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.
Science Fiction has a long history of including in stories fantastic materials with incredible properties, often with no basis in fact. These range from the arenak of Doc Smith to the scrith of Niven. 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 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 and may even be 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) generally are made by assembling atoms into a perfect matrix in a liquid or vapor phase deposition process. Now that I've explained my units, I can report that the figures for perfect iron are 4,600,000, 4,600,000, and 660,000 in Newtons per square centimeter for tension, compression and shear, respectively. The values for a typical mild steel, such as is normally used for 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 and 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.) 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, the inclusions of alloying materials also provide spots where corrosives - water being a major one - can chemically attack the metal. Pure iron is very resistant to corrosion, which is why wrought iron is used for lawn furniture. 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 for corrosion about thirty years back. He says if they had known it was made of wrought iron beforehand they wouldn't have bothered.) 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 the forms of both diamond and graphite. 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.
Perfect diamond is a another real material, but there is a theoretical material which is far stronger. It, too, uses carbon, but in the form of 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 hard. (My thanks to Dr. John Brantley for telling me about this.)
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. Such as to geosynchronous orbit.
So, how long before we ride cables of perfect iron or diamond 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 of perfect materials without having to actually make something perfect.
One of the more promising is vapor phase deposition of diamond. This was originally developed to create substrates for electronic circuitry in integrated chips. Diamond is the best normal 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. 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 which 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, with the metal substrate etched out, and we have Beanstalk material. (And 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 real material that comes in a hollow fibre form is buckytubes, or single-walled carbon nanotubes (SWCN). These have theoretical maximum yield strengths of 30,000,000 16,100,000 and 16,100,000, with values of 12,000,000, 7,000,000 and 7,000,000 being more likely. That's pretty respectable. Density is 1.300, which is even better. Buckytubes should also be easier to make than either diamond tubes or perfect iron wires. One NASA study developed details of making and deploying a Beanstalk using buckytubes. The only showstopper is motivation.
| Yield Strengths for Various Materials | |||||||||||||
| 01/16/2003 | |||||||||||||
| Yield Strengths | |||||||||||||
| (N/cm^2) | Failure | Specific | Melting | Specific | |||||||||
| Density | Temperature | Tensile | Point | Heat | |||||||||
| Material | Type* | Tensile | Compressive | Shear | Tested | g/cm^3 | (kelvins) | Strength | (kelvins) | (J/kg-k) | |||
| A 286 | T | Y | 7.944 | ||||||||||
| ABS HT9000 | G | 6200 | 8100 | Y | |||||||||
| ABS polymer | G | 6200 | 8500 | Y | 1.050 | 370 | 59 | ||||||
| ABS-Polycarbonate alloy | G | 6300 | Y | 370 | |||||||||
| Acetal homopolymer | T | 9900 | 15600 | Y | 1.050 | 375 | 94 | ||||||
| Acetal homopolymer HV (Delrin 900 & Tenac 7010) | T | 6800 | 10300 | Y | 1.420 | 48 | |||||||
| Acetal polymer (POM) | G | 15200 | Y | 1.420 | 107 | ||||||||
| Acrylic | G | 7600 | Y | 380 | |||||||||
| Acrylic-styrene-acrylonitrile (ASA) | G | 4200 | Y | 380 | |||||||||
| Aerogel, typical silica | T | 1600 | Y | 370 | |||||||||
| Alloy 253MA | T | 71,700 | Y | ||||||||||
| Alloy 600 | T | 66,200 | Y | ||||||||||
| Alloy 601 | T | 70,300 | Y | ||||||||||
| Alloy 800H | T | 56,600 | Y | ||||||||||
| Alumina (AD-995) | T | 11,900 | 260,000 | Y | 0.003 | 39667 | |||||||
