Novel devices for blocking electromagnetic interference

Two separate research groups have recently developed devices to block electromagnetic radiation, which could be useful for strengthening wireless connections and securing mobile communications against intrusion.

Electronic devices are found throughout our homes, on factory floors and in medical facilities, and electromagnetic shielding is often used to prevent electromagnetic radiation from one device from interfering with another. But such shielding — which is also used in the military to keep equipment and vehicles hidden from the enemy — can also block the optical communication channels needed for remote sensing, detection or operation of these devices. A shield that can block interference but allow for optical communication channels could thus help to optimise device performance in a variety of civilian and military settings.

Silver mesh

Researchers from Zhejiang University have now experimentally demonstrated a mechanically flexible silver mesh that is visibly transparent, allowing high-quality infrared wireless optical communication and efficiently shielding electromagnetic interference in the X band portion of the microwave radio region. Their work has been described in the journal Optical Materials Express.

“Many conventional transparent electromagnetic interference shields allow only visible light signals through,” said research team leader Liu Yang. “However, visible wavelengths are not well suited for optical communication, especially free-space — or wireless — optical communication, because of the huge amount of background noise.”

The researchers designed their silver mesh with a very simple structure — a repeating square grid pattern applied to a transparent and flexible polyethylene substrate. The continuous grid structure makes the mesh very flexible by releasing stress during bending. Because the transparency of the silver mesh is primarily determined by the opening ratio, a measure of the size of the holes in the mesh, it is independent of the incident light wavelength.

“A large opening ratio, for example, is beneficial for a high broadband transparency and low haze, but is detrimental to high conductivity and thus electromagnetic shielding performance,” Yang said. “Because the physical parameters for our mesh can be easily optimised by changing the grid period, line width and thickness, it is easier to achieve well-balanced optical, electrical and electromagnetic properties compared with what is possible with other kinds of transparent conductive films such as silver nanowire networks, ultrathin metallic films and carbon-based materials.”

To demonstrate their new technology, the researchers fabricated a silver mesh onto a polyethylene substrate. The mesh had a grid period of approximately 150 μm, a grid line width of approximately 6 μm and a thickness that ranged from 59 to 220 nm. This was then covered with a layer of 60 μm-thick polydimethylsiloxane. The resulting film showed high transmission for a broad wavelength range from 400 to 2000 nm and sheet resistance as low as 7.12Ω/sq, allowing a high electromagnetic shield effectiveness up to 26.2 dB in the X band. The researchers also showed that the film could shield low-frequency mobile phone signals.

“We take the advantage of the ultrabroad transparency and low haze of a metallic micromesh to demonstrate efficient electromagnetic shielding, visible transparency and high-quality free-space optical communication,” Yang said. “Sandwiching the mesh between transparent materials improves the chemical stability and mechanical flexibility of the silver mesh while also imparting a self-cleaning quality. These properties will enable our silver mesh to be applied widely both indoors and outdoors, even on corrosive and free-form surfaces.”

The researchers acknowledged that their work is only a prototype demonstration, so there is much room for improvement. For example, using more conductive materials would improve the electromagnetic shielding effectiveness, and materials that are more transparent and have a lower haze could improve not only the visible transparency but also the free-space optical communication quality.

They are also exploring mid-infrared transparent conductive materials, which would extend the free-space optical communication to longer wavelengths where atmospheric interference is reduced and higher communication quality can be achieved. For commercialisation, the mesh would also have to be more practical to install and less expensive.

Thin film device

A second device, fabricated by spray coating, has been designed to block electromagnetic radiation with the flip of a switch. The breakthrough, enabled by versatile two-dimensional materials called MXenes, was made by researchers from Drexel University and has been published in the journal Nature Nanotechnology.

MXene is a unique material in that it is highly conductive — making it well suited for reflecting microwave radiation that could cause static/feedback or diminish the performance of communications devices — but its internal chemical structure can also be temporarily altered to allow these electromagnetic waves to pass through. This means that a thin coating on a device or electrical components prevents them from emitting electromagnetic waves, as well as being penetrated by those emitted by other electronics. Eliminating the possibility of interference from both internal and external sources can ensure the performance of the device, but some waves must be allowed to exit and enter when it is being used for communication.

The Drexel team had previously demonstrated that the two-dimensional layered MXene materials, when combined with an electrolyte solution, can be turned into a potent active shield against electromagnetic waves. Their latest discovery shows how this shielding can be tuned when a small voltage — less than that produced by an alkaline battery — is applied.

“Without being able to control the ebb and flow of electromagnetic waves within and around a device, it’s a bit like a leaky faucet — you’re not really turning off the water and that constant dripping is no good,” said team leader Professor Yury Gogotsi. “Our shielding ensures the plumbing is tight, so to speak — no electromagnetic radiation is leaking out or getting in until we want to use the device.”

The key to eliciting bidirectional tunability of MXene’s shielding property is using the flow and expulsion of ions to alternately expand and compress the space between material’s layers, like an accordion, as well as to change the surface chemistry of MXenes. With a small voltage applied to the film, ions enter — or intercalate — between the MXene layers altering the charge of their surface and inducing electrostatic attraction, which serves to change the layer spacing, the conductivity and shielding efficiency of the material. When the ions are de-intercalated, as the current is switched off, the MXene layers return to their original state.

The team tested 10 different MXene–electrolyte combinations, applying each via paint sprayer in a layer about 30 to 100 times thinner than a human hair. The materials consistently demonstrated the dynamic tunability of shielding efficiency in blocking microwave radiation, which is impossible for traditional metals like copper and steel. The device sustained the performance through more than 500 charge–discharge cycles.

“These results indicate that the MXene films can convert from electromagnetic interference shielding to quasi-electromagnetic wave transmission by electrochemical oxidation of MXenes,” the study authors wrote. “The MXene film can potentially serve as a dynamic EMI shielding switch.”

For security applications, Gogotsi suggests that the MXene shielding could hide devices from detection by radar or other tracing systems. The team also tested the potential of a one-way shielding switch, which would allow a device to remain undetectable and protected from unauthorised access until it is deployed for use.

“A one-way switch could open the protection and allow a signal to be sent or communication to be opened in an emergency or at the required moment; this means it could protect communications equipment from being influenced or tampered with until it is in use,” Gogotsi said. The next step for the team is to explore additional MXene-electrolyte combinations and mechanisms to fine-tune the shielding to achieve a stronger modulation of electromagnetic wave transmission and dynamic adjustment to block radiation at a variety of bandwidths.

“Dynamic control of electromagnetic wave jamming has been a significant technological challenge for protecting electronic devices working at gigahertz frequencies and a variety of other communications technologies,” Gogotsi said. “As the number of wireless devices being used in industrial and private sectors has increased by orders of magnitude over the past decade, the urgency of this challenge has grown accordingly. This is why our discovery — which would dynamically mitigate the effect of electromagnetic interference on these devices — could have a broad impact.”

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