08 Mar 2017
Materials & Manufacturing
Did you Know that your Mobile Devices Cause Electromagnetic Interference?
Electromagnetic interference (EMI) can cause many problems to equipments, communication, living organisms, etc. Your mobile electronic devices is an emission sources of EMI. This article explains what researchers at École de technologie supérieure (ÉTS) of Montreal are proposing to reduce / eliminate EMI.
Editor’s note: today is the international women’s day. During the entire week, the Substance team has decided to pay tribute to women in research by promoting a selection of articles written by women. We hope you will enjoy our selection.
Today’s piece is an article written in July 2015 by Nicole R. Demarquette.
For the last few years, mainly due to information and communications technology development, a new environmental problem related to electromagnetic radiation has emerged. Especially in large urban centers, mobile electronic devices such as cell phones, laptops and tablets, have become portable sources of electromagnetic interferences.
These electromagnetic interferences (EMI) can hinder the operation of surrounding electronic equipment or cause damage to living organisms. As a result, extensive research has been conducted towards the development of EMI shielding materials that decrease electromagnetic wave propagation from one region to another. Traditionally, EMI shielding is achieved using electrical conductors or magnetic materials such as metals. Unfortunately, metallic materials present the disadvantage of a high rigidity, density and are prone to corrosion. As an alternative, lots of efforts have been spent towards the development of polymeric shielding materials that present a lower density, a low cost and good processability. However, most polymers are electrical insulators, a property that makes them nearly transparent to electromagnetic waves. Therefore, in order to overcome this limitation, electrically conductive polymer composites were prepared by dispersing in a polymeric matrix conductive fillers such as copper, nickel, intrinsically conducting polymers, or carbon particles in suitable concentrations.
The success of a conductive polymer composite to be used as EMI shielding material relies on a proper dispersion of the particles within the matrix. For the polymer to be conductive, it is necessary that the particles form a physical path that will enable the conduction of electricity. Figure 2 shows in a schematic way the formation of conductive paths in electrically conductive polymer composites. This is achieved once the amount of particles reaches a certain value, known as percolation threshold. The percolation threshold can be affected by a number of factors inherent to the type of polymer and conductive particle used to obtain the composite, such as polymer conductivity (conductive polymers may be used for such purposes, although they are not easily processed), filler aspect ratio, specific surface area, surface conductivity. It also depends on the morphology of the composite, which can be tailored during processing. The morphology of the composite, which can be described by the state of distribution and dispersion of the conductive filler within the matrix, depends on the processing conditions to obtain the composites, the rheological properties of the polymers and the interactions polymer/filler (polymer/filler interactions).
As a general rule, the amount of fillers in composite materials should always be as small as possible. In this context, the use of carbon nanoparticles, such as graphene and carbon nanotubes, could be and (an) advantageous option. These nanoparticles present some aspects such as huge specific area and high aspect ratio, which can reduce the amount of filler needed to achieve the percolation threshold needed for the material to be efficient as EMI shielding. Furthermore, carbon nanotubes and graphene also exhibit extraordinary mechanical properties, and high thermal and electrical conductivity.
However, dispersing carbon nanoparticles within a molten polymer using conventional polymer processing techniques is not a piece of cake. Carbon nanoparticules, as any other nanoparticles, tend to form large agglomerates and the 3D network that is necessary to obtain the percolation threshold for electrical conductivity is seldom obtained. Therefore, the difficulty in dispersing carbon nanoparticles is a limiting factor for the use of carbon nanocomposite as EMI materials.
Block copolymers formed of immiscible blocks are interesting materials that can be nanostructured. Depending on the size of the block and their affinity, they can present a whole range of morphologies. They can be used as templates to tailor the location of carbon nanoparticles and therefore obtain a successful EMI material. Moreover, these materials often present properties of thermoplastic elastomers material that present a two-phase morphology structure made up of soft and hard blocks. These materials exhibit mechanical properties similar to crosslinked rubbers, but can be processed and recycled as thermoplastic materials.
In our research group, we work more specifically with the styrene-ethylene/butylene-styrene (SEBS) block copolymer. One of the biggest advantages in using this material is the fact that through applying appropriated methods and processing parameters, the SEBS morphology may allow a suitable nanoparticle dispersion and distribution through this matrix. Moreover, by controlling the two-phase morphology it is possible to provide a selective location of the nanoparticle in one of the phases. This selective localization also decreases the amount of filler required to achieve the desired properties. Figure 3 shows a scheme regarding the preferential location of carbon nanoparticles in one of the SEBS blocks.
As a result, our research group is obtaining good values of EMI shielding. To exemplify, with 15 wt. % of a carbon nanoparticle in SEBS nanocomposites, it was already possible to achieve an effectiveness of 99.9 % in electromagnetic interference shielding. And with a mixture of two different carbon nanoparticles it was possible to reach 99.5 % of shielding effectiveness with only 10 wt. % of fillers. Taking into account that using other fillers such as intrinsically conducting polymers it is necessary a filler content of more than 30 wt. %, and that when metallic particles are used the composite mechanical properties generally drastically decrease, our achieved results are even more interesting. Under those circumstances, the preliminary results show the promising potential of the obtained material for commercial applications.
This research is being developed in a partnership between ÉTS, Canada, and the Federal University of Santa Catarina, Brazil.
Guilherme M. O. Barra
uilherme M. O. Barra is a professor at the mechanical engineering department at Universidade Federal de Santa Catarina (UFSC), Brazil. He is specialized in polymer blends and composites.