The frequency domain (0.1 to 10 THz) between microwave and infrared regions of electromagnetic spectrum (EM) is known as terahertz (THz) region. In the past decade, THz spectroscopy and non-destructive testing (NDT) have attained much interest due to recent revolution in optics and THz devices. THz technology finds a wide application domain including defence, security [1, 2], pharmaceutical industry [2, 3], NDT [2, 4, 5] etc. Recent applications include THz-stealth satellites, inter-satellite THz communication systems, THz thermal detectors, EMI shielding, THz radar systems etc with increased focus on wireless communication (beyond 5G) in civil domain and counter stealth in defence domain [6–12]. Considering the recent progress in the THz radar field, THz counter stealth is very crucial in defence sector. THz radars have more potential than regular radars considering their capability of generating a wide range of continuous wave (CW) frequencies as well as nano and pico second pulses which enable them to overcome the stealth advantage of an aircraft due to its narrow-band radar absorbing coatings [13, 14]. With the enhanced use of THz technology, current research is inclined towards THz counter stealth technologies and addressing the problem of increased density of THz radiation. Wideband THz absorbers are attracting the scientific fraternity as a solution [12–17].
Currently EM absorption and electromagnetic interference (EMI) shielding (absorption as well as reflection) approaches are used for attenuating undesired EM radiation [12, 18]. For EMI shielding applications, the transmission loss is of prime focus while for stealth applications (absorbers), reflection loss (of test specimen with a metal backing) is focussed primarily. EM absorbing materials attenuate the incident EM wave by converting them into heat energy. Various efforts are being done worldwide for the development of THz absorber materials with wide absorption bandwidths and strong absorption intensity [12, 19]. Many groups have provided metamaterials based absorbers and reported very sharp absorption dips enabling them to be used for narrowband applications as they work on resonance phenomenon [20–24]. Wideband THz absorptive coatings were also developed but being fragile in nature are not useful for field applications [24, 25]. Conductive fillers based THz absorber composites have gained much attention during the last decade in which THz absorption is governed by various factors like dispersion, aspect ratio and intrinsic conductivity of the filler [26]. Their properties can be tuned with the change in the filler loading [27].
Carbon based materials are prominent in EM absorption owing to various factors like electrical conductivity, chemical inertness, improved strength and durability, low production cost, flame retardancy, requirement of low filler loading, existence of various allotropic forms exhibiting diversified properties, UV absorption etc [25, 27, 28]. They possess electrical properties ranging from conductors to insulators e.g. two allotropic forms of carbon i.e. graphite and diamond have complementary properties in THz domain- graphite being opaque (good conductivity) while diamond being transparent (good insulator) [28].
Carbon based materials are being explored in various forms for THz absorber applications e.g. carbon based polymer composites, carbon thin films, carbon based foams, polymer derived carbon etc. [29]. A lot of work has been done on THz absorbers based on carbon thin films including carbon nanofibers [25], free standing carbon film (MWCNT based) [30], stackedgraphene[31, 32] etc. Huang et al. studied graphene foam and multiwalled carbon nanotubes / multiwalledgraphene foam based THz absorber [33]. S. Venkatachalam et al. studied polymer derived carbon obtained from thermal conversion of polymers for THz absorber applications [34]. Most of the work remains concentrated around carbon based polymer composites due to their light weight, corrosion resistance, flexibility, ease of processing [35]. Carbon fillers of various shape and structures have been explored like carbon nanotubes (CNT) [36], carbon black [36], carbonnanofibers (CNF) [25, 26], graphite particles [37],graphene [31], carbonnanowhiskers [38], onion-like carbon (OLC) [39] etc. Previously, magnetic materials (e.g ferrite, iron particles etc) were also mixed alongwith conducting fillers to enhance EM absorption properties, but it also resulted in added weight penalty restricting their usage in defence applications [40].
In the current work, we have fabricated mechanically stable multilayer polymer composites with various carbon fillers with glass fiber as reinforcement and epoxy as matrix. Carbon veil (CV), chopped carbon fiber (CCF) and milled carbon fiber (MCF) are used as fillers and loaded at varying percentages. The prepared polymer composites were tested for absorption of THz radiations in wideband frequency region. 10–12 dB reflection loss was achieved for CCF as well as MCF loaded polymer composite sample for 0.75–2.25 THz frequency domain. CV based composite has shown the best performance with a reflection loss of 10–20 dB (high absorption upto 99%) in the frequency range of 0.5–2.25 THz showing its capability to replace fragile wideband THz absorptive coatings as well as metamaterials based narrow band THz absorbers owing to its high mechanical strength & broadband THz absorption.