Electromagnetic properties of Ni–Zn–Cu ferrite as magnetic sheet in wireless charging system

To obtain high-performance magnetic sheet with high initial permeability and low magnetic loss for wireless charging system, Ni–Zn–Cu ferrite ceramics with a chemical formula of Ni_0.6 − xZn_0.4Cu_xFe₂O₄ (x = 0–0.30) were prepared by traditional solid reaction-sintering method. As expected, the doped Cu²⁺ ions could effectively reduce the sintering temperature and increase the density of the ferrites. With increasing doping amount of Cu²⁺, the coercivity, saturation magnetization, and Curie temperature of the resultant ferrites decreased. With x = 0.15, the sample presented the maximum initial magnetic permeability and highest Q-factor. On the basis of the measured magnetic parameters, the ferrites were examined as magnetic sheet in wireless charging system by digital simulation, while the structural parameters of the system were considered and the coefficients of hysteresis loss, eddy current loss, and excess loss were quantitatively separated under sinusoidal excitation. Specifically, because of its lower core loss, the optimal ferrite (x = 0.15) would perform better with much improved transmission efficiency than that without the doping of Cu²⁺, especially when the transmitting and receiving coils were settled more apart, which might be a good candidate for wireless charging system, and the proposed simple wireless charging system can be used as a general strategy for the evaluation of magnetic sheets.