Preparation And Investigation Of A Broadband Metamaterial Absorber With Thermal Scalability

Microwave absorbing materials can effectively absorb and attenuate electromagnetic waves,and have been extensively applied for electromagnetic interference shielding and stealth.The traditional microwave absorbing materials can have broadband absorption by tailoring the electromagnetic parameters（includingε_randμ_r）and the thickness.Researches on the metamaterial absorbers provide a facile way to achieve broadband absorption.For some purposes,they are required to be used at high temperatures.The dielectric properties of the substrate and the electrical conductivity of the electric resonator in a metamaterial absorber can still exist at elevated temperatures and are expected to be in anticipation of metamaterial absorbers designed for high-temperature microwave absorption applications.Based on the lossy resistive frequency selective surfaces（RFSS）and equivalent circuit model,the feasibility of a metamaterial absorber to achieve broadband absorption with thermal scalability was verified,and then a single square-loop shaped broadband metamaterial absorber based on the RFSS using the SiC_f/NAS glass composite with good mechanical strength as the dielectric substrate was proposed and applied for load-bearing and microwave absorption at elevated temperatures.The main conclusions are shown in the following contents:（1）A single square-loop-shaped metamaterial absorber named Type I consists of a lossy resistive frequency selective surface on a dielectric substrate and a metallic ground plane.The top RFSS layer,made of conductive composites,was prepared by3D pringting using a mixture of graphite microflakes and WJ-3 silicone resin.The metamaterial absorber tested at 400°C obtained an absorption of over 85%in the X band.However,after multiple tests at 400°C,obvious cracks on the RFSS arrays occurred and some RFSS arrays even dropped off due to the thermal pyrolysis of the silicone resin,resulting in the deterioration of the broadband absorption.A single square-loop-shaped metamaterial absorber named Type II was manufactured by screen printing on the quartz glass with the conductive paste that was the mixture of graphite microflakes and water glass.The RFSS layer was smooth and dense,and good conductive networks were formed.When the volume fraction of graphite was 37.95vol%,the sheet resistance of the RFSS layer was approximately 40Ω/sq at room temperature,and it gradually decreased with the increase of graphite content and measurement temperatures.The metamaterial absorber had a broadband absorption of more than 90%covering the X-band frequency range at room temperature and 400°C,and the measurement matched well with the simulation results.The good absorptivity at high temperatures was attributed to the weak temperature dependence on the sheet resistance of the RFSS layer and the dielectric properties of the quartz substrate.Simulation results also show that the majority of energy was dissipated by ohmic loss of the electric resonator which was higher than that by the dielectric loss of the quartz substrate.（2）Due to the oxidation of graphite at high temperatures,the Type II broadband metamaterial absorber works with thermal scalability to 400°C.To comply with harsh conditions,another single square-loop-shaped metamaterial absorber with scalable high-temperature absorption was prepared by screen printing on the quartz glass with the conductive paste which was composed of commercial Mo Si₂ powder,aluminum powder,and water glass.With Al powder content of 11.51 vol%,the sheet resistance of the RFSS layer was approximately 40Ω/sq.The sheet resistance increased with the increasing measurement temperatures and the temperature coefficient of electrical resistivity was shown to be positive experimentally.The experimental results show that the reflection coefficient was less than-10 d B in the frequency band near 7 to 11 GHz from room temperature to approximately 600°C,resulting in broadband absorption of over 90%.They agreed with the simulation results reasonably well subject to small uncertainty in the sheet resistance.After three cycles’measurements at 600°C,the absorption changed little due to the thermal stability of the metamaterial absorber.The validation of a simple equivalent RLC model,in which good estimation of the frequency response and bandwidth of RFSS structures was obtained to explain the simulation results.（3）For the purpose of load-bearing and high-temperature broadband absorption,La₂Ti₂O₇ interphase was then prepared on the SiC fibers by dip-coating and the SiC_f/NAS glass composites were also fabricated.The La₂Ti₂O₇ coating was uniformly distributed on the surface of the SiC fibers,but the tensile strength of the coated fibers was slightly decreased.A weak interphase was formed between the SiC fiber and glass matrix due to the presence of La₂Ti₂O₇ coating,which improved the flexural strength of the composites.The fiber debonding and pull-out show that the SiC fibers had a good toughening effect.The room-temperature complex permittivity of the composites increased and the dielectric loss was higher than that of the pristine SiC fiber due to the generation of free carbon in the SiC fibers at high-temperature heat treatments.Both the dielectric constant and the dielectric loss increased with the increasing measurement temperatures owing to the higher electrical conductivity of free carbon.The composites had a narrow-band absorption in the X-band by tuning the thickness.According to the equivalent circuit model,the relationship between the equivalent inductance and the equivalent capacitance was determined,and the equivalent resistance dependence on the resonant frequency was further studied.Thus,a dual-band metamaterial absorber with broadband absorption was proposed.The feasibility of the design was verified by preparing a single square-loop metamaterial absorber with the as-received SiC_f/NAS glass composite.Simulations demonstrate that the reflection coefficient of the absorber is less than-10 d B from room temperature to elevated temperatures,resulting in a broadband absorption of more than 90%in the X-band,which is a promising candidate for use in high-temperature microwave absorption applications.