Ultra-porous alumina for microwave planar antennas

Abstract

AbstractWe report on the experimental study of ultra-porous alumina (UPA) of transition phases γ and θ, which we fabricate by oxidation of laminated metallic aluminum followed by chemical and thermal treatments. Its morphology of a nanostructured monolith with up to 99% porosity brings together several crucial advantages with respect to existing aluminum oxides: an ultra-low density, the potential of refractive index gradients, and a chemically tunable hydrophilic character. Its extremely low permittivity measured here for the first time, εr ≈ 1.2 at 130–165 GHz, makes UPA a promising substrate material for planar electromagnetic components in this microwave spectral range of great interest for the cosmic microwave background observation. The dielectric loss tan δ ≈ 10−3, although being relatively low, can be further reduced after a chemical treatment that conveys hydrophobic character to the UPA constituting nanofibers.