Carbon nanofibers (CNFs)

Car­bon nanofibers is a super­ma­te­r­i­al. It is stronger, more elas­tic, and lighter than steel. It con­ducts heat and elec­tric­i­ty bet­ter than met­als. And it can be used to thou­sand­fold the sur­face of mate­ri­als. Let’s take a clos­er look at these tiny fel­lows. What is a car­bon nanofiber (CNF)? A car­bon nanofiber (CNF) is a car­bon-made mate­r­i­al so thin that…

Car­bon nanofibers is a super­ma­te­r­i­al. It is stronger, more elas­tic, and lighter than steel. It con­ducts heat and elec­tric­i­ty bet­ter than met­als. And it can be used to thou­sand­fold the sur­face of mate­ri­als. Let’s take a clos­er look at these tiny fellows.

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.

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

Scan­ning elec­tron micro­scope image of a “for­est” of car­bon nanofibers.

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 net 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.
A pre­pared sub­strate is placed in the CCVD reac­tion chamber.
  1. 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.
  2. 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.
  3. 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.
  4. 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.

Schemat­ic dia­gram show­ing how car­bon nanofibers (CNFs) are grow­ing in a Plas­ma Enhanced Chem­i­cal Vapor Depo­si­tion (PEVCD) reac­tion chamber.

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. 

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