Measurement and Analysis of Low Frequency Magnetic Shielding Performance of Open-Cell Nickel Foam

Abstract

Low frequency magnetic field in the range from 10 to 500 kHz is between very-low and mid-high frequencies, mainly coming from industrial equipments such as antenna, battery, inverter, etc. In some shielding applications simultaneously demanding for ventilation and cooling, high-performance shielding materials with a porous structure are necessary. However, the investigation of these materials in this band was rarely reported. Open-cell nickel foam has excellent electrical conductivity and magnetic permeability, which makes it a potential new kind of shielding material in the field of low frequency magnetic shielding. It is generally believed that, in this band, materials shield the magnetic field mainly through two mechanisms, i.e., flux shunting and eddy current cancellation. With the equation of shielding effectiveness (SE) for a cylindrical shell, SEs of copper foil and Permalloy were calculated. The results indicated that the effect of flux shunting can be ignored, which was further verified by the results of Helmholtz coil test. Thus, only eddy current cancellation mechanism was considered to establish a shielding model, in which the flat open-cell nickel foam was assumed to be a group of coils and the eddy current was assumed to flow along the conductive paths composed of the coils. The prediction of the model was verified by the measured results of a window test method. Both the experimental and theoretical results revealed that, SE is enhanced with the decrease of the average pore size of the foam because the number of conductive paths in the foam increases. In addition, the calculated results of SE indicated that the open-cell nickel foam has no obvious superiority compared to a fully dense nickel strip of the same areal density. However, the structure characteristics of open-cell foam make it definite to find applications simultaneously asking for shielding, ventilating and cooling.