Friday, June 5, 2009

Strange Carbon Allotropes, Part 1

Besides diamonds, carbon has another dozen or so allotropes, all of which are also very strange. This first part includes the massed molecules; the second part will include smaller carbon groups.

Lonsdaleite, also called hexagonal diamond (diamond is normally cubic), is an sp3-hybridized compound (like diamond) with similar characteristics. It forms when graphite-containing meteorites impact the earth. The heat and force of the impact turns the graphite into diamond-like material that still retains the hexagonal shape of the graphite. Although natural samples only have a Mohs hardness of 7-8 due to impuritites, compared to 10 for diamonds, pure sample could theoretically be 58% ahrder than diamond.

Graphite, better known as pencil lead, is formed of layers of hexagonal sheets that slide easily over each other. this allows it to slide onto paper, or over each other for lubricants. Whereas diamond is one of the strongest known solids, graphite is one of the weakest. It has some applications in high-temperature crucibles. Pyrolytic graphite, which is artificially produced, has some covalent bonds across the sheets. This make sit stronger for uses like rocket nozzles, as well as making it diamagnetic, and thus able to levitate stably over a block of magnets.

Although the layered sheets of graphite make it very weak, graphene, a single hexagonal sheet of carbon, has incredible strength and amazing properties. It's incredibly strong for just a single layer of atoms; in fact, it's the strongest material known. It's got very strange electrical, quantum, and semiconducting properties that I can't even begin to understand, much less simplify and explain. It shows promise for semiconductor devices like field-effect transistor, intergrated circuits, gas sensors (since its entire area is exposed), transparent electrodes for LCDs, biodevices, and ultracapacitors capable of holding farads of charge in components that currently hold just microfarads.
The craziest thing about graphene, though, is its reflective ability. It reflects 2.3% of light (exactly: 2.29253%), a very high amount for a single layer of atoms. However, the even cooler thing is that number, 0.0229, is exactly equal to pi times a, the fine structure constant.

Glassy carbon is an irregular form of carbon that combines glassy and ceramic properties with those of graphite. Like other glassy substances, it exhibits conchoidal fracture.

Amphorus carbon is carbon that has absolutely no regular bonding, exhibits 'dangling' bonds, and is not as glassy as glassy carbon. Impure forms of it include coal and soot.

Tetrahedral amphorus carbon, also known as diamond-like carbon, is a general term for 7 different forms of amphorus carbon that exhibit diamond-like properties. It contains high numbers of the sp3 bonds seen in diamond. It's very hard and mostly used for coating for drill bits, molds and dies, engine components for racing vehicles, and as protection on the platters of hard drives.

Activated carbon is charcoal, mostly pure carbon, that has been processed to increase its surface area - to about 500 square meters per gram. It's used in gas purification, filtering, and removing pollutants from water systems.

Chaoite, aka white carbon, is a grey-white allotrope of carbon that may or may not exist - the experimental evidence is uncertain. If it exists, and most chemists think it doesn't, then it may be a form of carbyne, a string of carbon atoms connected by alternating single and triple bonds.

Carbon nanofoam is 'a low-density cluster-assembly of carbon atoms strung together is a loose three-dimensional web' (quote from wikipedia). Each cluster contains about 400 carbon atoms and the overall structure, which has negative curvature due to the inclusion of heptagons in the web, is like an aerogel, but with 1% the density - only about 3 times that of air. It's a poor electrical conductor - unlike carbon aerogels - but below -183° C (its Curie point) it is highly magnetic, and even at higher temperatures it can be attracted by magnets.

Carbon nanotubes are cylindrical tubes formed of hexagonal carbon structures. The tubes are formed entirely of sp2 bonds. They have incredible strength and exhibit unusual effects, such as concentric nanotubes sliding and rotating within each other, in essence forming nearly perfect bearings. They have unusual electrical properties, including high current-carrying capacity and semiconductor effects that vary depending on the structure of the nanotube.
Their potential applications are nearly endless; however, their toxicity is not yet fully determined and may be significant. Their hight tensile strength leads to uses including sports equipment, bulletproof vests, tear-resistant clothes, better concrete, long-lasting flywheels for energy storage, and the 'space elevator'. Electrical applications include artificial muscles, 'buckypaper' for heat sinks and Faraday cages, conductive films, motor brushes, magnets, optical ignition of small explosive charges, solar cells, ultracapacitors, superconductors, displays, and transitors, as well as high-speed field-effect transistors and diodes.
They may be useful for air, water, and hydrogen filters, as well as 50GHz mechanical oscillators, membranes, and surfaces slicker than teflon.

All information from Wikipedia.

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