Tikalon Blog by Dev Gualtieri

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Under Pressure

June 13, 2011

After my post-doc , I interviewed at the company for which I worked until my retirement, thirty years later. This company, at that time characterized as a "sleepy little chemical company," had decided to invest in research to reinvent itself with new product lines. To that end, it had hired some very prominent scientists in the early 1970s to staff a materials research center, and I was recommended there for an interview by a colleague of one of the managers.

Part of the employment process was an interview with the director , Jack Gilman (December 22, 1925 - September 10, 2009). The company was engaged in research on the production of amorphous metals on an industrial scale, one of Jack's pet projects,[2] so we talked about those for a while. Jack was insistent that these metals had no order at all, whether long-range order or short-range order .

I was likewise insistent that short-range order must exist, but perhaps it was just too hard to detect with the instrumentation he had at the time. I still got the job. My conjecture was proven about five years later, when short range order was discovered in metallic glasses.

Atoms of metal will arrange themselves into orderly crystalline structures unless you try really hard to do otherwise. One interesting halfway-state between an ordered crystal and a completely amorphous solid is the quasicrystal , in which atoms have a regular arrangement when viewed locally, but their lattice doesn't have translational symmetry ; that is, their lattice structure is not periodic .

The first published example of a quasicrystal was an aluminum alloy with 14% manganese content.[1] Not surprisingly, this material was formed by rapid cooling, which presumably doesn't give the atoms sufficient time to arrange themselves into their most favored position; that is, a regular crystal. This discovery was significant, since the Al-Mn quasicrystalline phase persisted after heating six hours at 300°C. Heating one hour at 400°C did result in conversion to an Al₆Mn crystal form.

Rapid cooling is the key to making quasicrystals, and its also the key to making amorphous metals. The rapid cooling task is made easier in alloy compositions at "deep" eutectics ; that is, eutectic compositions that solidify at a temperature much lower than their constituents.

Heating, of course, will allow amorphous metals to revert to their preferred crystalline state. You can even perform a partial annealing to get small crystals in an amorphous matrix, something which allows important modification of a material's magnetic properties .

As recent research shows, there's another, more complicated, way. Application of high pressure will likewise cause an amorphous-to-crystalline phase transition in a particular metallic glass; and by high pressure, I mean really high. It takes 250,000 bars of pressure (250,000 times atmospheric pressure at sea level ), or 25 gigapascals .[3-5]

Diamond Anvil Pressure Cell

Diamond anvil pressure cell.

(Via Wikimedia Commons)

The usual way to apply very high pressure to materials is through a diamond anvil cell , as shown in the above figure. Diamond is used because of its hardness , and also because its crystal structure allows an easy fabrication of a pointed shape in which an applied force can be converted to a high pressure because it's acting on just a small area.

An international team of researchers from the US and the People's Republic of China have reported in Science on their experiments that transformed the amorphous metallic glass phase of Ce ₇₅Al₂₅ to a face-centered cubic crystal by applying pressure.[3] Their studies indicated that the applied pressure causes an electronic transition in cerium that reduces the size of the cerium atoms and allows formation of the crystal lattice. The mechanism that they elucidated means that application of pressure will crystallize just a few special metallic glass compositions.

The most interesting revelation in the research paper is that some strange sort of long range order apparently does exist in this metallic glass. When the researchers crystallized portions of a centimeter-long ribbon of the metallic glass, the crystallized regions had the same orientation.[4] The ribbon form of the metallic glass indicates that it was fabricated in a wheel-casting operation. The idea that rolled materials retain texture, which I wrote about in a previous article (Secure Labeling, April 12, 2011), seems to apply here as well.

As I wrote at the start of this article, I worked at a laboratory that did a lot of work in metallic glasses. I didn't research the manufacture of these materials myself, but I did work on one interesting application, anti-theft tags . The magnetic properties of metallic glasses were ideal for anti-theft tagging in the age before RFID , and one thing that I developed was a method of deactivating the tags after purchase. Surprisingly, this involved radio frequencies , but I can't go into details.

The title of this article, "Under Pressure," is also the title of a 1981 song by David Bowie and Queen .

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