With the popularity of 5G communication technology and the use of a large number of high-power electronic devices, more and more electromagnetic pollution threatens people's physical and mental health and information security, which promotes excellent electromagnetic shielding materials in the future production and life will be more Wide and large number of applications. Generally speaking, high electrical conductivity will give the material excellent electromagnetic shielding performance. Although the metal material has excellent electrical conductivity and electromagnetic shielding performance, considering its high density and modulus, it is difficult for the metal material to be applied alone. Wearable devices, software robots, and flexible sensors require highly stretchable elastomer materials. At present, the preparation of stretchable elastomer materials with electromagnetic shielding efficiency generally adopts the method of blending conductive fillers (metals, graphene, carbon tubes, MXene, etc.) and polymer elastomers into composite materials, but there are 3 methods in this method The bigger problem:
1) Under the condition of low conductive filler content, the composite material has poor conductivity and low electromagnetic shielding effectiveness;
2) The high conductive filler content will cause the mechanical properties of the composite material to drop sharply;
3) The increase in the distance between the conductive fillers during the material stretching process leads to a decrease in electrical conductivity, which in turn leads to a reduction in electromagnetic shielding effectiveness.
In response to these three application problems, recently, the team of Professor Wang Hong of Southern University of Science and Technology and the team of Professor Wang Qing of Pennsylvania State University used a simple template-impregnation method to prepare a liquid metal (LM) skeleton / silicone rubber three-dimensional composite with a double continuous structure Elastomer (3-D LM). Under the premise of ensuring excellent and stable tensile properties (maximum strain 300% cyclic stretching 50 times, maximum strain 250% stretching 10000 times, no significant decrease in mechanical properties), 3-D LM high content liquid metal alloy (30 vol%) gives the composite elastomer high electrical conductivity and excellent electromagnetic shielding performance in the frequency range of 2.65–40 GHz, and both increase with increasing tensile strain: the composite elastomer film is gradually stretched until the strain reaches 400%, the conductivity is gradually increased from 5.3 × 105 S m−1 to 1.1 × 106 S m−1, and the corresponding electromagnetic shielding efficiency is increased from 34.5 dB to 86.2 dB, which is comparable to metals of similar thickness. The research results were published in "Advanced Materials" as a paper entitled "Highly Stretchable Polymer Composite with Strain-Enhanced Electromagnetic Interference Shielding Effectiveness" (see the original text link after the article).
1. Preparation of composite elastomer
In this study, the easy-to-process liquid eutectic gallium-indium alloy (EGaIn, containing 74.5 wt% Ga and 25.5 wt% In) was selected as the electromagnetic shielding host material, as shown in Figure 1 a), the EGaIn alloy was first cast into the porous In the sugar cube, the sugar in the composite is dissolved to obtain a porous EGaIn alloy skeleton (the surface of the EGaIn alloy will be oxidized in the air to form a thin and hard oxide layer, which can support the entire skeleton without collapsing), Then, the silicone rubber resin is impregnated into the EGaIn framework to form a composite elastomer. The results show that the morphological structure of the EGaIn alloy skeleton is maintained after adding the polymer compound, and a bi-continuous structure is formed between the two. The method is simple and easy to operate, does not require special processing equipment, and can produce composite elastomers of various shapes and structures on a large scale.
2. Mechanical properties of composite elastomer
Stable mechanical properties are the basis for the use of materials. The tensile test results show that 3-D LM composite elastomers still have excellent and stable mechanical properties even when the filler volume fraction is up to 30%. The average elongation at break of 3-D LM is close to 510%, even higher than 465% of pure elastomer, and can be repeatedly stretched under deformation conditions within 400%. The elastic modulus is between 30–60 kPa. The toughness is 2170J m−2, similar to human skin. In order to explore the excellent performance of 3-D LM, the researchers also prepared composite elastomers with several conventional structures and compositions (0–3 LM: containing 30 vol% LM microparticles; 1–3 LM: containing 30 vol% LM micrometers Fiber; composite elastomer containing 5 vol% graphene, carbon nanotubes, and Ti3C2Tx, respectively). Lateral comparison results show that 3-D LM elastomer exhibits better flexibility, stretchability and recoverability under different tensile strain conditions than other composite and pure elastomers. As shown in Figure 2, the elastic body is cyclically stretched. After the first cycle with a maximum strain of 400%, the 3–D LM has only 6.3% inelastic deformation, while the 0–3 LM and 1–3 LM are inelastic Deformation reached 33.9% and 24.5% respectively. After the end of the subsequent 50 cycles of maximum deformation of 300%, the inelastic deformation value of 3–D LM is almost unchanged while the inelasticity of 0–3 LM and 1–3 LM The deformation value increased by 10.7% and 9.1%, respectively.
