STUFF MATTERS: Chapter 8

CHAPTER 8: UNBREAKABLE
• On carbon and its many allotropes
• The word diamond is derived from the Greek adamas, meaning “unadulterated” or “unbreakable”.
• BONDING STRUCTURE OF DIAMOND:  Two of the 6 electrons of a carbon atom do not participate in the “atom’s chemical life” as it interacts with other atoms.  The four electrons do “make the difference between the graphite of a pencil and the diamond of an engagement ring”.  In diamond these four electrons are each shared with another one of the 4 electrons of another carbon atom, with the four bonds arranged in a tetrahedral geometry around each carbon atom,
• BIGGEST EXTRATERRESTIAL AND TERRESTIAL DIAMONDS:  The biggest diamond yet discovered has a size five times that of the earth and was detected in the constellation of Serpens Cauda in the Milky Way where it is orbiting a pulsar star called PSR J1719-1438.  On earth, the biggest one was found about a mile down and is about the size of a football and weighs a half kilogram.
• PROEPRTIES OF DIAMOND:  The structure of diamond gives rise to its unique properties:
o Its strength, hardness, and stability are due to the stable configuration of electrons that are essentially “locked in place”.
o It is transparent because the electrons do not absorb in the visible range. It also has a high optical dispersion capability which allows it to split light into its constituent wavelengths.
• RELATIVE STABILITY:  Although diamonds give the impression of stability, it’s the other allotrope of carbon, graphite that is actually more thermodynamically stable although because diamond is kinetically stable, it will take billions of years for the conversion from diamond to graphite can happen.
• STRUCTURE OF GRAPHITE:  The structure of graphite is quite different from that of diamond even though they both consist purely of carbon atoms. In graphite, the carbon atoms are bonded in a planar hexagonal arrangement forming layers that are relatively more weakly held together compared to the bonds between the carbon atoms.  The bond between the carbon atoms within a hexagon of 6 bonded carbon atoms is stronger than the C-C bond in diamond.  The “softness” of graphite derives from the relatively weak attractive forces between layers of hexagonal planes that allow the planes to move relative each other (causing the ease with which one can write with graphite, depositing layers of the hexagon planes stripped from the rest).
• STRONG AND WEAK BONDS IN GRAPHITE:  The author explains that the strength of the C-C bond in graphite derives from the fact that the four electrons of each C atom are shared with only two other C atoms.  However, this leaves no shared electrons between layers, weakening the “bond” between these layers.  These layers are held together by the weaker van der Waal’s forces generated by “fluctuations in the electric field of molecules”.  These are the bonds that are breached when graphite is held firmly down and slid across a piece of paper, stripping layers of the graphite.
o Graphite is an electrical conductor because partial electron density from the outer electrons shared among three carbon atoms forms a “sea of electrons” (delocalized in standard chemical parlance ) between the layers that are mobile and can carry an electric current.  In diamond, all the electrons are “locked-up (localized) between C atoms and not sufficiently mobile to carry an electric current.
o Edison used graphite as a lightbulb filament because of its high melting point, glowing “white hot without melting when a high current passes through it”.
o Graphite has a metallic luster because the “sea of electrons also act as electromagnetic trampoline for light” and this reflected light is what gives shininess to graphite.
• COAL FORMATION:  Unlike soot that is formed by simply burning carbon-based material, coal is produced by millions of years of exposure to extreme heat and pressure and geological processes that give coal its unique structure and properties.  Coal starts off as a form of peat (brown, soil-like material consisting of decayed vegetable organic matter as defined by the Oxford American Dictionary) which turns into lignite (“soft brownish coal showing traces of plant structure, OAD) then becomes bituminous coal (semi-solid, oily mixture) which hardens into anthracite coal (hardened, almost pure carbon, OAD), the final precursor to coal which has the hexagonal planes of graphite.  At each step, the C content is concentrated into a dense structure after volatile compounds containing S, N, and O evaporate.
