Around us, electromagnetic waves are everywhere. When the mobile phone has no signal, I worry about not being able to find the electromagnetic wave; when I turn on the microwave oven, I am afraid that the electromagnetic wave released will "burn out" us. In the field of electromagnetic wave applications, the enhancement and weakening of electromagnetic waves are two inseparable "enemy".
The transmission of electromagnetic waves is bound to be accompanied by crosstalk between electromagnetic waves and the surrounding environment. The stronger the signal, the greater the interference. Therefore, how to eliminate electromagnetic wave interference has become an important issue in the field of electromagnetic wave applications.
How to isolate electromagnetic waves? Don’t let out, don’t let in, and there is no going back
The scenarios of electromagnetic interference problems can be roughly divided into the following three types: (a) non-leakage type; (b) non-transmission type; (c) there is no return type.
a. The scenario requires that the electromagnetic waves generated by the electromagnetic wave source cannot leak outside the box (ie a certain area); b. The scenario requires that the electromagnetic waves outside the box cannot enter the inside of the box; c. The scenario requires that the electromagnetic waves generated inside or outside the box reach the surface of the box. After the box, the reflected electromagnetic wave (shown by the blue arrow in Figure 1c) is also required to be zero (as small as possible). Among them, the C-type scene requires the most difficulty.
The actual application scenario may be a combination of three forms, which is more complicated. In layman's terms, the interference removal process of electromagnetic waves is the isolation process of electromagnetic waves.
Absorbing materials: the "core force" of electromagnetic wave isolation
The isolation of electromagnetic waves is mainly achieved by absorbing materials. Scene a and scene b are conventional modes of electromagnetic wave isolation. Electromagnetic interference generated by conventional mobile phone screens and digital camera circuit boards belongs to this type (as shown in Figure 2), mainly through conductive materials (copper foil, aluminum foil, conductive Polymers, graphene, etc.) and high permeability materials (FeSiAl, silicon steel, etc.) can basically meet the application requirements.
Scene c is a complex mode of electromagnetic wave isolation. Most of the absorbing materials commonly used in this kind of scenes are composite materials, such as carbonyl iron, carbon materials, ferrites, polymers and other composite materials can be used as absorbing materials for scene c. Application requirements for various specific scenarios.
In practical applications, the absorbing material must have the four characteristics of small (thin) material (coating), low density (light), large absorption band (wide), and high absorption strength (strong). How to broaden the absorbing material The absorption frequency band of is the focus of current absorbing materials. Commonly used electromagnetic wave frequency bands are widely used from meter wave (~MHz) to millimeter wave (~GHz) to terahertz electromagnetic wave (~THz).
At present, advanced communication technology represented by 5G communication technology has expanded the communication frequency band from 700 MHz to 6 GHz and even millimeter waves. On the one hand, the increase in operating frequency has compressed the communication base station to the size of a suitcase, and the interference between electromagnetic signals has increased; on the other hand, signal receiving ends such as mobile phones need to add new antennas, which further reduces the design space of the receiving end. The interference between signals is further enhanced. The emergence of these practical applications not only requires the absorbing material to be thinner, but also requires the absorbing material to have a wider absorption band.
In order to make a kind of absorbing material meet the needs of multi-frequency electromagnetic wave absorption as much as possible, broadband absorbing materials have become the first choice. However, this is not an easy task.
New material, multilayer structure absorbs broadband electromagnetic waves
In order to solve the broadband absorption problem of absorbing materials, researchers from the Ningbo Institute of Materials, Chinese Academy of Sciences designed and prepared a multilayer structure that can absorb broadband electromagnetic waves through digital simulation technology, as shown in Figure 3. The multi-layer structure utilizes the high conductivity of carbon nanotubes and the conductive grid formed in the composite film to realize the frequency separation of electromagnetic waves, and realize broadband absorption of electromagnetic waves through sub-frequency absorption of electromagnetic waves.
The principle of electromagnetic wave absorption is: the absorption layer 1 is designed to absorb high-frequency electromagnetic waves, the absorption layer 3 is designed to absorb low-frequency electromagnetic waves, and the absorption layer 2 is designed to isolate high-frequency electromagnetic waves from low-frequency electromagnetic waves after simulation calculations. The isolation layer to achieve sub-band absorption of electromagnetic waves. Therefore, the key to this structure lies in the design of the electromagnetic wave isolation layer.
This work is based on the high conductivity of carbon nanotubes and the special microstructure of the conductive network, which can realize electromagnetic wave frequency isolation, so as to realize the simultaneous absorption of long-band and short-band electromagnetic waves. In the end, the absorption of nearly 13GHz bandwidth (covering the three mainstream frequency bands of C, X, and Ku) was achieved with a thickness of 2.4mm, and the electromagnetic wave energy absorption intensity reached more than 90%.
This multi-layer structure greatly broadens the electromagnetic wave absorption band, and at the same time the thickness is thinner than materials of the same performance. It can be expected that if the thickness of the absorption layer 1 and the absorption layer 3 can be further reduced, the overall thickness of the multilayer structure can also be further reduced, which can better meet the miniaturization application requirements of electronic equipment and devices. If the controllable modulation of the electromagnetic wave filter layer can be realized, and the isolation of more frequency bands can be realized, a wider frequency band of electromagnetic wave absorption can be realized, thereby meeting the needs of broadband applications.