Transparent Aluminum: A Cutting Edge Material

One of the more recent technologies to emerge in the ever-changing world of advanced building materials is transparent aluminum. It has long been the ambition of architects and engineers to find a material that combines the strength and hardness of metal with the crystal-clear purity of glass. Such a “clear metal” could be used, for example, to construct towering glass-walled skyscrapers whose outer skins are structurally self-supporting, with an absolute minimum of secondary supporting members. Transparent aluminum is a cutting edge-material already being used in specialty architectural applications including blast-resistant and infrared-optical windows.

What is Transparent Aluminum?
Known commercially as ALON, transparent aluminum is made of aluminum oxynitride, a combination of aluminum, oxygen and nitrogen. Before it can end up as a hard transparent armor plate, it begins as a powder. This powder is then molded, subjected to high heat and baked, just as any other ceramic is baked. Once baked, the powder liquefies and then quickly cools into a solid, which leaves the molecules loosely arranged as if still in liquid form. The resulting rigid crystalline structure of the molecules provides a level of strength and scratch resistance that’s comparable to rugged sapphire. Additional polishing strengthens the aluminum alloy and also makes it extremely clear.

Chemical Composition
Transparent aluminum is a transparent polycrystalline ceramic with a cubic spinel crystal structure. ALON is produced by combining aluminum, oxygen, and nitrogen. Its chemical formula is Al₂O₂₇N₅.

Physical Properties
• Hardest polycrystalline transparent ceramic commercially available.
• 80% transparent in the near-ultraviolet, visible and infrared regions.
• Best material available material in terms of optical quality.
• It is three times harder than steel and four times as hard as conventional silica glass of the same thickness.
• Corrosion and oxidation resistant.
• High flexural strength.
• Good compressive strength.

Manufacturing Process
ALON production begins by introducing a mixture having aluminum oxide and carbon into a chamber, agitating the mixture within the chamber, and heating the mixture to make aluminum oxynitride. The next step involves pressurizing the aluminum oxynitride powder to 15,000 pounds per inch in rubber molds submerged in hydraulic fluid. The obtained material, which is molded and opaque, is heated to 2000-deg C and kept at this temperature for two days. Once cooled, ALON emerges transparent and is then ground and polished for extra clarity and strength.

Applications of Transparent Aluminum
Although the material has exceptional mechanical and optical properties, at present it is prohibitively expensive to produce. For this reason its current application in construction is relatively limited, constrained to specialty applications including bullet-proof glass, blast-resistant windows, and infrared-optical windows.

It is thought that as the material becomes more mainstream, production costs will become less prohibitive. In turn this could lead to its application in more commonplace uses. Specifically, due to its exceptional strength and impact-resistance characteristics, in the near term it’s well positioned to emerge as a superior alternative to conventional safety glass (i.e. tempered or laminated glass) in building applications. In the longer term, it could be a material poised to allow towering buildings whose skins are made of only transparent metal.

For Further Reading:
• Science Daily has published an article, “Transparent Aluminum Is New State Of Matter”, found at: https://www.sciencedaily.com/releases/2009/07/090727130814.htm
• An article, “What is Transparent Aluminum?” can be found at TheConstructor.org. The link is: https://theconstructor.org/building/building-material/transparent-aluminium/561733/
• Surmet, a producer of transparent aluminum, has published several articles about ALON at: http://www.surmet.com/technology/alon-optical-ceramics/