The reason for the difference in mechanical properties of different composite elastomers is the difference in alloy structure. Compared with 3-D LM, the specific surface area of the alloy in the 0–3 LM and 1–3 LM systems using microparticles and fibers as fillers is more Large, so the proportion of oxide layer on the surface will be more. On the one hand, more hard oxide layers will increase the rigidity of the whole system and reduce the flexibility; on the other hand, a larger specific surface area will increase the interface between the alloy and the polymer. During the stretching process, the hard and brittle oxide layer is easy Fracture, the exposed alloy is quickly oxidized to form a new oxide layer, causing mechanical properties to gradually decline as the number of stretching increases. The 3-D LM elastomer alloy skeleton as a coherent whole can also be maintained under high tensile state, the specific surface area is small, the proportion of hard oxide layer is small, it is not easy to break, and it can be easily removed after stress Restore to the original state.
3. Electromagnetic shielding performance of composite elastomer
Most conductive filler / polymer composite elastomers have a decrease in conductivity due to the increasing distance between the fillers during the stretching process, which will also reduce their electromagnetic shielding performance, but for 3-D LM composite elastomers, the results On the contrary, as the tensile strain increases, the electromagnetic shielding effectiveness of the 3-D LM elastomer is higher. Excellent electrical conductivity is still the source of 3-D LM's excellent electromagnetic shielding performance. Compared with the carbon material composite elastomer whose conductivity gradually decreases during the stretching process and the similar change law of 3-D LM, the absolute value difference is 14-17 In terms of the magnitude of 0–3 LM and 1–3 LM, 3-D LM has a very high electrical conductivity. During the process of stretching to a maximum strain of 400%, the electrical conductivity gradually increases from 5.3 × 105 S m−1. Increased to 1.1 × 106 S m−1. For electromagnetic waves in the frequency range of 2.65–40 GHz, the 3-D LM elastomer shows excellent shielding effectiveness, as shown in Figure 4 b) and c), during the process of gradual stretching to 400% deformation (thickness from 2mm Becomes 1mm), the average shielding effectiveness of 3-D LM elastomer is gradually increased from 34.5 dB to 86.2 dB (more than 99.999999% of the electromagnetic wave energy is shielded), and the cycle is 10000 times under the condition of the maximum tensile strain of 250%, Its electrical conductivity and shielding effectiveness can be well maintained.
The electrical conductivity that gradually increases during stretching is due to the change in the morphology and structure of the alloy skeleton during stretching. After stretching, the alloy will extend in the direction of stretching, increasing the current density in that direction. At the same time, the researchers also found that the size of the alloy skeleton after stretching was reduced from 100 um to 55 um, which increased the specific surface area by about 82%, which was beneficial to the multiple reflection and scattering of electromagnetic waves and further increased the shielding effectiveness.
The researchers prepared a conductive liquid alloy metal skeleton / polymer composite elastomer with a bi-continuous structure through an easy-to-operate template-impregnation method. The elastomer has excellent electromagnetic shielding performance in a wide electromagnetic wave frequency band, especially in After stretching and thinning, the electromagnetic shielding performance can be further improved, its value is close to the shielding effectiveness of the metal film of the same thickness, and the mechanical properties can be well maintained after repeated cyclic stretching under high tensile strain conditions, the research results It is expected to be used in the fields of flexible robots, wearable devices and flexible sensors.
Original link: https://onlinelibrary.wiley.com/doi/10.1002/adma.201907499