What is a carbon nanofiber (CNF)?

A car­bon nan­ofiber (CNF) is a car­bon-made mater­i­al so thin that its dia­met­er is meas­ured in nano­met­ers. Its length is many times longer than its dia­met­er. Car­bon nan­ofibers have dia­met­ers in the range of 5–500 nm and lengths in the range of 1–200 μm.

Ultra slim

A nano­met­er (1 nm) is one-thou­sandth of a micro­met­er (0.001 µm). That’s so tiny that only three free car­bon atoms fit on a straight line of that length.¹ To get an idea of how extremely small that is, let’s ima­gine a hair strand. If you split it length­wise, and then split the two halves again, and then repeat this over and over again until you have 70,000 strains from that single strand, then each of them is about 1 nano­met­er thick.²

Super strong

Car­bon nan­ofibers is ten times stronger than the strongest steel in the world (mar­aging steel) and up to a hun­dred times stronger than ordin­ary steel.

The tensile strength of car­bon nan­ofibers is a mind-bog­gling 30 GPa.

Sup­pose you have a round bar with a dia­met­er of 1 cm and the same tensile strength as car­bon nan­ofibers. Let’s assume you some­how man­age to hang a mid-size car from its end. Then you add anoth­er, and yet anoth­er, and so on until the round bar breaks. Can you guess how many cars were hanging from the round bar before it broke? Two hun­dred! That’s how strong car­bon nan­ofibers are.

There­fore, car­bon nan­ofiber is an excel­lent build­ing block at the micro­scop­ic level. It can be used as a sup­port or as a rein­force­ment bar (“rebar”) for mater­i­als that become brittle at small sizes. They can also be used as tiny spacers between lay­ers of mater­i­als. Or as needles that make micro­scop­ic holes in membranes.

Stiff as a board

If you were going to bend a car­bon nan­ofiber, think again. It requires four times more force to bend or stretch car­bon nan­ofibers than steel. And the little fil­let will return to its ori­gin­al shape as soon as you release it—like a rub­ber band.

The Young’s Mod­u­lus of car­bon nan­ofibers var­ies between 80 and 800 GPa.

Lightweight

Although car­bon nan­ofiber is much stronger than steel, it weighs only a quarter of steel.

The dens­ity of car­bon nan­ofiber is between 1.3 and 2 g/​cm³.

Ima­gine a steel cube with sides 10 cm long and a car­bon nan­ofiber cube of the same size. If you put them on sep­ar­ate scales, the one with the steel cube will read 8 kg, while the one with the car­bon fiber cube will read less than 2 kg.

Excellent heat conductor

Sil­ver and cop­per are the best heat-con­duct­ive metals. Yet they take a beat­ing com­pared to car­bon nan­ofibers, which can be made to con­duct heat more than sev­en times better.

The thermal con­duct­iv­ity of car­bon nan­ofibers var­ies between 20 and 3,000 W/(mK) depend­ing on how they are manufactured.

That excel­lent heat con­duct­iv­ity can be used to solve one of the biggest prob­lems as more and more tran­sist­ors are squeezed onto a chip: Heat dis­sip­a­tion. A chip can become madly hot, short­en­ing its lifespan and increas­ing the risk of fail­ure. But the heat can effect­ively be dis­sip­ated by car­bon nan­ofibers between the chip and the cap­sule enclos­ing it.

Electrical conductor

It’s not just heat that car­bon nan­ofibers con­duct effi­ciently, but also elec­tri­city. Again sil­ver and cop­per are the metals that con­duct best. Car­bon nan­ofiber is not quite as good, fall­ing between iron and steel.

The elec­tric­al con­duct­iv­ity of car­bon nan­ofibers var­ies from 5⋅10² to 10⁷ S/​m. That is, the res­istiv­ity var­ies between 0,1 µΩm and 2 mΩm.

The good con­duct­iv­ity, com­bined with the small size, opens up the pos­sib­il­ity of con­nect­ing bio­sensors made by car­bon nan­ofibers dir­ectly to indi­vidu­al nerve cells. Car­bon nan­ofibers can also be used on chips instead of sol­der­ing cop­per con­tacts and conductors.