• BLACK DIAMOND?  “Jet”, the most aesthetically appealing type of coal because of its hardness and black metal-like luster, is a type of coal derived from fossilized monkey puzzle trees.  It has been called “black amber” because of its “similar triboelectric properties”:  the ability to create static electricity and make hair stand on end.
• BURNING DIAMOND?  In 1772, Antoine Lavoisier, investigating the properties of carbon, succeeded in burning a piece of diamond upon heating it until it was red hot.  However, instead of making soot, the diamond just simply became a gas and the diamond disappeared.  Upon heating in a vacuum to eliminate air, the diamond upon heating simply turned into pure graphite.  These experiments, of course, led to speculations and, later, attempts to reverse the process: turn graphite into diamond.
• SYNTHETIC DIAMOND:  In 1953, finally, a process was developed to turn graphite into synthetic diamond.  The much faster transformation from fossil-based graphite to the diamond structure causes impurities and defects that give color to synthetic diamond unlike real diamond formed from millions of years of heat and pressure.  The hardness of synthetic diamond, however, makes it useful for tools for cutting through granite and other hard rocks.
• LONDALEITE, PURE CARBON HARDER THAN DIAMOND:  In 1967, a different allotrope of C found to be 58% harder than diamond was discovered in the Canyon Diablo meteorite.  It was named lonsdaleite with a structure like graphite’s hexagonal layers but arranged in three-dimension.  Very few samples exist as it takes the heat and pressure of a meteorite impact for this allotrope to form.
• CARBON FIBER COMPOSITE, ANOTHER SYNTHETIC CARBON:  Carbon fiber is manufactured by “spinning graphite” to arrange the hexagonal layers lengthwise into fibers.  The fibers are arranged into sheets and rolled up retaining the material strength within the sheets.  The fibers are then epoxy sealed to replace the weaker can der Waals force between the fibers. The resulting composite benefits from the strength within the hexagonal layer leading to a material that is strong and light (high strength-to-weight ratio) and can be made stiff.
• LIGHT BUT STRONG:  Using a bicycle made from carbon fiber in 1996, Chris Boardman demonstrated the record for the fastest human speed achieved under his own power, reaching a speed of 56.375 km/hr.
• OTHER USES OF CARBON FIBER:  Carbon fiber composite has also been used to provide structure for artificial limbs.
• BUCKYBALLS, A FOURTH TYPE OF CARBON: In 1985, Harry Kroto and others discovered a fourth carbon allotrope formed in the flame of a candle.  This allotrope contains 60 carbon atoms, with the hexagonal sheet arranged in a spherical shape.  The name buckminsterfullerene was given to this fourth type after Buckminster Fuller , the architect who designed geodesic domes with the same hexagonal sections.
• CARBON NANOTUBES, THE FIFTH TYPE:  The name carbon nanotubes derived from this group of C atoms to self-assemble into nanometer-sized tubes.  The hexagons of C atoms are arranged in such a way that there are no van der Waal’s forces present and that the entire network of hexagons are held together by the same strong bond between C atoms within the hexagon.  (They are just like the carbon fibers but, in this case, the fibers are held together by this stronger C-C bond rather than the epoxy).  As a result, these carbon nanotubes have been found to have the highest strength-to-weight ratio of any known material on earth.
• 2-DIMENSIONAL GRAPHENE, THE PROSPECTIVE WONDER MATERIAL:  Graphene was first identified as a distinct material with potentially special properties by Andre Geim’s group at the University of Manchester.  Graphene is a single-atom hexagonal sheet of C atoms which can be derived by using tape to separate sheets of hexagonal structures from a thin sample of graphite until only one layer is left.  Some of these record-breaking distinctions of graphene include
o being the thinnest but strongest and stiffest known material on earth
o fastest thermal conductor
o ability to conduct more electricity, faster and with less resistance
o ability to allow Klein tunneling, “an exotic quantum effect in which electrons within the material can tunnel through barriers as if they were not there”
o it only retains these properties as long as it remains a 2-dimensional material (while it cannot be made thinner, add another layer on top and it just becomes graphite again)