Nitrate salt-halloysite nanotube (HNT) composite phase change materials for thermal energy storage : The feasibility of material fabrication by using HNT as skeleton substance and its thermal properties

This work concerns the fabrication of a salt based form-stable phase change composite by employing halloysite nanotube (HNT) as ceramic supporting material (CSM). The HNT has been broadly reported for preparation of low-temperature organic composites but very little has been done in medium and high temperature thermal energy storage fields. Herein we demonstrate for the first time with the use of HNT as skeleton substance for the fabrication of salt composite by cold compression and hot sintering technology. The feasibility of material fabrication is confirmed by evaluating a case containing a salt of sodium nitrate as phase change material and HNT as CSM. The effect of preheating treatment of HNT on the microstructure and phase change behaviours as well as cycling performance of the composite is investigated. The results show that HNT can be wetted by the liquid nitrate salt at elevated temperatures, verifying the viability of the present strategy by using HNT as SSM for the synthesis of salt composite. A stable chemical structure is achieved in the salt-HNT composite, which confirms a preeminent chemical stability and compatibility between the HNT and nitrate salt. The hollow tubular structure of HNT could not only be able to provide extra surface for accommodating liquid salt to form a dense composite, but also strengthen the thermal stability of the nitrate salt. The preheating treatment of HNT at 300 °C and 500 °C respectively causes the occurrence of dehydration and dehydroxylation process, which in turn leads to the morphology and microstructure change of HNT, and hence affects the cycling performance of the composite. In comparison with the composite made from HNT preheated at 500 °C where a serious pulverization and deformation has been observed after 100 thermal cycles, the composite containing HNT preheated at 300 °C presents more stable structure and excellent cycling performance, revealing that the HNT is only suitable for the accommodation of inorganic salt that having application temperature limit less than 500 °C