Navy

Containment of molten aluminum

Coating material prevents wetting in crucibles and other containment vessels

Materials

Illustration of a used Fabmate crucible with spinel-coated rims. Note the straight lines of aluminum at the edge of the spinel film, indicating non-wetting behavior.

Thermal evaporation is a very common method of depositing thin films of aluminum. In this technique, an aluminum shot is placed into a crucible that is typically made of graphite, a refractory metal, or an oxide, and heated using either an electron beam or resistive coils.

Unfortunately, molten aluminum has a strong propensity to wet most crucible materials and is highly corrosive to refractory metals, such as tungsten and molybdenum. As the molten aluminum creeps during the deposition, it can wet out onto other system elements, solidify on the backside of the crucible and cause thermal shock and cracks, corrode resistive heating elements, and change the thermal conduction properties of the crucible.

This problem is typically managed by frequently replacing crucibles, leading to excess cost and material waste. But Navy researchers have developed a method to contain molten aluminum within a crucible using non-wetting barrier layers. The method uses magnesium aluminum (MgAl2O4) deposited using RF magnetron sputtering onto the crucible with an energy density of ˜9 W/cm2 and a pressure of 1-10 mT with the crucible held at room temperature for an estimated spinel film thickness of 500-1000 nm. This non-wetting material is chosen due to its surface energy that results in a large contact angle when in touch with molten aluminum. As the molten aluminum wets out its container, the material is stopped at the edge of the MgAl2O4 and will not advance further, prolonging crucible lifetime and reducing source material waste. The technique is applicable to containers of any size and shape.

Instead of MgAl2O4, a variety of other materials can be used, including but not limited to oxides, such as Al2O3, or nitrides, such as AlN and BN, or carbides, such as SiC. Likewise, instead of sputter deposition, other deposition methods can be used, such as chemical vapor deposition, thermal spray, or thermal evaporation, as well as dip coating or spray coating of precursors followed by thermal treatment.

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