Soaking Up Hydrogen

A silvery-white metal named after the goddess of wisdom. It doesn't tarnish in air. Mixing it with gold produces the white gold used in jewelry. Closely related to platinum, it's used in dentistry, watchmaking, surgical instruments and electrical contacts.

The metal is palladium, number 46 on the periodic chart, and its most remarkable property, actually, is none of the above. Palladium soaks up hydrogen like a sponge -- that is, if you can imagine a sponge that soaks up hundreds of buckets of water. At room temperature and atmospheric pressure, palladium can absorb up to 900 times its own volume of hydrogen. "That means," says Khalid Mansour, "that if you were to pump hydrogen into a bottle, it would take enormous pressure to store the same amount easily absorbed in a palladium bed of the same volume."

This makes palladium an efficient, safe storage medium for hydrogen and hydrogen isotopes, such as tritium, a byproduct of nuclear reactions. Nevertheless, the microscopic details of how this absorption process works are poorly understood. Better understanding of what happens at the molecular level, such as phase changes that occur as hydrogen fills the metal -- forming a metal hydride -- and the effect of defects in the material, could give clues to designing metal hydrides that perform better. Such research also bears directly on technologies that use metal hydrides in fuel cells and batteries.

At Westinghouse Savannah River Company (WSRC) in South Carolina, Mansour (now at Cray Research), Ralph Wolf and their colleagues Myung Lee and Clemson University physicist John Ray are working to fill-in the gaps of knowledge. "Basically," says Mansour, "the experimentalists did things by alchemy. We know that palladium has this remarkable ability to soak up hydrogen, but there's a lot we don't understand. What we need is physical understanding of the underlying behavior at a microscopic level. And that's where we come in."

The researchers are using the CRAY T3D at the a new approach to modeling the key thermodynamical property, chemical potential. Applying this method on the T3D allows them to model a much larger number of atoms, and they're getting results that, for the first time, make it possible to directly compare simulations with observed experimental data.

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Onset of the beta phase in palladium hydride at 300 degrees Kelvin. This phase change occurs as the concentration of hydrogen atoms (yellow) in the palladium (purple) increases. At early stages (the alpha phase), hydrogen atoms randomly populate small interstices in the lattice structure. At a critical point, the lattice expands, allowing hydrogen to cluster at higher density, as visualized here. This image shows the lattice from the (001) direction.

Researchers: Ralph Wolf, Westinghouse Savannah River Company; Khalid Mansour, Cray Research
Hardware: CRAY T3D
Software: user-developed code
Keywords: Hydrogen, palladium, tritium, helium, absorption, metal hydride, fuel cells, batteries, alchemy, chemical potential, thermodynamics, atoms, grain boundaries, dislocated material, crystal structure, embedded atom method (EAM), volume expansitivity, lattice, fracturing, alloys, nuclear energy.

Related Material on the Web:
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits