### Electromagnetic Shielding

October 17, 2016

Technology is driven by consumer demand, and that's why cellphones continue to evolve into the miraculous devices that we have today. It's said that the Internet, and the previous technology of video cassette players, advanced more quickly because of the demand for pornography.

In the past, when endless warfare was a way of life, materials science was driven by the need for better swords, and shields as protection from them. Prehistoric shields were made from available materials, such as wood and animal hides, but they were wrought from metals as metallurgy progressed (see figure).

Shield image from Alexander Pope's 1720 translation of Homer's Iliad, vol. 5, p. 171, as digitized by the British Library and found at Wikimedia Commons. Greek text and English translation from Project Perseus of Tufts University. (Click for larger image.)

Shortly after radio waves were discovered, it was realized that it was sometimes necessary to shield against them. Since an electromagnetic wave is a combination of an electric field and a magnetic field, it can be attenuated by shielding just one, or the other, of its components. The first effective shields short-circuited the electric field component using a Faraday cage, invented by Michael Faraday in 1836. A Faraday cage is just a box formed from conductive sheets or wire mesh.

Faraday's invention predated Maxwell's electromagnetic theory of 1873 and the generation and detection of radio waves by Heinrich Hertz in 1887. At its conception, the Faraday cage was designed to protect sensitive electrical measurements from external electric charge. After Faraday's time, the Faraday cage served as a shield for radio interference.

In most cases, you don't need that thick of a conductive material to have a Faraday cage. The well-known skin effect describes the tendency of alternating electric currents, such as those of radio waves, to have their highest density at a conductor's surface. Each conductor has a characteristic skin depth at a given frequency, and about two-thirds (1 - 1/e) of electric currents are concentrated above that depth. The skin depth falls with frequency, and currents are attenuated to about 45 dBm below their surface values at ten skin depths.

Copper skin depth as a function of frequency.

Just a small thickness of copper or similarly conductive material shields against cellphone frequencies, which are 800 MHz and above.

(Plotted using Gnumeric)

Magnetic materials will act as electromagnetic shields, with their effectiveness scaling as the square root of the permeability. Materials such as nickel that are both electrically conductive and magnetic offer both types of shielding in a single material (see graph). Because of their high permeability and low coercivity, ferrite materials are more effective than other magnetic materials at high frequencies.

Shielding effectiveness of aluminum flake and nickel-coated graphite when used in a polymer composite.

(Plotted using Gnumeric from data in ref. 1.[1]

Since wireless devices have migrated to higher frequencies to attain a wider transmission bandwidth for higher data rates, the electromagnetic shielding mantra for the thickness of an effective shield has become, "How low can you go?". Scientists at the Korea Institute of Science and Technology (Seoul, Republic of Korea), the University of Science and Technology (Daejeon, Republic of Korea), and Drexel University (Philadelphia, Pennsylvania), have developed a nanomaterial, a thin film of titanium carbide of a class of materials known as MXenes, that acts as an effective electromagnetic shield.[2-4] MXenes get their name from their similarity to graphene, an archetypal 2-D material, since they are two-dimensional metal carbides and nitrides.[2]

As Babak Anasori, a research assistant professor at Drexel and a co-author of the paper describing this research, explains
"As technology evolves and electronics become lighter, faster and smaller, their electromagnetic interference increases dramatically... Internal electromagnetic noise coming from different electronic parts can have a serious effect on everyday devices such as cell phones, tablets and laptops, leading to malfunctions and overall degradation of the device."[3]

While conventional shielding is effective, most such shields are heavy, and we all want our cellphones to be as light as possible. Aerospace electronics requires shielding as well, and weight reduction is a prime aerospace consideration. The MXene material can be combined with a polymer solution and used as a spray coating to add shielding to component cases.[3]

A flexible, free-standing film of MXene shielding material.

(Drexel University image.)

The research team tested MXene shields in a thickness range from several micrometers up to 45 micrometers in order to assess the thinnest films capable of shielding.[3] They found that micrometer thickness MXene films compete with aluminum and copper foils; and, at 8 micrometers, the MXene shields block 99.9999% of radiation (40 dBm attenuation) at cellphone frequencies, performance that requires millimeter thickness carbon composite sheets.[3] A 45 μm MXene film of Ti3C2 exhibited 92 dBm attenuation at these frequencies.[2]

MXenes function so well as electromagnetic shields because of their high electrical conductivity (4600 siemens per centimeter) and two-dimensional structure that allows a shield built from layers.[3] The layered structure allows multiple internal reflections of the radiation, and these reflected waves are bounced around until they're absorbed (see figure).[3] The fundamental research of MXene properties was funded by the National Science Foundation.[3]

Shielding principle of the MXene layered structure. The structure absorbs and traps electromagnetic radiation. (Drexel University image.)

### References:

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