Thermal and Crystallization Properties of Carbon-nanotube Doped Phase Change Materials for Variable-load Heat Exchanger

Phase change materials (PCMs) are emerging as efficient energy management materials for various applications ranging from building materials to heat exchangers. Paraffin waxes are versatile PCMs with attractive benefits such as high latent heat, chemical inertness, low vapor pressure, and good thermal stability. Additionally, wax-based PCMs can be emulsified, providing enhanced surface area for efficient heat transfer. Nevertheless, a significant drawback of paraffin waxes is low thermal conductivity, making heat storage and release slow. To address this limitation, carbon nanotubes (CNTs), which have high thermal conductivity, are proposed as dopants in paraffin waxes. The PCMs need to go through many heating and cooling cycles (>1000) in PCM heat exchangers. The sedimentation/aggregation of CNTs can impact the dispersant of CNTs due to repeated heating and cooling cycles in the PCM heat exchanger. The dispersion of CNTs in PCMs changes the thermal conductivity and could change the phase transition properties of PCMs. The doping could also decrease the melting and increase the onset of crystallization. Here, doped elements work as seeding for crystallization. The dispersion characteristic of CNTs after several thermal cycles is unknown. Not much fundamental understanding exists on the nucleation and crystallization behavior of these advanced PCMs. We propose to exploit the effect of thermal cycles on the dispersion of doped CNTs in commercial paraffin wax. We synthesize CNT-doped PCM (paraffin wax) in bulk and microcapsules forms with desired thermal conductivity. The dispersion pattern of doped CNTs varies in microcapsules compared to bulk samples. We take two approaches to characterize the phase transition properties due to the thermal cycles of CNT-doped paraffin wax: 1) a bulk phase measurement of the melting and crystallization points and heat of fusion of CNT-doped wax samples using DSC (differential scanning calorimetry), and 2) a parallel microfluidic technology and microscopy methods to directly visualize the stochastic nature of the onset of crystallization and melting of microcapsules of CNT-doped paraffin wax due to thermal cycles. This fundamental investigation of crystallization and thermal properties of CNT-based PCMs broadly impacts the technology and engineering in areas as diverse as building materials, electronic cooling devices, and heat exchangers.