Through the Looking Glass

Monday, July 1, 1996
Hans Huiberts normally works on esoteric problems in the field of superconductivity--the phenomenon by which some substances suddenly lose all electrical resistance and become perfect conductors. While studying the superconducting properties of hydrogen combined with other elements, he and some physicist colleagues at the Free University in Amsterdam accidentally discovered a metal compound with a remarkable dual nature: with a little chemical tweaking, the compound changes from a shiny metallic mirror into a transparent window.

The physicists made their chance discovery while working with a thin sheet--just a few millionths of an inch thick--of yttrium coated with palladium. Yttrium is a common ingredient in superconducting compounds, and the palladium was supposed to prevent it from reacting with oxygen in the air. The palladium layer was permeable to hydrogen atoms, though, which are smaller than oxygen molecules. When the physicists exposed the sheet to hydrogen gas, they got a surprise: the shiny metal turned transparent within a few seconds.

The Dutch researchers don’t know for certain how this effect takes place, but they do have a theory. The electrons in yttrium are normally free to absorb light and reemit it or pass it along to the free electrons of neighboring atoms. These free electrons make yttrium a good reflector as well as a good conductor of electricity. But when hydrogen is added to yttrium, Huiberts believes, yttrium’s free electrons bond with electron-craving hydrogen atoms. Yttrium’s electrons can then no longer absorb or radiate light, so light passes right through the material.

The process is also reversible, says Huiberts. When the physicists pumped hydrogen out of the jar containing the yttrium-palladium sample, it once again became reflective. That’s very different from a chemical reaction that is always in one direction; here you can go two ways, Huiberts says. The reversibility of the process, he explains, will make it much easier to find practical applications. Although such applications are probably still years away, Philips, the Dutch electronics firm, has bought the patent rights to the material.
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