Due to the importance of satellite communication, electromagnetic compatibility in the Ka
band (26 to 37 GHz) has recently become an important concern. The band overcrowding requires enhancing electromagnetic interference (EMI) shielding effectiveness (SE), i.e., development of novel coatings, shields, and filters that prevent degradation of the performance of the systems operating in densely populated EM environment [1
]. It is worth noting that compared to conventional metal-based EMI shielding materials, using carbon-based conducting composites is advantageous for satellite applications because of their low weight, small thickness, and flexibility [3
]. These include polymer composites containing exfoliated graphite, graphene nanoplatelets, carbon black, carbon fibers and nanofibers, carbon nanotubes (CNT), and carbon onions. Shielding effectiveness of these carbon-based coating has been extensively investigated in the last decade (see reviews [3
] and the references therein).
The EMI shielding effectiveness of a material is defined as SE (dB) = 10 log (Pt
], where Pt
are the transmitted and incident electromagnetic powers, respectively. Thus, the magnitude of the SE is determined by the material transmittivity, which depends on the absorption, reflection, and scattering losses of the EM energy. In homogeneous materials, absorption and reflection losses dominate the SE. The absorption-related losses in conventional metals are determined by the relationship between the metal thickness and the skin depth, which decreases with the frequency [6
]. The reflection occurs due to the impedance discontinuity at metal-air interface. The reflection losses decrease at higher frequencies since material impedance increases. The absorption mechanism predominates when the coating thickness is comparable with the skin depth or at sufficiently high frequencies when the conductivity decreases [6
]. Thus, conventional metallic coating being much thinner than EM skin depth should, strictly speaking, be transparent to microwave radiation.
Breakthrough in the EMI technology has been recently made by Bosman et al. [7
]. Using a simple equivalent transmission line model for the thin film as a lumped resistor they demonstrated that an ultrathin film may absorb up to 50% of the incident power despite the fact that its thickness is only a small fraction of the skin depth [7
Very recently, we have demonstrated [8
] that the pyrolytic carbon (PyC) films with thickness of several tens of nanometers satisfy the requirements imposed by the theory [7
]. Specifically, the PyC film thickness is much smaller than the skin depth, which is much smaller than the wave length. Thus these films should allow one to achieve high SE. We showed in [8
] that sheet resistance of these nanometrically thin films is close to that of multilayer graphene flakes [9
] and carbon nanotubes [11
], which have already displayed unique EMI shielding ability [3
]. However, in contrast to graphene and carbon nanotubes, a catalyst-free synthesis allows one to deposit PyC films directly on both dielectric and metallic substrates of arbitrary size and shape opening a new route towards fabrication of ultrathin EMI protective coatings with enhanced shielding effectiveness.
In this paper, we study experimentally the EMI shielding ability of an ultrathin PyC film in Ka
band (26 to 37 GHz). The thickness of the film is 25 nm, which is close to the PyC skin depth at 800 nm [13
]. We demonstrate that despite the fact that the film is several thousand times thinner than the skin depth of conventional metals (aluminum, copper) in this frequency range, it can absorb up to 38% of the incident radiation.
The paper is organized as follows: the details of sample preparation and microwave (MW) measurements are given in the ‘Methods.’ Experimental data together with their physical interpretation are collected in the ‘Results and discussion.’ The ‘Conclusion’ summarizes the main results as well as some important possible applications of the functional properties of PyC films.