Triple-band passive high-temperature sensor using a circular-ring metamaterial absorber on oxide–oxide CMC substrates (25–1200 °C)

This study presents a triple-band oxide–oxide ceramic metamaterial absorber designed as a passive wireless high-temperature sensor. The metasurface consists of two cascaded circular platinum ring resonators printed on Axiom oxide–oxide ceramic matrix composite (CMC) substrates with a continuous platinum ground plane. Free-space, focused-beam measurements from room temperature to 1200 °C reveal a repeatable and nearly linear frequency downshift (up to 0.96 MHz/°C) due to the temperature-dependent permittivity of the ceramic substrates. Three Axiom CMCs (AX7900EF13, AX7900EF20, and AX7800EF13) were analyzed to compare trade-offs among frequency stability, thermal sensitivity, and thermal stability. These structures operate in the C-band (4–8 GHz), X-band (8–12 GHz), and Ku-band (12–18 GHz). At room temperature, experiments showed that the AX7900EF20 sample displayed three absorption peaks at 6 GHz, 10.9 GHz, and 14.6 GHz, with peak absorptivities of 93%, 91%, and 99%, respectively. Thermal sensitivity tests indicated that the AX7900EF13 had the highest overall sensitivity of –0.9638 MHz/°C in the 12–15 GHz range. The AX7800EF13 sample demonstrated the lowest frequency-shift rates, suggesting greater thermal stability, with slopes of –0.7311 MHz/°C, –0.4801 MHz/°C, and –0.3277 MHz/°C for the first, second, and third resonances. The results confirm the feasibility of oxide–oxide CMC metasurfaces for high-temperature, passive, contactless sensing in harsh environments such as aerospace, energy, and manufacturing systems.