I report here first on new forms of carbon with amazing properties that promise in time a host of inventions. While the carbon-to-carbon bonds can be understood from the quantum nature of the atom, these materials have additional properties predicted from the quantum mechanics of one- and two-dimensional materials. (I'll describe the predictions without getting into the theory.) I conclude with a success story of one application that extensively uses an advanced carbon-fiber composite technology that may someday incorporate some of these materials. It also uses a package of controls and communications equipment that relies heavily on the quantum devices that are described in the next chapter.

GRAPHENE AND RELATED FORMS (ALLOTROPES) OF CARBON

Graphene

Graphene is the basic structural configuration for many forms of carbon: some fullerenes, including nanotubes, and the more familiar graphite, charcoal, and soot. In graphene, carbon atoms are at the vertices of a honeycomb-like arrangement with chemical bonds between them. The bonding carbon atoms form a sheet of connected hexagons all in a single plane, as shown in the scanning probe microscope image of Figure 22.1(a). (Note that boron nitride and other compounds can also be formed in monolayer sheets.)

The effective thickness of the graphene sheet is 3.4 × 10^–10 meters, that is, 0.34 nanometers, where a nanometer is one billionth of a meter (about one millionth the thickness of a dime. Graphene is one hundred times stronger than the comparable thickness of a (hypothetical) sheet of the strongest high-carbon steel. The spacing between carbon atoms in the hexagons of the sheet is 0.14 nanometers, about the same as the spacing between the densely packed carbon atoms in diamond (a measure that is used for the effective diameter of the carbon atom, as shown in Table D.1.) So graphene is very compact in the plane of the sheet. Note that its thickness (given above) is about 2.5 times the carbon-to-carbon spacing. (The stability and strength of graphene, and that of the carbon nanotubes to be described below, are due to carbon's sp² hybrid of s and p spatial states [bonding as described in Chapter 15].)

Fig. 22.1. (a) Graphene magnified approximately twenty million times in this surface probe image. (Image from Wikipedia Creative Commons; file:Graphene SPM.jpg; author: US Army Materiel Command. Licensed under CC BY 2.0.) (b) Model of buckminsterfullerene at a similar scale. (Image from Wikipedia Creative Commons; file:C60 Buckyball.gif; author: Saumitra R. Mehrotra and Gerhard Klimeck. Licensed under CC BY-SA 3.0.) (c) A carbon nanotube with a nanobud at similar scale. (Image from Wikipedia Creative Commons; File:Nanobud.jpg; author: Arkady Krasheninnikov. Licensed under CC BY-SA 3.0.)

Graphene had been observed under electron microscopes in 1962, but it was not studied further.1 However, as a result of its rediscovery in 2004, Andre Geim and Konstantin Novoselov in 2010 received the Nobel Prize in Physics “for groundbreaking experiments regarding the two-dimensional material graphene.”

Graphene, per pound, is the strongest material ever tested. As mentioned in Geim's Nobel announcement, a graphene hammock one meter square that would support a 4 kilogram (8.8 lb.) cat would weigh less than one of the cat's whiskers.²

Wikipedia notes that by the end of 2014, the global market for graphene had reportedly reached just $9 million, mainly to the battery, energy, semiconductor, and electronics industries.³ This is small compared to the billions that are apparently being spent to search out new applications for this substance (see below). To get a sense of the effort, note that a recent publication for graduate students and professionals entering the field uses some 450 pages just to catalogue and briefly describe work as of 2013 relating to this substance and its potential applications.⁴

In a 2014 article in Scientific American, Geim describes graphene as one component in the fabrication of stacks or sandwiches with other monolayer materials, including boron nitride and molybdenum or tungsten disulfides.⁵ These composites can have special properties. He suggested possibly even ambient temperature superconductivity, but he remarked that as yet no “killer” applications have emerged (despite that graphene can be manufactured in sheets of hundreds of square meters). But perhaps that is about to change.

