Science Communications - Publicity for Technologya


carbon black magic

Should the periodic table ever stage a magic show, carbon is sure to dazzle the audience.  In its pure molecular forms carbon is variously, hard and soft, black and sparkling, long and short, conductive and non-conductive.  It is the element of life and health, love and death, food and heat, soot and gems.  It resides in the food we eat, the clothes we wear, the fuel we burn, the soil we till, and most importantly, in the bodies that we inhabit. 


Carbon’s ubiquity comes in large part from its four valence electrons, which present the element with a great number of opportunities to bond with other atoms.  Throughout the kingdom of chemistry, carbon is renowned for its gregarious and accommodating nature.  And although carbon’s strong and gentle bonds with other atoms make the miracle of life possible, it is also unique among the elements for its ability to make long, complex molecules of itself alone.


As Mother Nature has it, electrons orbit atomic nuclei in a series of concentric shells or clouds.   As the number of electrons increases from one element to the next, they occupy the shells in a prescribed order and are limited by each shell’s preordained electron capacity.   For an atom to be completely happy, all its shells should be completely full.  This blissful state of fullness is achieved by sharing outer shell electrons with other atoms.  Carbon has six electrons and two electron shells.  In accordance with Mother Nature’s book of magic, the two shells have room for a total of ten electrons: two in the first shell and eight in the second.  Carbon’s first shell is filled to capacity by two of its electrons.  That leaves four electrons in the outer, or valence, shell.  Since carbon’s valence shell has room for eight electrons, but only four taking up residence, carbon atoms are happy to bond with neighboring atoms in order to share their valence electrons.   As electron bonds go, carbon bonds are strong.  Carbon-to-carbon bonds are even stronger as a consequence of the element’s ability to create hybrid bonds with other carbon atoms in which two or three pairs of electrons may occupy a single shared orbit. 


Carbon comes in four allotopes, or types.  They are amorphous carbon, graphite, diamond and fullerenes.  (Actually, there’s a fifth in the laboratory, but we’ll talk about that some other time.)  Carbon black is amorphous carbon; it has no regular molecular structure.  Carbon black can be made by burning natural gas, oil, wood, vegetables or other organic matter and is used mainly as a pigment and as an additive in rubber to extend tire wear.    Graphite, the lead in pencils and solid lubricant, has a repeating flat hexagonal structure, very much like chicken wire.  Graphite is slippery and transfers to paper because it is comprised of flat layers separated by small, weakly bonded spaces that permit the layers to slide across each other.  Graphite’s loose structure allows it to conduct electricity.  Graphite has been used for years in battery and arc lamp electrodes.  Diamonds have a very rigid cubic structure that gives all the electrons a permanent place of residence.  As a consequence, diamonds are the hardest of all elements and are electrical insulators.


As a matter of practice, carbon synthesis by humans predates recorded history.  Prehistoric charcoal production promoted the advent of the Iron Age by facilitating the refinement of iron-bearing ores into metallic iron.  For the Caesars, carbo meant charcoal.  It is all but impossible to look at the world, either ancient or modern, and not find carbon in one form or another.  Although we have known about carbon for millennia, it is clear that it has more tricks up its sleeve than we can imagine.  For example, buckminsterfullerenes are a novel set of nanoscale carbon structures, the first of which was discovered in 1985 by a group of scientists including the recently deceased legend of nanoscience, Richard E. Smalley.  Called fullerenes for short, their physical properties make them the proverbial top hat, cape and wand for an anxious world of materials scientists.


Fullerenes are named after the renowned architect, inventor and author, R. Buckminster Fuller, who received a patent for his now-famous geodesic structures in 1965.  At base, Fuller's structures exploit the fact that ounce-for-ounce triangles are stronger than rectangles.  The upshot was the invention of a structural system reputed to be the world's strongest and, contrary to conventional intuition, that became stronger with increased size.  If you look at an illustration of a fullerene, you will notice six-sided (hexagonal) and five-sided (pentagonal) structural elements - no triangles. However, chances are that in less than two minutes of doodling you can prove to yourself that you can make both hexagons and pentagons out of triangles.  Surely Fuller had no clue as to the existence of three-dimensional polygonal carbon molecules when he invented his dome system.   But the ideal structural form of his namesake molecules gives testament to his design genius.  Fuller borrowed a trick from Mother Nature’s book of magic more than two decades before anybody knew it was a trick in the first place.


