Once confined to the world of Star Trek, transparent aluminum is now very much a reality, and can have significant real-world applications.
Transparent aluminum, also known as aluminum oxynitride, is a transparent polycrystalline ceramic with a cubic spinel crystal structure made of nitrogen, oxygen and aluminum.
It is optically transparent in the near-ultraviolet, visible and infrared regions. It is four times harder than fused silica glass, 85% harder than sapphire and 15% harder than magnesium aluminate spinel. The material remains solid up to 1200°C (2190°F). It has good corrosion resistance and resistance to damage from radiation and oxidation. It is about three times harder than steel of the same thickness.
Domes, tubes, transparent windows, rods and plates can be produced from this material using conventional ceramic powder processing methods. Methods for manufacturing transparent aluminum remain refined. The cost of this material is similar to that of synthetic sapphire.
The mechanical properties of transparent aluminum are shown in the following table:
The fabricated ceramic material is subjected to heat treatment at elevated temperatures followed by the process of grinding. The material is then polished to obtain transparency. It loses transparency at around 2100°C (3812°F). The processes of grinding and polishing mainly enhance the impact resistance and the resulting material is harder than sapphire by 85% and magnesium aluminate spinel by 15%.
However, transparent aluminum produced by conventional methods has a high porosity and hence low transparency. Lee et al from Yeungnam University proposed a manufacturing method that solves this in 2010.
In this method, a sintering additive is added to a source powder composed of less than 0.5 wt.% of MgO. The source powder is then presintered at temperatures ranging from 1550 to 1750°C (2822 to 3182°F) to obtain a cubic-phased polycrystalline aluminum oxynitride ceramic having a relative density of more than 95%. The source powder can be again sintered at 1900°C (3452°F) in order to further increase the relative density of the material. As a result, the porosity is eliminated and the transparency increases to more than 95%.
Some of the applications of transparent aluminum include the following:
- Various defense applications like Recce sensor windows, transparent armor, windows for laser communications and specialty IR domes with different shapes that include hemispherical and hyper-hemispherical domes
- Semi-conductor related applications
- Insulators and heat radiation plates
- Optoelectronic devices
- Metal matrix composites
- Power and multichip modules
- Translucent ceramics
- High temperature materials and heat sinks
- Break rings
- Thermally conductive filler
- Integrated circuit packages and substrates
Since the discovery of transparent aluminum ceramics in the early 1950s, many researchers have been involved in the study of transparent aluminum ceramics. Their work reports a number of challenges to be resolved for obtaining transparent aluminum ceramics with high transmittance rate, which includes the following:
- Minimizing impurities
- Eliminating micropores
- Controlling grain boundaries.
Although several research works suggest methods to improve transmittance of transparent aluminum by controlling the average grain size under 1 µm in specific wavelength, the grain sizes could not be decreased effectively into a scale that is very much smaller than the wavelength of visible light, using current techniques.
As a result, the transmittance of the material is dramatically reduced with the decreasing wave length in visible light range. Therefore, the problem of birefringence in transparent aluminum could not be resolved essentially. In addition, the cost of producing transparent aluminum is very high when compared to that of other transparent ceramics.
Sources and Further Reading