Nano-PCMs for electronics cooling applications

The solid-liquid phase change process represents an interesting possibility to reject even high heat loads, especially when they are intermittent. The term Phase Change Materials (PCMs) commonly refers to those materials that use the solid-liquid phase change process to adsorb and then release heat loads. Amongst the available PCMs, paraffin waxes have been found to exhibit many desirable characteristics, such as high latent heat, low vapour pressure in the melt, they are chemically inert and stable, and non-toxic. However, they also have a very low thermal conductivity and a high volume change during the melting process. Thus, heat transfer enhancement techniques are required for their possible implementation in PCM passive electronics cooling devices, especially in case of intermittent operations. The present work aims at investigating a new challenging use of aluminium oxide (Al2O3) nanoparticles to enhance the thermal properties (thermal conductivity, specific heat, and latent heat) of pure paraffin waxes to obtain a new class of PCMs, the so-called nano-PCMs. The nano-PCMs were obtained by seeding different amounts of oxide nanoparticles in two paraffin waxes having melting temperatures of 45 and 55 °C. The thermophysical properties such as specific heat, latent heat, and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCMs. Finally, a numerical comparison between the use of pure paraffin waxes and the nano-PCMs obtained in a typical electronics passive cooling device was developed and implemented. The heat sink is an aluminium box of 40x40 mm and 50 mm high, which is filled up with the PCMs and then heated from the bottom to simulate a hot spot to be cooled. Different heat flow rates were imposed 10, 20 and 30 W; the results are presented in terms of melting time and maximum junction temperature. A numerical model is accomplished to simulate the heat transfer inside the cavity with PCM and nano-PCM. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures and stream function were reported. Moreover, a comparison with experimental results was also performed.