New route to creating complex architectures from graphite powder

Graphite has been an important material to humanity for millennia. It has allowed us to make our mark – first, on pottery and later, on sheets of paper. But graphite’s industrial application doesn’t end at writing and drawing. Its layered, 2D structure makes it a key ingredient in countless lubricants. Its thermal and chemical stability help it withstand extreme conditions – from crucibles that hold molten metal, to the heart of a nuclear reactor. The material’s high electrical conductivity sees it used in batteries and in electrical contacts for devices, and graphite inks are making flexible, printed electronics possible.

Polymers loaded with graphite have also been successfully 3D printed via fused filament fabrication and direct ink writing (DIW) techniques. But the resulting composites tend to feature < 50 wt% graphite, and so their thermal and electrical properties fall short of those of pure graphite. A group of researchers from Rice University are now trying to tackle this challenge. Writing in Carbon [DOI: 10.1016/j.carbon.2021.05.003], they report on the development of a colloidal ink that can be used to fabricate complex 3D architectures that are ~ 97 % graphite.

They achieved this by adding small quantities (3 – 10 wt%) of a silicate nanoclay to their graphite flakes suspended in water, resulting in an ink that could be printed by DIW. Its viscosity was significantly higher than that of a pure graphite ink, which made it suitable for extrusion through a small nozzle at relatively low print pressures and ambient temperatures. In contrast, the graphite-only ink clogged up the nozzle, and separated under pressure. The colloidal ink’s shear-thinning behaviour combined with the nozzle design also worked to align the graphite flakes during extrusion.

The nanoclay acted as a binder for the graphite flakes, allowing the flakes to be uniformly distributed throughout the ink.  Its presence increased the storage modulus of the ink, and the effect of this could be seen in the resulting 3D structures. Despite their complexity, all of the printed objects were self-supporting and structurally sound. After drying in air, they retained their original shape and form. Under structural tests, the objects were found to behave plastically, with an increased ability to absorb energy when compared to unmodified graphite.

The authors used DIW and their modified ink to print graphite moulds which they then successfully used to cast tin. They also printed a graphite circuit connected to several LEDs to demonstrate its electrical performance. And they produced a prototype heating element capable of boiling water when a voltage was applied to it.

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S.M. Sajadi, S. Enayat, L. Vásárhelyi, A. Alabastri, M. Lou, L.M.Sassi, A. Kutana, S. Bhowmick, C. Durante, Á. Kukovecz, A.B. Puthirath, Z. Kónya, R. Vajtai, P. Boul, C.S. Tiwary, M.M. Rahman, P.M. Ajayan. “Three-dimensional printing of complex graphite structures”, Carbon, Volume 181 (2021), 260-269 DOI: 10.1016/j.carbon.2021.05.003