Chemical engineers make diverse range of thermally conductive materials using single method
Joule heating coupled with an increasing density of components mean that our electronic devices rely heavily on thermal management in order to operate. From polymer coatings that protect circuit boards, to conductive pastes that bridge the gap between devices and heat sinks, there are already countless commercial products that can redirect and dissipate heat effectively. But new ideas are always being developed, and in the research world, polymer-based composites with high thermal conductivity (TC) are attracting a lot of attention. They are cheap, lightweight, and often easily processed, and their thermal performance can be tuned via selection of their individual components. For example, graphene nanoplatelets (GNP) have remarkably high in-plane TC (2000 W m- 1K- 1), which suggests they would be effective at dissipating heat in the in-plane direction, e.g., as seen in the conformal coating of PCBs.
Improving TC in the cross-plane direction – as required for thermal interface materials – can be a challenge for GNP-based composites. Classic preparation methods involve multiple days and many complex steps. A group of Israeli chemical engineers has proposed an alternative, rapid approach; to combine GNP with a high-volume filler (HVF) to produce conductive composites that, they say, yield cross-plane thermal conductivity values that meet the demands of the electronics industry. They’ve published their findings in the latest issue of Carbon [DOI: 10.1016/j.carbon.2023.118440].
The hybrid filler approach used by this team is based on the excluded volume effect, whereby the presence of a micrometre-sized HVF reduces the volume available for the conductive nanofiller (in this case, GNP), increasing its effective concentration. They chose to test both a thermally conductive HVF – diamond powder (40–60 μm in diameter) – and a thermally insulating one – glass spheres (9–13 μm in diameter). These fillers were mixed with epoxy resin in a planetary ball mill, before a hardener was added.
Previous research had indicated that GNP-loaded composites compressed before curing tend to exhibit enhanced thermal performance when cured. The team wanted to investigate what role compression might have on their hybrid GNP-HVF composite. They prepared non-compressed samples by casting the wet mixture into silicone moulds and curing them in-situ. Other samples were subjected to pressure (in the range of 20–100 bars) using a hydraulic press, before curing. Additional composite samples were made using a commercial thermal paste in place of the epoxy resin.
They found that for a fixed volume of GNP (7%), the hybrid composites that contained the glass HVF had a slightly higher thermal conductivity than the composite with the diamond HVF. A non-compressed GNP diamond sample with a total filler concentration of 56 vol% was seen to have a higher in-plane TC than cross-plane TC (TC-/TC= = 0.35). When compressed at 100 bar, the bulk thermal conductivity was greatly enhanced, reaching 9.26 W m−1 K−1; a value that surpasses most commercial thermal interface materials (typical range: 0.5 – 6 W m−1 K−1). The increased pressure also led to a cross-plane TC of 12.3 W m−1 K−1, putting it within the range required by the electronics industry for thermal interface materials (Typical TC-: 10 – 25 W m−1 K−1). They attribute this to the GNP particles being pushed between the diamonds, causing them to tilt towards the compression vector, improving TC in that direction. Compressing the ‘commercial’ composite at 50 bar showed a similar trend. TC= decreased and TC- increased, leading to a substantially increased conductivity ratio (from 0.80 to 1.7).
The authors conclude that the excluded volume approach they’ve developed “…constitutes a facile and effective means for increasing the TC of composites. Importantly, the results of this study show that our methodology is both generalizable and industrially relevant.”
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Gal Shachar-Michaely, Noam Lusthaus, Lev Vaikhanski, Gennady Ziskind, Yachin Cohen, Oren Regev. “Pressure-induced tuning of thermal transport in carbon-based composites: Directional control of heat dissipation,” Carbon 215 (2023) 118440. DOI: 10.1016/j.carbon.2023.118440