Thermal Conductivity of Phase Changing Materials Doped with Carbon Nanotubes

Abstract

Phase changing materials (PCM) can accumulate and release a great quantity of energy at changing the temperature in result of the phase transition. This permits one to use PCMs as a basis of thermal accumulators storing the thermal energy at elevated temperatures and releasing it at lowering the temperature below the phase transition point. Worldwide usage of PCMs in building technology and other fields is hindered by a rather low thermal conductivity coefficient of the most PCMs which makes the response of the relevant thermal accumulators too slow and limits the possibilities of application of such devices. This drawback can be overcame through doping a PCM with particles having high thermal conduction coefficient. The present article contains results of experimental and theoretical investigations of heat propagation in PCM doped with carbon nanotubes the thermal conduction coefficient of which exceeds that of the most PCM by 4 – 5 orders of magnitude. Paraffin П-2 have been used as PCM. The experiments performed demonstrate a 2 – 3 times enhancement of thermal conductivity and 16 orders of magnitude enhancement of the electric conductivity because of doping paraffin with 10% multi-walled nanotubes. The propagation of both heat and electric current has a percolation character, so that nanotubes form conductive paths at exceeding some concentration of the dopant. The heat propagation process was modelled through the solution of the non-stationary heat conduction equation with taking into account the sorption of heat due to the phase transition. The calculations performed for composite materials with the varied thermal characteristics of a material imply that the characteristic heating time is proportional to the value of the melting enthalpy and inversely proportional to the value of the heat conduction coefficient.