Thermal conductivity enhancement in organic phase change material (phenol-water system) upon addition of Al2O3, SiO2 and TiO2 nano-inclusions

Abstract

In spite of the immense industrial benefits of phase change materials (PCM) in energy conservation and management, the practical applicability of organic PCM is severely limited due to its low thermal conductivity. We report enhanced thermal conductivity during the first order liquid-solid phase transition in, phenol-water system, loaded with four different nano-inclusions, viz., α-Al2O3, SiO2, hydrophobic SiO2 and TiO2. Nano-inclusion loaded PCM showed a large thermal conductivity enhancement, which was attributed to the formation of nano-crystalline needle like microstructure in the PCM, during freezing, which squeezes the nano-inclusions towards the grain boundaries, thereby forming a quasi 2D network of high thermal conductivity percolation pathways. In the solid state, thermal conductivity enhancement is found to increase with concentration of the nano-inclusions due to the formation of larger well dispersed aggregates. Atomic force microscopy images, show an increased contact area for closely packed larger aggregates at higher concentration of the nano-inclusions. Our results show lower thermal conductivity enhancement for the PCM loaded with hydrophobic silica, which is attributed to the higher interfacial resistance of the hydrophobic interfaces. Addition of carbon black nano powder to the nano-inclusion loaded PCM showed a synergistic increase in thermal conductivity due to the fractal aggregates of carbon black that act as volume filling agents and reduce the inter-aggregate gaps, thereby enhancing the conductive pathways. Our studies show the nano-inclusion assisted thermal conductivity enhancement in phenol-water system as a PCM with tunable phase transition temperature for room temperature thermal energy storage applications.