Surface multiplier

An excit­ing applic­a­tion for car­bon nan­ofibers is as sur­face coat­ings to increase the con­tact area. By grow­ing car­bon nan­ofibers dir­ectly on a sur­face, the con­tact area can be increased tens of thou­sands of times.

Ima­gine a square sur­face with a width and height of one mil­li­meter. It can eas­ily hold 100,000 rows and as many columns of car­bon nan­ofibers being 5 nano­met­ers in dia­met­er and 50 micro­met­ers in length. The sur­face area of those car­bon nan­ofibers increases the total sur­face area 7 855 times. In oth­er words, with car­bon nan­ofibers, you can eas­ily shrink an area of 88 × 88 mil­li­meters to just 1 × 1 millimeters.

Increas­ing the sur­face area many thou­sand­folds is bene­fi­cial in vari­ous applic­a­tions. One example is car­bon nan­ofibers coated with titani­um on the sur­face of a titani­um implant. The sur­face area of the implant will increase mak­ing it easi­er to join the bone. Anoth­er example is the mini­atur­iz­a­tion of capacitors.

Carbon nanofibers at the atomic level

What makes CNF so spe­cial is how the car­bon atoms are arranged. To under­stand how we must first review some basic atom­ic theories.

Bohr’s clas­sic­al mod­el of an atom describes it as a nuc­le­us sur­roun­ded by shells of elec­trons. Each shell has room for a fixed num­ber of elec­trons. The inner­most shell has two loc­a­tions. The fol­low­ing shell has room for eight elec­trons. And so on. In most cases, the shells fill up from the inside out. When a shell is com­plete, it is said to be closed. If it’s not com­plete, it is, of course, called open.

Elec­trons in the out­er­most shell are called valence elec­trons. Only noble gases have valence elec­trons in a closed shell. They are super stable. All oth­er sub­stances have valence elec­trons in an open shell, and these can do all sorts of crazy things.

For example, a valence elec­tron in an open shell can attract anoth­er atom, and in exchange for it also serve as the valence elec­tron for the oth­er atom. In this way, the two atoms will share an elec­tron. This cre­ates a bond between the two atoms. This type of bond, where the atoms share one or more elec­trons equally, is called a cova­lent bond.

A car­bon atom has six elec­trons: Two are in the inner­most elec­tron shell (which is there­fore closed), and four are in the shell out­side (which is there­fore open). So the car­bon atom has four valence elec­trons but “needs” four more to close the out­er shell. There­fore, free car­bon atoms do not become long-lived; they soon find oth­er atoms to cre­ate cova­lent bonds with.

A car­bon atom can fill the out­er shell by a valence elec­tron, form­ing a cova­lent bond with anoth­er car­bon atom. These, in turn, can do the same. Sup­pose this is repeated over and over again. In that case, we get a molecule made up of car­bon atom after car­bon atom. The prop­er­ties of these car­bon atoms depend on how they arrange themselves.

Allo­tropy is the phe­nomen­on of cer­tain ele­ments that can exist in dif­fer­ent forms called allo­tropes. Car­bon has many allotropes.

A three-dimen­sion­al array of car­bon atoms, where each bond has the same angle to its neigh­bors, is called a dia­mond.

Car­bon atoms that form hexagon­al rings, where adja­cent rings share sides and are in the same plane, are called graphene. It looks like a sheet of chick­en wire where the knots are car­bon atoms, and the threads between them are cova­lent bonds.

Graph­ite is lay­ers upon lay­ers of graphene. The lay­ers are held togeth­er by quantum dynam­ic inter­ac­tions between atoms in adja­cent lay­ers called van der Waals force.

Car­bon atoms that form five‑, six- or sev­en-sided rings, where adja­cent rings share sides but are not in the same plane, are called fullerene. These occur in many dif­fer­ent forms, many of which have their own names. In a sense, graphene is just a spe­cial case.

A fullerene that looks like a tube, pos­sibly closed at the end, is said to be a single-wall car­bon nan­otube (SWCNT) or car­bon nan­otube (CNT) for short. A multi-wall car­bon nan­otube (MWCNT) con­sists of one SWCNT enclos­ing anoth­er SWCNT, pos­sibly enclos­ing a third SWCNT, and so on. The nes­ted SWCNTs are “glued” to each oth­er by the van der Waals force.

