stickmaker | JOHT 10A: Hanging Tough

The Joy of High Tech

Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. And if you find a mistake, please let me know about it.

Material World: Hanging Tough

Science Fiction has a long history of including in stories mention of fantastic materials with incredible properties, often with no basis in fact. Usually what makes these substances stand out is their strength. Such materials range from the arenak of Doc Smith to the scrith of Niven, and probably beyond for a considerable distance in both directions. (Jonathan Swift's 1726 satirical novel _Gulliver's Travels_ mentions an adamantine material, which is used by the people of the flying island of Laputa, for example.) Sometimes, though, a story will contain mention of something fantastic, but real. One of the most persistent of these is perfect, monocrystalline iron.

I haven't been able to trace the first usage of this material in SF, but I do know it was used in van Vogt's _Slan_. It has also been used by Larry Niven several times, and was a main element (pun intended) in his _Descent of Anansi_. This material was first made and tested in laboratories in the Twenties, and it wouldn't surprise me if the earliest use in a work of fiction came shortly afterwards. The stuff is a natural for SF stories, especially those focusing on technology and invention. Perfect iron is roughly one hundred times as strong as common steels, and four times as strong as the very best, super-exotic steels. Yet it retains much of the elasticity of wrought iron, enabling it to flex and absorb impacts without damage. Fantastically hard, chemically resistant, cheap if we can learn how to make it in large quantities, it is perfect for orbital tethers, though not quite good enough for a Beanstalk.

Measuring and describing in a meaningful way the properties of materials is a complicated business. You have hardness, elasticity, three types of yield strength, three types of ultimate strength and so forth. I'm going to concentrate on the yield strength, which is the maximum value before permanent deformation. I will restrict this discussion to values for tension, compression and shear, and I'm going to use units of Newtons per square centimeter, which is a bit unconventional but understandable by anyone in the materials testing business, or who is familiar with the metric system. This is greatly simplifying the study of materials properties, but do you _really_ want to know what Young's Modulus measures? I didn't think so...

As mentioned above, perfect iron has been made and tested in the laboratory... in laboratory quantities. Whiskers (to the materials engineer, a whisker is a fibre with no imperfections) were made by assembling atoms into a perfect matrix in a liquid or vapor phase deposition process. (There may be other methods.) Now that I've explained my units, I can report that the figures for perfect iron are approximately 4,600,000 for tension, 4,600,000 for compression, and 660,000 for shear in Newtons per square centimeter, respectively. The values for a typical mild steel, such is normally used for beams or concrete reinforcement in structures, are 46,300, 46,300, and 38600. An excellent commercial steel would have typical values of 463,400, 463,400 and 380,000.

Why is this perfect iron so much stronger than normal iron, or even steel? The secret is that there are no disruptions in the crystal structure of the metal to create weak spots. A chain is only as strong as its weakest link, after all. A casting or forging is only as strong as its biggest flaw. That's part of the reason why we add carbon and other elements to iron to make steel. (Another reason is to increase the hardness, workability, chemical resistance, etc.) These additions help reduce the size and number of and bridge the gaps caused by atomic misalignments which occur during the normal processes of iron and steel manufacture. In fact, many of the steps used in making steel are designed to reduce the number, size and detrimental effect of such flaws.

Unfortunately, these alloying materials also provide their own weak spots, places where corrosives - water being a major one, if only because it can carry so many others, such as salt - can chemically attack the metal. Pure iron is very resistant to corrosion, though wrought iron also has traces of silicon which helps with that. The Roebling bridge over the Ohio River doesn't really need to be painted, since it is made of wrought iron. (One of my materials instructors in college was involved in testing this structure, something he related in a class I took in the late Seventies.) A bad paint job can actually accelerate corrosion, since paint that doesn't bond properly to the structure will separate from it, leaving a gap between paint and metal. Water and de-icing salt make their way into this gap and stay there, working on the metal for long periods, unseen. However, if a bureaucrat decides something needs painting...

Of course, there are other materials besides iron, and many of them have also been tested in the form of perfect whiskers. The most impressive of these is carbon, in various forms. Perfect diamond is a another real material. Perfect diamond whiskers have values of 20,500,000, 20,200,000, and 12,100,000 N/cm^2 in tension, compression and shear. That's quite impressive. Hardness is closely related to tensile strength, so you can see why diamond is so hard.

There are other forms of carbon which have impressive test results. Graphene is a sheet of carbon atoms in a hexagonal arrangement (13,050,000, 36,100 and 6,000,000 N/cm^2). Fold those into nanotubes, or Buckytubes, and you have something even stronger, due to the elimination of edge effects. (Perfect single-walled nanotubes: 30,000,000, 16,100,000 and 16,100,000. Best produced so far: 2,200,000 for tension; other values not reported. Minimum needed for a beanstalk: 13,000,000, 7,000,000 and 7,000,000. Double-walled and other carbon nanotube configurations can be even stronger, but are denser.)

There is a theoretical - and probably impossible - material which is far stronger than any of these. It, too, uses carbon, but in the form of stand-alone, benzine-like rings. These are looped through each other in a three-dimensional matrix, and the impressive figures (1.0 X 10^15 (that's a 1 followed by 15 zeroes), 9.3 X 10^14, and 9.3 X 10^12 N/cm^2) for the yield strengths come from the fact that not only is deformation resisted by the normal molecular bonds, but by the mutual repulsion of the shared electron clouds around the rings. As you can imagine, this also makes the material extremely rigid, and very hard.

There's more to a material than just strength, of course. Hardness is important for resistance to wear, and as mentioned above is directly proportional to tensile strength. There's also density. A long cable must be able to support both its own weight plus a useful load. Iron and steel have a density ranging from roughly 7.1 to 8.0 grams per cubic centimeter; diamond of a little over 3.5. A load/mass factor can be calculated simply by dividing the tensile strength by the density. This gives values of 585,200 for perfect iron and 5,758,400 for perfect diamond. You can see from this that diamond is much more desirable for long cables. Like to geosynchronous orbit.

So, how long before we ride cables of perfect diamond or Buckytubes to a space station? Probably quite a while. It is just too hard to make long, perfect cables with current techniques. However, if we are willing to settle for less than perfection, there are several ways to get most of the potential benefits of perfect materials without having to actually make something perfect.

One of the most promising is vapor phase deposition of diamond. This was originally developed to create substrates for electronic circuitry in integrated chips. Diamond is an extremely good conductor of heat, and an excellent electrical insulator, making it ideal for this purpose. The process is simple, at least in theory. Carbon is vaporized in a vacuum chamber and allowed to settle onto a suitable material. Do this right and you get a uniform, near-flawless layer made of a single diamond crystal. Something easier to make is amorphous diamond, which is as good for most purposes. However, you can do more with this process than make thin, flat sheets.

One Japanese company is already marketing surgical instruments with bonded diamond coatings on the cutting edges. This produces blades that are incredibly sharp, very resistant to wear (to put it mildly) and only slightly more expensive than ordinary stainless steel. Several chemical research companies are experimenting with diamond-coating fine wires. Tests have shown that the resulting diamond coating approaches the strength of perfect diamond whiskers. All we have to do is set up a process to make these diamond-coated wires in continuous lengths, remove the substrate (perhaps by etching for a metal, or just leave it, if it's something like a high-strength graphite strand) and we have Beanstalk material. (Keep in mind that Robert L. Forward assigns using diamond as a structural material to the category of "indistinguishable from magic." ;-)

Hollow tubes are actually structurally superior to solid wires in many applications. Another advantage for Buckytubes. Single-walled carbon nanotubes have a density of around 1.3 grams/cubic centimeter.