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Part XXIV |
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Nanotubes
The diameter of a human hair is equal to 50,000 SWNTs lying side by side. Although the tensile strength of SWNTs hasn't actually been measured, it is expected to be 50 to 100 times that of steel. At the same time, nanotubes weigh only one-sixth as much as the equivalent amount of steel. Ropes of parallel multiple SWNTs are expected to be the strongest fibers in existence, and perhaps the strongest fibers that are even theoretically possible. In the 1970s, science fiction author Arthur C. Clarke in "The Fountain of Paradise" envisioned a cable joining the earth to a geostationary satellite orbiting 22,000 miles above. At that time no material was conceivable that could support its own 22,000-mile-long weight. Now, however, Daniel Colbert of Rice University, TX, thinks nanotubes might fill the bill. Such a cable would allow payloads to be raised to a satellite by electric elevator, without the use of rockets. Nanotubes generally have roughly hemispherical caps at both ends. Methods have been devised for removing these caps and filling the tubes with various substances to alter their physical, chemical, and electrical properties:
Nanotube lengths are now under some control, ranging from less than 100 nanometers to a claimed length of 3 cm. Refined computational techniques have allowed Charles W. Bauschicker, Jr., of NASA Ames Research Center, CA, to study the possible use of a short C200 nanotube to store hydrogen. Such studies are of interest for potential alternate-fuel systems. Shown below is a computer model of C200H220:
Han, Globus, Jaffe, and Deardorff visualize nanotubes with projecting substituents used as gears or rack-and-pinion mechanisms:
Recent successes in attaching substituents to SWNTs lend some credence to such conjectural concepts. The present author, however, cautions against the growing tendency to interpret computer monitor displays as visions of reality. In the meantime, in the seemingly contradictory words of Richard C. Smalley, codiscoverer of buckminsterfullerene and Nobel Prize laureate: "Think nano, think big!". Smalley's group at The Center for Nanoscale Science & Technology, at Rice University, Houston, TX, by 1998 was able to offer 90%-pure SWNTs for sale at a price of $2,000 per gram ($906,000 per pound). Minimum order: $500.00. They were churning out roughly 60 g of "buckytubes" per week. At the same time, Professors Robert C. Haddon and Peter C. Eklund at the University of Kentucky, Lexington, had founded CarboLex, Inc., to develop the production of nanotubes and its engineered derivative materials. A new American Chemical Society journal, Nano Letters, is available free on the Internet at http://pubs.acs.org/NanoLett. Articles can be downloaded in HTML or PDF format. Unclassifiable trivia Did you ever wonder why beer has never been sold in clear bottles? Denis De Keukeleire of Ghent University, Belgium also wondered and explored the matter. When exposed to light, substances called humulones present in hops are photochemically converted to 3-methylbut-2-ene-1-thiol. Diligent followers of this Series can readily identify this chemical as: (CH3)2C=CHCH2SH The same compound is found in skunk secretions. The odor and taste of lightstruck beer are accurately described as "skunky." As little as 1 ng per liter (1 part per trillion, 1:1,000,000,000,000, 1/8 teaspoon in 165,000,000 gallons) can ruin the taste of beer! Thus brown or green bottles are required. Beer producers have now found a way to hydrogenate the humulones in beer to circumvent their photochemical conversion. You can look forward to buying beer in clear glass or plastic containers, sometime in the future. Source: Chemical & Engineering News 79 (22), 72 (5/28/01). The author asks readers to suggest topics for future installments. Please email your suggestions and/or questions to cclaff@cs.com. |
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arbon nanotubes, first characterized in 1991 by Sumio Iijima at NEC Corporation, Japan, can be visualized as sheets of graphite rolled up into cylinders. The first nanotubes were multilayered, consisting of multiple coaxial cylinders. In 1993, single-walled nanotubes (SWNTs) consisting of a single cylinder of hexagonally and/or pentagonally linked carbon atoms were discovered:
