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Composites made from epoxy resin containing graphene could be used to shield electronic devices from electromagnetic radiation and dissipate excess heat in these devices at the same time. This is the new finding from researchers at the University of California, Riverside (UCR) who have tested composites containing different thicknesses of few-layer graphene fillers. The best materials boast a thermal conductivity, K, of around 8 W/m/K (which is 35 times larger than the matrix material on its own) while providing a total electromagnetic interference shielding, SEtot, of 45 dB in the important X-band frequency range (of between 8.2 GHz to 12.4 GHz).
“Heat and electromagnetic radiation are inevitably produced in electronic devices, especially those operating at high frequencies,” explains research team leader Alexander Balandin, who is in the Department of Electrical and Computer Engineering at UCR. “As electronic devices become ever smaller and operate at higher and higher frequencies, they generate even more heat and electromagnetic waves. These not only degrade the devices themselves (EM waves also produce heat), but they can adversely affect neighbouring electronics systems. EM radiation might also be dangerous for human and animal health and the environment.”
The problem of excess heat is usually solved by using interface materials with a high thermal conductivity that dissipate this heat. And EM shielding materials are the answer to blocking EM radiation. “These two types of materials often have very different characteristics, however – an excellent shielding material can be a poor heat conductor while an efficient thermal interface material is usually an insulator, which means that EM waves pass right through it,” says Balandin. “This means that both types of material need to be employed in the same device, which adds to complexity and cost.”
The UCR researchers have now found that composites containing the “wonder material” graphene can block EM radiation while dissipating excess heat. “Surprisingly, we discovered that the graphene composites can block EM energy even below the so-called percolation threshold, and remain electrically insulating (which is an important property for a thermal interface material).” Electrical percolation is the term used to describe composites in which electrically conductive filler particles form a continuous network, allowing for electrical current to flow.
Team members Fariborz Kargar and Zahra Barani of the UCR Phonon Optimized Engineered Materials (POEM) Center, led by Balandin, prepared epoxy-resin composites containing a high loading fraction of few-layer graphene fillers (FLG). They processed the material in their lab to determine the optimum aspect ratio, lateral dimensions, and thickness of the fillers. For EM shielding applications, for example, fillers with high aspect ratios are best, and for thermal applications optimum lateral dimensions and thickness are required, says Balandin.
They then prepared two batches of the composites using fillers with very different thicknesses. In the first batch, referred to as GF-A, the lateral dimensions of the FLG fillers ranged from 1.5 to 10 microns while their thicknesses were between 0.35 to 12 nm, which corresponds to 1 to 40 graphene monolayers, respectively. In the second batch (GF-B), the lateral dimensions were between 2-8 microns, but the thicknesses were much smaller – ranging from 0.35 to 3 nm, corresponding to 1-8 graphene monolayers, respectively.
The researchers found that the best composites had an efficient total electromagnetic interference shielding SEtot of 45 dB, in the X-band frequency range while simultaneously providing a high thermal conductivity, K, of around 8 W/m/K. “Our results also show that graphene composites can block more than 99.998% of high-frequency EM radiation,” says Balandin.
“Electromagnetic shielding requires interactions of the EM waves with the charge carriers inside the shielding material so that the EM radiation is either reflected or absorbed,” he explains. For this reason, the shielding material must be electrically conductive or contain electrically conducting fillers. Graphene is a good conductor of electricity, which allows fillers made from this material to reflect and absorb EM waves. It is also a good conductor of heat – thanks to the unique properties of phonons (quanta of crystal lattice vibrations) in 2D materials. Our group discovered this property back in 2008.”
The UCR team, reporting its work in Advanced Electronic Materials 10.1002/aelm.201800558, says that it is now busy testing out its graphene composites as protective coatings in real-world heat-generating electromagnetic devices.
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