A Measurement Technique for Infrared Emissivity of Epoxy-Based Microwave Absorbing Materials

Abstract

Infrared (IR) emissivity is a critical parameter for modeling and predicting heat transfer by radiation. Microwave absorbing materials, having a high emissivity in the microwave spectrum, are crucial in a wide array of applications, such as electromagnetic interference mitigation, stealth technology, and microwave remote sensing and radiometer calibration. Accurate knowledge of the thermal properties of these materials is necessary for efficient design and optimization of these types of systems. Typical microwave absorbing materials consist of a dielectric epoxy material impregnated with a lossy material, such as iron or carbon. We study a novel cryogenically compatible epoxy-based absorber material that has been loaded with varying concentrations of carbonyl iron powder (CIP). We study six materials with CIP concentrations of 0%, 5%, 10%, 20%, 30%, and 50% by tap volume. We use a commercial IR camera with sensitivity in the range 7.5-13 μm to measure the radiance of the samples and a waterbath IR blackbody at ten temperatures between about 19 °C and 45 °C. A linear Deming fitting is performed, considering uncertainties in both the measured parameters, and the slope of the linear fit is shown to be the IR emissivity, averaged over the spectral response of the camera. The emissivity ranges between 0.868 and 0.757, decreasing monotonically as a function of iron carbonyl concentration between 0% and 50%. The uncertainty of the emissivity determination method is derived and presented. The uncertainty of the presented method is shown to be no larger than 3.3% for all measured samples.