Study Of Flexible Infrared Absorbing Material Based On Multilayer Structure

In recent years,infrared frequency selective absorbing materials have shown extensive application potential in various fields of civil and military applications,such as passive radiative cooling,infrared camouflage,and infrared detection.Generally speaking,the frequency selectivity is achieved through various micro/nano geometric structural designs and thus highly depending on micro/nano-processing technologies such as photolithography.However,these methods have high costs and unstable performance.Therefore,this thesis proposes infrared selective absorbing materials based on multilayer film structures.Firstly,the 8-20μm high absorption is realized in multilayer dielectric film structure for passive radiation cooling.Secondly,by inserting an ultra-thin metal layer into the dielectric structure,5-8μm high absorption is achieved for infrared camouflage.Finally,visible/infrared compatible camouflage design is implemented using hole structure,while infrared reflection control is explored based on rough surfaces.The main work and innovations include the following aspects:(1)According to the equivalent medium theory and impedance matching theory,a multi-layer dielectric thin film model is designed and optimized to obtain 8-20μm absorption with an average value of 90.2%.The structure contains alternating Si3N4(400 nm)and SiO2(50 nm)thin films,and the metal substrate is Al with a thickness of 100 nm.The magnetron sputtering conditions of SiO2 include vacuum degree 1×10-4 Pa,argon pressure 0.5 Pa,and sputtering power 150 W,while that of Si3N4 are vacuum degree 1×10-4 Pa,argon pressure 1.0 Pa,and sputtering power 200 W.The parameters of electron beam evaporation deposition of Al are vacuum degree 1×10-4 Pa,and beam current 150 mA.The spectral properties of the prepared structure are consistent with the design,as the average reflectivity in the visible light band is 98.6%and the average infrared absorption rate is 86.8%.The temperature drop of the hard substrate sample reaches 4 degrees centigrade,and the temperature drop effect of the PET substrate sample reaches 8 degrees centigrade.(2)Based on the impedance matching theory and combined with the high loss characteristics of ultra-thin metal films in the infrared band,a 5-8μm absorption structure is obtained with average absorption rate 83.1%and absorption bandwidth 1.9μm.The resulting optimization design concentrates,the in-band energy in the ultra-thin Ag layer to achieve high absorption,while the out-band energies are concentrated in the Ge dielectric layers to achieve high reflection.Finally,5-8μm infrared non-detection band shows high absorption for radiation cooling,while 3-5μm and 8-14μm infrared detection band possess high reflection for infrared camouflage.(3)A hole-array structure is designed on the top of the above-mentioned Ge/Ag/Ge structure to enhance visible light absorption and achieve the effect of visible/infrared compatible camouflage.Considering the needs for infrared directional reflectivity suppression,roughness of the thin films are changed by coating on rough substrates.At the same time,low emissivity materials are used to achieve low infrared emissivity.Due to the convenience brought by a non-rigid thin film system,it is possible to achieve both flexible and large area applications of our infrared absorbing materials.