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Transparent Aluminum

June 4, 2015

Among the many accomplishments of recently deceased Leonard Nimoy (March 26, 1931 - February 27, 2015) is directing credit for the fourth Star Trek Movie, "Star Trek IV: The Voyage Home." This movie is an unusual one in the Star Trek franchise, since it's very nearly a comedy. It certainly was popular, with gross receipts slightly more than $100 million, about four times the production cost.[1]

As an example of comedy in this film, ship's engineer, Scotty, played by James_Doohan (March 3, 1920 - July 20, 2005), attempts to use a 1980s era desktop computer by talking into a mouse. After discovering that the computer will accept commands only via keyboard, he adeptly inputs the structure of "transparent aluminum" into a CAD program.

aluminum, of course, is not transparent, so "transparent aluminum" must be aluminum modified in some way. One way to do this while still keeping the material chemically aluminum is to create a (fictional) allotrope. Just as the diamond allotrope of carbon is transparent while graphite is black, this fictional allotrope of aluminum could be transparent.

Such an allotrope of aluminum does not exist, but we can still render aluminum transparent by combining it with other elements. Aluminum oxynitride and the sapphire crystalline form of aluminum oxide are both transparent. Sapphire is commonly used as a watch crystal.

Trigonal crystal structure of sapphire (corundum)

The arrangement of atoms in a unit cell of sapphire.

Sapphire has
trigonal crystal structure, represented by the hexagonal cell shown.

The space group is designated R3c

(Illustration by the author using Inkscape.)

Sapphire (Al2O3) has a hardness of 9 on Mohs hardness scale, so it's nearly as hard as diamond (Mohs scale 10). Aluminum oxynitride, which has a cubic spinel crystal structure, has a hardness of 7.7 on Mohs scale, a melting point of about 2150 °C, and a compressive strength of 2.68 GPa. In 1980, James W. McCauley and Normand D. Corbin patented a process for making aluminum oxynitride,[2] and Corbin has written a review article on its properties.[3]

Figure caption

Phase Diagram of the aluminum nitride-aluminum oxide system, showing the phase region of aluminum oxynitride.

Click for larger image.

(Via Google Patents.)[3)]

The McCauley-Corbin patent is assigned to the US Army for the principal reason that the military has always been interested in bullet-proof windows; and, bullet-proof items, in general. This interest has extended to the the US Navy, which has been doing research in ways to make a different aluminum oxide into hard, transparent windows. The Naval material is spinel, a combination of magnesium oxide (MgO) and aluminum oxide (Al2O3) forming MgAl2O4.

Like sapphire and aluminum oxynitride, spinel has a high hardness (Mohs scale nearly 8), a high melting point (2135 °C), and a high Young's Modulus (276 GPa). As the figure shows, it's just as optically transmissive as the others in the mid-infrared. The Navy material is made from a powder precursor that's hotpressed in a vacuum to form transparent sheets that weigh much less than conventional bulletproof panes. The material is also suited for consumer applications, such as display screens.

Optical transmission of sapphire, aluminum oxynitride, and spinel

Optical transmission of sapphire, aluminum oxynitride, and spinel. The mid-infrared transmittance of spinel allows its use as a protective window for lasers. (Graphed by the author from available data using Gnumeric.)

In conventional spinel synthesis, the starting materials are melted and then poured into a mold. The problem for spinel is that the melting point is extremely high, so the melting crucible material and the mold material must have an even higher melting point and be non-reactive at those temperatures. Says Jas Sanghera of the Naval Research Laboratory (NRL), who led the research effort,
"A big problem with growing crystals is that you have to melt the starting powder at very high temperatures, over 2000 degrees Celsius... the molten material reacts with the crucible, and so if you're trying to make very high quality crystals, you end up [with a] huge amount of defects."[4]

Instead, NRL used a sintering aid - a "glue" that helps stick solid ceramic particles together - and hotpressed the materials at a lower temperature in a vacuum. This process is limited only be the size of your press, so it's scalable to very large pieces. The NRL team was able to make transparent pieces up to eight inches in diameter in their own laboratory using the equipment shown in the photograph.[4]

Hotpress at NRL

Hotpress and vacuum chamber used for making transparent spinel windows.

(Photograph by Jamie Hartman, U.S. Naval Research Laboratory.)

Magnesium oxide and aluminum oxide are abundant materials, which keeps the cost down. These materials, however, must be very
pure, since impurities will reduce optical transparency.[4] Sintering is a common process in ceramics manufacture, but NRL's advance in spinel fabrication came through identification of the specific sintering aid, lithium fluoride, and finding that careful mixing of the precursor powders is important.[4] I wrote about mixing in several previous articles (Graphene Production, May 26, 2014, Blending, March 11, 2008, and Blending (Part II), March 12, 2008).

Forming an aluminum oxynitride window (NRL)

The "green state" of the spinel precursor is actually pink, as the left image shows. The right image is an example of the final transparent spinel product. (left and right photographs by Jamie Hartman, U.S. Naval Research Laboratory)

The spinel product is not a glass; rather, it's a polycrystalline ceramic material. For this reason, it has a much higher fracture toughness.[4] A crack at the surface will likely generate a chip, rather than cracking the material through its depth. The final shape need not be flat, since it will conform to the mold, so domes are easy to fabricate.[4] NRL has licensed the technology to a company that's scaled the process to to 30-inch plates, and Sanghera is interested in using the spinel material as sensor windows on satellites.[4]


  1. Star Trek IV: The Voyage Home (1986, Leonard Nimoy, Director) on the Internet Movie Database.
  2. Normand D. Corbin, "Aluminum oxynitride spinel: A review," Journal of the European Ceramic Society, vol. 5, no. 3 (1989), pp. 143-154.
  3. James W. McCauley and Normand D. Corbin, "Process for producing polycrystalline cubic aluminum oxynitride," US Patent No. 4,241,000 (December 23, 1980).
  4. Kyra Wiens, "Transparent Armor from NRL; Spinel Could Also Ruggedize Your Smart Phone," Naval Research Laboratory Press Release, April 23, 2015.

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