| The widespread application of electromagnetic technology in communication,detection and other fields has facilitated people’s lives and improved military defense capabilities.However,with the miniaturization and integration of electronic equipment at an increasingly high level,more electromagnetic interference and thermal radiation are formed in a narrow space,which would seriously hamper normal operation of the above equipment.On the other hand,advances in radar detection technology have led to a decline in survivability of high-speed weapons.Some high-speed weapons can cause a significant increase in surface temperature during use,thus deteriorating the service environment of microwave absorbing materials.It is particularly important to ensure electromagnetic safety of equipment in high-temperature environment.High-temperature microwave absorbing materials can be a good solution to the above problems.According to the Landau-Lifshitz-Gilbert（LLG）equation,Debye theory and Arrhenius formula,electromagnetic parameters and microwave absorbing properties of materials are very sensitive to temperature,so it is difficult to maintain the desired microwave absorption properties for materials over a wide temperature range.Meanwhile,the complex application environments also place higher demands on the multi-functional and intelligent level of high-temperature microwave absorbing materials.Carbonyl iron as a typical magnetic absorber can provide effective magnetic loss and dielectric loss at the same time,which not only facilitates to reduce thickness,but also provides multiple ways to modulate microwave absorbing properties in a wide temperature range.Therefore,carbonyl iron with different morphologies is used as the main absorber in this paper.Based on the study of microwave absorption properties and loss mechanisms in a wide temperature range,the three-dimensional,two-dimensional and one-dimensional structural templates are constructed to realize ordered distribution of carbonyl iron,which induces structural magnetic loss and optimizes magnetic loss at high temperatures.At the same time,structural templates are used to improve dielectric loss of composites,achieving the regulation of microwave absorbing performance over a wide temperature range and the optimization of thickness,effective absorption bandwidth and temperature stability.In addition,based on the synergy of structural templates and carbonyl iron,this paper further develops multi-spectral stealth and temperature monitoring.The main research achievements are as follows:Firstly,carbonyl iron/resin coating is used as study object to reveal that the main reason for a decrease in magnetic loss of coating with increasing temperature is an increase in damping of magnetic moment progression and the blue shift of exchange resonance frequency.Metal-insulator-metal junction as structural unit is established according to the distribution characteristics of carbonyl iron in matrix.Based on the fluctuation tunneling theory,the effect of tunneling current on dielectric loss of coating is investigated by one-dimensional electron tunneling calculations.Temperature-sensitive tunneling current allows dielectric loss of coating to increase with temperature,offsetting the negative impact of decrease in magnetic loss on coating.A coating of 0.9 mm thickness obtains the best reflection loss of-12.78 d B at 300℃,while the effective absorption bandwidth below-10 d B reaches 8.0 GHz.In order to obtain stable microwave absorbing properties at high temperatures,three-dimensional network structure of carbon foam is used as template to realize three-dimensional ring distribution of carbonyl iron.This distribution induces a structural magnetic loss by magnetic-field convergence effect while retaining the intrinsic magnetic loss,the maximum improvement in magnetic loss being 24%.Meanwhile,the three-dimensional continuous-conductive path of carbon foam reduces the number of interfaces and increases conductance loss of composites by up to 120%.The conductivity-temperature characteristics of conductive path can be tuned by carbonization temperature of carbon foam.The synergistic optimization of magnetic and conductance losses allows carbonyl iron/carbon foam composites to maintain an effective absorption bandwidth of around 8.48 GHz in the temperature range of 25～300℃.In order to further simplify structural template and optimize impedance characteristics of microwave absorbing materials,Fe3O4-modified glass fiber cloth is used as two-dimensional structural template to realize orientation distribution of carbonyl iron.The optimized mechanism of this distribution structure on microwave loss characteristics,impedance characteristics and microwave absorbing properties in a wide temperature range is investigated.It is found that the orientation distribution of carbonyl iron along glass fibers not only reduces reflection area of microwave to enhance impedance characteristics by 14.3%～22.3%,but also increases the number of eddy current centers from one to four,enhancing the interaction between carbonyl iron and microwave and maximum 18%increase in magnetic loss.In a temperature range of 25～400℃,carbonyl iron/glass fiber cloth shows a maximum effective bandwidth of 5.82 GHz and an optimum reflection loss of-21.50 d B.The low thermal conductivity of carbonyl iron/glass fiber cloth also provides good thermal infrared stealth protection for objects within room temperature～254.5℃.In order to meet development demands of multi-function and intelligence,inspired by the dynamic adjustment mechanism of beetle elytra color,carbonyl iron is distributed in one-dimensional channel structure of carbonized wood to obtain multi-functional microwave modulator.Through electromagnetic simulation studies,it is found that spaced arrangement structure formed by carbonyl iron and carbonized wood in the horizontal direction can not only dissipate electromagnetic wave,but also break the translational invariance of electromagnetic properties.The radiation direction of reflected wave is modulated by phase mutation,enabling microwave modulator to have two protection mechanisms of electromagnetic wave absorption and radiation deflection.In a temperature range of 25～400℃,microwave modulator shows a maximum effective absorption bandwidth of 5.20 GHz and a maximum electromagnetic protection efficiency of 97%.In addition,the low thermal conductivity and monotonic conductivity-temperature characteristics of microwave modulator are used to further develop its functions of thermal infrared stealth in complex environments and real-time monitoring of operating temperature. |
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