Electromagnetic shielding (EMI) is very important in the field of portable electronic equipment, wireless communication, aerospace, military and medical equipment, and is an indispensable means to ensure the normal operation of equipment. With the development of smart lighting and wearable devices, there is an urgent need for materials with small size, light weight, good flexibility, strong hydrophobicity, and good EMI performance. However, most of the current EMI materials are mainly metal, which are heavy and have poor toughness.
In response to this demand, researchers have developed many functional conductive nanocomposites or components, such as materials based on carbon nanotubes (CNT), graphene, transition metal carbides (MXene) and metal nanofibers. Among them, the introduction of microporous structures in EMI materials can effectively improve the shielding efficiency (SE) by increasing the number of reflections of incident electromagnetic waves. This method is more and more popular. The normalized ratio SE value (SSE) of this material ) It even reached an amazing 136752 dB · cm2 · g-1.
Although the EMI material of the microporous structure has excellent electromagnetic shielding performance, its thickness reaches millimeters or even centimeters, and its mechanical properties are poor, and its flexibility is insufficient, which seriously hinders practical applications.
Based on the above analysis, the research group of Professor Jing Wang of the Federal Institute of Technology in Zurich designed a metal-encapsulated polymer nanofiber microporous membrane, which is only a few microns thick, but has excellent electromagnetic shielding performance: SSE reaches an amazing 232860 dB · cm2 · g-1, the SE of 2.5μm thick film is as high as 60 dB, and the SE can still reach 53 dB after 1000 times of bending, far exceeding the industry requirement of 20 dB. At the same time, this material has a density of only 1.6 g · cm-3, which is slightly heavier than A4 paper, and also has antibacterial and waterproof functions. It is a very high-performance EMI shielding material with great application prospects.
Preparation and characterization of metal-coated microporous membrane
The researchers first prepared 1.2 μm and 2.5 μm thick nylon microporous membranes by electrospinning. The multilayer microporous membranes were hot-pressed at 200 ° C and 2 tons of pressure for 20 min to melt and crosslink to obtain hot-pressed membranes. , Which helps to improve the mechanical strength of the film. To better cover the microporous membrane with metal, the researchers polymerized a layer of polydopamine (pDA) on the surface of the film, and then immersed the film in a solution containing Cu2 + or Ag + ions, using borane dimethylamine (DMAB) or Glucose is used as a reducing agent to deposit Cu2 + or Ag + onto the microporous membrane to prepare metal-coated nanofiber membranes.
The researchers carried out structural characterization of the microporous membrane, and found that after hot-press crosslinking, there was a binding point between the nanofibers. After being wrapped with metal, the metal nanolayers were firmly bonded, which significantly reduced the metal layer in the film. Contact resistance. The introduction of the pDA layer realizes the continuous and uniform growth of Cu nanoparticles on the fibers, which is conducive to the metal tightly wrapping the nanofibers. Before and after metal cladding, the average diameter of nylon nanofibers is 84 nm and 86 nm, respectively. The fiber structure of metal cladding is critical to the high electrical conductivity and high EMI performance of the film.
The microporous structure of the film and the strong bond between the metal and nanofibers make the film highly resistant to mechanical deformation and excellent bendability. After 1000 bends, the electrical conductivity of the film did not change, indicating that the adhesion of the metal layer to the nylon nanofibers was very strong.
The temperature distribution of the film is very uniform. The 4 × 4 cm2 film can reach 30 ° C at a low voltage of 1V, which is very suitable for flexible wearable electric heaters for outdoor use. After the metal is wrapped, the contact angle of the film increases from 30 ° to 110 °, which turns the hydrophilic film into a hydrophobic film while also having excellent antibacterial properties and breathability.
EMI performance of copper-coated microporous membrane
The researchers studied the EMI performance of metal-coated microporous membranes. It was found that by adjusting the reaction time, the metal content on the nanofibers can be changed, thereby optimizing the conductivity and EMI performance of the material: after the metal coating reaction for 1 h, the conductivity of the material is only 7.18 S · cm-1, in the X-band ~ 12.4 GHz) SE value is 15 dB; after the reaction time is extended to 1.5, the film thickness reaches 2.5 μm, the material conductivity is increased to 7870 S · cm-1, SE is as high as 60dB, far exceeding the industry requirements of 20 dB, It can attenuate 99% of incident electromagnetic waves. With the extension of the reaction time, the density of the material has increased. After 2 hours of reaction, the density of the material is only 1.6 g · cm-3, which is slightly heavier than A4 paper.
Prolonging the time of electrospinning can increase the thickness of the film. The thicker the film, the better the reflection frequency of incident electromagnetic waves and the EMI performance. When the film thickness increases from 1.2 microns to 5.0 microns, SET and SEA increase significantly.
The researchers studied the EMI performance of the thin film in various frequency ranges (X, Ku, Q, THz band). Compared with other materials, the materials prepared in this paper showed excellent performance in a wider frequency range. The film with a thickness of 2.5 μm still maintains 53 dB after 1000 bends, showing excellent stability.
The researchers also prepared Ag-coated microporous membranes with a conductivity of 31200 S · cm-1 and SE values as high as 55.1 to 77.6 dB at a thickness of 2.5 to 5.0 μm. However, at the same thickness, the performance of Ag thin films The Cu film is similar, which is higher than the CuDF-coated PVDF film.
The research team of Professor Jing Wang of the Federal Institute of Technology in Zurich has designed a light-weight, flexible and ultra-thin metal-coated nanofiber film that can be used as an EMI material. Compared with traditional materials, the thickness of this material is only 2.5 μm, the electrical conductivity reaches 7870 S · cm-1, and the SE is as high as 60dB, far exceeding the industry requirements of 20dB, which can attenuate 99% of the incident electromagnetic waves, while the density It is only 1.6 g · cm-3, which is slightly heavier than the A4 value. After 1000 times of bending, the SE of Cu-coated nanofiber film is still as high as 53 dB, showing excellent flexibility and performance retention rate. This film not only has excellent EMI performance, but also has multiple functions such as antibacterial and waterproof. It is widely used in smart lighting and wearable devices.