)2CoCl4-(1-C18H37NH3)2CoCl4 systems were characterized over the entire composition range using differential scanning calorimetry and X-ray diffraction. The phase diagram constructed from the experiments indicated one stable intermediate phase, (1-C10H21NH3)-(1-C18H37NH3)CoCl4, at a mass fraction wC10Co=52.51%, two invariant three-phase equilibria, and two eutectoid temperatures, which are assigned to e1 at (347±1) K, for the eutectoid point with wC10Co=38.50%, and e2 at (343±1) K, for the eutectoid point with wC10Co=69.86%. These three clear solid-solution ranges are α-phase on the left, β-phase on the right, and γ-phase in the middle of the phase diagram. The (1-CnH2n+1NH3)2CoCl4 and their binary-mixture systems as phase change materials have phase-transition temperatures in the range 340-370 K, and transition enthalpies in the range 2.13 and 141.12 J∙g-1, between two polymorphic crystal forms.