A carbon nanotube is stronger than steel. A rope of them is not — and the reason is worth understanding

Every few years a headline announces that carbon nanotubes are a hundred times stronger than steel and asks why we are not yet building bridges, aircraft or space elevators out of them. The number in that headline is real. It has been measured, repeatedly, and it is not marketing. The reason it has not become a cable is far more interesting than a story about slow engineering, and understanding it tells you most of what you need to know about how nanomaterials behave when you try to scale them from one object to a trillion.

The short version: strength at the scale of a single tube and strength at the scale of a rope are two different quantities, and the second one is not set by the first.

What a nanotube actually is

Take a single sheet of graphene — one atom thick, carbon atoms arranged in a hexagonal lattice, the material we wrote about at twenty years old — and roll it into a seamless cylinder a nanometre or two across. That is a single-walled carbon nanotube (SWCNT). Nest several of those cylinders inside one another like a telescope and you have a multi-walled carbon nanotube (MWCNT), typically tens of nanometres in outer diameter.

The bonds along the wall are sp² carbon–carbon bonds: the same bonds that make graphene and diamond exceptional, arranged so that a load applied along the tube's axis is carried by the covalent lattice itself rather than by anything weaker. This is why the numbers are what they are. Individual nanotubes have measured Young's moduli close to 1 TPa and tensile strengths in the range of tens of GPa — the classic measurements on MWCNT outer shells landed between roughly 11 and 63 GPa — against about 0.4 to 2 GPa for structural and high-strength steels. Nanotubes do this at a density near 1.3–1.4 g/cm³, where steel sits at 7.8. Per unit weight, nothing else measured comes close.

There is a second property that matters as much commercially, and it is electrical. How the graphene sheet is rolled — the chirality, written as a pair of indices (n,m) — decides whether the resulting tube is metallic or semiconducting. Roughly a third of the tubes in an unsorted batch conduct like a metal and two thirds behave as semiconductors. For a bulk conductive additive that mixture is irrelevant. For anyone trying to build transistors, it is the central problem of the field, because a batch containing metallic tubes contains shorted devices, and separating them at scale and at purity remains hard.

Where the strength goes

Now try to make something you can hold. A nanotube is a few micrometres to a few millimetres long. A cable is metres. So the load has to travel from one tube to the next, over and over, and that is the moment the physics changes.

Within a tube, force is carried by covalent bonds. Between tubes, there are no bonds at all — only van der Waals attraction, the same weak, non-bonded interaction that makes a gecko's foot stick and graphite pencils write. Nanotubes are extremely good at bundling themselves together through it, which is why they arrive as tangled agglomerates rather than as free tubes, but as a mechanism for transferring stress it is feeble compared with the wall it is transferring stress to. Pull hard enough on a bundle and the tubes do not break. They slide.

The same thing happens inside a multi-walled tube, and it has a name: sword-in-sheath failure. The outer shell takes the load, breaks, and the inner shells simply telescope out of it, contributing almost nothing. A ten-walled tube is not ten times as strong as a one-walled tube; in tension it is close to one wall doing the work while nine ride along.

Three consequences follow, and together they account for the whole gap between the headline and the cable.

• Length dominates. If load transfer relies on friction along an overlap, then the longer the overlap the more force it can carry before slipping. This is why growing longer tubes matters far more than growing stronger ones, and why millimetre-scale tubes are treated as a serious achievement.
• Alignment matters almost as much. A tube lying at an angle to the load carries a fraction of it. Real fibres contain waviness, loops and misalignment, all of which are strength that never arrives.
• Defects set the ceiling. Strength is decided by the worst point along a tube, not the average. A single vacancy or a pentagon–heptagon pair in the lattice is where fracture starts, and long tubes have more chances to contain one.

Put together, macroscopic CNT fibres reach single-digit GPa in the best laboratory results and less than that commercially — excellent by the standards of engineering fibres, particularly on a per-weight basis, but one to two orders of magnitude below the individual tube. The gap is not incompetence. It is the difference between a covalent bond and a frictional contact.

