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.
A Simple Wireless Power Charging Antenna System: Evaluation of Ferrite Sheet
We report a simple wireless power charging (WPC) antenna system (50 mm <inline-formula> <tex-math notation="LaTeX">$\times40$ </tex-math></inline-formula> mm <inline-formula> <tex-math notation="LaTeX">$\times0.1$ </tex-math></inline-formula> mm) for use in simulating power transfer efficiency (<inline-formula> <tex-math notation="LaTeX">$\eta $ </tex-math></inline-formula>) with and without ferrite and a metallic object (battery case), thereby identifying suitable magnetic parameters and ferrite sheet thickness. The simulation results suggest that magnetic loss tangent (tan <inline-formula> <tex-math notation="LaTeX">$\delta _{\mu }$ </tex-math></inline-formula>) and permeability (<inline-formula> <tex-math notation="LaTeX">$\mu '$ </tex-math></inline-formula>) need to be less than 0.05 and higher than 125 at 13.56 MHz, respectively, to achieve at least 75% of the transfer efficiency (<inline-formula> <tex-math notation="LaTeX">$\eta _{\mathrm {max}}$ </tex-math></inline-formula>) of the WPC antenna without ferrite and the metallic object. Accordingly, we have fabricated Ni<sub><italic>x</italic></sub>Zn<sub>0.85–<italic>x</italic></sub>Cu<sub>0.15</sub>Fe<sub>2</sub>O<sub>4</sub> (x = 0.32 – 0.38) spinel ferrites using a one-step sintering process and obtained relatively high <inline-formula> <tex-math notation="LaTeX">$\mu '$ </tex-math></inline-formula> of 169 and low tan <inline-formula> <tex-math notation="LaTeX">$\delta _{\mu }$ </tex-math></inline-formula> of 0.1 with a nickel (Ni) concentration of 0.38. This magnetic loss is still too high to achieve 75% of the <inline-formula> <tex-math notation="LaTeX">$\eta _{\mathrm {max}}$ </tex-math></inline-formula> value. In order to further reduce the magnetic loss, a two-step sintering process was used. The two-step sintered Ni<sub>0.38</sub>Zn<sub>0.47</sub>Cu<sub>0.15</sub>Fe<sub>2</sub>O<sub>4</sub> shows <inline-formula> <tex-math notation="LaTeX">$\mu '$ </tex-math></inline-formula> of 132 and a tan <inline-formula> <tex-math notation="LaTeX">$\delta _{\mu }$ </tex-math></inline-formula> of 0.03 at 13.56 MHz, respectively. Therefore, this ferrite meets criteria identified by the transfer efficiency simulation and is a good candidate for 13.56-MHz wireless power transfer charging antenna system. In addition, our designed WPC system is capable of evaluating other magnetic materials that are suitable for WPC antenna applications.
Influence of V2O5 on the Magnetic Properties of Nickel-Zinc-Copper Ferrites
Nickel-zinc-copper ferrites are used for manufacturing multilayer chip inductors. They can also be used to realize integrated transformers but in this case, low core losses are also required. Their sintering temperature (around 900degC) is low but not enough to allow co-firing with conventional silver inks for making the internal conductor. Their sintering temperature can be lowered by using sintering aids like low melting point oxides: B<sub>2</sub>O<sub>3</sub>, Bi<sub>2</sub>O<sub>3</sub> or V<sub>2</sub>O<sub>5</sub>. Previous papers showed that V<sub>2</sub>O<sub>5</sub> was effective for lowering the sintering temperature but only for low permeability ferrites (mu < 30) and for low power applications. In this paper, we report the effect of V<sub>2</sub>O<sub>5</sub> upon the magnetic properties of some NiZnCu ferrites with permeability higher than 300 and with low core losses.