Long Lines

Technology Review reports on progress in Making Carbon Nanotubes into Long Fibers

A new method for assembling carbon nanotubes has been used to create fibers hundreds of meters long. Individual carbon nanotubes are strong, lightweight, and electrically conductive, and could be valuable as, among other things, electrical transmission wires. But aligning masses of the nanotubes into well-ordered materials such as fibers has proven challenging at a scale suitable for manufacturing. By processing carbon nanotubes in a solution called a superacid, researchers at Rice University have made long fibers that might be used as lightweight, efficient wires for the electrical grid or as the basis of structural materials and conductive textiles.

Yep. It could be a very good replacement for copper or aluminum wires. And the base material is rather abundant. Coal mines are full of it. On the other hand petroleum or natural gas might be easier to process.

But we are not quite there yet.

So far, the group has made fibers that are highly conductive but not as strong as other carbon materials. Pasquali says the strength of the fibers could probably be improved tenfold by using longer carbon nanotubes. "We're now working on a project for making electrical transmission lines," says Pasquali. "Metallic nanotubes conduct electricity better than copper, they're lighter, and they fail less often."

One important hurdle for large-scale manufacturing of carbon nanotubes remains: Today, there aren't any good methods for making the nanotubes themselves in large, pure batches. In order to make nanotube transmission lines, for example, the Rice group would need to start with a large batch of nanotubes containing all metallic nanotubes and no semiconducting ones. Last month, chemists at the Honda Research Institute published a paper in Science describing a method for making large amounts of metallic nanotubes that Pasquali says is promising. "For transmission lines you need to make tons, and there are no methods now to do that," he says. "We are one miracle away."

And that miracle may have already happened.

What remains to be done after the breakthrough: making enough Carbon Nanotube (CNT) wire to build a test section into the grid. Developing methods for joining the wire to other non CNT segments of the grid. Developing methods for joining CNT segments. Testing it against weather and lightning strikes. And at least 10,000 other details (hand tools among them) will need to be worked out including crew training. Miracles take time to unfold.

And power lines might not be the prime candidate. Lowering the weight of wiring harnesses in automobiles might be a more favorable initial application since weight reduction is worth real money.

H/T GPecchia at Talk Polywell

Carbon Chips

Carbon is going to be the next big thing in computer chips.

Carbon—the basis of all organic compounds—seems destined to displace silicon as the material of choice for future semiconductors. According to researchers, various structures based on the element that sits just above silicon on the Periodic Table can surpass silicon's abilities in thermal performance, frequency range and perhaps even superconductivity.

"Of the carbon technologies, diamond is probably the closest [to commercialization] at this time, as work in diamond has been taking place for 15 years or longer," said Dean Freeman, senior analyst at Gartner Inc. "Most of the others still have a ways to go."

Three-dimensional carbon—diamond—offers 10x the heat dissipation of silicon, according to suppliers currently hawking 40nm to 15µm diamond films on silicon wafers. Two-dimensional carbon—3-angstrom-thick monolayers called graphene—could dismantle silicon's roadblock to terahertz performance by attaining 10x the electron mobility of silicon.

Likewise, one-dimensional carbon—1nm-diameter nanotubes—could solve digital silicon's speed woes. Nanotubes will appear first as printable "inks" that are 10 times faster than competing organic transistors.

Meanwhile, zero-dimensional carbon—60-atom, hollow spheres of carbon called fullerenes—could answer silicon's inability to attain high-temperature superconductivity. Tightly packed fullerenes intercalcated with alkali-metal atoms superconduct at 38K.

Over the next few years, carbon process technologies will become available to replace nearly every circuit material in use today: conductors, for interconnecting devices; semiconductors; and insulators, for isolating devices. But how quickly the industry embraces the carbon-based materials, especially during uncertain economic times, remains to be seen.

So how about some details on how the work is progressing? I have them. Lets look at some Buckyball research first.

Add a drop of oil to buckyballs, and they join together to form wires like strings of pearls.Finally! Something useful from buckyballs.

Junfeng Geng at the University of Cambridge, in the U.K., and buddies have found a way to polymerize these microballs so that they line up into buckywires.

The trick that Geng and co have found is a way to connect two buckyballs together using a molecule of 1,2,4-trimethylbenzene--a colorless aromatic hydrocarbon. Repeat that and you've got a way to connect any number of buckyballs. And to prove it, the researchers have created and studied these buckywires in their lab, saying that the wires are highly stable.

Buckywires ought to be handy for all kinds of biological, electrical, optical, and magnetic applications. The gist of the paper is that anything that traditional carbon nanotubes can do, buckywires can do better. Or at least more cheaply.

