Can one carbon material be both ultra-hard and conductive?
Squeezing and heating glassy carbon in a narrow temperature window produced a carbon composite of tiny diamonds in conductive graphene that is nearly as hard as diamond yet still conducts electricity.
Source
Ultrastrong conductive in situ composite composed of nanodiamond incoherently embedded in disordered multilayer graphene
Study at a glance
- Design
- Other — Glassy carbon compressed at 25 GPa and heated to three temperatures; recovered composites characterised structurally, mechanically and electrically, plus molecular-dynamics compression of nanopillars.
- N
- No single N: three main composite samples (made at three temperatures) plus repeat syntheses; simulations used nanopillars of two diameters.
- Population
- Nanodiamond / disordered multilayer graphene (ND/DMG) carbon composites synthesised from glassy carbon
- Outcome
- Knoop hardness, Young's modulus, micropillar compressive strength and elastic strain, electrical conductivity, and simulated shear-band behaviour
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors compressed glassy carbon to 25 GPa and heated it to temperatures between 1,050 and 1,150 degrees C for one hour, producing three composites with increasing diamond content. They imaged the structure with X-ray diffraction, Raman spectroscopy and atomic-resolution electron microscopy, then measured hardness by indentation, strength by compressing micron-sized pillars, and electrical resistivity from 4 to 300 K. Molecular-dynamics simulations compressed model nanopillars with and without embedded nanodiamonds to explain the mechanics.
What they found
Diamond content rose with synthesis temperature (about 20%, 50% and 70%), and nanodiamond grains averaged about 4.8 nm, joined to the graphene matrix through an irregular, incoherent interface of mixed sp2/sp3 bonds. Hardness climbed to 53 GPa for the most diamond-rich composite, exceeding cubic boron nitride, and micropillars reached compressive strengths up to 54 GPa with roughly 10% elastic strain before fracture. Room-temperature conductivity stayed in the range of about 670 to 1,240 S/m because the graphene matrix provides conducting paths. Simulations showed that nanodiamonds block the spread of shear bands through the graphene, explaining the higher strength.
The limits
What it doesn't show
The results come from a small number of millimetre-sized samples made in a large multi-anvil press, so it is unclear whether the material can be produced at useful scale or cost. The micropillar tests probe micron-sized volumes, which often show higher strength than bulk pieces, and fracture toughness is not reported. The simulations use idealised nanopillars about ten nanometres across and a very fast strain rate, so they illustrate a mechanism rather than reproduce the measured numbers. The claim that the composite is the hardest and strongest carbon/carbon composite rests on comparison with literature values measured by other groups and methods.
Key terms
- Glassy carbon
- A disordered, non-graphitising form of carbon made of short, curved graphene fragments; here it is the starting material.
- sp2 and sp3 bonding
- The two main ways carbon bonds: sp2 gives flat, conductive graphene-like sheets; sp3 gives the rigid, insulating 3D network of diamond.
- Incoherent interface
- A boundary where two phases join irregularly without a matching crystal lattice, here via random covalent bonds between nanodiamond and graphene layers.
- Knoop hardness
- Hardness measured from the size of an elongated diamond-tip indentation under a known load.
- Shear band
- A narrow zone where plastic deformation concentrates; its spread through a material usually precedes failure.
Flashcards
0 of 10 answers reviewed
Research intelligence for this paper
See its role on concept claims, tensions it is part of, placement history, and related discoveries.
Quiz yourself
What happened to diamond content as the synthesis temperature was raised from 1,050 to 1,150 °C?
Common questions
Why is it unusual for a material to be both very hard and conductive?
Hard materials like diamond and most ceramics have tightly bound electrons and are insulators, while conductive metals and graphite are relatively soft. This composite combines an insulating hard phase with a continuous conductive matrix.
How can a composite containing insulating diamond conduct electricity?
The diamond grains are only a few nanometres across and are surrounded by disordered multilayer graphene, which forms connected conductive pathways around them.
Why does adding nanodiamonds make the material stronger?
In the simulations the nanodiamonds act as obstacles that stop shear bands from running through the softer graphene, and the covalent bonds at the interface transfer load.
More on Elasticity and mechanical metamaterials