Using carbon-fibre-reinforced polymer (CFRP) composites for electromagnetic interference (EMI) shielding has become a rapidly emerging field. This state-of-the-art review summarises all the recent research advancements in the field of electromagnetic shielding properties of CFRP composites, with exclusive attention paid to experimental work. It focuses on (1) important mechanisms and physical phenomena in the shielding process for anisotropic carbon-fibre composites and (2) shielding performance of CFRP materials as reported in the literature, with important performance-affecting parameters. The key properties which directly influence the shielding performance are identified, the most critical being the carbon-fibre concentration along with length for discontinuous carbon-fibre-filled polymers and the lay-up for continuous carbon-fibre-reinforced composites. The effect of adding conductive inclusions such as metal or carbon nanotubes is also reviewed. It is emphasised that processing conditions are strongly linked with the shielding properties of a composite. This is a first review, which covers all the recent advancements in the field of shielding properties of carbon-fibre-reinforced composites, with detailed analysis of factors influencing these properties and clear distinction between continuous and discontinuous reinforcement. It is shown that CFRP composites make a good candidate as an EMI shielding enclosure material.
Rapid industrialisation and digitalisation in the modern world has brought about a growth in the application of electronic devices across a broad spectrum of commercial, industrial and military sectors. Application areas range from scientific instruments and commercial electronic devices, such as mobile phones, medical apparatus and industrial robots, to military and aerospace products, including communication and navigation systems. Increased electromagnetic (EM) radiation emissions from this multitude of electronic devices have raised the problem of electromagnetic interference (EMI), defined as the effect of unwanted energy emitted by electrical circuits under operation, which is demonstrated by performance perturbation or even complete breakdown of the surrounding electrical system [1,2,3,4,5,6]. Modern electronic devices are densely integrated and operate at relatively low voltage, hence are particularly vulnerable to the perturbation caused by EMI. The problem is addressed by applying a suitable shielding barrier, which often comes in the form of enclosures carrying the sensitive equipment. EMI shielding, defined as a practice of blocking or reducing electromagnetic field penetration, is accomplished with the help of conductive and magnetic materials. Depending upon the material characteristics, shielding barriers minimise signal transmission by reflecting an oncoming wave at the front face, absorbing and dissipating oncoming radiation inside the material, or a combination of both [7,8,9,10]. Shielding is quantified by the shielding effectiveness (SE) parameter, expressed in decibels, as the ratio of the incident to transmitted EM power or field intensity. Important aspects of the shielding enclosure design process and relevant strategies can be followed [11, 12].
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As an EM plane wave of a specific frequency impinges on a barrier with intrinsic impedance lower than the wave propagation medium impedance, it forces charges in the material to oscillate at the same frequency as the incident wave. The oscillating charges behave like an antenna, inducing superficial high-frequency alternating currents, which generate a counteracting electric field. This field weakens or cancels the original incident field of the incident wave and appears as reflected power, with minimum energy losses. Reflection thus requires the presence of charge carriers, such as electrons or holes, in the material and is the dominant mode of shielding for highly conductive metals. According to the transmission line theory, the magnitude of the reflection loss SER is dependent on the degree of impedance mismatch between the wave propagation medium (\(\eta _o\)) and shielding material (\(\eta _s\)):
For radio frequency and microwaves, which are the main concern in terms of electromagnetic interference, the attenuation takes place due to the currents induced in the structure and magnetic and electric dipoles in the material interacting with the wave. In particular, two types of absorption loss can be distinguished, namely Ohmic and polarisation loss. The former corresponds to the Joule heating loss when the energy is dissipated via induced electron current flow in phase with the applied electric field. The dissipation of energy by nomadic charges takes place through various mechanisms such as conduction, hopping and tunnelling and is enhanced by electrical connectivity within the material, which provides a continuity of electric field lines. Polarisation loss comes from attenuation of energy through rotation and reorientation of dipoles (rather than motion of electrons or ions), thus improving with the increase of the dielectric constant or magnetic permeability of the material. The incident EM wave is annihilated by dipoles as they are forced to vibrate back and forth by the oscillating electric field. When the frequency of the electric field grows, the dipoles are not able to orient themselves fast enough to respond to the applied electric field. Polarisation sites lead to the formation of local fields and a lag displacement current relative to the conduction current and as a result, dielectric loss. Absorption loss is thus frequency-dependent and affected by the atomic structure of the shield material. Microstructural features within the material, such as functional groups, defects and interfaces will all affect the formation of polarisation sites.
