microcellular foam, radar absorbing materials, thermoplastic polyurethane, foam morphology, electromagnetic properties M icrocellular thermoplastic polyurethane foams are examined as absorbing materials in the X-band (8.2-12.4 GHz) frequency range by means of experiment. In this work, we aim to establish relationships between foam morphology including cell size and air volume fraction and electromagnetic properties including absorption, transmission and reflection quality. Nanocomposites based

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... n thermoplastic polyurethane containing carbon black were prepared by coagulation method. In this procedure 15 wt% carbon black-containing nanocomposite was converted to microcellular foams using batch foaming process and supercritical carbon dioxide as physical foaming agent. The morphology of the foams were evaluated by scanning electron microscopy. S-parameters of the samples were measured by a vector network analyzer (VNA) and the effect of morphological parameters such as cell size and air volume fraction on the absorbing properties were investigated. We also established structure/properties relationships which were essential for further optimizations of the materials used in the construction of radar absorbing composites. Foaming reduced the percolation threshold of the nanocomposites by reducing the average distance between nanoparticles. Foaming and dielectric constant reduction reduced the reflection of the samples significantly. The increase in air volume fraction in the foam increased absorption per weight, due to multiple scattering in the composite media. The sensitivity of electromagnetic wave due to the variation of cell size is strongly weaker than that of the variation of air volume fraction. Electromagnetic properties of the microcellular foams deviated a little from effective medium theories (EMTs). Air volume fraction of the cells were a function of cell size and smaller cells showed higher absorption.