Processing Compressed Expanded Natural Graphite for Phase Change Material Composites

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2 Scopus Citations

Abstract

Phase change materials (PCMs) are used in various thermal energy storage applications but are limited by their low thermal conductivity. One method to increase conductivity involves impregnating organic PCMs into highly porous conductive matrix materials. Of these materials, compressed expanded natural graphite (CENG) matrices have received the most attention. Despite this attention, the effect that CENG processing has on PCM saturation and overall matrix thermal conductivity has not been fully investigated. Therefore, the effect of the heat treatment process used to expand intercalated graphite flakes is evaluated here. Higher heat treatment temperatures yielded higher saturation rates and overall saturation at similar matrix porosities. For example, increasing temperature from 300°C to 700°C resulted in approximately 60%–70% increase in pore saturation after 100 minutes of soaking. The exposure time to heat treatment had less of an effect on PCM saturation. The exposure time had negligible effect above 30 min and above 500°C heating temperatures. However, because the expanded graphite was found to oxidize around 700°C, the use of longer exposure time in manufacturing applications can be beneficial if a shortened impregnation time is needed. Heat treatment conditions did not impact thermal conductivity. The composite latent heat of fusion was also reduced approximately proportionally to the PCM mass fraction. A local maximum in axial thermal conductivity was observed at around 83% porosity, which is similar to previous studies. The observed conductivity at this maximum was a factor of 81 times greater than the conductivity of the PCM.

Original languageAmerican English
Pages (from-to)1213-1226
Number of pages14
JournalJournal of Thermal Science
Volume32
Issue number3
DOIs
StatePublished - 2023

Bibliographical note

Publisher Copyright:
© 2022, National Renewable Energy Laboratory under exclusive licence to Science Press, Institute of Engineering Thermophysics, CAS.

NREL Publication Number

  • NREL/JA-5500-83852

Keywords

  • composite matrix
  • compressed expanded natural graphite
  • phase change materials
  • porous material sorptivity
  • thermal conductivity enhancement
  • thermal energy storage

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