Activated carbon, porous materials not unlike the charcoal used for
barbecuing, performs important industrial functions such as filtering
air, removing toxic vapors, and purifying our food and beverages (e.g.,
sugar, molasses, vodka).
For that reason, a far-flung collaboration of scientists (the Universities
of Missouri and New Mexico, the CNRS lab in France, the Universidad
de Alicante in Spain, the Air Force Research Lab, and Los Alamos) set
out to learn more about the internal structure of the material. To their
surprise they discovered a fractal network of uniform channels, what
is perhaps the first documented pore fractal.
The researchers (contact Peter Pfeifer, email@example.com, 573-882-2335)
take simple olive pits, "char" them (burn them into charcoal),
and then treat them in steam at 750 C. How ironic that in this case
water, normally used to put out fire, here sustains combustion by providing
oxygen to burn with surface carbon.
What happens is not the removal of layer after layer or the carving
of holes of various sizes but instead the local etching and collapse
of pore walls to form channels of uniform size, about 2 nm wide. This
oxidation process will then abruptly branch in a new direction.
When it's all over the solid is riddled with a maze governed by a fractal
geometry. Scattering x rays from the material establishes a "fractal
dimension" of nearly 3, meaning that surface of the internal pore
network practically fills all the inside space.
The fractal nature of solid shapes has been measured many times, but
this might be the first time a fractal mapping has been performed for
the empty space inside a void, namely the nanopore network. (For comparison
of pore, surface, and solid fractals, see figure.)
The surface area of this great inland realm works out to about 1000
square meters (or one football field) per gram.
The researchers expect that methane and other fuels could be stored
in this kind of structure (the molecules are readily taken up into the
branching alleyways by the weak attraction of induced electric dipole
"van der Waals" forces), and at pressures much less than the
200 atm needed to store methane in steel cylinders. Gas separation can
also be accomplished because the narrow channels are negotiated more
easily by some molecular species than others. Electricity storage might
be accomplished by building capacitors enhanced by intermediate layers
of activated carbon networks filled with an ionic conducting fluid.
al., Physical Review Letters, 18 March 2002.)