9. Ultrastrong Fibers

All of this must seem as a far cry from the days when new materials meant bronze, or stainless steel or Bakelite casings or cement. But this survey will conclude by returning to such traditional structural roles of materials. We will always need to fabricate structures that execute some passive function without snapping, cracking, crumbling, wearing away, or collapsing. Technological advances pose new challenges to the strength, toughness, and resilience of materials, and nowhere more evidently so than in space engineering. These demands may be usefully illustrated by taking a leap into the future: one that admittedly may never materialize, but which at least has a good pedigree, as it comes from the fertile mind of Arthur C. Clarke.

In his novel The Fountains of Paradise, Clarke posited the Space Elevator: a platform positioned in geostationary orbit around the Earth, tethered to the ground by a long, superstrong cable. Space hardware is shuttled up to the platform via an elevator, from where it can be launched into space with a fraction of the fuel requirements needed to escape the Earth's gravity from ground level. As most of a rocket's mass consists of the engines and fuel needed for this ascent, the savings imparted by the Space Elevator are substantial.

But what manner of thread could be relied upon to tie a platform, possibly manned, to the Earth's surface? Today's materials science provides two candidates, and both are forms of pure carbon.

The chemical bond between two carbon atoms is one of the strongest and stiffest known: it is ultimately responsible for the tremendous hardness of diamond (although the relationship between bond strength and hardness is by no means simple, or even fully understood). Diamond's sibling allotrope graphite, in contrast, has a reputation for weakness: its flaky layers can be rubbed off simply by the passage of pencil over paper, and for this reason, graphite makes a good lubricant. But the weakness is all in one direction. Graphite consists of sheets in which carbon atoms are linked into adjoining hexagons, like chicken wire. The bonding between the sheets is weak, since there are no bonds "left over" on each carbon atom; so the sheets slide easily over one another. But the bonding within the sheets is very strong. It is just that we do not get to see this, because the stacks of sheets form tiny crystallites with little cohesion between them. The potential strength of graphite-like (graphitic) carbon is evident, however, in carbon fibers, where the sheets are all oriented and are linked by a few strong bonds.

In 1991, the ultimate carbon fiber was discovered: sheets of graphite-like carbon rolled up on themselves into tubes just a few nanometers in diameter. The first of these "carbon nanotubes" were many-layered: tubes inside tubes, like Russian dolls, each one separated from its neighbors by the same distance that divides the flat sheets in graphite. But nanotubes have since been made that have only a single layer, and it is now possible to exercise at least a little control over the width of the tubes.

Figure 21. Carbon Nanotubes are Hollow Cylindrical Structures of Pure Carbon, Linked into the Hexagonal Sheets Characteristic of Graphite

The carbon atoms in nanotubes are linked into hexagons, which are arrayed around the side of the tube in a spiral fashion. The properties of the tubes can depend very sensitively on this spiral arrangement-on the pitch, for example. Certain spirals confer electrical conductivity, while others give insulating nanotubes. This opens up the possibility of using carbon nanotubes as "molecular wires," thinner than the thinnest wires that can be carved lithographically into metal or semiconductor films. And theories predict that the electrons, confined essentially to one dimension in these tiny wires, will exhibit unusual quantum-mechanical behavior as a result of this reduced dimensionality. Moreover, if the nanotubes can be modified at certain locations, perhaps by introducing kinks or dopant molecules into the framework, the electronic properties might be altered in ways that become useful for electronic engineering. Transistor-like and rectifying behavior has already been seen in carbon nanotubes.

There are many other possible applications of nanotubes that exploit their small size, hollow nature and electrical conductivity. But perhaps the most dramatic application could take advantage of the fact that, as an essentially crystalline form of pure graphitic carbon, they should be extremely strong, and also very stiff. Experiments have provided some justification for the belief that nanotubes are at least as stiff as diamond, and several times more so than steel. Lacking the imperfections that exist in conventional carbon fibers (which are orders of magnitude larger), carbon nanotubes may be the strongest of all human-made fibers.

One of the challenges in putting these exceptional properties to use, however, is that of growing nanotubes to great lengths-typically, they are no longer than a micrometer or so, and capped with a hemisphere or polygon of carbon hexagons and pentagons. The dream is to understand the formation process well enough that nanotubes could be grown to any length, like spaghetti feeding continuously from an extruder.

Yet nanotubes are not the only contender for the Space Elevator's tether-for strong fibers can be fabricated from carbon's other form, diamond. Synthetic diamond has been known since the 1950s, and most of that currently used as an industrial abrasive and in cutting tools is created by exposing carbon-rich material to very high temperatures and pressures. Yet diamond can also be grown at low pressure from a carbon-containing gas, typically methane fragmented into free radicals by heat or radio waves. This is a form of chemical vapor deposition (CVD), and it may be used to deposit thin films of diamond on a substrate. Diamond films grown by CVD are generally polycrystalline: a mass of tiny grains fused together. Diamond coatings of this sort offer to confer wear-resistance and good frictional characteristics on machine parts.

Diamond wires, meanwhile, can be made simply by depositing CVD diamond onto metal wires. Iron dissolves in diamond to form a carbide; but other metals, such as titanium and molybdenum, provide suitable substrates.

Figure 22. A Diamond-coated Metal Wire Represents a Very Strong Fiber,
which could be Used to make Ultra-tough Fiber-reinforced Composites
If the wire is coiled, diamond coating produces a hollow diamond tube

Used in fiber composites, these diamond fibers can engender a much greater stiffness than that offered by conventional "stiff" fibers such as silicon carbide. A diamond-fiber/titanium alloy has even been proposed as the fabric of future spacecraft by one of the scientific advisers to the Star Trek series, blurring the lines between fact and fiction. And diamond tubes, made by coating coiled wire with a diamond film, could contain an air-curing adhesive that would provide a self-healing capability to damaged fibers. Whether even this will keep a Space Elevator in place remains to be seen.

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