Infrared properties of Pt/Al2O3cermet films

Abstract

The room-temperature transmittance and front reflectance of mid- and near-infrared radiation (400–15 000 ${\mathrm{cm}}^{\mathrm{-}1}$) by thin Pt/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ cermet films prepared by electron-beam evaporation onto sapphire substrates were measured using a Fourier-transform spectrometer. The high value of the dc percolation threshold ${\mathit{f}}_{\mathit{c}}$ (0.50≤${\mathit{f}}_{\mathit{c}}$≤0.59) for the Pt/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ system is evidence for correlations in the positions of the particles that can be described by coated-grain topologies. The data were compared with the predictions of five effective-medium models, which feature different microstructural topologies and values of ${\mathit{f}}_{\mathit{c}}$. Published data on the dielectric functions of the component materials were used in the modeling. The Maxwell-Garnett and Bruggeman models do not describe the data adequately. A simplified version of a model by Sheng (${\mathit{f}}_{\mathit{c}}$≃0.455) provides an improved description. The best agreement is achieved for two models with adjustable, high values of ${\mathit{f}}_{\mathit{c}}$. We conclude that an effective-medium theory is able to describe the infrared optical properties of a cermet system over a wide range of composition if proper account is taken of both the microstructure and the value of ${\mathit{f}}_{\mathit{c}}$.