Physics News Update no. 578

The American Institute of Physics Bulletin of Physics
Number 578, February 27, 2002

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.)

Using Clocks in Space to Search for New Physics

Einstein's theory of relativity holds several things sacred. One is the idea that if you rotate a particle or object, or boost it up to a high velocity, the laws of physics affecting the object should stay the same. This is called Lorentz invariance.

But in some "extensions" of the standard model of particle physics, interactions of particles with certain hypothetical universal fields (very roughly analogous to the way in which Higgs bosons are supposed to make some particles massive) might lead to subtle violations of Lorentz invariance.

In a new paper Alan Kostelecky of Indiana University and his colleagues show how this can happen, and how such a violation could be detected in clock-comparison experiments now being readied for the International Space Station (ISS).

In general an atomic clock works by shooting microwaves into a sample of cooled cesium atoms and reading out the microwave-absorption frequency which corresponds to a specific quantum transition for electrons in the cesium atoms. The microwave frequency setting is used to define the "second."

If one can cool the atoms to lower temperatures (thus reducing the blurring caused by their movement) or observe them for longer periods, the precision of the whole readout process (and the standardization of the second) would improve.

The world's best clock, NIST F-1, currently has an uncertainty of one part in 1015. It achieves this by chilling Cs atoms in a trap and then gently boosting them upwards. Where they reach the top of their trajectory (subject always to the attraction of gravity) and are at their slowest is where they are subjected to the microwave bath.

A related apparatus mounted on the ISS could gain in precision because the atoms would never fall (at least not relative to the atom trap setup) and could be sampled for longer periods. The goal is to have several such "space clocks" in orbit within a few years (see, for example, NIST website).

According to Kostelecky (kostelec@indiana.edu, 812-855-1485) certain Lorentz-violation effects, expected to show up as a tiny shifting of an atom's energy level, would be more readily accessible in space thanks to the speeds, rotation rates, and clock orientations available on space platforms (see animations).

With sensitivities in space comparable to those in Earth-based experiments, the expected tests of Lorentz-violating effects would be measured with uncertainties at the level of parts in 10²⁷. (Bluhm et al., Physical Review Letters, 4 March 2002.)

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