Formulation and thermophysical characterization of oleic acid-functionalized nano-ZnO composite phase change materials

Efficient thermal energy storage materials are crucial for advancing low-temperature waste heat recovery and thermal management systems. This study investigates the formulation of a nanocomposite by dispersing functionalized ZnO nanoparticles within a lauric acid phase-change material (PCM) to enhance its thermal performance. ZnO nanoparticles are synthesized via microwave-assisted precipitation, offering a low-temperature, time-efficient alternative to conventional furnace synthesis, and phase purity is confirmed by X-ray diffraction. Unlike traditional dispersion techniques that introduce ZnO into molten PCM via surfactants, this work employs a surface-functionalization strategy whereby ZnO nanoparticles are functionalized with oleic acid prior to dispersion in LA to improve homogeneity. The study aims to distinguish the effects of free oleic acid addition from those of prior oleic acid functionalization by examining five formulations: Lauric Acid, lauric acid + oleic acid, lauric acid + ZnO, lauric acid + oleic acid + ZnO, and lauric acid + oleic acid-functionalized ZnO. All ZnO-containing composites are prepared with a nominal 4 wt% ZnO-based additive loading; in the functionalized system, this comprises a ZnO core (∼2.68 wt%) and grafted oleic acid (∼1.32 wt%), as determined by thermogravimetric analysis. Functionalization is carried out at pH 9 in aqueous and methanol media and is confirmed by Fourier transform infrared spectroscopy and thermogravimetric analysis. Field-emission scanning electron microscopy confirms the nanoscale morphology of both pristine and functionalized ZnO. Their thermal behavior is evaluated through differential scanning calorimetry, heat transfer experiments, and thermal conductivity measurements. The results reveal a reduction in melting temperature and latent heat upon additive incorporation. Oleic acid-functionalized ZnO reduces the latent heat by about 25%, while retaining useful heat-storage capacity and improving the composite's measured thermophysical response. Thermal conductivity enhancements of 31% and 25% are observed for non-functionalized and functionalized nanocomposites, respectively. Although the oleic acid-functionalized ZnO composite exhibits lower latent heat and slightly lower thermal conductivity than the non-functionalized composite, it delivers the highest volumetric heat capacity and the lowest operating temperature under forced-convection heat-transfer testing, with this improved performance achieved despite a lower effective ZnO content, indicating improved nanoparticle utilization. These findings further suggest that ZnO functionalization improves particle-matrix compatibility and enhances overall device-level thermal regulation.