Development and investigation of salt based composite phase change material containing diatomite as skeleton substance by cold sintering technology for medium and high temperature thermal energy storage

This work concerns the development of a nitrate salt based form-stable composite phase change material by cold sintering technology with the employment of diatomite as skeleton substance. Such fabrication strategy has never before been reported in the literature and we demonstrate for the first time the feasibility of salt-diatomite composite production through a cold sintering process by means of sodium hydroxide solution as sintering additive. The microstructural characteristics and development mechanism, chemical and physical compatibility, mechanical strength, phase change behaviour, thermal and shape stability of the composite are investigated to reveal the structure formation and densification mechanism of salt and diatomite. The results show that the composite could be sintered at a temperature of 180 °C, far lower than that required in the traditional hot sintering approach. Due to the congruent solubility of the salt and diatomite in aqueous alkali, a rigid and dense structure induced by a so-called dissolution-precipitation process could be generated in the composite, which provides solution to accommodate the salt, endowing the composite with splendid capacity to maintain the structure stability and restrain the liquid leakage over the phase change process. An excellent chemical and physical compatibility has been achieved between the salt and other components, and over 60 wt% salt could be encapsulated in the composite by which a melting temperature around 278 °C and an energy storage density over 650 kJ/kg at a temperature range of 50–600 °C are attained. The results also reveal that the composite exhibits preeminent mechanical strength and excellent thermal cycling performance. For a given cycling temperature range of 50–500 °C, a stable macroscopic shape with a compressive strength over 210 MPa could be achieved in the composite even after 100 times of thermal cycles.