Effect of Modified Vermiculite on the Interface of a Capric Acid-expanded Vermiculite Composite Phase Change Material with Phase Transition Kinetics

Abstract

A new type of capric acid (CA)-acid expanded vermiculite (AEV) composite phase change material (PCM) with improved adsorption ability and interface adhesive strength was developed. Through the analysis of non-isothermal phase transition kinetics, modified vermiculite was observed to change and affect the phase transformation mechanism of the composite. AEV was treated with hydrochloric acid to improve the specific surface area and micro-pore structure. The surface area measured by BET increased from 81.94 m2/g for expanded vermiculite (EV) to 544.13 m2/g for AEV. CA-EV and CA-AEV composite PCMs were prepared by direct impregnation. The non-isothermal phase transition isotherms of CA-EV and CA-AEV were recorded by DSC at different heating rates (1, 5, 10, 15, and 20 ℃/min), which indicated that the phase transition rate increased with the heating rate and the phase transition process changed. Kinetics parameters were analyzed by a double extrapolation method. The activation energy (E) under the original state (Eα→0) of CA-AEV and CA-EV was 1 117 kJ/mol and 937 kJ/mol, respectively, and 1 205 kJ/mol and 1 016 kJ/mol under the thermal equilibrium state (Eβ→0). The most probabilistic mechanism function of CA-AEV satisfied G(α)=α2/3, which followed the Mample special rule, and the function of CA-EV satisfied G(α)=[(1+α)(1/3)−1]2, which followed the anti-Jander function.