Effects of Nb2O5 on DC-Bias-Superposition Characteristic of the Low-Temperature-Fired NiCuZn Ferrites

The influences of Nb2O5 addition on the microstructure and magnetic properties, especially DC-bias-superposition characteristic of the low-temperature-fired NiCuZn ferrites, were investigated. It was found that Nb ions entered into the lattice when Nb2O5 content was less than 0.3 wt%, after that, a second phase, bismuth iron ( zinc or nickel) niobium oxide appeared. The second phase consumed some sintering aid Bi2O3 and had obvious influences on sintered density, grain size and magnetic properties, such as permeability of the ferrites. All of the sintered density, saturation magnetic flux density and average grain size gradually decreased with Nb2O5 content. Initial permeability first increased, and then continuously decreased with Nb2O5 content. The sample with 0.1 wt% Nb2O5 displayed the highest initial permeability, which was mainly due to the decrease of magnetocrystalline anisotropy constant. Under relatively low superposition magnetic field H <= 375 A/m, a higher initial permeability favored to obtain a higher incremental permeability. However, when the superposition magnetic field continued to increase, some samples with a lower initial permeability displayed a higher incremental permeability. The normalized incremental permeability was mainly determined by the coercivity of the samples. A higher coercivity favored the attainment of a better DC-bias-superposition characteristic on normalized permeability. So the sample with 0.5 wt% Nb2O5, which displayed the largest coercivity, had the best performance. Possible mechanisms contributing to the above results were discussed in detail.