In a late 2015 article titled “Introducing the Micro-Super-Capacitor: Laser Etched Graphene Brings Moore's Law to Energy Storage,” Maher El-Kady and Richard Kaner describe the work of a group at UCLA toward what seems a possible breakthrough technology.⁶ Capacitors, the charge-storing components of electronic circuits, are the only circuit elements (along with batteries) that have not kept pace with the ongoing miniaturization that has allowed the development of small modern user-friendly electronic devices. The authors describe a graphene-based two-dimensional approach that would allow the integration of miniature, flexible, high-energy capacitors into modern solid-state electronic devices. The group is also developing battery/capacitor hybrids, with the aim of reducing the size of the battery as well. Commercial applications are now being explored by Nanotech Energy, a Los Angeles–based startup.

Katherine Bourzac7 describes the use of graphene as a substrate layer upon which other electronic materials can be thinly deposited to make flexible devices, including liquid crystal displays (LCDs) that have flexed a thousand times without degradation. (The displays on our cell phones are LCDs.)

In its pure form, graphene has an exceptionally high electrical conductivity. While this is very good for some applications, it can be a problem for others. John Pavlus describes an effort at IBM ($3 billion in 2014) to explore the use of new materials, primarily graphene, to go beyond our present silicon-based semiconductor technology. “Graphene transistors have been built that operate 100s to 1,000s of times faster than top silicon devices, at reasonable power density, and below 5nm where silicon goes quantum.”⁸ (“Goes quantum” refers to being on such a small scale that the wavefunction of electrons in neighboring parts of the circuit begin to significantly overlap in a way that effectively shorts across circuit elements.) But graphene lacks a band gap (see Chapter 17) and so behaves as a metal rather than as a semiconductor. It can't turn off a current as a transistor does, and so it cannot encode digital logic. (A transistor current is turned on and off to physically store a bit of information, a 1 or a 0, as described in Chapter 8.) Pavlus notes that carbon nanotubes (to be described below) can have a small band gap and be semiconducting. Individual tubes show a fivefold improvement over silicon. But they are fragile and disturbances can remove the band gap.

Note that graphene is the only solid form of carbon in which every atom is available for reaction from two sides. It has a very high opacity (an ability to absorb radiation). And, as a monolayer, its properties are extremely sensitive to its surroundings. Performance can be degraded by impurities, but this opens possibilities for its use in sensors.

Graphite has been found in nature and used for various purposes as early as 4000 BCE⁹ It is composed of graphene sheets stacked one atop the other, but slightly shifted so that the carbon atoms in the second sheet lie above the middle of the hexagons in the first sheet, the carbon atoms of the third sheet line up over the carbon atoms in the first sheet, and so on. Because the bonds between the sheets are relatively weak, the sheets find it easy to slide over one another. That is why graphite is “slippery” and sheets or groups of sheets can rub off, a property that has been conveniently exploited in the manufacture of pencils.

Fullerene

Fullerenes are a family of geometric structures having a monolayer surface of atoms (or molecules) somewhat like that shown for graphene. One of these structures is the “buckyball,” which has the type of configuration shown in the lines or seams of a soccer ball or the support structure of the large spherical geodesic dome at the entrance to Disney World's EPCOT Center. Another geometric form of the fullerene is the nanotube, to be described just ahead.

Figure 22.1(b) shows a model of the buckminsterfullerene C₆₀, prepared first in the laboratory and named “in homage of” Buckminster Fuller, who first started constructing the geodesic domes that this structure resembles. In C_60, carbon atoms are at the vertices of “rings” of hexagons surrounding rings of pentagons (rather than having only the rings of hexagons, as in graphene). There are sixty carbon atoms in the structure. The spacing between the center of one ring and the center of the next is, on average, 0.14 nanometers, about the carbon-to-carbon length described for graphene.

The buckyball type of fullerene has also been produced with complexes of carbon and other atoms at the vertices. Various types of buckyballs have found application in the medical field for gene and drug delivery and in contrast agents for x-Ray and MRI medical diagnostic imaging.

Although first prepared in the laboratory in 1985, fullerenes have since been detected in nature and even in outer space. According to astronomer Letizia Stanghellini, “It's possible that buckyballs from outer space provided seeds for life on earth.”¹⁰

Carbon Nanotube

The carbon nanotube (CNT) can be grown as a tube by various means in the laboratory, but it is also found in less-regular forms in ordinary flames produced by burning ethylene, benzene, and methane and in soot in indoor and outdoor air. The CNT can be thought of as a very long strip of graphene that has rolled itself about its long axis to form a tubular, seamless monolayer. The rows of hexagonal rings can just circle the tube's long axis or spiral around it. There are many variations beyond these structures, including, for example, a nanotube within a nanotube within a nanotube. Depending on the structures and variations, different electrical, thermal, optical, tensile, and compressive properties are obtained.

