INFRARED PROPERTIES OF PT/AL2O3 CERMET FILMS

The room-temperature transmittance and front reflectance of mid- and near-infrared radiation (400-15 000 cm-1) by thin Pt/Al2O3 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 f(c) (0.50 less-than-or-equal-to f(c) less-than-or-equal-to 0.59) for the Pt/Al2O3 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 f(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 (f(c) congruent-to 0.455) provides an improved description. The best agreement is achieved for two models with adjustable, high values of f(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 f(c).