For purposes of simplicity, fullerenes are either spherical or tubular.  Buckyballs are comprised of a specific number of carbon atoms in a specific configuration of hexagons and pentagons that make up the shell of the ball.  Shaped like a soccer ball, Carbon 60 (C60), the most frequently mentioned buckyball, has sixty carbon atoms positioned at the corners of 20 hexagons and 12 pentagons. Carbon 70 (C70) has 10 more pentagons than its sixty-atom cousin and is oblong like a rugby ball.  Other configurations, both simpler and more complex, exist.  Buckyballs are prized for their thermal and electrical conductivity, as well as their capacity to carry other molecules, either chemically tethered outside their shells or mechanically captured inside.  They can also be used as wheels or rollers for nanomachines.


True to their name, buckytubes, or nanotubes, are tubular.  They can be single walled (SWNT) or multi-walled (MWNT).  Single wall nanotubes have a single wall or shell and are difficult to manufacture.  Multi-walled nanotubes can be scrolled so that one long sheet of graphite is coiled up like an old parchment, or concentric -- like a retractable tube, or combinations of both.  Multi-walled nanotubes are easier to produce than their single-walled brethren.


In all fullerenes every carbon atom shares three electron bonds with three neighboring carbon atoms in the hexagon/pentagon lattice.  Obviously, there's one electron left over, so one of the electrons from each of the atoms becomes delocalized, or set free, to roam about the polygonal lattice with no single place to call its own. 


Because the materials characteristics of nanotubes are largely functions of structure, it is important to have a consistent way to describe a given tube's structure. In response to that issue, scientists have come up with a theoretical way of unrolling a nanotube, laying it flat as though it were a sheet of graphite and analyzing its geometry to determine whether it is one of three geometric types: armchair, zigzag, and chiral.  The names of the three types describe a discernable pattern within the polygonal lattice.  Put simply, if the imaginary polygonal lattice were laid out as a rectangle, so that the tops of the polygons made a horizontal row, then: armchair nanotubes would be rolled like a cigarette, from one side of the rectangular sheet toward its opposing side; zigzag nanotubes would be rolled like a cigar, from one corner to its opposite and; chiral nanotubes would be rolled and twisted, something like a hand-rolled cigarette.   


Anticipation for carbon fullerenes is high in the fields of electronics, polymers and metals.  Carbon nanotubes are: 100 times as strong and one-sixth the weight of steel; in some cases extremely conductive; in others semi-conductive, depending on their chiral “twist;” flexible; have good electron field emission properties; and a high aspect ratio – that is, they are long and thin.  For materials scientists and product developers, the location of such a set of properties in one place is as close to magical as science gets.


But alas, the story is not all magic. Obstacles to full-scale commercialization of fullerenes abound.  A lack of commercial-scale availability, the inability to measure and inspect more than an extremely small fraction of any production lot and high cost combine to create an unreceptive marketplace for fullerenes.  Although a few fullerene manufacturers claim kilos-per-day production output, many measure it in grams per day.  Depending on product type, size, purity and quantity, prices run from less than $10 to more than $900 dollars per gram. 


Low capacity and high prices create a chicken-and-egg conundrum for fullerenes.  Product developers have no incentive to tinker with expensive materials to improve or invent products for which those same materials are in short supply.  What is more, so far, fullerenes have not enabled a “killer app.” Nonetheless, a handful of manufacturers make fullerenes every day.  And researchers are working on ways to use them.   The pressure is on for higher capacity and lower prices.


As with all magic tricks the outlook for fullerenes is filled with both hazard and hope.  The cape is draped across the top hat.  Carbo the Great is waving his wand.  He’s pulled the rabbit out a thousand times before.  Will he be able to do it a thousand-and-one?  He puts down the wand and mutters, “Where did I put that magic book?”. 


This column first appeared in Tom Imerito's column, Views FromThe Bottom on nanotechnology.com

© Copyright 2005, Thomas P. Imerito / dba Science Communications


Bookmark and Share

 
©2009 Science Communications
thomas@science-communications.com