MIXING METALS FOR MAGIC MATERIALS: Analysis of Binary Eutectic Alloys for Metamaterial Applications

Uses of metamaterials, also known as negative index materials (NIMs), are wide ranging and include lenses that have a resolution beyond the diffraction limit, incredibly small antenna, and cloaking devices for optical, infrared, and microwave wavelengths.

The unique properties of metamaterials are not found in any naturally occurring material. By having simultaneously negative values for ε [electrical permittivity] and μ [magnetic permeability], a metamaterial can have a negative index of refraction over a certain frequency band. The unique properties of negative index materials emerge chiefly from their highly ordered structure.

Binary eutectic alloys have the potential to be used as a metamaterial if their intrinsic material properties are properly selected and the eutectic has the proper microstructure. The eutectic microstructures of interest for this work are the lamellar structure, with one phase being electrically conducting and the other phase being insulating or semiconducting, and rod microstructure with conducting rods in an insulating or semiconducting matrix.

The use of directional solidification of a eutectic alloy in order to create the highly ordered structure necessary for a metamaterial is a new frontier. The selection of an appropriate alloy for this application is the purpose of this work.

Two classes of materials were investigated: conducting metal + rare earths and semiconductor + conducting metal. Oxidation of the rare earths can form the necessary insulating layer. Eutectic alloys were created in an arc melter and analyzed using metallography and electron dispersive x-ray spectroscopy.

While compounds with rare earths can provide needed properties, preventing oxidation of the rare earths long enough to allow good mixing with the conducting metal proved difficult. Germanium with 9.2 at% titanium seems a good candidate based on structure as it forms conducting rods in a matrix of primarily germanium. Silver with 10 at% lanthanum forms a lamellar structure and has a highly conducting phase layered with a phase that has the potential to become insulating.

Future work should involve directional solidification of a eutectic alloy to further investigate its potential as a metamaterial.