| The rapid popularization of electronic communication equipment has brought a lot of convenience to human society.However,the stray electromagnetic waves emitted by the equipment interfere with each other and affect the normal work of electronic equipment.Electromagnetic shielding material can block the transmission of electromagnetic wave,realize the attenuation of electromagnetic wave,and then ensure the smooth operation of electronic equipment,which has become the research focus in the field of electromagnetic interference protection.With the development of electronic equipment towards lightweight and miniaturization,higher requirements are put forward for the occupied volume and shielding performance of electromagnetic interference shielding materials.Compared with traditional metal shielding materials,polymer based electromagnetic shielding composites have the advantages of low density,easy processing and easy regulation of properties.They have attracted more attention in the research and application field of electromagnetic interference shielding materials.With the continuous improvement of chip integration and the complexity of internal electronic components,single function electromagnetic shielding materials often cannot meet the packaging requirements.In the face of different application scenarios and electromagnetic shielding solutions,electromagnetic interference shielding materials need to add more targeted functions.In the research work of this paper,firstly,the structure-activity relationship between the morphology of fillers and the conductivity and electromagnetic interference shielding properties of composites is explored and studied.And then,based on the application scenario of conformal shielding,it is proposed that through the complementary advantages of different functional fillers and resin matrix,combined with multi-layer structure design,the composites have excellent electromagnetic shielding properties,excellent tensile conductivity stability and self-healing properties respectively,Realize the multifunction of composite materials.The main research contents are divided into the following three parts:(1)Preparation and properties of Ag/thermoplastic polyurethane composite electromagnetic interference shielding filmUsing conductive silver powder(Ag)with different morphology as filler and thermoplastic polyurethane(TPU)as polymer matrix,the composite electromagnetic shielding film of silver powder and thermoplastic polyurethane(Ag/TPU)was prepared by solvent blending and scraping.By adjusting the thickness of the composite film and the filler filling rate,different morphologies(flake,spherical,dendritic)and sizes(1?40)were studied under the same thickness and conductivity.The structure-activity relationship between the conductive network formed by silver powder fillers with different morphologies and the electromagnetic interference shielding properties of composite films was established.The results show that the electromagnetic interference shielding performance of the conductive network constructed by the smaller flake silver powder filler is better.In the X-band,the 3B-Ag/TPU film can reach 60 d B when the filling rate is 58 wt%,and the electromagnetic interference shielding efficiency is improved more significantly in the higher frequency band,up to 72 d B in the K-band.(2)Preparation and properties of Ag/liquid metal/thermoplastic polyurethane composite electromagnetic interference shielding filmA three-layer sandwich structure composite film(Ag/TPU-LM/TPU-Ag/TPU,abbreviated as Ag/LM/TPU)with upper and lower layers of Ag/TPU and middle layer of LM/TPU was prepared by scraping and coating step by step after using flake silver powder and liquid metal(LM)with flow characteristics as conductive filler and solvent mixing with TPU matrix respectively.When the composite film is at low tensile strain(< 100%),the Ag/TPU conductive layer on the upper and lower layers is used as the main conductive path to ensure the conductivity of the composite film.With the increase of tensile ratio,the conductivity of Ag/TPU conductive layer gradually decreases.Due to the good fluidity of liquid metal at room temperature,LM/TPU intermediate layer forms a new conductive network to ensure that the composite film has excellent conductivity in the initial state and high deformation stretching(300%-600%).When the multi-layer filler content of the composite film is 80%-90%-80%,its elongation at break can reach more than 600%.In the initial state without stretching,the shielding efficiency of Ag/LM/TPU composite film is 110 d B in the X-band frequency range.When the tensile deformation is 200%,it still has excellent electromagnetic interference shielding performance of more than 90 d B.In the verification of three-dimensional conformal shielding application model,Ag /LM/TPU composite film shows excellent tensile conductivity stability.Compared with pure silver powder film,Ag/LM/TPU composite film with sandwich structure has no electromagnetic signal leakage due to the fracture of conductive layer in the cladding process,indicating that it has good potential in the application of three-dimensional conformal electromagnetic interference shielding.(3)Preparation and properties of graphene/carbon fiber/thermoplastic polyurethane composite electromagnetic interference shielding filmGraphene(G)and carbon fiber(CF)were used as conductive fillers.After solvent mixing with TPU matrix respectively,a three-layer sandwich structure composite film(G/TPU-CF/TPU-G/TPU,abbreviated as G/CF/TPU)with upper and lower layers of G/TPU and middle layer of CF/TPU was prepared by step scraping method.Microwave excited self-healing is a fast and effective method.However,polymer conductive composites with high conductivity are easy to absorb heat and burn under microwave excitation.By designing multilayer composite films with conductivity gradient,the compatibility of microwave self-healing and electromagnetic interference shielding can be realized.The G/TPU layer with low conductivity releases heat due to absorbing microwave energy,which melts the thermoplastic TPU matrix and endows the G/CF/TPU composite film with self-healing performance.By changing the content of carbon fiber in the middle layer,the conductivity of the film is regulated.After the threelayer composite film with 40 wt% carbon fiber content is damaged and treated with microwave for 5 s,it can recover more than 50% of the mechanical properties,conductivity and more than 90% of the electromagnetic interference shielding performance.After recovery,the electromagnetic interference shielding efficiency can reach 30 d B and the near-field shielding performance can also reach 35 d B. |
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