The lesson generalises well beyond nanotubes: a nanomaterial's headline property is a property of one object. Whether any of it survives into a bulk material is decided by the interfaces between objects, and interfaces are almost always the weakest thing in the system.

What is being done about the gap

The work aimed at closing it attacks exactly the three points above, and it is genuinely progressing.

Longer tubes from floating-catalyst chemical vapour deposition, where fibre is spun continuously from the reaction zone rather than assembled from powder afterwards, produce the strongest fibres made so far. Densification — drawing the fibre through a solvent, or twisting it — squeezes tubes into closer contact and increases the frictional area available for load transfer. Cross-linking goes further and tries to replace friction with chemistry: ion or electron irradiation creates covalent bridges between adjacent tubes, which raises strength up to a point and then begins damaging the walls that supply the strength in the first place, a trade-off that is still being mapped. Polymer infiltration takes the composite route instead, using a matrix to shuttle stress between tubes.

None of these has produced a material at the individual-tube number, and the honest assessment is that none is close. A space elevator tether, for the record, needs a specific strength beyond anything demonstrated in a real fibre, and it is not a useful benchmark for anything happening this decade.

What nanotubes are genuinely good at today

Here is the part that gets less attention than the strength story, and it is where the tonnage actually goes.

Conductive additives in lithium-ion battery electrodes are the largest real application by volume. Replacing carbon black with a small fraction of a percent of nanotubes gives a percolating network of long, thin conductors instead of a crowd of touching spheres, which means the same electrical connectivity from far less inactive material — and every gram not spent on conductive additive is a gram available for active material. Single-walled tubes matter especially in silicon-containing anodes, where the electrode swells and contracts dramatically on every cycle and a flexible network survives that movement in a way a brittle one does not.

Electrostatic discharge control and conductive plastics. A few tenths of a percent of MWCNT turns an insulating polymer into a controlled-dissipative one without the loading — and the loss of mechanical properties — that carbon black demands. This is unglamorous, high-volume and thoroughly commercial: fuel lines, wafer-handling parts, automotive components painted electrostatically.

Thermal interfaces and heat spreading, exploiting axial thermal conductivity that is very high along a tube, provided the material is aligned so the tubes actually point from hot side to cold side.

Reinforcement in composites, where the target is usually not raw strength but toughness, fatigue life, damage tolerance and crack bridging in an epoxy or a carbon-fibre laminate. A small addition that stops a microcrack propagating is worth more in service than a large addition that raises a datasheet number.

Field emission, sensors and transparent conductors, each a real niche. And in the laboratory, CNT transistors — genuinely promising for a post-silicon future, and genuinely blocked on the purity and placement problems that chirality creates.

How to read a nanotube specification

If you are buying this material rather than reading about it, "carbon nanotubes" is not a specification. Six things decide whether a given grade will work in your application, and reputable suppliers publish all of them.

1. SWCNT or MWCNT. These are different materials at different prices, by roughly two orders of magnitude. Single-walled tubes are used at very low loadings for electrical percolation; multi-walled ones are the general-purpose workhorse.
2. Diameter, length and aspect ratio. Aspect ratio governs percolation threshold — the loading at which a conductive network first forms — and it is the number to compare between grades.
3. Purity, and specifically residual catalyst metal. Tubes are grown on iron, cobalt or nickel nanoparticles, and what is left behind matters for electrochemical applications, where a trace transition metal is a side-reaction site, and for biomedical ones, where it confounds toxicity results. Ask for TGA for carbon purity and ICP for metals.
4. Defect density, via Raman spectroscopy. The ratio of the D band near 1350 cm⁻¹ to the G band near 1580 cm⁻¹ is the standard quality indicator; a low D/G means a more perfect lattice. For single-walled material the radial breathing mode also reports diameter.
5. Dispersion state and functionalisation. This is the one most often underestimated. Nanotubes arrive agglomerated, and a poorly dispersed batch performs worse than a lower grade properly dispersed. Ask what the supplier recommends — surfactant, dispersant, masterbatch, pre-dispersed paste — and note that aggressive sonication cuts tubes shorter, destroying the aspect ratio you paid for. Carboxyl or amine functionalisation improves dispersion and matrix bonding at the cost of introducing sidewall defects.
6. Form supplied. Dry powder, aqueous or NMP dispersion, masterbatch, buckypaper and aligned array are different products with different handling requirements, and the dispersion is often the sensible purchase even at a premium.

