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Absorbing Hydrogen Turns Palladium Into A Quasi Liquid

Palladium can absorb vast quantities of hydrogen but materials scientists have now discovered that this process has an extraordinary effect on the metal

  • November 19, 2010

Here’s a curious experiment involving palladium, the rare silvery metal well known for its ability to absorb hydrogen. When it is saturated, the ratio of hydrogen to palladium can be as high 0.6, which is why the metal is used to filter and even store hydrogen.

It’s easy to imagine that the movement of hydrogen atoms in and out of the metallic lattice has little effect on the material. But that turns out to be wrong, as Akio Kawasaki at the University of Tokyo and friends discovered when they decided to test the idea.

Materials scientists have known for some time that palladium expands when it absorbs hydrogen and shrinks during desorption. What they hadn’t known until now is the toll that this process takes on the metal.

Kawasaki and co attached a rectangular plate of palladium about the size of of stick of gum to the side of a chamber so that it stuck out horizontally. They then heated it to 150 degrees C and hung the weight of an apple on the end of plate. Finally, they pumped hydrogen into the chamber and waited while the metal absorbed it.

To their surprise, the palladium immediately drooped under the weight and continued to droop as the hydrogen was pumped out of the chamber and the gas was desorbed. (In contrast, when they hung the plate vertically with the weight hanging beneath, there was almost no stretching at all.)

There’s no escaping the conclusion that hydrogen somehow robs palladium of its strength but in a very specific way.

That’s a somewhat unexpected result but one that Kawasaki and co think they can explain.

In its pure state, the palladium lattice has a face centre cubic structure but this has to change to allow so much hydrogen on board. Materials scientists know that when this a happens, it can adopt two other structures known as alpha and beta phases as well as a mixture of these phases.

Kawasaki’s conclusion is that during this change, the metal atoms are neither held in a rigid solid structure nor able to move in an entirely random way either. This makes it a little like a liquid. In fact, physicists call this type of material a quasi-liquid.

So what they have is a material that they can change into a quasi-liquid at will. That should peak the interest of materials scientists. The next stage will be to study the change using various techniques such as x-ray diffraction and perhaps NMR which should reveal what happens to a substance as it morphs from a solid to a quasi-liquid.

As for applications, just where such a quasi-liquid could be put to good use isn’t clear. Suggestions in the comments sections please.

Ref: arxiv.org/abs/1011.2776: Anomalous Deformation Of Palladium Plates By A Small Gravitational Force During Hydrogen Absorption And Desorption

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