Preparation of paraffin/porous TiO2 foams with enhanced thermal conductivity as PCM, by covering the TiO2 surface with a carbon layer

In the present study, porous TiO2 foams (PTF) were prepared by particle-stabilized emulsion method. The samples having solid contents ranging from 20 wt.% to 35 wt.% were prepared and successfully impregnated with paraffin without using a surfactant. SEM images showed that the prepared PTFs had three-dimensional interpenetrating structures with good porosities. The pore structure and morphology of PTF was markedly influenced by the solid content. With increase in solid content, porosity decreased, along with a decrease in the homologous encapsulation ratio of paraffin. Differential scanning calorimetry (DSC) evaluated the thermal properties of the paraffin/PTF (PTFP) composites. DSC results showed that with a solid content of 20 wt.%, the paraffin adsorption reached 62 vol.% and latent heat of the composite PCM was 94.05 J/g after 200 melting/freezing cycles. TGA results showed that the form-stable composite PCMs had good thermal stabilities. In order to further enhance the thermal conductivity and improve the adsorption capacity of PTF, sucrose was added to the emulsion and carbonized in situ to form carbon/PTF (PTFC). This could also be successfully impregnated with paraffin to obtain the form-stable composite paraffin/PTFC (PTFCP). The TEM images and results of XPS analysis confirmed that the surfaces of TiO2 particles were covered with a 2 nm thick carbon film (C/TiO2). The thermal conductivity of PTFCP increased from 0.302 W/m K to 1.059 W/m K compared to PTFP, along with a further improvement in adsorption capacity. Furthermore, the results of FT-IR analysis and thermal cyclic tests showed that the form-stable composite PCMs had good chemical stability and thermal reliability after 200 melting/freezing cycles. Therefore, the prepared form-stable composite PCMs having excellent pore structures, thermal properties, thermal reliabilities, and chemical stabilities are promising PCM candidates for heat energy storage applications.