Finally, we have arrived at the car­bon nan­ofiber (CNF), which can be seen as a “mod­i­fied MWCNT” where each tube is tightened at one end so that they take the shape of a plate, cup, or cone before they are stacked one inside the other.

How are CNFs manufactured?

Com­mer­cial fab­ric­a­tion of car­bon nan­ofibers is often done with Cata­lyt­ic Chem­ic­al Vapor Depos­ition (CCVD, or simply CVD). The basic idea is to apply so much energy to a car­bon-based gas, in the form of heat­ing or light­ning dis­charges, that the valence elec­trons are torn loose, releas­ing car­bon ions that can depos­it on sur­faces pre­pared with cata­lyt­ic metal.

Smol­tek has developed a pat­en­ted solu­tion to con­trol with extreme pre­ci­sion where the released car­bon atoms settle and how they are formed into car­bon nan­ofibers. This gives us a unique oppor­tun­ity to tail­or car­bon nan­ofibers with desired prop­er­ties. We have also developed the tech­no­logy to use com­par­at­ively low tem­per­at­ures (375 °C), which allows our man­u­fac­tur­ing tech­nique to be used in pro­duc­tion lines in the semi­con­duct­or industry.

The fol­low­ing is a sim­pli­fied descrip­tion of our man­u­fac­tur­ing pro­cess, which we call SmolGROW:

1. The sub­strate on which the car­bon nan­ofibers will grow, e.g., sil­ic­on wafer, is pre­pared by depos­it­ing vari­ous mater­i­als that form an under­lay­er on which the car­bon nan­ofibers will grow.
2. A cata­lyst is depos­ited as dots or pads where car­bon nan­ofibers will grow on top of the under­lay­er. These facil­it­ate a con­trolled growth of indi­vidu­al nano­struc­tures in pre­cise locations.
3. The sub­strate is put into a react­or cham­ber, which is her­met­ic­ally sealed and emp­tied of air, mak­ing a vacu­um inside.

4. A car­bon-based gas is intro­duced into the react­or cham­ber along with oth­er gases that facil­it­ate the reac­tion. Typ­ic­ally, acet­ylene is used to grow the fibers and the ammo­nia to clear excess deposition.
5. Inside the cham­ber, a huge dif­fer­ence in elec­tric­al voltage cre­ates an arc of light (elec­tric­al dis­charge). This heats the gas so that elec­trons are sep­ar­ated from the nuc­lei and can move freely. The res­ult is a soup, called plasma, of elec­trons and ions. The dis­charges are repeated sev­er­al times per second to main­tain the plasma. One of the cru­cial prop­er­ties of plasma is that the elec­trons have the energy equi­val­ent to sev­er­al thou­sand degrees Celsi­us. At the same time, the rest of the gas is rel­at­ively cool.
6. The elec­trons’ energy induces the depos­ition of car­bon on the cata­lyt­ic dots and pads, which form a car­bon nano­struc­ture. Smol­tek can cre­ate vari­ous car­bon nano­struc­tures by con­trolling the depos­it, includ­ing car­bon nan­ofibers with desired properties.
7. When the car­bon nan­ofibers have reached the desired length, the pro­cess is stopped, and the remain­ing gases are ven­ted out of the cham­ber again. We are left with the sub­strate with the car­bon nanofibers.

Smoltek’s tech­no­logy works not only with car­bon nan­ofibers. It is pos­sible to cre­ate oth­er car­bon nano­struc­tures, like car­bon nan­otubes (CNTs), and also use mater­i­als oth­er than carbon.

Want to know more?

Do you want to learn more about car­bon nan­ofibers and how they can be used in your applic­a­tion? Con­tact us and let’s have a chat.

1. If you think of a free car­bon atom as a hard-sphere, its dia­met­er is 0.34 nm. Its van der Waals radi­us is 0,17 nm. ↩︎
2. The thick­ness of a strand of hair var­ies from 0.017 to 0.18 mm, but most com­mon is around 0.07 mm. ↩︎