Every spacecraft ever launched has been designed with thermal management in mind. From passive systems like heat sinks, insulation and coatings, to active systems such as cryocooler and Peltier cells, engineers use a variety of approaches to ensure that every critical component operates within its ideal temperature range. One class of materials that play a particularly important role in managing heat loads are thermal interface materials (TIMs). Their job is to enhance the thermal coupling between components, by improving the efficiency of heat transfer across interfaces. Commercial, space-qualified TIMs tend to be composites, consisting of a flexible polymer-based matrix mixed with a thermally-conductive filler. But though effective for many applications, few are conductive enough to be used in demanding, high-heat flux devices.
A group of researchers from the Beijing Institute of Spacecraft System Engineering may be close to changing that. Writing in a recent issue of Carbon [DOI: 10.1016/j.carbon.2021.11.039], they say that their new TIM has “a perfect interface heat transfer capability both on the ground and in space.” At the heart of the composite is milled mesophase pitch-based carbon fibre (CF), which, when carefully aligned, is known to have excellent thermal properties. They combined this, in varying ratios, with high purity alumina (Al2O3) particles, before mixing both into a two-part silicone rubber. Before curing, they placed the mixture into a strong magnetic field (9T) to orient and align the fibres. The cured composite TIM could then be characterised, and its performance compared to a fibre-free composite sample (0 vol% CF) with identical dimensions.
SEM imaging showed that in all samples, the alumina particles were uniformly dispersed throughout the silicone (this ratio was fixed at 1:1). And in samples that contained anything up to 20 vol% CF, the fibres were aligned and oriented to match that of the magnetic field. Beyond that, the alignment was seen to decrease, which, the authors suggest, results from interactions between the fibres at high loading fractions.
In terms of thermal performance, the aligned CFs were shown to have a significant impact. At 0 vol% CF, the thermal conductivity of the alumina/silicone rubber composite was found to be 1.59 W/m.K. But at 20 vol% CF, the value was 17 times higher – the thermal conductivity reached a peak of 26.49 W/m.K.
This highly conductive sample also fared well in terms of vacuum outgassing – an important metric for understanding how the gases trapped in solids can be released in the vacuum conditions of space. Measurements of both the total mass loss and the condensable volatile matter showed that the material met the requirements defined in industry standards. It also outperformed three existing TIMs – CHO-CHERM 1671, Tpli 200 and SIL PAD 200 – in those same tests. And finally, the material’s heat transfer capacity was measured in ground tests and in an on-orbit test. And in both cases, the temperature gradient across the sample was small (< 5 °C), suggesting that heat flow enabled by the novel TIM was highly efficient. The authors conclude that their composite “can potentially be applied as [an] elastomeric TIM for spacecraft thermal control.”
This story first appeared here: https://www.materialstoday.com/carbon/news/cf-rubber-composite-shows-potential/
Original research papwr ($): Qi Wu , Wenjun Li , Chang Liu , Yawei Xu , Guoguang Li , Hongxing Zhang , Jinyin Huang, Jianyin Miao. “Carbon fiber reinforced elastomeric thermal interface materials for spacecraft,” Carbon 187 (2022) 432-438. DOI: 10.1016/j.carbon.2021.11.039