Clathrate hydrate of tetra n-butyl ammonium fluoride (TBAF) is a promising thermal storage material, because this clathrate can be produced from TBAF solutions at temperatures between 0 and 27 °C under the atmospheric pressure. In this study, melting characteristics of the TBAF clathrate hydrate were investigated by using a differential scanning calorimeter (DSC). Four types of endothermic peaks were observed in DSC curves during melting, when the TBAF clathrate was produced from TBAF solutions at initial concentrations between 10 and 55 wt%. One of these peaks cannot be explained by the existing phase diagram. This endothermic peak suggests an unknown crystal structure of TBAF clathrate that coexists stably with TBAF solutions at a concentration higher than 50 wt%. X-ray diffraction data of TBAF clathrate also supported the existence of this unknown crystal structure. Enthalpy changes corresponding to the four types of endothermic peaks in DSC curves were evaluated. The results showed that the TBAF clathrate can be used as a thermal storage material at 29 °C, which is nearly equal to the congruent melting point of TBAF clathrate.
