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A broadband metamaterial absorber intended for applications at elevated temperatures was designed and fabricated. Its performance was measured over a temperature range from room temperature to approximately 400 °C. The absorber consists of a lossy resistive frequency selective surface (RFSS) on a dielectric substrate and a metallic ground plane with high-temperature stability scalable to approximately 400 °C. The top RFSS layer, made of conductive composites, was prepared using a mixture of graphite microflakes and water glass, and it showed good chemical and thermal stability up to 400 °C. Its sheet resistance was adjustable over a wide range with different proportions of graphite flakes and decreased gradually with increasing temperature. The metamaterial absorber demonstrated broadband absorption with a reflection coefficient of less than −10 dB in the X-band frequency range (8.2–12.4 GHz), resulting in the absorption of more than 90%. The influence of testing temperature on the reflection coefficient was correlated with the variation in the sheet resistance. The proposed metamaterial absorber is a promising candidate for use in high-temperature microwave absorption applications.

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