The Experimental Exploration of Embedding Phase Change Materials With Graphite Nanofibers for the Thermal Management of Electronics

Phase change materials PCMs are materials that undergo aphase transformation, typically the solid-liquid phase transforma-tion, at a temperature within the operating range of the thermalapplication. The latent heat absorption inherent in the phasechange process results in the maintenance of a constant operatingtemperature during the melt process. In transient applications,PCMs can thus be used to absorb heat and maintain operation at aspecified temperature. PCMs have been shown to be effective intransient thermal abatement by slowing the rate of temperatureincrease during transient operation 1 .While basic PCM systems have proven to be effective in lowvolume applications 2–12 , in larger volumes, the low thermalconductivity of the PCM for example, 0.2 W/m K for tricosaneimpedes the thermal performance. The low thermal conductivitycreates a high conductive thermal resistance and leads to the iso-lation of the melt process near the heat source. Pal and Joshi 13numerically analyzed the melting of PCM using a uniformly dis-sipating flush mounted heat source in a rectangular enclosure andestablished that for low thermal conductivity PCMs, melting islocalized near the heat source, whereas for higher conductivity,heat is more effectively distributed throughout the mass. Krishnanet al. 14 studied a hybrid heat sink/paraffin combination for usein electronics cooling applications, finding that paraffin alone isunsuitable for transient heating applications due to its low thermaldiffusivity. Therefore, for high power applications the design mustbe adapted to facilitate more effective heat flow into the PCM.The PCM is typically contained within a sealed container mod-ule located adjacent to the heat source. The PCM can melt as itabsorbs heat and then resolidify at the end of a power cycle withinthis container module. In some cases, embedded finned heat sinks 15–19 or metallic foams 20–22 have been used to facilitate theheat penetration from the module walls into the contained PCMby providing a heat flow path to the module center and thus en-suring effective heat absorption through an even melt process.However, the use of embedded heat sinks and metallic foams hasseveral significant disadvantages, including added weight, dis-placed PCM, and the difficulty of manufacturing foams in thickenough layers for larger modules. This project investigates the useof graphite nanofibers suspended within the PCM to increase ther-mal performance without significantly increasing module weightor size.One of the most commonly studied PCMs is paraffin wax. Par-affin waxes in general are inexpensive, thermally and chemicallystable, and have a low vapor pressure in the melt 23 . In thisproject, graphite nanofibers are mixed uniformly into a paraffinwax blend with a melt temperature of 56°C and the thermal per-formance of the system is quantified.Graphite nanofibers GNFs generally have diameters of2–100 nm and lengths of up to 100 m 24 . The advantage ofusing GNF as the conductivity enhancer is that they exhibit highsurface area 25 and possess thermal properties, which are of thesame order of magnitude of carbon nanotubes 24 , but with asignificantly easier and less expensive production process 25 .The suspension of graphite nanofibers in the PCM is expected toimprove the thermal diffusivity and thus the thermal performanceby reducing the bottlenecking of heat flux at the source. The em-bedding of graphite nanofibers will accomplish this through in-creased conductivity of the composite material and possiblythrough an additional nanofluid-type enhancement effect throughBrownian motion of the particles when suspended in the liquidphase. This will be accomplished with low fiber loading levels,thus preserving a maximum volume for PCM and maximizing thepossible heat absorption and duration of melt process.The GNFs used in this study are grown through the catalyticdeposition of hydrocarbons and/or carbon monoxide over metalcatalysts in a reducing atmosphere using a process previously de-scribed 25 , which will be thus only covered in summary here.The carbon precipitates as graphite, which initially encapsulatesthe metal particle. The catalyst particle is “squeezed” through,leaving a perfectly formed graphite plane. As each graphite planeis formed, the fiber grows longer along an axis extending out-wards from the metal catalyst particle. Through precise manage-ment of the deposition process, the resulting orientation of these