Electromagnetic interference shielding using electrospun fibers

Electromagnetic frequencies (EMF) are often generated by electrical appliances and this may cause electromagnetic interference (EMI) on sensitive electronics. It is important to ensure that any EMF emission is kept to a minimal so that it does not interfere with other electrical devices and there are regulatory guidelines and requirements by the major economies on this. There are several ways to control EMI and having an EMI shielding is one option where electropun fibers have been tested for.

EMI shielding is usually carried out by having a barrier made of conductive or magnetic materials to block out the electromagnetic field. Electrospun nanofibers made from conductive inorganic material, polymer material or composite can be made into a mesh to function as the barrier. Several researches have already demonstrated conductivity of electrospun mesh [Read article] and this has raised the possibility of using the conductive mesh as an EMI shield. Electrospun nanofibers mesh is an interesting material for EMI shielding as high aspect ratio or network of interconnected fibers provide a conductive pathway. The mesh is also very light and the pore sizes between the fibers are typically small which may provide an effective barrier to EMF. Various forms and methods of incorporating and using electrospun fibers have been tested for EMI shielding potential.

A commonly method of applying an EMI shield is to coat the enclosure with a layer of conductive paint. Based on a similar concept, Lee et al (2009) manufactured electrospun Fe nanofibers through reduction of Fe salt in polyvinyl pyrrolidone (PVP)/Fe salt nanofibers. The Fe nanofibers were later grinded to form short fibers and mixed uniformly into an Epoxy matrix and tested for EM characteristic for the purpose of shielding against high frequency EM radiation. Their study showed that the Fe nanofibers fillers with its high aspect ratio improved the permittivities and permabilities compared with spheres or particle fillers which are important for improving high frequency EM wave shielding. Further tests were carried out by Lee and Choa (2012) on electrospun FeNi alloy nanofibers and compared with FeNi nanoparticles embedded within epoxy matrix. Without breaking up the nanofibers into short strands, their study showed that the power loss of EM waves passing through commercial sheets with FeNi nanoparticles decreases at frequency beyond 12 GHz while the power loss for composite containing FeNi nanofibers continue to increase up to 20 GHz.

Instead of having short conductive nanofibers as fillers in a composite matrix, electrospun fibrous mesh may be used directly as shielding material. Wei et al (2011) used metal sputtering method to coat Cu onto electrospun polyurethane nanofibers to test for EMI shielding effectiveness. With increasing Cu layer, the resistivity of the mesh reduces and the shielding effectiveness increases. Kim et al (2012) tested the mechanism of EMI shielding by metal coated electrospun nanofibers. They found that the Cu coated nanofiber mat provided shielding through absorption of the EMF with reflection as the secondary shielding mechanism. This tendency of nanofiber mat shielding through absorption was attributed to the interconnected nanofibrous network that cause the wave to scatter within the material upon impact which prevented them from escaping. Selection of material for coating is important as their studies showed that Nickel layer provided no shielding while silver coating provided the best result. With increased metal layer from 50 nm to 200 nm, the shielding effectiveness increases. Ramlow et al (2022) demonstrated the electromagnetic (EM) shielding by pyrolyzed electrospun polysilazane/polyacrylonitrile (PAN) fibers to form silicon carbonitride (SiCN) fibers. The SiCN fibers were able to shield more than 50% of incident EM wave energy from 100 MHz to 4.5 GHz which includes VHF, UHF, and L-, S-, and part of the C-band. The addition of PAN to polysilazane solution was found to reduce the diameter of the resultant fibers compared to electrospun pure polysilazane solution giving polysilazane/PAN derived SiCN fibers a greater porosity. The presence of PAN also helps to increase the carbon content of the fibers which enhances the electrical conductivity of the mesh. Salem et al (2023) investigated the use of electrospun polyvinylidene fluoride (PVDF) and barium hexaferrite (BHF) fibers as X-band (frequency spectrum, spanning from 8.2 GHz to 12.4 GHz) electromagnetic shielding. PVDF and BHF powders were blended to form a homogeneous solution before electrospinning to form PVDF/BHF fibers. A measure of Reflection Loss (RL) in electrospun PVDF/BHF fiber membrane gave a value less than -10 dB which showed that the material was able to capture at least 90% of electromagnetic waves. The reduced RL may be attributed to higher surface area of fiber form and interfacial polarization at the boundary between PVDF and BHF particles. Measuring the radiation shielding parameter for gamma-ray in the energy range of 0.015 - 15 MeV in PVDF/BHF fiber, the mean free path (MFP) range from 0.014 to 8.828 cm².g^-1, the half value layer was 0.009 to 6.119 cm².g^-1 and linear attenuation coefficient (µ) of 73.960 to 0.113 cm².g^-1. Therefore, the fiber composite has the potential to be used for protection against electromagnetic radiation (EMR) from electronic devices.

Electrospun fibers may also be used as a carrier for EM shielding nanoparticles. Yang et al (2022) used electrospinning to construct a FeCoNi magnetic alloy embedded in a 1D carbon matrix framework to facilitate electromagnetic (EM) wave attenuation in low-frequency (2-6 GHz) microwave absorption. FeCoNi nanoparticles were first synthesized and loaded in a polyacrylonitrile (PAN) solution for electrospinning into nanofibers. Carbonization was carried out to form the FeCoNi carbon fibers. The absorption coefficient of FeCoNi/CF was significantly greater than pure carbon fibers within the 2 - 6 GHz band. The effective absorption band was at 1.3 GHz where the reflection loss value was greater than -4 dB and at a thickness of only 2 mm. Instead of loading the polymer solution with nanoparticles, Zhang et al (20024) loaded polyvinyl alcohol (PVA) solution with nickel acetate tetrahydrate for electrospinning into fibers followed by carbonization to form Ni/C nanofibers. The Ni/C nanofibers were subsequently blended with molten paraffin wax, solidified and grounded into powder before molded into a ring-shaped specimen for testing. Optimal microwave absorption with minimum reflection loss (RL) value of -30.6 dB and effective absorption bandwidth of 5.96 GHz was achieved with 50 wt% paraffin wax, Ni/C nanofibers and 3 mm thickness. It is hypothesised that the microwave-absorption was from several mechanisms including dipole polarization and interfacial polarization. Dipoles, unlike free electrons, cannot move freely in an external electric field. Hence dipoles electromagnetic wave energy from the environment to reorient and polarize. In a composite material such as Ni/C, the difference in their dielectric properties led to accumulation and uneven distribution of space charges at the interface. This results in electromagnetic wave energy loss which is known as interface polarization. Another mechanism is the electromagnetic waves cause induced currents passing through the material and produce the Joule effect where the electromagnetic waves convert to thermal energy. The Ni nanoparticles encourage the scattering of electromagnetic waves which may produce interference phase cancellation. Interconnected network of fibers also increases the pathways for electron movements and transfers leading to conduction loss.