Ferromagnetic materials and metallic conductive materials are commonly used as shielding materials. Conductive shielding materials are suitable for shielding lowfrequency magnetic fields (below 100 kHz). Their principle of operation is to utilize the high magnetic permeability of ferromagnetic materials to guide magnetic lines of force through highly permeable materials and reduce the magnetic flux density in nearby spaces, thereby achieving magnetic shielding. Commonly used ferromagnetic materials include pure iron, silicon steel, permalloy (an iron-nickel alloy), etc. Permalloy exhibits significantly superior electromagnetic shielding performance compared to other materials. Permalloy has three main components: 78% Ni, 65% Ni, and 50% Ni. Among them, the magnetic permeability of 78% Ni permalloy is much higher than that of the other two, reaching the order of magnitude of 3×10^-1 to 2×10^-1. Permalloy is sensitive to stress, and its magnetic properties are greatly influenced by heat treatment. However, the materials provided for use are not heat-treated, so it is necessary to understand and master the heat treatment process when using them. The newly emerging iron-cobalt (FeCo) alloy [51] and iron-aluminum alloy [51] are also soft magnetic alloy materials that can be used for shielding low-frequency magnetic fields.
Ferromagnetic materials are not suitable for shielding high-frequency electromagnetic fields due to their low conductivity. On the other hand, good metallic conductors possess high conductivity and are suitable for shielding both high and low-frequency electromagnetic fields as well as electrostatic fields. In electromagnetic shielding, conductivity serves as the primary criterion for selecting shielding materials. Table 2 presents the conductivity and magnetic permeability of some metals. The most commonly used materials include steel plates, galvanized steel sheets, copper plates, aluminum plates, and other materials with good conductivity. While metallic shielding materials exhibit excellent mechanical properties, they also have significant limitations such as high density, susceptibility to corrosion, and difficulty in processing.
Surface coating thin film shielding material
This type of material is a shielding material that achieves shielding by attaching a conductive layer to the surface of insulators such as plastics, primarily relying on reflection loss. Common preparation methods include chemical gold plating, vacuum spraying, sputtering, metal melting spraying, and metal foil bonding. This type of surface conductive film shielding material generally has advantages such as good conductivity and obvious shielding effect. However, its disadvantage is that the adhesion of the surface conductive film is not high, which easily leads to peeling, and the secondary processing performance is poor