Influences of High Calcination Temperature on Densification and Magnetic Properties of Low-Temperature-Fired NiCuZn Ferrites
Traditional low-temperature-fired NiCuZn ferrites (900 °C sintered) were generally prepared at relatively low calcination temperatures to increase the activity of the calcined powders and promote sintering. However, in this study, we changed the calcination temperature from 700 °C to 1000 °C to investigate the influences of high calcination temperature on the densification characteristic and magnetic properties of the low-temperature-fired NiCuZn ferrites. It was found that although a high calcination temperature decreased the activity of the calcined powder, the NiCuZn ferrite could attain a higher sintered density with an adequate sintering aid. Furthermore, a uniform microstructure with small average grain size was obtained for the sample calcined at 1000 °C with 1 wt% Bi2O3 sintering aid. It also displayed high performances on Q-factor and permeability. Hence, choosing a high calcination temperature and adding more sintering aid was also an option for obtaining high performance and low-temperature-fired NiCuZn ferrites.
Grain growth, densification and magnetic properties of NiZn ferrites with Bi2O3 additive
The grain growth, densification and magnetic properties of Bi2O3 doped NiZn ferrites prepared by a solid-state reaction method were investigated. Addition of a moderate content of Bi2O3 additive, to NiZn ferrite, due to its liquid phase sintering enhanced the diffraction intensity while excessive content of Bi2O3 produced a thick liquid phase layer at the grain boundaries and retarded mass transfer during sintering. Thus the x-ray diffraction peaks exhibited reduced intensity. In samples with higher additive content, Bi2O3 segregated at the grain boundaries as a liquid phase layer. This phenomenon could be confirmed by backscattering electron image. The grain growth of Bi2O3 doped NiZn ferrites was discussed by using the liquid phase sintering mechanism. Bi2O3 promoted grain growth via liquid phase sintering, and the addition of the optimum content of Bi2O3 could enhance the density of the sample and reduce its porosity. The NiZn ferrite samples containing x = 0.20, 0.08 and 0.04 sintered at 1180 °C, 1220 °C and 1250 °C, respectively, exhibited high permeability (μi) and low core losses (PL) at 50 kHz and 150 mT. The losses here were composed mainly of hysteresis loss, both eddy current loss and residual loss were ignored. The addition of proper content of Bi2O3 can improve the saturation induction (Bs) and reduce the coercivity (Hc) by liquid phase sintering.
Women's choice in the gender and ethnicity of her obstetrician and gynaecologist
Objective: To ascertain women's preferences in the gender and ethnicity of her obstetrician or gynaecologist and to determine whether there were differences in preferences among different ethnic and age groups.