Preparation of paraffin/expanded vermiculite with enhanced thermal conductivity by implanting network carbon in vermiculite layers

Paraffin/expanded vermiculite with modified porous carbon layer was prepared in the study. In the composite phase change materials (PCM), expanded vermiculite was impregnated with sucrose in the layer and then carbonated in situ in order to enhance thermal conductivity and improve the adsorption capacity of expanded vermiculite, which acted as a carrier material in the preparation of the form-stable PCM. SEM images showed the morphology of expanded vermiculite had been changed after carbonation. A series of micro-pores had been formed among the layers of expanded vermiculite with the diameter ranging from several microns to 120 μm. Thermal properties of the composite PCM were determined by differential scanning calorimeter (DSC) analysis. According to DSC analysis results, when the paraffin adsorption reached 53.2 wt.%, latent heats of the composite PCM were respectively 101.14 J/g at the freezing temperature of 48.85 °C and 103 J/g at the melting temperature of 53.1 °C. TGA results showed that the form-stable composite PCM presented good thermal stability. Due to the formation of carbon structure in the layer of expanded vermiculite, the thermal conductivity of the supporting materials was improved, thus slightly decreasing the extrapolated onset melting temperature of pure paraffin. Furthermore, the results of FT-IR analysis and thermal cycling tests showed that the form-stable composite PCM had good chemical stability and thermal reliability after 200 melting/freezing cycles. The form-stable composite PCM with good thermal properties, thermal reliability, and chemical stability is the promising PCM for the low-temperature thermal energy storage applications.