Functional phase change composites with highly efficient electrical to thermal energy conversion

Abstract

Electrothermal functional phase change materials (PCMs) have poor mechanical properties and low thermal conductivities (k). To address this problem, a new electrothermal functional composite PCM, denoted PEG2000-CaCl2/CNTs, was synthesized in one step by ligand substitution using a polyethylene glycol with a molecular weight of 2000 (PEG2000) as the PCM, carbon nanotubes (CNTs) as a k- and electrical conductivity (σ)-enhancing framework material and Cl−1 as the ligand. The PEG2000-CaCl2/CNTs composite PCM was characterized in situ by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. The experimental results demonstrated the following. The coordinate covalent bonds in the PEG2000-CaCl2/CNTs composite PCM enhanced the mechanical performance of the material. The compressive strength of the composite PCM sample with 20 wt% CNTs exhibited excellent compressive strength at 80 °C. Adding CNTs to the PEG2000 PCM at a ratio of 20 wt% increased k by 252% and reduced the electrical resistivity from 9500 to 90 Ω∙m. The energy stored in the composite PCM could be triggered and released under relatively low voltages (1.5–2.0 V). This result could significantly reduce energy consumption. Additionally, after 100 melt and crystallization cycles, the DSC curve of the composite PCM changed by less than 3.0%. After 50 electrical-to-thermal energy conversion cycles, the heat storage/release curves of the composite PCM changed by less than 5.0%. The synthesized functional composite PCM exhibits excellent σ, k and thermostability and exceptional mechanical properties, and it opens up new applications for PCMs.