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.
Design, Fabrication, and Characterization of a Low-Temperature Curable Magnetic Composite for Power Electronics Integration
To simplify the integration process of embedding magnetic components in power electronics converters, we fabricated a magnetic-filled-benzocyclobutene composite that can be cured at temperatures below 250 °C without pressure. The magnetic fillers used in the formulation were a round-shaped particle of permalloy and a flake-shaped particle of Metglas 2705 M. To guide the formulation, we first constructed 3-D finite-element models of the composite consisting of periodic unit cells of magnetic particles and flakes in the polymer matrix and used Ansoft Maxwell to simulate magnetic properties of the composite. Then, flowable pastes of the composite with varying amounts of Metglas in the magnetic fillers up to 12.5 wt% were prepared, and toroid cores were poured and cured at 250 °C. Subsequently, magnetic properties of the cores, i.e., complex permeability and core loss density, were measured. We found that the real part of the composite’s relative permeability increased with Metglas addition, reaching a value of 26 at 12.5%. However, the core loss data at 1 and 5 MHz showed that the addition of Metglas flakes also increased the core loss density. The measured properties were consistent with the Maxwell simulation results. Microstructures of the cores were examined by scanning electron microscopy. We found that the magnetic particles were uniformly dispersed and isolated by the polymer, which explained the high electrical resistivities of the composite and relatively low core loss densities due to suppression of inter-particle eddy-current losses.
Interfacial diffusion between Ni–Zn–Cu ferrite and Ag during sintering
The interfacial diffusion between Ni–Zn–Cu ferrite and Ag during sintering in the temperature range of 850–950°C was investigated utilizing the scanning electron microscopy-energy–dispersive X-ray analysis technique. It was found that diffusion increased with an increase in the sintering temperature. Furthermore, some impurities in the ferrite powders, especially SiO2 and NH4Cl, promoted the interfacial diffusion. The interfacial diffusion of Ag can be explained by the liquid-phase formation at the grain boundary in the sintered ferrite. © 1998 chapman & Hall