Figure 22.1(c) shows a model of a short section of a single carbon nanotube of the “ring circling” type with a fullerene “bud” attached (via covalent bonds). These buds may prevent slippage between nanotubes to improve the mechanical properties of bundles of nanotubes in the high-strength composite structures to be discussed just ahead.

Publication of a “discovery” of nanotubes by Sumio Iijima of NEC (Previously Nippon Electric Company) in 1991 produced a “flurry of excitement” about this new material, though a string of observations by investigators in various countries had been reported earlier, starting with work in the Soviet Union by Radushkevich and Lukyanovich published in the Soviet Journal of Physical Chemistry (in Russian) in 1952. Nanotubes have also been found in nature, in charcoal, and in soot.

Carbon nanotubes have been made as narrow as one nanometer in diameter (“nanotube” is fitting), the span of about seven carbon atoms. Like the buckyballs, they aren't visible without the aid of an electron microscope. But these tubes can be made millions of times longer than their diameters, so we have tubes perhaps ten atom widths in diameter extending to lengths on the order of the thickness of a dime. Some nanotubes have been constructed with length-to-diameter ratios of 132,000,000-to-1, a larger ratio then for any other material.¹¹

Though half as strong as graphene, individual carbon nanotubes are still three hundred times as strong as high-carbon steels, and they will stretch by up to 5 percent rather than break under excessive strain (unlike graphene, which is a bit brittle). Most synthesized CNT arrays are hydrophobic (they repel water), but with the application of a low voltage they can become hydrophilic (attract water). Carbon nanotubes are frequently referred to as one-dimensional electrical conductors. Depending on how the rows of carbon rings circle or spiral around the long axis of the tube, the nanotube can be either electrically conducting or electrically semiconducting. The theoretical maximum conductance of a single-walled tube is described as resulting from the tube acting as “a ballistic quantum channel.”12 (A theory-based claim of “intrinsic superconductivity” is in dispute.) In theory, the conducting nanotube can carry a current density one thousand times that of metals such as copper. And at normal room temperatures, the CNT is expected to conduct heat along its long axis almost ten times better than copper, but transverse to that long axis the CNT is a good thermal insulator.

These properties have suggested a host of potential applications. I mention a few here. Note that a high current capacity is desirable for many applications, and a CNT-copper composite has been shown to carry one hundred times the current of pure copper or gold. Coating military aircraft with radar-absorbing CNTs may enhance their stealth capabilities. Toward computing, a nanotube-integrated memory circuit was made as early as 2004, but regulating the conductivity of the nanotube proved to be difficult. (From my experience, it would seem that the exceptional heat conduction of the CNT could help to solve one of the biggest problems limiting the compactness and capacity of computers: removing the heat produced in the switching of the billions of circuit elements in the processor and memory components of the computer.) CNTs are being looked at for improved electrodes in lithium batteries. Solar cells are being developed using a combination of buckyballs and CNTs, the former to trap electrons and the latter to conduct them away to deliver electrical power. CNTs have also been used as tips for scientific-force microscope probes and in medicine to provide a scaffolding for bone growth. And, in what might be an inadvertent and outmoded use, we note that CNTs have been found in Damascus steel from the seventeenth century, perhaps accounting for the legendary strength of the swords that were made from that material.¹³

Nanotubes are just one of a number of structures involved in the development of devices on a nanoscale. The inherent strength of CNTs makes them particularly attractive for this type of application. One experimental use for the nanotubes is for nanoscale bearings, where one nanotube, repelled to the center inside of a larger-diameter nanotube, can rotate essentially without friction. This property has been used to create the world's smallest rotational motor.14

Finally, CNTs may be useful in helping to solve the biggest problem related to solar and wind power: that these sources generate power intermittently and at times when it may not be needed. Ways are sought to store the energy produced during the “down” periods, so that it can be used when and where it is needed. One method is to use the electrical power produced from these sources to electrolyze water into hydrogen and oxygen, so that the hydrogen can be stored and transported. For automobiles in particular, safe storage at ambient temperatures is desired. One way to achieve this is to have the hydrogen molecules or atoms attach themselves to the surface of solid materials in a way that lets them detach for direct use in combustion engines or for conversion back, via fuel cells, into electrical energy to power electric cars. CNTs would offer an enormous surface area (per pound of material) for this kind of attachment. For this and many of the applications that I have mentioned, the challenge may be to produce CNTs of sufficiently high purity at sufficiently low cost.