The exciting thing about this breakthrough is the potential to grow buckywires on an industrial scale from buckyballs dissolved in a vat of bubbling oil. Since the buckywires are insoluble, they precipitate out, forming crystals. (Here it ought to be said that various other groups are said to have made buckywires of one kind or another, but none seem to have nailed it from an industrial perspective.)

There is more. Go have a look.

Next there is a most interesting substance called graphene, which is a very thin layer of carbon. Its properties can be tuned by applying an electric field to the material.

Semiconductors, for example, can be turned off because of a finite bandgap between the valence and conduction electron bands.

While a single layer of graphene has a zero bandgap, two layers of graphene together theoretically should have a variable bandgap controlled by an electrical field, Wang said. Previous experiments on bilayer graphene, however, have failed to demonstrate the predicted bandgap structure, possibly because of impurities. Researchers obtain graphene with a very low-tech method: They take graphite, like that in pencil lead, smear it over a surface, cover with Scotch tape and rip it off. The tape shears the graphite, which is just billions of layers of graphene, to produce single- as well as multi-layered graphene.

Wang, Zhang, Tang and their colleagues decided to construct bilayer graphene with two voltage gates instead of one. When the gate electrodes were attached to the top and bottom of the bilayer and electrical connections (a source and drain) made at the edges of the bilayer sheets, the researchers were able to open up and tune a bandgap merely by varying the gating voltages.

The team also showed that it can change another critical property of graphene, its Fermi energy, that is, the maximum energy of occupied electron states, which controls the electron density in the material.

"With top and bottom gates on bilayer graphene, you can independently control the two most important parameters in a semiconductor: You can change the electronic structure to vary the bandgap continuously, and independently control electron doping by varying the Fermi level," Wang said.

For those of you not conversant with transistor design and terms like band gap and Fermi energy you might find this book helpful. It is a history of physics and includes the work by Bell Labs on the transistor.

Crystals, Electrons, Transistors



Cross Posted at Classical Values

Understanding Before Voting

The Climate Bill passed the House 219 to 212. Which is a fairly slim margin since the minimum required for a House majority is 218 votes. Now the Senate has to go over it. It will be interesting to see how the Senators from Illinois (a coal state - I met my mate in Carbondale, Illinois) vote.

H/T Watts Up With That?

Cross Posted at Classical Values

Graphene Is Strange

NIST and Georgia Tech have been looking at graphene (a single layer of carbon atoms in a hexagonal grid pattern) and they are seeing some strange behavior.

Graphene’s exotic behaviors present intriguing prospects for future technologies, including high-speed, graphene-based electronics that might replace today’s silicon-based integrated circuits and other devices. Even at room temperature, electrons in graphene are more than 100 times more mobile than in silicon.

Graphene apparently owes this enhanced mobility to the curious fact that its electrons and other carriers of electric charges behave as though they do not have mass. In conventional materials, the speed of electrons is related to their energy, but not in graphene. Although they do not approach the speed of light, the unbound electrons in graphene behave much like photons, massless particles of light that also move at a speed independent of their energy.

This weird massless behavior is associated with other strangeness. When ordinary conductors are put in a strong magnetic field, charge carriers such as electrons begin moving in circular orbits that are constrained to discrete, equally spaced energy levels. In graphene these levels are known to be unevenly spaced because of the “massless” electrons.

When something so unusual comes up in a lab it means a lot of potential applications that can't even be imagined now. One of the reasons for the lack of imagination is that such a material was not even suspected so why would you even spend any time thinking about what you could do with such a material? However, with the current research mapping out such properties the floodgates are open.

The Georgia Tech/NIST team tracked these massless electrons in action, using a specialized NIST instrument to zoom in on the graphene layer at a billion times magnification, tracking the electronic states while at the same time applying high magnetic fields. The custom-built, ultra-low-temperature and ultra-high-vacuum scanning tunneling microscope allowed them to sweep an adjustable magnetic field across graphene samples prepared at Georgia Tech, observing and mapping the peculiar non-uniform spacing among discrete energy levels that form when the material is exposed to magnetic fields.

The team developed a high-resolution map of the distribution of energy levels in graphene. In contrast to metals and other conducting materials, where the distance from one energy peak to the next is uniformly equal, this spacing is uneven in graphene.

The researchers also probed and spatially mapped graphene’s hallmark “zero energy state,” a curious phenomenon where the material has no electrical carriers until a magnetic field is applied.

Expect to hear more about this material and its close relative, carbon nanotubes, in the not too distant future. I can't wait to find out what the applications might be.