Carbon nanostructure stronger than diamond

Materials World magazine
3 Jun 2020

A super-light carbon nanostructure stronger than diamond can now finally be fabricated, claim scientists State-side. 
Shardell Joseph catches up with them.  

A nanometre-sized carbon structure, produced with a closed-cell plate architecture, is said to outperform diamond for its ratio of strength to density. Building on a previous theoretical study, researchers from the University of California, Irvine (UCI), USA, believe they have proved manufacturing capability of these nanolattices and achieved the highest strength ever thought possible for a highly porous material. 

The design is said to increase performance of cylindrical beam-based architectures by up to 639% in strength and 522% in rigidity. ‘The plate-nanolattices not only reach both the theoretical limit for stiffness and yield strength, but are also the strongest and stiffest architected materials created to date, even surpassing bulk diamond in specific strength,’ says Cameron Crook, Graduate Student at UCI and Lead Author on the research.

Crook outlines how the foundation for the work started with a previous paper by one of the Co-authors, Jonathan Berger. ‘He proposed a novel plate lattice that theoretically reached the bounds for stiffness of a voided material,’ Crook explains. ‘However, this was never demonstrated experimentally until this work, due to the complexity of the plate structure and manufacturing limitations on a lattice composed of closed voids.’ 

The team is using a complex 3D printing process known as two-photon polymerisation direct laser writing. It works by focusing a laser inside an ultra-violet, light-sensitive, liquid resin droplet, making the material a solid polymer where molecules are simultaneously hit by two photons.

The significantly increased strength of the plate-nanolattices is being achieved by combining a highly efficient plate topology known as the cubic+octet and nanoscale plates. This results in a ceramic-like material with little to no defects, which would limit their strength on a macroscopic scale.

Tiny holes are then included in the plates, removing excess resin from the material. Lastly, the lattices undergo pyrolysis, heated at 900°C in a vacuum for an hour. This results in a cube-shaped lattice of glassy carbon. 

‘No current manufacturing techniques currently permit closed voids for a topology such as the cubic+octet,’ says Crook. ‘This necessitated the addition of holes at the centre of plate walls to allow removal of trapped resin during printing.
‘We demonstrate that these holes do not significantly reduce the mechanical properties and that these structures still attain the upper bounds for stiffness and yield strength.’ 

The team will now look at designing new manufacturing processes that can fabricate this topology with nano or micro-size features on a large scale. 

‘Of secondary importance, is improving the constituent material. The constituent material of this study was pyrolytic carbon due to limitations on the available materials in the current processing route,’ Crook explains. 

‘However, one could conceivably see a significant increase in stiffness and strength with a cubic+octet composed of tungsten carbide, or perhaps, far in the future, a plate-lattice composed of single-layer graphene plates.’