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