Macro-encapsulation of metallic phase change materials with sacrificial density less than 3.5 % for medium-temperature heat storage

Abstract

To solve the problems of volume thermal expansion and high chemical activity of metallic phase change materials (PCMs), this paper successfully prepared millimeter/centimeter-size durable Bi@Void@Kaolin macrocapsules. Fortunately, carbon powder clusters from the thermal decomposition of adhesive CMC gathered on the core PCMs, cleverly adding a reducing atmosphere into the capsules. In particular, the buffer voids of macrocapsules were created from the pores and gaps between the accumulated Bi powders, and the method of improving the bulk density of powder cores as core precursors by the cold isostatic pressing (CIP) technology solves the problem of excessive cavity volume. Thermal analysis indicates that after CIP treatment, the heat storage density of the Bi@Kaolin macrocapsule with the unpressurized powder core volume accounting for 50% can be increased by 18.4%–275.0 J/cm3, which is about 2.5 times the heat storage density of the common medium-temperature heat storage materials - concrete and NaCl, under the working temperature range of 240–300 °C. Solid-liquid PCMs need to sacrifice their density in exchange for the necessary cavity volume, therefore the “actual volumetric heat storage density” of PCMs should be specified. The results show that the proportion of sacrificial density of Bi PCM (i.e. the cavity rate of Bi-based macrocapsule) can reach the optimal value of 3.5%, its actual volumetric heat storage density can reach 558.9 J/cm3 at 240–300 °C. Surprisingly, all the Bi@Kaolin macrocapsules showed excellent cycling durability and thermal stability during 500 cycles under air atmosphere and subsequent 24 h overload-temperature oxidation tests.