It looks like a tube made of glass but it’s actually aluminum. Well, aluminum with an asterisk beside it — this is not elemental aluminum but rather a material made using it.
We got onto the buzz about “transparent aluminum” as a result of a Tweet from whence the image above came. This Tweet was posted by [Jo Pitesky], a Science Systems Engineer at the Jet Propulsion Lab in Pasadena. [Jo] reported that at a recent JPL technology open house she had the chance to handle a tube of material that looks for all the world like a section of glass tubing, but was billed as transparent aluminum. [Jo] tweeted this because it was an interesting artifact that few people get to play with and she’s right, this is fascinating!
The the material itself is intriguing, and I immediately had practical questions like what is this stuff? What is it good for? How is it made? And is it really aluminum rendered transparent by some science fiction process?
Can Aluminum Be Transparent?
As with many things in life, the answer to that question is, “It depends.” In this case, it depends on how you define aluminum. Or more precisely, it depends on what your expectations are for a material that purports to be aluminum. Regular old aluminum is an abundant metal with all the expected properties of metals — electrically and thermally conductive, ductile, malleable, and lustrous. You can melt it and cast it into useful shapes, beat it flat into a foil to wrap a sandwich, or crush an empty can made from it against your forehead, if you’re so inclined.
If you’re expecting transparent aluminum to have all of those properties, you’ll be disappointed. Although she doesn’t identify the material specifically, the material [Jo] got to handle was most likley not a metal at all, but a ceramic called aluminum oxynitride, composed of equal parts aluminum, oxygen, and nitrogen and known by the chemical formula AlON.
Aluminum oxynitride ceramics have been around since the 1980s, so it’s not new stuff by any means. Coincidentally, AlON development was underway more or less at the same time that Star Trek IV: The Voyage Home was being produced; it was from the now classic scene from that film were Scotty uses a mouse as a microphone in an attempt to trade the formula for “transparent aluminum” for sheets of plexiglass that AlON and similar transparent ceramics get their colloquial name.
What’s It Good For?
Despite clearly not being a metal — and not a glass either; glasses are amorphous solids, while ceramics are crystalline — AlON and the other transparent ceramics that have been developed since have some amazing properties. AlON, marketed as the uncreatively named ALON by its manufacturer, Surmet Corporation, is produced by sintering. Powdered ingredients are poured into a mold, compacted under tremendous pressure, and cooked at high temperatures for days. The resulting translucent material is ground and polished to transparency before use.
Aside from being optically clear, ALON is also immensely tough. Tests show that a laminated pane of ALON 1.6″ thick can stop a 50 caliber rifle round, something even 3.7″ of traditional “bullet-proof” glass can’t do. ALON also has better optical properties than regular glass in the infrared wavelengths; where most glasses absorbs IR, ALON is essentially transparent to it. That makes ALON a great choice for the windows on heat seeking missiles and other IR applications.
On the downside, ALON is expensive — in the armored glass market, it’s about 5 times the price of traditional laminated glass. But it has so many benefits, not least of which is superior scratch resistance, that for some applications it’s the material of choice. Chances are good that increased demand for the material will drive costs down, and it may not be long before Gorilla Glass is replaced by transparent aluminum smartphone screens that might actually damage the pavement when you drop your phone.
So, sorry Scotty — there’s no such thing as transparent aluminum metal. But the stuff we’re calling transparent aluminum is just as fascinating and just as sci-fi as it sounds. Isn’t material science cool? If you have other interesting materials like AlON that we should dig into, let us know about it in the comments below.