Handling deserves its own paragraph, because dry nanotube powder is a fibrous, low-density, respirable material and the precautionary case for treating it seriously is well established. Long, rigid, biopersistent fibres are the shape that raises the greatest concern in inhalation toxicology, and NIOSH has set a recommended exposure limit of 1 µg/m³ of elemental carbon as an 8-hour time-weighted average for carbon nanotubes and nanofibres — a low number that tells you how the question is regarded. Handle dry powder in an enclosure or fume hood, use appropriate respiratory protection, avoid dry sweeping, and read the SDS before the first bottle is opened rather than after. Dispersions and masterbatches substantially reduce the exposure route, which is a good reason to prefer them where the process allows.

Why it matters for students and researchers

Carbon nanotubes are an unusually honest teacher, because the field has already lived through its own hype cycle and come out the other side with real products. The strength story did not deliver the cable; the electrical and electrochemical story quietly built an industry. Anyone entering nanomaterials research now gets to study a complete example of the difference between a property and an application.

The open problems are also unusually well defined, which is a gift when choosing a project. Chirality-selective growth and separation at scale is the barrier standing between CNT electronics and manufacturing. Load transfer at the tube–tube and tube–matrix interface is a mechanics problem with decades of work behind it and no closed solution. Scalable synthesis of long, low-defect tubes is a reactor engineering problem as much as a chemical one. And the toxicology of fibre length, rigidity and biopersistence is a live public-health question with regulatory consequences.

The other reason this field rewards close reading is that the distance between a laboratory result and a purchasable grade is wide, and it is documented mainly in the journals rather than in the brochures. A measured D/G ratio, a stated length distribution and a dispersion protocol that survives peer review will tell you more about a material than any adjective on a product page.

Frequently asked questions

Are carbon nanotubes really a hundred times stronger than steel?

An individual tube, measured along its axis, genuinely reaches tens of GPa at roughly a fifth of steel's density, so on a strength-per-weight basis the comparison holds. A macroscopic fibre made of those tubes does not, because the load has to cross tube-to-tube interfaces held together by van der Waals attraction rather than covalent bonds. Both statements are true; they are statements about different objects.

What is the difference between single-walled and multi-walled tubes?

An SWCNT is one rolled graphene cylinder, typically around a nanometre across; an MWCNT is several nested cylinders, typically tens of nanometres across. Single-walled material has better intrinsic electronic properties and reaches percolation at far lower loadings, but costs one to two orders of magnitude more and is harder to disperse. Multi-walled material is the practical choice for most bulk applications, and in tension it suffers sword-in-sheath failure where only the outer wall carries load.

Why does dispersion matter so much?

Because nanotubes bundle themselves. Van der Waals attraction along a long tube adds up to a firm grip, so the material arrives as agglomerates, and an agglomerate contributes nothing that a free tube would. A well-dispersed lower-grade batch routinely outperforms a poorly dispersed premium one, which is why suppliers sell pre-dispersed pastes and masterbatches, and why sonication protocols need care — too much shortens the tubes and destroys the aspect ratio the application depends on.

Are carbon nanotubes dangerous?

They require respect rather than alarm. The concern is specific and shape-based: long, rigid, biopersistent fibres are the geometry associated with the greatest risk in inhalation toxicology, and NIOSH's recommended exposure limit of 1 µg/m³ elemental carbon reflects that. Short, tangled or well-dispersed material in a matrix presents a very different exposure profile from dry, respirable powder. Engineering controls, correct PPE and reading the SDS are the answer, and they are the same answer that applies to any fine powder in a laboratory.

Where would I actually encounter one today?

Most likely in a battery. Nanotubes are now a standard conductive additive in lithium-ion cathodes and in silicon-containing anodes, which puts them in electric vehicles and consumer electronics at meaningful volume. After that, in antistatic and conductive plastics, in thermal interface materials, and as a toughening additive in composites — none of them the cable from the headline, all of them real.