Optical characterization of thin layers grown on metal components

Abstract

In this work a new method for calculating the optical properties of an absorbing film grown on substrates, including the real and imaginary part of the index of refraction is shown. In particular the thermal radiation of a growing oxide-film on a heated steel specimen is measured by a CCD camera with a near infrared (1000nm) band-pass filter. The observed radiation-signal shows significant temporal patterns due to interferences in the growing oxide film. Under the assumptions of known specimen temperature, constant optical properties (dielectric functions) of each layer and semitransparent films, it is possible to develop a model by which variations of the resulting emissivity with varying film-thickness can be explained. From this model, which is derived from the calculation of the reflectance of a thin absorbing film on an absorbing substrate, the complex index of refraction of the film can be determined without explicit knowledge of the optical constants of the respective layers. Since a matrix-camera is used to monitor the process changes of the emissivity over time, spatial information may also be derived. In this way it is possible to detect spatial inhomogeneities in the film and to determine the cause (either inhomogeneous growth rates or spatial variations of material properties). In addition, these results can be used for emissivity correction in non-contact thermal imaging. This method is not limited to oxide films and can be used for other heat treatment processes that deposit semitransparent films as well.

Highlighting of Critical Experimental Data for SOFC Modeling That is Missing From the Literature and Potential of N-IR Thermography for SOFC Study

“…Instead, temperature changes were isolated in order to state where NiO oxidation "exothermic reactions" and carbon deposition "endothermic reactions" were occurring. It has been shown on a number of occasions [27,33,34] that emmisivity changes may be used in order to characterize materials, material growth and material morphing.…”

Noise effect on the interpolation equation for near infrared thermography

“…Furthermore, they deliver images with a high spatial resolution which is suitable for studies on materials with non-uniform surfaces. These thermal imagers are then widely used for thermal imaging applications [1], for radiance temperature measurements [2], for multi-spectral measurements [3,4] and for true temperature field measurements [5].…”