In all analytical and numerical models, it is assumed that parallel fibres are uniformly distributed in the matrix. However in practice the carbon fibres are undulating and contact points between fibres are randomly distributed depending on factors such as reinforcement weave pattern, fibre volume fraction and manufacturing conditions [115]. The effect of fibre array irregularities on the shielding is still not well known, hence any theoretical and computational predictions must be verified experimentally for a specific material. There exist few analytical models [116, 117], which allow us to make initial estimations of SE for a multilayer CFRP without costly and time-consuming experiments and simulations, which are valid only under very specific conditions. Using an effective media approach, where the mixture of materials in the composite is homogenised, Angulo et al. [116] developed an analytical model for predicting the SE of carbon-fibre composites with sheet square resistance and panel thickness being the only input parameters. The model is valid for unidirectional (UD) and cross-ply (CP) configurations only, since dominant intermodal conversions for other ply orientations are not taken into account. A model for the plane wave SE of anisotropic laminated composites was developed by Lin et al. [117], where each individual lamina is regarded as a homogeneous and anisotropic sheet with known electrical parameters. By constructing the solution to the Maxwell equation in each lamina, imposing boundary conditions on the electric and magnetic fields at the interface and assuming each lamina is electrically thin, the relationship between the incident and transmitted electromagnetic waves can be derived. The empirical formula allows us to estimate the SE of anisotropic laminated composites as a function of overall thickness, stacking sequence, electrical properties of each ply and incident radiation orientation relative to the laminate. The model is applicable only in a low frequency regime, where the plies are electrically thin and fibre separation is only a small fraction of the radiation wavelength.
The investigation of the effect of laminate stacking sequence on the SE has so far attracted great attention [188,189,190,191,192,193,194,195,196,197,198,199,200]. This is not surprising as the orientation of fibres in the laminate plies, relative to the polarisation of the incident radiation, has the biggest impact on the SE, incomparable to any other parameter. Most of the investigations [190, 197,198,199,200] followed ASTM D4935 standard, in which the tested sample is illuminated by an EM plane wave in transverse electromagnetic (TEM) mode, with a radially polarised electric field and in a similar number of studies [189, 191, 192, 194,195,196], a rectangular waveguide transmission line with UD polarisation was used. In one study [193], a nested reverberation chamber (NRC) method was employed, which exposes the tested sample to a random, multi-mode EM field with varying incident angles and polarisation. The majority of the host laminates [191, 193,194,195, 197,198,199,200] did not consist of more than six plies in unidirectional (UD) [190, 191, 193, 198,199,200], cross-ply (CP) [190, 193, 194, 197,198,199,200], quasi-isotropic (QI) [193, 194] and multidirectional (MD) [190, 194, 197, 198, 200] configurations and in most cases [190, 193,194,195, 197,198,199,200], various lay-ups were compared within a single study. All the experimental results and accompanying details have been summarised in Table 5.
Although the uniform characterisation of the shielding properties of CFRP materials is far from being achieved on a consistent basis, several key issues that directly follow from the literature review can be highlighted and serve as a starting point in the design process of a composite shielding material. In the case of short carbon-fibre-filled composites, the length of the carbon fibres is the most crucial parameter, which enables them to create multidimensional conductive networks with lower filler amounts. It is therefore of paramount importance to choose the appropriate composite manufacturing method and processing conditions, which allow us to preserve the carbon-fibre length. The most critical aspects concern the minimisation of the shear stress applied to the fibres during mixing, whilst providing uniform dispersion in the polymer for a given filler quantity. The optimisation must therefore be made between the mixing method, applied torque, processing temperature, matrix viscosity and addition of chemicals such as lubricants and coupling agents. For multilayer, continuous carbon-fibre-reinforced composites, the lay-up has the greatest influence on the SE, with CP and QI configurations performing much better than UD when illuminated with the most common unpolarised electromagnetic wave. This is attributed to the coincidence of the fibre orientation with electric field direction, facilitating the current flow and hence the interaction with the EM wave. A less pronounced improvement is achieved by increasing the number of plies. Multilayer composites may also be constructed with other forms of carbon-fibre reinforcement such as mats, veils and felts, which provide an interesting alternative given their good electrical properties, due to greater systematisation and uniformity in the structure, whilst reducing costs by using discontinuous carbon fibres. Recycled carbon fibres can also be used for such architectures. To further improve the electrical and thus shielding properties of CFRP, carbon fibres can be coated with metals or combined with conductive inclusions, such as metal particles or carbon nanotubes. The degree of enhancement in such composites depends on the overall content of conductive inclusions, the method of their incorporation and composite configuration. A graph summarising all factors affecting the SE of carbon-fibre-reinforced composites is shown in Fig. 14. 2ff7e9595c
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