Current applications mainly use bulk carbon nanotubes (masses of unorganized fragments of nanotubes) added into carbon-fiber composites for improvements in mechanical, electrical, and thermal properties of the bulk product. Composites containing CNTs have been used, for example, for added lightweight strength in bike components. (Note that carbon-fiber composites, even without CNTs, already have mechanical properties superior to the best of steels.)

LARGER-SCALE APPLICATIONS

The Space Elevator

Before leaving CNTs, I mention what I call a “way out” (pun and two meanings intended) potential application. (I don't expect you to take this too seriously.)

Carbon nanotubes may be the only material strong and lightweight enough to enable the construction of a “space elevator.” This device would allow payloads to be lifted up to an altitude of 22,000 miles (the altitude of stable Earth orbit) or beyond, there to be released for whatever missions may be involved. (This would save a lot of rocket fuel, and maybe rockets as well.)

The elevator was first suggested by Konstantin Tsiolkovsky, one of the founding fathers of rocketry and astronautics,15 in 1895 (in his “Speculations about Earth and Sky and on Vesta”).¹⁶ He suggested using a tower. However, since 1959, most ideas have involved tensile structures. And that is where the CNT comes in: the application depends on a lightweight and very strong cable, and the only possibly viable prospect to date is one constructed of carbon nanotubes.

The elevator car would be hooked onto the middle of a cable between an orbiting “counterweight” in the sky (the “sky hook”?) and an anchor station somewhere on the equator. The counterweight would ride substantially above the 22,000-mile altitude, being thrown outward by the centrifugal force of the earth's rotation and kept from flying away into space by the equal inward centripetal downward pull of the cable, plus gravity. (The net of the downward gravitational force minus the upward centrifugal force is referred to as the force of the “apparent gravitational field.”)¹⁷ As the cable is paid out from the anchor station, the anchor (and the elevator hooked on far below it) would rise into space. To bring the elevator back down for a reloading, the anchor station would reel it in. The anchor, lower now but still well above the stable Earth orbit and pulling strongly upward, would be ready to lift the elevator again with its new payload.

It's time to come down to Earth. Well, almost.

Carbon-Fiber-Reinforced Polymer Composites¹⁸

Carbon fibers are about five thousand times the diameter of nanotubes, but still pretty small, about one-tenth the thickness of a human hair. They may be produced, for example, using yarns wound from rayon, from which all constituents except the carbon are driven off. In one approach for making the composite, carbon-fiber yarns can then be woven into fabrics, which are cut to size, layered in an appropriate mold, and impregnated with an epoxy resin. These composites have already been used in a host of applications, including sporting goods and automobiles. But at the turn of the century, another application was on the drawing boards at Boeing.

The Dreamliner19

Boeing Company may have staked its future on carbon-fiber-reinforced polymer (CFRP) technology in developing its model 787 Dreamliner, the most advanced new generation in commercial aircraft, as it entered commercial service in 2011.²⁰ Following tests performed earlier in relation to military aircraft design, wings and fuselage are no longer made from sheets of aluminum but instead use CFRPs to form a super-strong lightweight component. By making this plane lighter and stronger and by using advances in control systems, Boeing has been able to improve range and fuel efficiency (by 20 percent) without increasing the size of the aircraft, bucking the historical trend. The longest-range variant of the 787 can fly over nine thousand miles (New York to Hong Kong) without refueling. As of March 2016, Boeing had orders for 1,139 aircraft from 62 customers. (The development and flight testing of the 787 has been beautifully presented in an IMAX 3-D movie called Legends of Flight.²¹)

But this is a competitive world. Responding to the threat that the Dreamliner posed to its business, Airbus in January 2015 introduced its A350, an aircraft of comparable capacity that is made 53 percent of composite structures, as opposed to Boeing's 50 percent. Variants of the two competing aircraft seat 225 to 350 passengers at a cost per aircraft ranging from $225 million to $356 million.²²