Fractal Carbon Nanopore Network

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, pfeiferp@missouri.edu, 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. (Pfeifer et al., Physical Review Letters, 18 March 2002.)