| Aluminum | A | 15,000 | 10,000 | Y | 2.720 | 55 | |||||||
| Aluminum | G | 57,000 | 34,000 | Y | 2.600 | 219 | |||||||
| Aluminum | G | 60,000 | 90,000 | Y | 2.600 | 231 | |||||||
| Aluminum | P | 185,000 | 262,000 | 2.600 | 712 | ||||||||
| Aluminum | T | 9,200 | 6,600 | Y | 3.900 | 24 | 880 | ||||||
| Aluminum 2014-T6 | A | 48,300 | Y | 2.796 | 173 | ||||||||
| Aluminum 6061-T6 | A | 31,000 | 20,600 | Y | 2.713 | 114 | |||||||
| Aluminum Bronze | G | 55,000 | Y | 7.778 | 71 | ||||||||
| Aluminum Bronze | E | 62,000 | Y | 7.778 | |||||||||
| Aluminum Bronze (5% to 7.5% Al) | E | 527,300 | 843,600 | Y | |||||||||
| Aluminum (B51S, NS 17305) YS+A220 | A | 30,000 | Y | ||||||||||
| Aluminum 5083 (MIL-A-46026) | E | 35,000 | Y | 2.657 | 132 | ||||||||
| Aluminum 7001-T6 | E | 67,500 | Y | 2.700 | 250 | ||||||||
| Aluminum alloy (96 Al - 4 Cu) | T | 24,000 | Y | 2.600 | 92 | ||||||||
| Aluminum alloy (96 Al - 4 Cu) | G | 41,500 | Y | ||||||||||
| Aluminum Oxide | P | 4,600,000 | 1,690,000 | ||||||||||
| Aluminum Weldalite 049-T81 | T | 46,000 | 270,000 | Y | 2.600 | ||||||||
| Aluminum, cast | E | 105,454 | 84,363 | Y | 2.700 | 391 | |||||||
| Aluminum, GIGAS24 or GIGSA30 | E | 70,000 | Y | 3.000 | 233 | ||||||||
| Aluminum/short alumina fibre metal/ceramic composite | T | 53,100 | Y | 3.800 | 140 | ||||||||
| Aluminum-lithium (Weldalite Al 2195) [100ksi] | E | 68,950 | Y | ||||||||||
| Americium | T | 13.670 | 1446 | ||||||||||
| Ampco No. 18 | T | Y | 7.584 | ||||||||||
| Antimony | T | Y | 6.690 | ||||||||||
| Asbestos | P | 586,000 | Y | 2.400 | 2442 | ||||||||
| Asbestos | T | 6,890 | Y | 3.800 | 18 | ||||||||
| Babbit (Lead/Tin) | G | 7,000 | Y | 2.400 | 29 | ||||||||
| Bakelite | T | 10,000 | 24,000 | Y | |||||||||
| Bamboo | T | 7800 | Y | ||||||||||
| Beanstalk (minimum) | 1,380,000 | ||||||||||||
| Beryllium | S | 330,000 | Y | 1.850 | 1784 | ||||||||
| Beryllium | E | 139,000 | Y | 1.800 | |||||||||
| Beryllium aluminum | E | 1.870 | 0 | ||||||||||
| Beryllium Copper | E | 138,000 | 66,000 | Y | |||||||||
| Beryllium IF-1, Foil Grade | T | 30,300 | 13,500 | Y | 1.844 | 164 | 1553 | 1925 | |||||
| Bone, long | G | 13,800 | Y | ||||||||||
| Borazon | G | 5,000,000 | Y | ||||||||||
| Boron | A | 347,600 | Y | 2.340 | 1485 | ||||||||
| Boron | E | 690,000 | Y | 2.450 | 2816 | ||||||||
| Boron | T | 138,000 | 310,000 | 12,000 | Y | 2.460 | 2350 | ||||||
| Boron Composite | T | 154,500 | 347,600 | 13,100 | 2.500 | 618 | |||||||
| Brass | E | 75,000 | Y | ||||||||||
| Brass (66% Cu - 34% Zn) | T | 48,300 | Y | 8.500 | 57 | ||||||||
| Brass (70% Cu - 30% Zn) | A | 55,000 | Y | ||||||||||
| Brass (70% Cu - 30% Zn) | E | 197,500 | 189,100 | Y | |||||||||
| Brass (83% Cu - 17% Zn) | E | 229,200 | 163,100 | Y | 8.500 | 270 | |||||||
| Brass (85% Cu - 15% Zn) | T | 41,500 | Y | 8.500 | 49 | ||||||||
| Brass, Red | T | Y | 8.747 | ||||||||||
| Brass, Yellow | T | Y | 8.498 | ||||||||||
| Brick (building) | T | 21,000 | Y | ||||||||||
| Brick (fireclay) | E | 105,500 | Y | 2.300 | 0 | ||||||||
| Bronze | T | Y | 2.100 | 0 | |||||||||
| Bronze (70% Cu - 30% Sn) | E | 39,400 | 1,033,400 | 85,100 | Y | 8.800 | 45 | ||||||
| Bronze (76% Cu - 24% Sn) | E | 154,700 | 801,400 | 225,000 | Y | 8.800 | 176 | ||||||
| Bronze (80% Cu - 20% Sn) | E | 232,000 | 548,300 | 398,600 | Y | 8.774 | 264 | ||||||
| Bronze (87% Cu - 13% Sn) | E | 206,700 | 372,600 | 242,500 | Y | 8.800 | 235 | ||||||
| Bronze (92% Cu - 8% Sn) | E | 200,400 | 295,300 | 307,200 | Y | 8.800 | 228 | ||||||
| Californium | T | 15.300 | 0 | 1173 | |||||||||
| Carbon (Buckytubes; Dyneema; B & C estimated ) | T | 300,000 | 140,000 | 140,000 | Y | 1.300 | 2308 | ||||||
| Carbon (Buckytubes; Single-Wall Nanotubes, or SWNT) | T | 1,960,000 | 920,000 | 920,000 | Y | 1.300 | 15077 | ||||||
| Carbon (Buckytubes; Single-Wall Nanotubes, or SWNT) | G | 6,300,000 | 2,900,000 | 2,900,000 | Y | 1.300 | 48462 | ||||||
| Carbon (Buckytubes; Single-Wall Nanotubes, or SWNT) | E | 13,000,000 | 7,000,000 | 7,000,000 | N | 1.300 | 2051 | 100000 | |||||
| Carbon (Buckytubes; Single-Wall Nanotubes, or SWNT) | P | 30,000,000 | 16,100,000 | 16,100,000 | N | 1.300 | 2051 | 230769 | |||||
| Carbon (Diamond) | P | 10,000,000 | 50,000,000 | 12,100,000 | Y | 3.515 | 28450 | 6.195 | |||||
| Carbon (Diamond, Type Ia) | E | 5,000,000 | 20,000,000 | 6,000,000 | Y | 3.515 | 14225 | 6.195 | |||||