实验制备了由镍泡沫、聚二甲基硅氧烷(PDMS)和少层石墨烯组成的镍基复合薄膜(NCF)太赫兹宽带吸波体，采用太赫兹时域光谱仪测试其吸收性能。结果表明，当镍泡沫厚度为0。5 mm、少层石墨烯质量分数为2%时，在0。3～3。5 THz范围内吸收率均高于0。9，且平均吸收率为0。944。此外，NCF在100℃和200℃的高温下，仍能保持稳定的吸收性能，平均吸收率分别为0。966和0。924。同时，测试了1 mm和1。5 mm NCF压至0。5 mm时的吸收性能，结果表明:NCF的吸收性能略有下降，但吸收率仍然保持在0。8以上;不同厚度的镍泡沫通过掺杂不同质量分数的少层石墨烯，可以增大吸收带宽。运用阻抗匹配理论和多重干涉理论对宽带吸收特性进行解释。这种NCF太赫兹宽带吸波体在太赫兹雷达隐身、宽带无线通信和太赫兹成像等领域具有潜在应用价值。
Preparation of Terahertz Broadband Absorber with Nickel-Based Composite Film
Objective With the rapid development of terahertz sources,highly sensitive detectors,and new terahertz functional devices,the practical application fields of terahertz technology are constantly expanding.Terahertz absorbers are of important application value in the fields of high-sensitivity terahertz detectors,terahertz radar stealth,and electromagnetic radiation protection,and they have become a research hotspot in the field of terahertz in the past decade.In this study,we report a nickel-based composite film(NCF)terahertz broadband absorber with an absorptivity of more than 0.9 in the range of 0.17 ‒ 3.5 THz,and it still has excellent absorptivity and absorption bandwidth under high temperature and compression.At present,the terahertz absorbers are mainly two-dimensional structures.We use a three-dimensional nickel foam to reduce surface reflection and find that it has a wider absorption bandwidth than two-dimensional structures.At the same time,compared with three-dimensional graphene absorbers and aerogel absorbers,NCFs are easier to prepare,with lower cost,and they can be applied on a large scale.Therefore,our work contributes to the design and fabrication of terahertz broadband absorbers.Methods We prepare NCF terahertz broadband absorbers from nickel foam,polydimethylsiloxane(PDMS),and few-layer graphene.First of all,we mix the main agent and curing agent of PDMS according to the ratio of 1∶10,put different mass fractions of few-layer graphene into PDMS to prepare PDMS/few-layer graphene mixtures,and stir magnetically for 30 min to mix PDMS and few-layer graphene evenly.Then,we use a pipette to add the mixture dropwise to the surface of the nickel foam and let it stand for 10 min to allow the mixture to fully enter the pores of the nickel foam.Subsequently,a homogenizer is used for spin coating at 500 r/s for 20 s,so as to evenly distribute the mixture in the nickel foam.The mixture is defoamed in a vacuum drying for 30 min.Finally,NCFs can be obtained by placing the PDMS on a thermostatic heating stage and curing the PDMS at 75℃for 30 min.A mixture of PDMS/few-layer graphene is thus prepared,and the mixture is added to nickel foam to obtain NCFs.The transmittance and reflectance of the NCFs are measured using a terahertz time-domain spectrometer(CCT-1800),and the absorption is calculated using a fast Fourier transform formula.We analyze the effects of PDMS/few-layer graphene mass fraction,temperature,compression,and nickel foam thickness on the absorption properties of NCFs.The characteristics of broadband absorption are discussed from the perspectives of impedance matching theory,electromagnetic wave theory,and multiple interference theory.Results and Discussions The absorptivity of 0.5 mm NCF(2%)exceeds 0.9(Fig.4)in the range of 0.3‒3.5 THz,and the average absorptivity reaches 0.944(Fig.5).At the same time,the effects of temperature,compression,and thickness on the absorption performance are studied,and the results show that the average absorptivity of NCF at 100℃reaches 0.966 when the mass fraction of few-layer graphene is 2%,which is 0.022 higher than that at room temperature(Fig.5).When the 1.5 mm and 1 mm nickel foam are compressed to 0.5 mm,the NCF still has excellent terahertz broadband absorption performance,and the absorptivity remains above 0.8 in the range of 0.3‒3.5 THz(Fig.7).In addition,the absorption bandwidth of NCFs with different thicknesses can be expanded by adjusting the mass fraction of few-layer graphene,and the qualified bandwidth of 1 mm NCF(0.5%)with an absorptivity of more than 0.9 is 0.2‒3.5 THz,which is 0.1 THz higher compared with the qualified bandwidth of 0.5 mm NCF(2%).The qualified bandwidth of 1.5 mm NCF(0.5%)with an absorptivity of more than 0.9 ranges from 0.17 THz to 3.5 THz,which is 0.13 THz higher than the qualified bandwidth of 0.5 mm NCF(2%).Conclusions In this paper,NCFs are prepared by using the spin coating method.There are three main reasons why NCFs have broadband absorption,Firstly,the three-dimensional porous structure facilitates good impedance matching,allowing as much of the incident terahertz wave to enter the absorber as possible.At the same time,terahertz waves will also be reflected and scattered many times in the three-dimensional porous structure,which prolongs the attenuation path of terahertz waves and enhances the attenuation ability of terahertz waves.Secondly,the interconnected nickel skeletons provide an efficient way for electron jumping and migration so that terahertz waves are consumed in the form of conductive losses.Finally,the addition of the mixture can provide a large number of heterogeneous interfaces(PDMS/nickel foam,PDMS/few-layer graphene,and few-layer graphene/nickel foam),where the accumulated charges result in the interface polarization loss because of their different permittivity.On the study surface,in nickel foams,broadband absorption of terahertz waves can be achieved by manipulating the PDMS/few-layer graphene mass fraction.
materialsnickel-based composite film(NCF)terahertz broadband absorbertemperature influencecompression influencemultiple interference theory
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