Polymeric materials are more advantageous to use as heat exchangers when compared to their metallic counterparts. Interestingly, despite the low cost and low weight associated with the former, their wide applicability in fields of water desalination, automotive control units, solar energy harvesting and micro-electronics cooling, has proven crucial. Unfortunately, polymeric materials have lower intrinsic thermal conductivity when compared to metals thereby limiting widespread application in thermal management technologies. Consequently, the quest to achieve high thermal conductivity in polymeric materials has led to groundbreaking research where the alignment of polymer chains has been proposed. Such works have shown that large enhancements in thermal conductivity can be achieved through the alignment of either the polyethylene chains or filler material. More so, alignments of both the polymer chains and the graphitic nanoplatelets has potential yield higher thermal conductivity values. However, the effectiveness of such simultaneous alignment in enhancing thermal conductivity has not been thoroughly explored.
Mortaza Saeidijavash and colleagues at University of Oklahoma performed an in-depth analysis of the effects of simultaneous alignment of polyethylene lamellae and graphene nanoplatelets on thermal conductivity of polyethylene/graphene composites. The researchers hoped to unearth the benefits that could be ripped from graphene nanoplatelets alignments as opposed to use of pure polyethylene. Their work is now published in the research journal, Nanoscale.
Briefly, the research team commenced their empirical procedure by producing graphene nanoplatelets through mechanical cleavage of raw graphite. They then obtained high density polyethylene. The team then proceeded to prepare the polyethylene/graphene nanoplatelets composites by melt compounding. The obtained nanocomposite was compression-molded so as to achieve composite samples of the desired thickness. The researchers then measured thermal conductivity, specific heat capacity and thermal diffusivity of both strained and unstrained composites.
From the various measurements undertaken, the authors of this paper observed that a large increase in the thermal conductivity of the aligned polyethylene/graphene nanoplatelets composite relative to un-oriented pure polyethylene was achieved. Interestingly, the rate of increase of the thermal conductivity with the applied strain for the pure polyethylene/graphene nanoplatelets composite with a tenth of graphene nanoplatelets by weight was noted to be almost a factor of two higher than the pure polyethylene sample. The researchers further found out that the aligned graphene nanoplatelets were three times more effective in enhancing the thermal conductivity as in the randomly oriented configuration.
University of Oklahoma researchers successfully demonstrated the effects of strain induced simultaneous alignment of polyethylene and graphene nanoplatelets on thermal conductivity enhancement of polyethylene/graphene nanoplatelets composite. In the study, both laser scanning confocal microscopy and polarized Raman spectroscopy have been effectively used to demonstrate the high level of graphene nanoplatelets alignment in strained samples. A comparison with aligned carbon nanotubes has showed that the aligned graphene nanoplatelets are twice as effective in enhancing thermal conductivity. These work therefore has illuminated further on the diverse potential of simultaneous alignment effects in enhancing thermal conductivity and also provided novel avenues for developing polymeric composites of high thermal conductivity.
“We could achieve a thermal conductivity of 5.9 W/mK at the strain of 400% and for 10 wt% GnP composition. It demonstrates 12-fold increase in thermal conductivity compared to unoriented pure polyethylene. This finding promises the potential of simultaneous alignment effects in providing new avenues for developing high thermal conductivity polymeric composites.” Said Mortaza Saeidijavash, first author of the study.
Mortaza Saeidijavash, Jivtesh Garg, Brian Grady, Benjamin Smith, Zheling Li, Robert J. Young, Fatema Tarannum, Nour Bel Bekri. High thermal conductivity through simultaneously aligned polyethylene lamellae and graphene nanoplatelets. Nanoscale, 2017, DOI:10.1039/C7NR04686C
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