Experimental investigation of stability and thermal conductivity of phase change materials containing pristine and functionalized multi-walled carbon nanotubes

The biggest challenge in dispersion of nanoparticles in phase change materials (PCMs) is the physical stability of these particles in PCM. Numerous studies have evaluated the effect of different parameters on the stability of nanoparticles in PCM, but the effect of PCM polarity has rarely been investigated. In this study, the stability of functionalized and pristine multi-walled carbon nanotubes (MWCNTs) in three different PCMs with various polarity levels was investigated. The utilized PCMs were paraffin wax (nonpolar), stearic acid (partially polar), and polyethylene glycol (polar). Two different methods of functionalization of MWCNTs, with stearic acid and hexadecyl amine, were used to compare their stability in PCMs. The FTIR analysis and FESEM images reveal that the surface modification reactions were done thoroughly and MWCNTs are well dispersed in PCM. The results showed that pristine MWCNT is more stable in nonpolar PCMs (paraffin and stearic acid), while the samples containing functionalized MWCNT or surfactant had higher stability in polyethylene glycol. The three most stable samples were used to measure their thermal conductivity and heat release/storage capability. The addition of nanoparticles to all kinds of PCMs led to higher rate of heat storage and release. Moreover, the thermal conductivity of all PCMs improved by the introduction of nanoparticles. In both liquid and solid phases, nanoenhanced PCM had higher thermal conductivity compared to pure PCM with stearic acid containing 1 mass% of pristine MWCNT having the highest rate of enhancement at 16.82% for solid phase.