Formation mechanism of n-octadecane @ calcium fluoride microencapsulated phase change material

Microencapsulating phase change materials (PCMs) with CaF2 as shell could obtain not only novel microencapsulated PCMs (MPCMs) but also potential luminescence materials with self-thermal regulation property. Prior to the preparation of good luminescent MPCMs with doped CaF2 as shell, the formation mechanism of the MPCMs should be well understood. In this work, n-octadecane (ODE) was selected as core to prepare CaF2 MPCMs and the formation mechanism was comprehensively investigated. The results revealed that the concentrations of the Ca2+ and the F− in the solution, which could be adjusted by the coordination equilibrium of Ca-citrate complex and the hydrolysis balance of NaBF4, played key roles in formation of the MPCMs. Moreover, the formation of the MPCMs was composed of five periods, including: formation of emulsion; formation of Ca-citrate intermediate microcapsules; induction of CaF2 shell; growth and aggregation of CaF2 shell and overgrowth of CaF2. Meanwhile, the results also revealed that prolonging the overgrowth period of CaF2 had significant effects on the morphology and properties of the prepared MPCMs. In addition, with the increase of the reaction temperature, the nucleation, growth and aggregation of CaF2 crystals could be accelerated, and the formation and growth periods of the CaF2 shell were shortened. This work is not only significant for the preparation and optimization of MPCMs with CaF2 as shell, but also can guide and promote the development of luminescent MPCMs.