The lightest shielding material in the world
A sample of the electromagnetic shielding material produced at Empa - a composite material made of cellulose nanofibers and silver nanowires. Picture: Empa
Empa researchers have succeeded in making aerogels usable for microelectronics: Aerogels based on cellulose nanofibers can effectively shield electromagnetic radiation in wide frequency ranges - and are unrivaled in their lightness.
Electric motors and electronic devices generate electromagnetic fields that sometimes need to be shielded so as not to affect neighboring electronic components or the transmission of signals. High-frequency electromagnetic fields can only be protected with conductive sheaths that are closed on all sides. Thin metal sheets or metalized foils are often used for this purpose. However, for many applications, such shielding is too heavy or too difficult to adapt to the given geometry. A lightweight, flexible and durable material with an extremely high shielding effect would be ideal.
Aerogels against electromagnetic radiation
A research team led by Zhihui Zeng and Gustav Nyström has now achieved a breakthrough in this area. The researchers use nanofibers made of cellulose as the basis for an aerogel, a lightweight, highly porous material. Cellulose fibers are obtained from wood and their chemical structure enables a wide range of chemical modifications. They are therefore a sought-after object of research. The decisive factor in the processing and modification of these cellulose nanofibers is that certain microstructures can be produced in a defined manner and the resulting effects can be interpreted. These relationships between structure and properties are precisely the research area of Nyström's team at Empa.
The researchers have succeeded in producing a mixture of cellulose nanofibers and silver nanowires to create ultra-light fine structures that provide excellent shielding against electromagnetic radiation. The sheer effectiveness of the material is impressive: with a density of just 1.7 milligrams per cubic centimeter, the silver-reinforced cellulose aerogel achieves more than 40 dB shielding in the frequency range of high-resolution radar radiation (8 to 12 GHz) - in other words: Almost all of the radiation in this frequency range is intercepted by the material.
Ice crystals control the shape
It is not only the correct mixture of cellulose and silver wires that is decisive for the shielding effect, but also the pore structure of the material. The electromagnetic fields are reflected back and forth within the pores and also trigger electromagnetic fields in the composite material that counteract the radiated field. In order to achieve pores of the optimum size and shape, the researchers pour the material into pre-cooled molds and allow it to freeze slowly. The growth of the ice crystals creates the optimum pore structure for attenuating the fields.
With this production method, the attenuation effect can even be specified in different spatial directions: If the material freezes out from the bottom to the top of the mold, the electromagnetic attenuation is lower in the vertical direction. In the horizontal direction - i.e. at right angles to the freezing direction - the attenuation is optimized. The shielding structures cast in this way are extremely flexible: even after being bent back and forth thousands of times, the attenuating effect is practically the same as with the new material. The desired absorption can even be slightly adjusted by adding more or less silver nanowires to the mixture, by the porosity of the cast aerogel and the thickness of the cast layer.
The lightest electromagnetic shield in the world
In another experiment, the researchers omitted the silver nanowires from the mixture and connected their cellulose nanofibers with two-dimensional nanoplatelets made of titanium carbide, which were produced using a special etching process. The nanoplatelets act like hard "bricks" that are bonded together with flexible "mortar" made of cellulose fibers. This mixture was also specifically frozen in cooled molds. In terms of material weight, no other material can achieve this level of shielding. This makes titanium carbide nanocellulose aerogel by far the lightest electromagnetic shielding material in the world.
https://www.empa.ch/
