Communications of the ACM
MXenes Shield EMI, More
A free-standing film of MXene (Titanium Carbide - Ti3C2Tx) functioning as a microwave disc resonator with an operational frequency of 3.0 GHz.
Credit: Omid Niksan
Radio frequency (RF) shielding today uses electromagnetic interference (EMI) barriers made from conductive and magnetic materials that direct EMI away from RF receivers. For instance, shielded wires are usually surrounded by a braided metallic sheath to prevent stray signals from being picked up, as if the internal signal-carrying wires were antennas.
In the extreme, a metallic box called a Faraday cage can completely shield all circuitry inside it.
Until now, such shields were bulky, expensive, scattered incident frequencies, and were always on. Thanks to the pioneering work of materials scientists such as Yury Gogotsi, Distinguished Drexel University professor at its Nanomaterials Institute and Bach Professor of Materials Science and Engineering, EMI shielding now is available at the nanoscale; it is inexpensive, absorbs incident frequencies, and can be electronically toggled on and off. A Faraday cage can be created by simply spray-painting the walls, floor, and ceiling of any room with "MXenes" (pronounced "max-eens").
"The RF-absorbing characteristics of MXenes adds to their intrigue as a novel new material," said Brendan DeLacy, founder and president of Ballydel Technologies Inc. (Wilmington, DE), which has three products based on MXenes under development. "What makes MXenes particularly attractive for EMI shielding applications is that they can not only absorption EM radiation, but can also be easily finely tuned to the frequencies being blocked."
Gogltsi and colleagues also claim to have proven that MXene utility reaches far beyond shielding, as evidenced by the more than 200,000 citations of their papers by researchers experimenting with a vast array of MXene-enabled applications. Drexel alone has 40 separate patented inventions based on the technology, with 20 more pending. Seven inventions involving 27 issued patents already have been licensed to a dozen companies, with the first commercial products due out by next year.
Also, the U.S. Air Force Research Laboratory acknowledges (but is mum about) the details of its classified development work with Drexel's patented inventions.
How It Works
"MXenes were discovered a decade ago, but the amazing electromagnetic properties of their two-dimensional structures were not immediately realized," said Gogotsi. "They are different from all other known materials, which accounts for their becoming the fastest-growing research material in the world. Their ability to manipulate electromagnetic signals extends even to visible wavelengths, but so far they have only been demonstrated for RF."
Besides being feather-light, yet providing more than 99.99% shielding capability, MXenes have also been found to effectively conduct electricity, tune the frequencies they affect, and enhance the integrity of communications signals better than traditional materials including copper circuit-board traces, resistors, capacitors, and inductors.
"In our lab, we are discovering that building the components used in high-frequency RF is incredibly simpler when using MXenes," said associate professor Mohammad Zarifi, Principal Researcher in Sensors and Microelectronics in the School of Engineering at Canada's University of British Columbia. "Instead of using copper, solder, and heavy circuit boards, we can use MXenes to greatly reduce the weight and size of communication equipment—lightening the load for soldiers in the field, for instance. Plus the development process is simpler and easier to fine-tune to perfection.
"First we simulate a circuit just as we do for traditional materials, but then we just paint on the simulation's specifications at room temperature. The electronic devices can be installed on thin feather-weight plastic substrates, and if any tuning needs to be done, we don't have to go back to the simulator, but just dab a bit of MXene on different components until it is tuned to perfection."
Unlike linear static electrical fields (that make your hair stand on end), MXenes are most effective on magnetically modulated electrical fields, such as the communications signals that depend on various types of modulation in order to encode the information being transmitted. As EMI shielding, MXenes insure error-free transmission, even in the presence of hackers trying to eavesdrop, jam, or commandeer radio control signals for they own nefarious purposes. Zarifi also claims they outperformed traditional RF antennas, oscillators, filters, and other RF components.
In more detail, MXenes improve the best qualities of transition metals (like titanium) by bonding them to carbon (carbides), nitrogen (nitrides), or both (carbonitrides). When synthesized in the laboratory, a pure metal (typically aluminum) is used to bond the MXene into what is called its MAX phase, albeit with the aluminum atom sticking out into the third dimension (3D). The trick Gogotsi and others scientists use to create a two-dimensional (2D) material is to use room-temperature hydrofluoric acid to etch off the aluminum atoms, thereby imparting remarkable electronic properties. What's more, the scientists claim, the resulting sheets need not be handled with care—like semiconductor monolayers—but rather can randomly flake off into various sizes. The flakes are then dispersed into a water-soluble paste which can be painted, sprayed, or spin-coated onto nearly any surface. The MXenes self-assemble as the solvent evaporates into monolayer-like structures; they do not have to be perfect, as the MXenes remain tightly bound to these surfaces with no need for an adhesive. Ordinarily several layers are successively applied, each self-assembling as its solvent evaporates, resulting in a typical 40-nanometer thickness for six or seven layers. To fine-tune its properties, more layers can be added.
Almost a quarter of the known elements on the periodic table can be combined to form thousands of different types of MXenes, yet to date only a small fraction of all the different formulations have been characterized. The number of properties, and their easy tuning, makes them "almost magical" when compared to the near-absolute perfection required by traditional electronic fabrication materials, equipment, and processes, according to Zarifi.
The combinations of novel properties so far discovered are manifold, according to these researchers. For instance, MXenes have been reported to be corrosion-resistant; instant death to bacteria touching some formulations but tolerant of bacteria in other formulations; able to reflect different frequencies depending on the angle of incidence; able to absorb tunable wavelengths between their layers; able to be formed into ultrafast sieves of salts and water pollutants, resulting ultra pure water without complicated machinery; tunable to be magnetic, ferromagnetic, or anti-ferromagnetic, and have the fewest atomic layers necessary to achieve the highest reversible-charge storage ability (for super-lightweight rechargeable solid-state batteries). All these properties and more are being developed into products, according to Gogotsi.
Current applications under development include supercapacitors, super-composites, super-filters, super-batteries, super-sensitive biosensors, spray-on antennas, spray-on optoelectronics, plasmonic metamaterials, and temperature-tunable superconductivity.
R. Colin Johnson is a Kyoto Prize Fellow who has worked as a technology journalist for two decades.
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